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Atmospheric Sounding from Fengyun-3C GPS Radio Occultation Observations: First Results and Validation

Atmospheric Sounding from Fengyun-3C GPS Radio Occultation Observations: First Results and... Hindawi Advances in Meteorology Volume 2019, Article ID 4780143, 13 pages https://doi.org/10.1155/2019/4780143 Research Article Atmospheric Sounding from Fengyun-3C GPS Radio Occultation Observations: First Results and Validation 1,2 2,3 2,4 Shuanggen Jin , Chao Gao , and Junhai Li School of Remote Sensing and Geomatics Engineering, Nanjing University of Information Science and Technology, Nanjing 210044, China Shanghai Astronomical Observatory, Chinese Academy of Sciences, Shanghai 200030, China University of Chinese Academy of Sciences, Beijing 100049, China Tianjin Richsoft Electric Power Information Technology Co., Ltd., State Grid, Tianjin 300000, China Correspondence should be addressed to Shuanggen Jin; sgjin@nuist.edu.cn Received 18 April 2019; Revised 11 June 2019; Accepted 3 July 2019; Published 4 August 2019 Academic Editor: Herminia Garc´ıa Mozo Copyright © 2019 Shuanggen Jin et al. ,is is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Carrying global positioning system (GPS) radio occultation (RO) receiver, Chinese meteorological satellite Fengyun-3C (FY-3C) was launched on September 23, 2013, which provides new observation data for observations and studies of weather and climate change. In this paper, the results of FY-3C GPS RO atmospheric sounding are presented for the first time, including high-order ionospheric correction, atmospheric parameters estimation, and evaluation by COSMIC and radiosonde observations as well as applications in estimating gravity wave activities. It is found that the effect of the ionospheric correction residual on the phase delay is below 20 mm, which has minimal impact on bending angle estimation and generates differences of about 1 K in the average temperature profile. ,e difference between FY-3C and COSMIC temperatures at all heights is within 1 C, and the tropopause temperature and height have a good consistency. Deviations from Radiosonde measurements are within 2 C, and the tropopause temperature and height results also have a strong consistency. Furthermore, global gravity wave potential energy is estimated from FY-3C GPS RO, exhibiting similar behavior to results derived from COSMIC radio occultation measurements. ,e mean value of the gravity wave potential energy near the equator is 10 J/kg and decreases toward the two poles while in the northern hemisphere, it is stronger than that in the southern hemisphere. Payload) launched in July 2000 provided GNSS occultation 1. Introduction data as well as Argentinian satellite SAC-C (Satellite de GPS radio occultation measurements can provide abundant Aplicaciones Cientificas-C) and US/German satellite GRACE atmospheric parameters like the atmospheric refractive index, (Gravity Recovery and Climate Experiment), and the de- pressure, and temperature, which can be used to study the viation of the inverted temperature profiles is less than 1 K atmospheric structure, variations, and dynamics of the Earth. [3, 4]. In addition, Hajj and Romans pointed out that GPS/ MET can be used for analysis of ionospheric E and F layers’ In 1995, the low-orbit satellite Microlab1 with GPS receiver was launched, and its measurement results showed that the electron density variations [5]. Jakowski et al. obtained the Earth’s atmospheric parameters could be inverted by re- electron density profile results of the ionosphere from ceiving GPS radio occultation signals, and the accuracy of the CHAMP’s radio occultation data [6]. In 2006, the United retrieving temperature is about 1 K [1, 2]. Subsequently, a States and Taiwan, China, jointly developed the COSMIC variety of low-orbit satellites carrying Global Navigation (Constellation Observing System for Meteorology Ionosphere Satellite System (GNSS) radio occultation receivers have been and Climate) mission with six small satellites, which can widely used for meteorology and atmospheric studies. For provide more than 2,500 occultation events per day. ,is example, the satellite CHAMP (CHAllenging Minisatellite greatly enhances the geographic coverage of the Earth’s 2 Advances in Meteorology atmosphere, which also provides rich data for the ionospheric the pressure, the neutral atmospheric temperature and and tropospheric applications [7–14]. pressure can be calculated. ,e most commonly used method to obtain the occultation bending angle profile On September 23, 2013, Chinese meteorological satellite FY-3C with a track altitude of 836 km and an orbital in- through LEO occultation Doppler shift or additional phase clination of 98.75 was launched successfully. FY-3C is the data is geometrical optics or wave optical method. second generation of China’s polar-orbit meteorological ,e relationship between atmospheric Doppler shift Δf satellites and belongs to the near-polar solar synchronous and the motions of GNSS satellite and LEO satellite can be orbit. It is equipped with a variety of payloads, such as the described by the following formula [1]: space environment detector, the solar radiation detector, the ⇀ ⇀ ⇀ ⇀ ⇀ ⇀ ⇀ (1) Earth radiation detector, the microwave radiometer, the λΔf � v · k − v · k − 􏼐v − v 􏼑 · k, t t r r t r microwave thermometer, the infrared spectrometer, and ⇀ ⇀ GNSS radio occultation sounder. ,erefore, this new FY-3C where λ is the signal wavelength; v and v are the velocity t r satellite will further improve the spatial and temporal res- vectors of GNSS satellite and LEO satellite, respectively; and ⇀ ⇀ olution of the GNSS radio occultation dataset and provide k, k and k are the unit vector of GNSS satellite to LEO t r abundant data for studies of the ionosphere, troposphere, satellite, the unit vector of GNSS satellite signal broad- and climate change. In this paper, the first atmospheric casting, and the unit vector of LEO satellite signal received, results are presented from FY-3C GPS RO data, including respectively. high-order ionospheric effects, atmospheric temperature As can be seen from Figure 2, if the occultation event is and pressure, and evaluation with COSMIC and radiosonde limited in the occultation plane, the bending angle α is the observations as well as applications in estimating gravity sum of the angles δ and δ . ,en, we assume that the re- t r wave potential energy. fractive indexes near the tangent point are the same value and follow the spherical symmetry, δ and δ also obey the t r 2. Data Processing and Methods shell rule [18], namely, 2.1. FY-3C GPS RO Data and Radiosonde Data. ,e radio a � n r􏼁 r sin θ + δ􏼁 � n r 􏼁 r sin θ + δ 􏼁 , (2) t t t t r r r r occultation receiver GNOS (GNSS occultation sounder) carried on FY-3C is the first satellite occultation receiver of where a is the impact parameter; n(r ) and n(r ) are the t r China. It can simultaneously receive the navigation signals refractive indexes of LEO satellite and GNSS satellite, re- of both GPS and BeiDou satellites for occultation obser- spectively; r and r are the orbital radius of LEO satellite and t r vations. ,e GNOS occultation receiver is equipped with the GNSS satellite, respectively; θ is the angle between the unit latest open-loop technology, which can reach 100 Hz in vector of GNSS-LEO line of sign and GNSS satellite orbit neutral atmospheric occultation detection, and the tracked vector; and θ is the angle between the unit vector of GNSS- signal can extend down to 1 to 2 km above the surface LEO line of sign and LEO satellite orbit vector. [15, 16]. FY-3C can provide GPS occultation events about Under the assumption that the refractive index exhibits 500 times per day. ,e GPS occultation data were released in stable stratification near the tangent point, we can get the August 2014, which can be obtained from the Fengyun relation between the bending angle α and the refractive meteorological satellites’ official website (http://satellite. index [1]: nsmc.org.cn), including the occurring time and duration 1 d ln n of occultation events as well as dual-frequency atmospheric √������ α(a) � −2a 􏽚 dx, (3) 2 2 x − a dx phase data. Detailed precise orbits for the FY-3C spacecraft and corresponding attitude data are described in Li et al. where n is the refractive index, and x is the integral variable [17]. of the impact parameter. ,rough the Abel transformation, ,e radiosonde data are from the IGRA (Integrated we can obtain the relation between the refractive index n and Global Radiosonde Archive) project of NOAA (National the bending angle. In the neutral atmosphere, the refractive Oceanic and Atmospheric Administration). ,e project index is a function of the atmospheric temperature, pressure, provided more than 2,700 radiosonde stations around the and water vapor. So by putting certain restrictions and world from 1905 to present. ,e data contain the location of conditions on the refractive index, we can obtain the neutral stations, the observation time, the sounding level above atmospheric parameters. Nowadays, there are several dif- stations, the temperature, the atmospheric pressure, etc. ferent RO processing systems [19], and here, we used the Figure 1 shows FY-3C occultation events and co-located Radio Occultation Processing Package (ROPP). radiosonde stations from September 21th to 28th, 2014. 2.3. High-Order Correction Algorithm. When the GNSS 2.2. GPS Occultation Data Processing. Firstly, the occultation signal propagates in the ionosphere and the neutral atmo- bending angle profile parameters are obtained by Doppler sphere, the phase delay caused by the atmospheric refraction shift or additional phase method. Secondly, the bending can be expressed as angle profile is transformed into the refractive index profile by the Abel transform. Using the relationship between the ΔL � L− ρ � 􏽚 n + n 􏼁 dl− ρ, (4) neu ion atmospheric refractive index and the temperature as well as Advances in Meteorology 3 80°N 40°N 0° 40°S 80°S 180°W 120°W 60°W 0° 60°E 120°E 180°E FY-3C Radiosonde Figure 1: FY-3C occultation events and co-located radiosonde stations from September 21th to 28th, 2014. the electron mass, ε is the dielectric constant, e is the α electronic charge, and θ is the angle between the spread t k direction of the signal and the direction of the geo- φ v t r δ magnetism. Simplifying equation (5), we get the following: δ r LEO GNSS θ k t 1 1 1 1 n � 1− X− XY cos θ− X X + Y (1 + cos θ) . 􏼔 􏼕 ion 2 2 4 2 t (7) ,e ionospheric delay can be further expressed as [21] q s r Figure 2: Schematic diagram for inversion of atmospheric bending I � 􏽚 n dl � − − − . (8) ϕ ion 2 3 4 angle using geometrical optics. f 2f 3f ,e quantities in equation (8) are calculated as follows: where ΔL is the phase delay, L is the phase measurement, ρ is C q � 􏽚 N dl, the geometric distance between GNSS satellite and LEO satellite, n and n are the refractive indexes in the neutral neu ion atmosphere and the ionosphere, respectively. And the s � C C 􏽚 N B cos θ dl, x y e 0 ionospheric refractive index n is associated with the ion electron density and the magnetic field intensity of the GNSS r � r + r + r , signal propagation path, which can be described by 1 2 3 Appleton–Hartree formula [20]: 3C r � 􏽚 N dl, 2X(1− X) 1 􏽱�������������� n � 1− , ion (5) 2 4 2 2(1− X)− Y ± Y + 4Y (1− X) t t l (9) 3C C x y 2 2 r � 􏽚 N B cos θ dl, 2 e e N X � , 4π ε m f 2 3C C x y e B r � 􏽚 N B dl, 3 e 0 0 Y � , 4 (6) 2πm f Y � Y sin θ, C � , 4π ε m Y � Y cos θ, where B is the magnetic field intensity, N is the electron 0 e C � . 2πm density, f is the frequency of the electromagnetic wave, m is 4 Advances in Meteorology ,e second term on the right side of equation (8) is the events, we mainly account for the second-order and the second-order ionospheric delay, and the third term is the third-order ionospheric delays, and the ionospheric cor- third-order ionospheric delay. Assessing and correcting the rection residuals. We evaluated all the second-order iono- effect of the ionospheric correction residual on an iono- spheric delays of FY-3C satellite neutral atmospheric spheric or atmospheric occultation involves solving the occultation events in September 2014. In the vicinity of the second-order and third-order ionospheric delays (the surface, the second-order delays are within ±15 mm. As the fourth-order term is very small and is therefore negligible). heights increase, the second-order delays also increase. At To estimate the high-order ionospheric delay of the altitudes above 90 km, the second-order ionospheric delays GNSS signal, the path information between the signal’s are about ±20 mm, and the average of the second-order emission position and the received position is needed. Born ionospheric delays at all heights is about ±7 mm. ,e sec- and Wolf proposed a signal propagation model in the ond-order ionospheric delays at 15 km, 45 km, and 90 km nonvacuum space—the optical path formula [18]: are shown in Figure 3. At the height of 15 km, almost all of → → → the second-order delays are less than 15 mm; at the height of d dr d r dn dr (10) 􏼠n 􏼡 � n + � ∇n, 45 km, there are less than 5% of the second-order delays ds ds ds ds ds greater than 15 mm; and when the height rises to 90 km, the ratio of the second-order delays over 15 mm is about 10%. where r is the Cartesian coordinate of the signal path, s is ,e residual effect of ionospheric delay after using the the length of the signal path, n is the refractive index, and ∇n dual-frequency correction is called the ionospheric correc- is the gradient component of the refractive index in the tion residual, and the statistic histograms are shown in Cartesian coordinate system. ,en, we can get Figure 4. At the height of 15 km, 98% of the residuals are less → → d r 1 dn dr than ±10 mm, at the height of 45 km, there are 95% of the � 􏼠∇n− 􏼡, (11) ds n ds ds residuals less than ±10 mm, but the ratio of the second-order ionospheric correction residuals within ±10 mm is less than where r contains three components (x, y, z), and if the 70% at the height of 90 km, which indicates that the effect of refractive index of each point and the gradient of the re- the ionospheric correction residual on the upper atmosphere fractive index ∇n are known, we can use the fourth-order is more pronounced. Since the top-to-bottom method is Runge–Kutta numerical integration method to numerically always used when we invert the refraction rate and the integrate each component and find the specific coordinates temperature, the ionospheric correction residual of the of each point in the signal path. upper atmosphere will affect the accuracy of the lower at- By the refractive index formula, the refractive index can mosphere. ,erefore, we usually limit the maximum height be obtained according to the meteorological elements of of inversion to weaken the effect of the upper atmosphere in each point in the signal path. In order to carry out the ray the data processing. tracing, we choose the neutral atmospheric model MSIS90 to In addition, the third-order ionospheric delays in the obtain neutral atmospheric parameters such as the atmo- measurement of neutral atmospheric occultation are also spheric pressure and temperature. ,e MSIS90 model in- estimated, and the values are very small. ,e maximum of tegrates the rocket mass spectrometer, the ground which is 0.5 mm and the average is only about 0.15 mm, noncoherent scattering radar, and satellite data to fit the which is only 1/40 to 1/30 of the second-order ionospheric global temperature and material density model at different delay. ,erefore, the second-order ionospheric delay is the locations, altitudes, and times [22]. ,e NeQuick2 model is main source of the ionospheric correction residual. used to obtain the ionospheric electron density information We further estimated the effect of the ionospheric under different solar activity cycles [23]. ,e NeQuick2 is an correction residual on the bending angle and temperature of ionospheric model developed by ICTP (International Center the neutral atmosphere. Here, the second-order ionospheric for ,eoretical Physics). In addition, the magnetic field delay is directly eliminated from the original observation, strength and three-dimensional components of arbitrary and then the inversion is performed according to the ion- point on the global are used from IGRF12 (International ospheric-free linear combination of both GPS frequencies to Geomagnetic Reference Field, 12th generation), which uses obtain the bending angle, the refractive index, and the the mass data of various observation methods to model the temperature of the atmosphere, which are then compared global geomagnetic data [24]. After the pressure, tempera- with the parameters obtained from the original data without ture, electron density, geomagnetic intensity, and other eliminating the high-order ionospheric delay. parameters are obtained through the models, we can use In order to verify the feasibility of correcting the high- numerical integration to obtain the GNSS signal path order ionospheric term by directly subtracting the second- according to equation (10) and then obtain the values of the order ionospheric delay from the original observation data, second-order and the third-order ionospheric delays from we compared the results from colocated occultation and the GNSS signal path according to equation (8). radiosonde measurements for 9 different radiosonde sta- tions (Figure 5). It was found that most temperature profiles obtained through the correction of high-order ionospheric 3. Results and Discussion delays are closer to the results of radiosonde, and the 3.1. Effect of High-Order Ionospheric Delays. In the estima- temperature measured by radiosonde is not affected by the tion of the ionospheric correction residual on occultation ionosphere. ,erefore, it is feasible to use this method to Advances in Meteorology 5 25 25 20 20 15 15 10 10 5 5 0 0 –20 –10 0 10 20 –20 –10 0 10 20 –20 –10 0 10 20 Second-order ionospheric Second-order ionospheric Second-order ionospheric delay (mm) delay (mm) delay (mm) (a) (b) (c) Figure 3: ,e statistics histograms of the second-order ionospheric delays at 15 km (a), 45 km (b), and 90 km (c). 40 40 40 30 30 30 20 20 20 10 10 10 0 0 0 –20 –10 0 10 20 –20 –10 0 10 20 –20 –10 0 10 20 Second-order residual Second-order residual Second-order residual ionospheric error (mm) ionospheric error (mm) ionospheric error (mm) (a) (b) (c) Figure 4: ,e statistic histograms of the second-order ionospheric correction residuals at 15 km (a), 45 km (b), and 90 km (c). effectively eliminate the effect of the high-order ionospheric From the statistics of the difference and the standard delays. deviation of the temperature profiles at the height of 10 km, Furthermore, 7,144 occultation events were estimated 15 km, 20 km, 25 km, 30 km, and 35 km (Figure 7), we find with and without eliminating the high-order ionospheric that the average differences between the FY-3C and COS- delays, which were divided into three groups according to MIC temperature profiles are within 1 C, and the standard the maximum residuals of temperature profiles between 2 K, deviation is about 2 to 3 C. In the refractive index curve, we 4 K, and 6 K. ,ere are 6,508 events located within 2 K, can also see the consistency of the refractive index obtained accounting for 91.1% of the total sample, 375 residuals of the by FY-3C and COSMIC occultation data. temperature are between 2 K and 4 K, accounting for 5.2%, In addition, we obtained and compared the tropopause and the residuals of the temperature between 4 K and 6 K are height and tropopause temperature from COSMIC and FY- 261 times, accounting for 3.7% of the estimated total sample. 3C occultation data, respectively (Figure 8). It was found And the average temperature residual of ionospheric cor- that there is a strong correlation of the tropopause height rection at all altitudes is about 1 K. between FY-3C data and COSMIC data, and the correlation coefficient is 0.94, indicating that they have similar capa- bility to retrieve the tropopause height. ,e tropopause 3.2. Comparison with COSMIC and Radiosonde temperatures from FY-3C data and COSMIC data also have a very strong correlation, with a correlation coefficient of 3.2.1. Comparison between COSMIC and FY-3C Data. In 0.96. order to verify the validity of FY-3C occultation data, we obtained COSMIC occultation data and looked for the overlapping occultation events from COSMIC and FY-3C 3.2.2. Comparison between Radiosonde and FY-3C Data. ° ° within 3 in latitude, 3 in longitude, and 3 hours. ,ere are In order to verify the accuracy of FY-3C occultation data, the totally 255 overlapping events from September 21th to 28th, independent radiosonde data are further used. We chose 2014. ,e profiles of four overlapping occultation events are radiosonde data from the IGRA program for the experiment, shown in Figure 6. ,e inversion results of the temperature and the criteria for co-located station observations is also set ° ° profiles from FY-3C occultation data are similar to those as 3 in latitude, 3 in longitude, and 3 hours. Figure 9 shows from COSMIC occultation data, but the temperature profiles the temperature profiles of four repeated observations. We of FY-3C are slightly smoother than those of COSMIC. find that the temperature profiles are very similar in the Samples (%) Samples (%) Samples (%) Samples (%) Samples (%) Samples (%) 6 Advances in Meteorology 50 50 50 40 40 40 30 30 30 20 20 20 –65 –54 –43 –32 –21 –10 –60 –51 –42 –33 –24 –15 –70 –54 –38 –22 –6 10 Temperature (°C) Temperature (°C) Temperature (°C) Radiosonde Radiosonde Radiosonde Non-calib Fengyun RO Non-calib Fengyun RO Non-calib Fengyun RO Calibrated Fengyun RO Calibrated Fengyun RO Calibrated Fengyun RO (a) (b) (c) 50 50 50 40 40 40 30 30 30 20 20 20 –85 –71 –57 –43 –29 –15 –60 –46 –32 –18 –4 –10 –65 –49 –33 –17 –1 15 Temperature (°C) Temperature (°C) Temperature (°C) Radiosonde Radiosonde Radiosonde Non-calib Fengyun RO Non-calib Fengyun RO Non-calib Fengyun RO Calibrated Fengyun RO Calibrated Fengyun RO Calibrated Fengyun RO (d) (e) (f) 50 50 50 40 40 40 30 30 30 20 20 20 –55 –42 –29 –16 –3 10 –75 –64 –53 –42 –31 –20 –75 –64 –53 –42 –31 –20 Temperature (°C) Temperature (°C) Temperature (°C) Radiosonde Radiosonde Radiosonde Non-calib Fengyun RO Non-calib Fengyun RO Non-calib Fengyun RO Calibrated Fengyun RO Calibrated Fengyun RO Calibrated Fengyun RO (g) (h) (i) Figure 5: Effect of the ionospheric correction residual on the neutral atmospheric temperature estimation at 9 radiosonde stations. region below 30 km, which indicates that FY-3C occultation In addition, we further compared the tropopause height data have a strong reliability. and tropopause temperature obtained from radiosonde and From the statistics of the difference and the standard FY-3C (Figure 11). ,e correlation coefficient of the tro- deviation of the temperature profiles at the heights of 10 km, popause height is 0.93, and the correlation coefficient of the 15 km, 20 km, 25 km, and 30 km (Figure 10), we find that the tropopause temperature is 0.95. It indicates that the tro- difference of the temperature between FY-3C and radio- popause height and temperature estimated from FY-3C are ° ° sonde is within 2 C, and the standard deviation is about 3 C. equivalent to that from radiosonde, which is consistent with ,e difference of the refractive index calculated by radio- the conclusion of Liao et al. [25]. sonde and FY-3C is very small with a relative error of less than 0.5%, and the standard deviation is less than 1.4%, 3.3. Gravity Wave Potential Energy from FY-3C GPS Radio which indicates that FY-3C occultation data have the same accuracy as radiosonde data in terms of statistics. Occultation. Considering the volume and spatial and Height (km) Height (km) Height (km) Height (km) Height (km) Height (km) Height (km) Height (km) Height (km) Advances in Meteorology 7 FY/20140926/0302/(–74.8, 33.6) FY/20140926/0442/(–48.0, –51.6) CM/20140926/0232/(–76.6, 34.8) CM/20140926/0357/(–50.2, –50.7) 100 60 0 0 –100 –50 0 50 –80 –60 –40 –20 0 20 Temperature (°C) Temperature (°C) FY-3C FY-3C COSMIC COSMIC (a) (b) FY/20140926/0426/(–43.5, –103.5) FY/20140923/0114/(22.6, 153.2) CM/20140926/0414/(–43.3, –101.6) CM/20140923/0240/(23.2, 153.5) 100 100 80 80 60 60 40 40 20 20 0 0 –100 –80 –60 –40 –20 0 –100 –80 –60 –40 –20 0 Temperature (°C) Temperature (°C) FY-3C FY-3C COSMIC COSMIC (c) (d) Figure 6: Comparison of the temperature profiles obtained from FY-3C and COSMIC occultation data. 35 35 30 30 25 25 20 20 15 15 10 10 –1 0123 –1 012 Temperature (°C) Relative refraction index (%) Diff Diff RMS RMS (a) (b) Figure 7: Statistics of temperature profiles and the relative refraction index obtained from FY-3C and COSMIC occultation data. Height (km) Height (km) Height (km) Height (km) Height (km) Height (km) 8 Advances in Meteorology Tropopause height Tropopause temperature corr coef = 0.94 corr coef = 0.96 –50 –60 –70 –80 5 –90 52 10 15 205 30 –90 –80 –70 –60 –50 COSMIC (km) COSMIC (°C) (a) (b) Figure 8: Comparison of the tropopause height (a) and temperature (b) obtained from FY-3C and COSMIC occultation data. FY/20140920/0154/(34.7, 118.7) FY/20140920/0256/(–50.7, –60.4) RS/20140920/0104/(36.7, 117.6) RS/20140920/0158/(–51.8, –58.5) 100 100 80 80 60 60 40 40 20 20 0 0 –100 –50 0 50 –100 –50 0 50 Temperature (°C) Temperature (°C) FY-3C FY-3C Radiosonde Radiosonde (a) (b) FY/20140923/0114/(22.6, 153.2) FY/20140925/1040/(43.9, 9.7) RS/20140923/0238/(24.3, 153.9) RS/20140925/0853/(45.4, 9.3) 100 100 80 80 60 60 40 40 20 20 0 0 –100 –50 0 50 –100 –50 0 50 Temperature (°C) Temperature (°C) FY-3C FY-3C Radiosonde Radiosonde (c) (d) Figure 9: Comparison of the temperature profiles obtained from FY-3C GPS radio occultation data and radiosonde data. FY-3C (km) Height (km) Height (km) FY-3C (°C) MSL altitude (km) Height (km) Advances in Meteorology 9 30 30 25 25 20 20 15 15 10 10 –2 0 24 –0.5 00.5 11.5 Temperature (°C) Relative refraction index (%) Diff Diff RMS RMS (a) (b) Figure 10: Statistics of the temperature profiles and the relative refraction index obtained from FY-3C occultation data and radiosonde data. Tropopause height Tropopause temperature corr coef = 0.93 corr coef = 0.95 30 –50 –60 –70 –80 5 –90 5 10 15 20 25 30 –90 –80 –70 –60 –50 Radiosonde (km) Radiosonde (°C) (a) (b) Figure 11: Comparison of the tropopause height (a) and temperature (b) obtained from FY-3C occultation data and radiosonde data. temporal distribution of FY-3C occultation data, the Earth is interception frequency of 0.1–0.5 ×10 MHz for bandpass meshed into a 90 × 90 ×1 grid with a spatial resolution of 2 filtering, and finally, the inverse transformation is performed in latitude, 4 in longitude, and a temporal resolution of to get the background temperature field. 1 month. ,e temperature profile is interpolated with a ,e atmospheric gravity wave is produced by tiny air resolution of 200 m, and the average is taken as the value of mass through reciprocating vibration under the influence the grid node. of gravity and buoyancy. We call this oscillation frequency First, we performed S-transform on each height of every buoyant frequency (Brumt–Vaisala frequency): latitude circle and extracted the components whose wave- number are of 0–6 to eliminate the interferences of some g zT g N � 􏼠 + 􏼡, (12) zonal disturbances, like the planetary wave [26, 27]. ,e T zz c method to separate disturbance temperature field from the background temperature field is to conduct continuous wavelet transform and then use the Butterworth filter with the where N is the buoyant frequency, g is the gravitational FY-3C (km) Height (km ) FY-3C (°C) Height (km ) 10 Advances in Meteorology 60°N 30°N 30°S 60°S 180°W 120°W 60°W 0° 60°E 120°E 180°W Longitude Figure 12: Global distribution of gravity wave potential energy from FY-3C occultation data at the height of 20–30 km in northern hemisphere winter from 2015 to 2016. acceleration, T is the background temperature field, and c is Furthermore, the gravity wave potential energy in the ° ° the heat capacity at constant pressure. ,e gravity wave eastern hemisphere (0 E–180 E) is more obvious than that potential energy can be described as in the western hemisphere, which may be due to the greater land area of the Eurasia in the eastern hemisphere. 2 2 At the same time, we can also find high values of gravity 1 g T (13) E (z) � 􏼠 􏼡 􏼠 􏼡 , p wave potential energy at the western side of the Rocky 2 N T Mountains, the northern side of the Himalayas in Eurasia in northern hemisphere summer, and on the west of the where E is the gravity wave potential energy and T is the Andes in northern hemisphere winter. To sum up, the gravity wave disturbance temperature. As for the processing gravity wave at the height of 15–20 km is basically excited mode of T , we adopted the method of Tsuda et al. and take by the terrain, and there are extreme values of the gravity 2 km as the window and 200 m as the step size to calculate wave potential energy at the topographic shear. the smoothing values in the window [28]: Terrain is one of the main factors that excite gravity wave potential energy [30]. Figure 14 shows the zonal 2 1 2 distribution of gravity wave potential energy at the lati- ′ ′ T � T(z) dz. 􏽚 (14) ° ° z − z z tude of 0 S–30 S in northern hemisphere summer. ,ere 2 1 1 are high values of the gravity wave potential energy below ° ° ° ° 25 km at the longitude of 120 W, 90 W, 50 W, and 10 E. Figure 12 shows the global distributions of gravity wave Examining terrain heights suggests that the high gravity potential energy obtained from FY-3C occultation data at wave potential energy values at longitude 120 W are likely the height of 20–30 km in northern hemisphere winter from caused by the topography of islands in the ocean, which 2015 to 2016. ,e white patches in the map are missing data confirms the conclusion of Alexander et al. [30]. ,e high due to the insufficient number of FY-3C occultation events. values at about 90 W are due to the Andes of South ,e results show that the average of the gravity wave po- America on the east, the high values at 50 W are due to the tential energy near the equator is the maximum with a value continental terrain of the South American continent, and of 10 J/kg due to strong convection near the equator and the high values near 10 E are due to the presence of the decreases toward the two poles, while the potential energy in African continent on the east. ,erefore, most gravity the northern hemisphere is stronger than that in the waves are excited by the landform and then spread up to southern hemisphere. Also, the gravity wave potential en- the height of about 25 km. ergy over the Eurasian continent is relatively higher. ,ese features are consistent with the excitation mechanism of gravity wave, such as the terrain, wind shear, and so on. 4. Summary Moreover, it is also consistent with other results [26, 27]. In In this paper, we analyzed the influence of high-order addition, the filtering methods or the density of the grid also ionospheric delay on atmospheric occultation and pro- affects the accuracy of the gravity wave parameter estimation posed a corresponding correction algorithm to eliminate [29]. the effect. ,e effect of the ionospheric correction residual Figure 13 shows the seasonal distributions of global on the phase delay is less than 20 mm, which has little gravity wave potential energy at the height of 15–20 km. effect on bending angle and about 1 K on average tem- High values of the gravity wave potential energy are basically perature profile. By comparing with COSMIC occultation distributed near the equator and it decreases toward the two data and radiosonde data, the difference of the temper- poles, which may be related to the convection near the ature between FY-3C occultation data and COSMIC equator. Latitude J/kg Advances in Meteorology 11 15 15 80°N 80°N 10 10 40°N 40°N 0° 0° 40°S 5 40°S 5 80°S 80°S 0 0 160°W 80°W 0° 80°E 160°E 160°W 80°W 0° 80°E 160°E Longitude Longitude (a) (b) 15 15 80°N 80°N 10 10 40°N 40°N 0° 0° 40°S 5 40°S 5 80°S 80°S 0 0 160°W 80°W 0° 80°E 160°E 160°W 80°W 0° 80°E 160°E Longitude Longitude (c) (d) Figure 13: Global distributions of gravity wave potential energy at the height of 15–20 km in northern hemisphere. Spring (a), summer (b), autumn (c), and winter (d). 30 3 120°W 90°W 60°W 30°W 0° 30°E Longitude (a) Andes Mount. 5 Africa 120°W 90°W 60°W 30°W 0° 30°E Longitude (b) ° ° Figure 14: Zonal distribution of gravity wave potential energy at the latitude of 0 S–30 S in summer of the northern hemisphere (a) and maximum terrain height (b). occultation data is within 1 C, and the difference between 3C occultation data, which are consistent with those FY-3C occultation data and the radiosonde data is within obtained by COSMIC occultation data. ,e high values of 2 C. ,is proves that the data from Chinese meteoro- gravity wave potential energy are basically distributed logical satellite FY-3C are highly reliable and can be used near the equator and decrease toward the two poles. Most for in-depth study of atmospheric science. Finally, the of the gravity waves below 25 km are mainly excited by the global gravity wave potential energy is obtained from FY- terrain. In the future, with further development and Latitude Latitude Height (km) Max orographic height (km) J/kg J/kg Latitude Latitude J/kg J/kg J/kg 12 Advances in Meteorology [7] S. Syndergaard, “On the ionosphere calibration in GPS radio improvement of more BeiDou constellations and GNOS occultation measurements,” Radio Science, vol. 35, no. 3, RO receiver, novel BeiDou RO observations and appli- pp. 865–883, 2000. cations are expected in atmospheric sounding [31]. [8] S. V. Sokolovskiy, C. Rocken, D. H. Lenschow et al., “Ob- serving the moist troposphere with radio occultation signals Data Availability from COSMIC,” Geophysical Research Letters, vol. 34, no. 18, pp. 266–278, 2007. ,e data used to support the findings of this study are [9] B. S. Potula, Y. Chu, G. Uma, H. Hsia, and K. Wu, “A global available from the corresponding author upon request. comparative study on the ionospheric measurements between COSMIC radio occultation technique and IRI model,” Journal Conflicts of Interest of Geophysical Research: Space Physics, vol. 116, no. A2, pp. 710–717, 2011. ,e authors declare that they have no conflicts of interest. [10] T. Tsuda, X. Lin, H. Hayashi, and Noersomadi, “Analysis of vertical wave number spectrum of atmospheric gravity waves in the stratosphere using COSMIC GPS radio occultation Authors’ Contributions data,” Atmospheric Measurement Techniques, vol. 4, no. 8, pp. 1627–1636, 2011. Shuanggen Jin, Junhai Li, and Chao Gao performed nu- [11] U. Foelsche, B. Pirscher, M. Borsche, G. Kirchengast, and merical studies and prepared a draft of manuscript. J. Wickert, “Assessing the climate monitoring utility of radio Shuanggen Jin and Chao Gao coordinated this research and occultation data: from CHAMP to FORMOSAT-3/COSMIC,” compiled the final form of manuscript. Terrestrial, Atmospheric and Oceanic Sciences, vol. 20, no. 1, pp. 155–170, 2009. [12] S. Jin and A. Komjathy, “GNSS reflectometry and remote Acknowledgments sensing: new objectives and results,” Advances in Space Re- ,is research was funded by the National Natural Science search, vol. 46, no. 2, pp. 111–117, 2010. [13] S. Jin, G. P. Feng, and S. Gleason, “Remote sensing using Foundation of China-German Science Foundation (NSFC- GNSS signals: current status and future directions,” Advances DFG) Project under contract #41761134092, Jiangsu Prov- in Space Research, vol. 47, no. 10, pp. 1645–1653, 2011. ince Distinguished Professor Project under contract [14] X. Zhao, S. Jin, C. Mekik, and J. 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Atmospheric Sounding from Fengyun-3C GPS Radio Occultation Observations: First Results and Validation

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Hindawi Advances in Meteorology Volume 2019, Article ID 4780143, 13 pages https://doi.org/10.1155/2019/4780143 Research Article Atmospheric Sounding from Fengyun-3C GPS Radio Occultation Observations: First Results and Validation 1,2 2,3 2,4 Shuanggen Jin , Chao Gao , and Junhai Li School of Remote Sensing and Geomatics Engineering, Nanjing University of Information Science and Technology, Nanjing 210044, China Shanghai Astronomical Observatory, Chinese Academy of Sciences, Shanghai 200030, China University of Chinese Academy of Sciences, Beijing 100049, China Tianjin Richsoft Electric Power Information Technology Co., Ltd., State Grid, Tianjin 300000, China Correspondence should be addressed to Shuanggen Jin; sgjin@nuist.edu.cn Received 18 April 2019; Revised 11 June 2019; Accepted 3 July 2019; Published 4 August 2019 Academic Editor: Herminia Garc´ıa Mozo Copyright © 2019 Shuanggen Jin et al. ,is is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Carrying global positioning system (GPS) radio occultation (RO) receiver, Chinese meteorological satellite Fengyun-3C (FY-3C) was launched on September 23, 2013, which provides new observation data for observations and studies of weather and climate change. In this paper, the results of FY-3C GPS RO atmospheric sounding are presented for the first time, including high-order ionospheric correction, atmospheric parameters estimation, and evaluation by COSMIC and radiosonde observations as well as applications in estimating gravity wave activities. It is found that the effect of the ionospheric correction residual on the phase delay is below 20 mm, which has minimal impact on bending angle estimation and generates differences of about 1 K in the average temperature profile. ,e difference between FY-3C and COSMIC temperatures at all heights is within 1 C, and the tropopause temperature and height have a good consistency. Deviations from Radiosonde measurements are within 2 C, and the tropopause temperature and height results also have a strong consistency. Furthermore, global gravity wave potential energy is estimated from FY-3C GPS RO, exhibiting similar behavior to results derived from COSMIC radio occultation measurements. ,e mean value of the gravity wave potential energy near the equator is 10 J/kg and decreases toward the two poles while in the northern hemisphere, it is stronger than that in the southern hemisphere. Payload) launched in July 2000 provided GNSS occultation 1. Introduction data as well as Argentinian satellite SAC-C (Satellite de GPS radio occultation measurements can provide abundant Aplicaciones Cientificas-C) and US/German satellite GRACE atmospheric parameters like the atmospheric refractive index, (Gravity Recovery and Climate Experiment), and the de- pressure, and temperature, which can be used to study the viation of the inverted temperature profiles is less than 1 K atmospheric structure, variations, and dynamics of the Earth. [3, 4]. In addition, Hajj and Romans pointed out that GPS/ MET can be used for analysis of ionospheric E and F layers’ In 1995, the low-orbit satellite Microlab1 with GPS receiver was launched, and its measurement results showed that the electron density variations [5]. Jakowski et al. obtained the Earth’s atmospheric parameters could be inverted by re- electron density profile results of the ionosphere from ceiving GPS radio occultation signals, and the accuracy of the CHAMP’s radio occultation data [6]. In 2006, the United retrieving temperature is about 1 K [1, 2]. Subsequently, a States and Taiwan, China, jointly developed the COSMIC variety of low-orbit satellites carrying Global Navigation (Constellation Observing System for Meteorology Ionosphere Satellite System (GNSS) radio occultation receivers have been and Climate) mission with six small satellites, which can widely used for meteorology and atmospheric studies. For provide more than 2,500 occultation events per day. ,is example, the satellite CHAMP (CHAllenging Minisatellite greatly enhances the geographic coverage of the Earth’s 2 Advances in Meteorology atmosphere, which also provides rich data for the ionospheric the pressure, the neutral atmospheric temperature and and tropospheric applications [7–14]. pressure can be calculated. ,e most commonly used method to obtain the occultation bending angle profile On September 23, 2013, Chinese meteorological satellite FY-3C with a track altitude of 836 km and an orbital in- through LEO occultation Doppler shift or additional phase clination of 98.75 was launched successfully. FY-3C is the data is geometrical optics or wave optical method. second generation of China’s polar-orbit meteorological ,e relationship between atmospheric Doppler shift Δf satellites and belongs to the near-polar solar synchronous and the motions of GNSS satellite and LEO satellite can be orbit. It is equipped with a variety of payloads, such as the described by the following formula [1]: space environment detector, the solar radiation detector, the ⇀ ⇀ ⇀ ⇀ ⇀ ⇀ ⇀ (1) Earth radiation detector, the microwave radiometer, the λΔf � v · k − v · k − 􏼐v − v 􏼑 · k, t t r r t r microwave thermometer, the infrared spectrometer, and ⇀ ⇀ GNSS radio occultation sounder. ,erefore, this new FY-3C where λ is the signal wavelength; v and v are the velocity t r satellite will further improve the spatial and temporal res- vectors of GNSS satellite and LEO satellite, respectively; and ⇀ ⇀ olution of the GNSS radio occultation dataset and provide k, k and k are the unit vector of GNSS satellite to LEO t r abundant data for studies of the ionosphere, troposphere, satellite, the unit vector of GNSS satellite signal broad- and climate change. In this paper, the first atmospheric casting, and the unit vector of LEO satellite signal received, results are presented from FY-3C GPS RO data, including respectively. high-order ionospheric effects, atmospheric temperature As can be seen from Figure 2, if the occultation event is and pressure, and evaluation with COSMIC and radiosonde limited in the occultation plane, the bending angle α is the observations as well as applications in estimating gravity sum of the angles δ and δ . ,en, we assume that the re- t r wave potential energy. fractive indexes near the tangent point are the same value and follow the spherical symmetry, δ and δ also obey the t r 2. Data Processing and Methods shell rule [18], namely, 2.1. FY-3C GPS RO Data and Radiosonde Data. ,e radio a � n r􏼁 r sin θ + δ􏼁 � n r 􏼁 r sin θ + δ 􏼁 , (2) t t t t r r r r occultation receiver GNOS (GNSS occultation sounder) carried on FY-3C is the first satellite occultation receiver of where a is the impact parameter; n(r ) and n(r ) are the t r China. It can simultaneously receive the navigation signals refractive indexes of LEO satellite and GNSS satellite, re- of both GPS and BeiDou satellites for occultation obser- spectively; r and r are the orbital radius of LEO satellite and t r vations. ,e GNOS occultation receiver is equipped with the GNSS satellite, respectively; θ is the angle between the unit latest open-loop technology, which can reach 100 Hz in vector of GNSS-LEO line of sign and GNSS satellite orbit neutral atmospheric occultation detection, and the tracked vector; and θ is the angle between the unit vector of GNSS- signal can extend down to 1 to 2 km above the surface LEO line of sign and LEO satellite orbit vector. [15, 16]. FY-3C can provide GPS occultation events about Under the assumption that the refractive index exhibits 500 times per day. ,e GPS occultation data were released in stable stratification near the tangent point, we can get the August 2014, which can be obtained from the Fengyun relation between the bending angle α and the refractive meteorological satellites’ official website (http://satellite. index [1]: nsmc.org.cn), including the occurring time and duration 1 d ln n of occultation events as well as dual-frequency atmospheric √������ α(a) � −2a 􏽚 dx, (3) 2 2 x − a dx phase data. Detailed precise orbits for the FY-3C spacecraft and corresponding attitude data are described in Li et al. where n is the refractive index, and x is the integral variable [17]. of the impact parameter. ,rough the Abel transformation, ,e radiosonde data are from the IGRA (Integrated we can obtain the relation between the refractive index n and Global Radiosonde Archive) project of NOAA (National the bending angle. In the neutral atmosphere, the refractive Oceanic and Atmospheric Administration). ,e project index is a function of the atmospheric temperature, pressure, provided more than 2,700 radiosonde stations around the and water vapor. So by putting certain restrictions and world from 1905 to present. ,e data contain the location of conditions on the refractive index, we can obtain the neutral stations, the observation time, the sounding level above atmospheric parameters. Nowadays, there are several dif- stations, the temperature, the atmospheric pressure, etc. ferent RO processing systems [19], and here, we used the Figure 1 shows FY-3C occultation events and co-located Radio Occultation Processing Package (ROPP). radiosonde stations from September 21th to 28th, 2014. 2.3. High-Order Correction Algorithm. When the GNSS 2.2. GPS Occultation Data Processing. Firstly, the occultation signal propagates in the ionosphere and the neutral atmo- bending angle profile parameters are obtained by Doppler sphere, the phase delay caused by the atmospheric refraction shift or additional phase method. Secondly, the bending can be expressed as angle profile is transformed into the refractive index profile by the Abel transform. Using the relationship between the ΔL � L− ρ � 􏽚 n + n 􏼁 dl− ρ, (4) neu ion atmospheric refractive index and the temperature as well as Advances in Meteorology 3 80°N 40°N 0° 40°S 80°S 180°W 120°W 60°W 0° 60°E 120°E 180°E FY-3C Radiosonde Figure 1: FY-3C occultation events and co-located radiosonde stations from September 21th to 28th, 2014. the electron mass, ε is the dielectric constant, e is the α electronic charge, and θ is the angle between the spread t k direction of the signal and the direction of the geo- φ v t r δ magnetism. Simplifying equation (5), we get the following: δ r LEO GNSS θ k t 1 1 1 1 n � 1− X− XY cos θ− X X + Y (1 + cos θ) . 􏼔 􏼕 ion 2 2 4 2 t (7) ,e ionospheric delay can be further expressed as [21] q s r Figure 2: Schematic diagram for inversion of atmospheric bending I � 􏽚 n dl � − − − . (8) ϕ ion 2 3 4 angle using geometrical optics. f 2f 3f ,e quantities in equation (8) are calculated as follows: where ΔL is the phase delay, L is the phase measurement, ρ is C q � 􏽚 N dl, the geometric distance between GNSS satellite and LEO satellite, n and n are the refractive indexes in the neutral neu ion atmosphere and the ionosphere, respectively. And the s � C C 􏽚 N B cos θ dl, x y e 0 ionospheric refractive index n is associated with the ion electron density and the magnetic field intensity of the GNSS r � r + r + r , signal propagation path, which can be described by 1 2 3 Appleton–Hartree formula [20]: 3C r � 􏽚 N dl, 2X(1− X) 1 􏽱�������������� n � 1− , ion (5) 2 4 2 2(1− X)− Y ± Y + 4Y (1− X) t t l (9) 3C C x y 2 2 r � 􏽚 N B cos θ dl, 2 e e N X � , 4π ε m f 2 3C C x y e B r � 􏽚 N B dl, 3 e 0 0 Y � , 4 (6) 2πm f Y � Y sin θ, C � , 4π ε m Y � Y cos θ, where B is the magnetic field intensity, N is the electron 0 e C � . 2πm density, f is the frequency of the electromagnetic wave, m is 4 Advances in Meteorology ,e second term on the right side of equation (8) is the events, we mainly account for the second-order and the second-order ionospheric delay, and the third term is the third-order ionospheric delays, and the ionospheric cor- third-order ionospheric delay. Assessing and correcting the rection residuals. We evaluated all the second-order iono- effect of the ionospheric correction residual on an iono- spheric delays of FY-3C satellite neutral atmospheric spheric or atmospheric occultation involves solving the occultation events in September 2014. In the vicinity of the second-order and third-order ionospheric delays (the surface, the second-order delays are within ±15 mm. As the fourth-order term is very small and is therefore negligible). heights increase, the second-order delays also increase. At To estimate the high-order ionospheric delay of the altitudes above 90 km, the second-order ionospheric delays GNSS signal, the path information between the signal’s are about ±20 mm, and the average of the second-order emission position and the received position is needed. Born ionospheric delays at all heights is about ±7 mm. ,e sec- and Wolf proposed a signal propagation model in the ond-order ionospheric delays at 15 km, 45 km, and 90 km nonvacuum space—the optical path formula [18]: are shown in Figure 3. At the height of 15 km, almost all of → → → the second-order delays are less than 15 mm; at the height of d dr d r dn dr (10) 􏼠n 􏼡 � n + � ∇n, 45 km, there are less than 5% of the second-order delays ds ds ds ds ds greater than 15 mm; and when the height rises to 90 km, the ratio of the second-order delays over 15 mm is about 10%. where r is the Cartesian coordinate of the signal path, s is ,e residual effect of ionospheric delay after using the the length of the signal path, n is the refractive index, and ∇n dual-frequency correction is called the ionospheric correc- is the gradient component of the refractive index in the tion residual, and the statistic histograms are shown in Cartesian coordinate system. ,en, we can get Figure 4. At the height of 15 km, 98% of the residuals are less → → d r 1 dn dr than ±10 mm, at the height of 45 km, there are 95% of the � 􏼠∇n− 􏼡, (11) ds n ds ds residuals less than ±10 mm, but the ratio of the second-order ionospheric correction residuals within ±10 mm is less than where r contains three components (x, y, z), and if the 70% at the height of 90 km, which indicates that the effect of refractive index of each point and the gradient of the re- the ionospheric correction residual on the upper atmosphere fractive index ∇n are known, we can use the fourth-order is more pronounced. Since the top-to-bottom method is Runge–Kutta numerical integration method to numerically always used when we invert the refraction rate and the integrate each component and find the specific coordinates temperature, the ionospheric correction residual of the of each point in the signal path. upper atmosphere will affect the accuracy of the lower at- By the refractive index formula, the refractive index can mosphere. ,erefore, we usually limit the maximum height be obtained according to the meteorological elements of of inversion to weaken the effect of the upper atmosphere in each point in the signal path. In order to carry out the ray the data processing. tracing, we choose the neutral atmospheric model MSIS90 to In addition, the third-order ionospheric delays in the obtain neutral atmospheric parameters such as the atmo- measurement of neutral atmospheric occultation are also spheric pressure and temperature. ,e MSIS90 model in- estimated, and the values are very small. ,e maximum of tegrates the rocket mass spectrometer, the ground which is 0.5 mm and the average is only about 0.15 mm, noncoherent scattering radar, and satellite data to fit the which is only 1/40 to 1/30 of the second-order ionospheric global temperature and material density model at different delay. ,erefore, the second-order ionospheric delay is the locations, altitudes, and times [22]. ,e NeQuick2 model is main source of the ionospheric correction residual. used to obtain the ionospheric electron density information We further estimated the effect of the ionospheric under different solar activity cycles [23]. ,e NeQuick2 is an correction residual on the bending angle and temperature of ionospheric model developed by ICTP (International Center the neutral atmosphere. Here, the second-order ionospheric for ,eoretical Physics). In addition, the magnetic field delay is directly eliminated from the original observation, strength and three-dimensional components of arbitrary and then the inversion is performed according to the ion- point on the global are used from IGRF12 (International ospheric-free linear combination of both GPS frequencies to Geomagnetic Reference Field, 12th generation), which uses obtain the bending angle, the refractive index, and the the mass data of various observation methods to model the temperature of the atmosphere, which are then compared global geomagnetic data [24]. After the pressure, tempera- with the parameters obtained from the original data without ture, electron density, geomagnetic intensity, and other eliminating the high-order ionospheric delay. parameters are obtained through the models, we can use In order to verify the feasibility of correcting the high- numerical integration to obtain the GNSS signal path order ionospheric term by directly subtracting the second- according to equation (10) and then obtain the values of the order ionospheric delay from the original observation data, second-order and the third-order ionospheric delays from we compared the results from colocated occultation and the GNSS signal path according to equation (8). radiosonde measurements for 9 different radiosonde sta- tions (Figure 5). It was found that most temperature profiles obtained through the correction of high-order ionospheric 3. Results and Discussion delays are closer to the results of radiosonde, and the 3.1. Effect of High-Order Ionospheric Delays. In the estima- temperature measured by radiosonde is not affected by the tion of the ionospheric correction residual on occultation ionosphere. ,erefore, it is feasible to use this method to Advances in Meteorology 5 25 25 20 20 15 15 10 10 5 5 0 0 –20 –10 0 10 20 –20 –10 0 10 20 –20 –10 0 10 20 Second-order ionospheric Second-order ionospheric Second-order ionospheric delay (mm) delay (mm) delay (mm) (a) (b) (c) Figure 3: ,e statistics histograms of the second-order ionospheric delays at 15 km (a), 45 km (b), and 90 km (c). 40 40 40 30 30 30 20 20 20 10 10 10 0 0 0 –20 –10 0 10 20 –20 –10 0 10 20 –20 –10 0 10 20 Second-order residual Second-order residual Second-order residual ionospheric error (mm) ionospheric error (mm) ionospheric error (mm) (a) (b) (c) Figure 4: ,e statistic histograms of the second-order ionospheric correction residuals at 15 km (a), 45 km (b), and 90 km (c). effectively eliminate the effect of the high-order ionospheric From the statistics of the difference and the standard delays. deviation of the temperature profiles at the height of 10 km, Furthermore, 7,144 occultation events were estimated 15 km, 20 km, 25 km, 30 km, and 35 km (Figure 7), we find with and without eliminating the high-order ionospheric that the average differences between the FY-3C and COS- delays, which were divided into three groups according to MIC temperature profiles are within 1 C, and the standard the maximum residuals of temperature profiles between 2 K, deviation is about 2 to 3 C. In the refractive index curve, we 4 K, and 6 K. ,ere are 6,508 events located within 2 K, can also see the consistency of the refractive index obtained accounting for 91.1% of the total sample, 375 residuals of the by FY-3C and COSMIC occultation data. temperature are between 2 K and 4 K, accounting for 5.2%, In addition, we obtained and compared the tropopause and the residuals of the temperature between 4 K and 6 K are height and tropopause temperature from COSMIC and FY- 261 times, accounting for 3.7% of the estimated total sample. 3C occultation data, respectively (Figure 8). It was found And the average temperature residual of ionospheric cor- that there is a strong correlation of the tropopause height rection at all altitudes is about 1 K. between FY-3C data and COSMIC data, and the correlation coefficient is 0.94, indicating that they have similar capa- bility to retrieve the tropopause height. ,e tropopause 3.2. Comparison with COSMIC and Radiosonde temperatures from FY-3C data and COSMIC data also have a very strong correlation, with a correlation coefficient of 3.2.1. Comparison between COSMIC and FY-3C Data. In 0.96. order to verify the validity of FY-3C occultation data, we obtained COSMIC occultation data and looked for the overlapping occultation events from COSMIC and FY-3C 3.2.2. Comparison between Radiosonde and FY-3C Data. ° ° within 3 in latitude, 3 in longitude, and 3 hours. ,ere are In order to verify the accuracy of FY-3C occultation data, the totally 255 overlapping events from September 21th to 28th, independent radiosonde data are further used. We chose 2014. ,e profiles of four overlapping occultation events are radiosonde data from the IGRA program for the experiment, shown in Figure 6. ,e inversion results of the temperature and the criteria for co-located station observations is also set ° ° profiles from FY-3C occultation data are similar to those as 3 in latitude, 3 in longitude, and 3 hours. Figure 9 shows from COSMIC occultation data, but the temperature profiles the temperature profiles of four repeated observations. We of FY-3C are slightly smoother than those of COSMIC. find that the temperature profiles are very similar in the Samples (%) Samples (%) Samples (%) Samples (%) Samples (%) Samples (%) 6 Advances in Meteorology 50 50 50 40 40 40 30 30 30 20 20 20 –65 –54 –43 –32 –21 –10 –60 –51 –42 –33 –24 –15 –70 –54 –38 –22 –6 10 Temperature (°C) Temperature (°C) Temperature (°C) Radiosonde Radiosonde Radiosonde Non-calib Fengyun RO Non-calib Fengyun RO Non-calib Fengyun RO Calibrated Fengyun RO Calibrated Fengyun RO Calibrated Fengyun RO (a) (b) (c) 50 50 50 40 40 40 30 30 30 20 20 20 –85 –71 –57 –43 –29 –15 –60 –46 –32 –18 –4 –10 –65 –49 –33 –17 –1 15 Temperature (°C) Temperature (°C) Temperature (°C) Radiosonde Radiosonde Radiosonde Non-calib Fengyun RO Non-calib Fengyun RO Non-calib Fengyun RO Calibrated Fengyun RO Calibrated Fengyun RO Calibrated Fengyun RO (d) (e) (f) 50 50 50 40 40 40 30 30 30 20 20 20 –55 –42 –29 –16 –3 10 –75 –64 –53 –42 –31 –20 –75 –64 –53 –42 –31 –20 Temperature (°C) Temperature (°C) Temperature (°C) Radiosonde Radiosonde Radiosonde Non-calib Fengyun RO Non-calib Fengyun RO Non-calib Fengyun RO Calibrated Fengyun RO Calibrated Fengyun RO Calibrated Fengyun RO (g) (h) (i) Figure 5: Effect of the ionospheric correction residual on the neutral atmospheric temperature estimation at 9 radiosonde stations. region below 30 km, which indicates that FY-3C occultation In addition, we further compared the tropopause height data have a strong reliability. and tropopause temperature obtained from radiosonde and From the statistics of the difference and the standard FY-3C (Figure 11). ,e correlation coefficient of the tro- deviation of the temperature profiles at the heights of 10 km, popause height is 0.93, and the correlation coefficient of the 15 km, 20 km, 25 km, and 30 km (Figure 10), we find that the tropopause temperature is 0.95. It indicates that the tro- difference of the temperature between FY-3C and radio- popause height and temperature estimated from FY-3C are ° ° sonde is within 2 C, and the standard deviation is about 3 C. equivalent to that from radiosonde, which is consistent with ,e difference of the refractive index calculated by radio- the conclusion of Liao et al. [25]. sonde and FY-3C is very small with a relative error of less than 0.5%, and the standard deviation is less than 1.4%, 3.3. Gravity Wave Potential Energy from FY-3C GPS Radio which indicates that FY-3C occultation data have the same accuracy as radiosonde data in terms of statistics. Occultation. Considering the volume and spatial and Height (km) Height (km) Height (km) Height (km) Height (km) Height (km) Height (km) Height (km) Height (km) Advances in Meteorology 7 FY/20140926/0302/(–74.8, 33.6) FY/20140926/0442/(–48.0, –51.6) CM/20140926/0232/(–76.6, 34.8) CM/20140926/0357/(–50.2, –50.7) 100 60 0 0 –100 –50 0 50 –80 –60 –40 –20 0 20 Temperature (°C) Temperature (°C) FY-3C FY-3C COSMIC COSMIC (a) (b) FY/20140926/0426/(–43.5, –103.5) FY/20140923/0114/(22.6, 153.2) CM/20140926/0414/(–43.3, –101.6) CM/20140923/0240/(23.2, 153.5) 100 100 80 80 60 60 40 40 20 20 0 0 –100 –80 –60 –40 –20 0 –100 –80 –60 –40 –20 0 Temperature (°C) Temperature (°C) FY-3C FY-3C COSMIC COSMIC (c) (d) Figure 6: Comparison of the temperature profiles obtained from FY-3C and COSMIC occultation data. 35 35 30 30 25 25 20 20 15 15 10 10 –1 0123 –1 012 Temperature (°C) Relative refraction index (%) Diff Diff RMS RMS (a) (b) Figure 7: Statistics of temperature profiles and the relative refraction index obtained from FY-3C and COSMIC occultation data. Height (km) Height (km) Height (km) Height (km) Height (km) Height (km) 8 Advances in Meteorology Tropopause height Tropopause temperature corr coef = 0.94 corr coef = 0.96 –50 –60 –70 –80 5 –90 52 10 15 205 30 –90 –80 –70 –60 –50 COSMIC (km) COSMIC (°C) (a) (b) Figure 8: Comparison of the tropopause height (a) and temperature (b) obtained from FY-3C and COSMIC occultation data. FY/20140920/0154/(34.7, 118.7) FY/20140920/0256/(–50.7, –60.4) RS/20140920/0104/(36.7, 117.6) RS/20140920/0158/(–51.8, –58.5) 100 100 80 80 60 60 40 40 20 20 0 0 –100 –50 0 50 –100 –50 0 50 Temperature (°C) Temperature (°C) FY-3C FY-3C Radiosonde Radiosonde (a) (b) FY/20140923/0114/(22.6, 153.2) FY/20140925/1040/(43.9, 9.7) RS/20140923/0238/(24.3, 153.9) RS/20140925/0853/(45.4, 9.3) 100 100 80 80 60 60 40 40 20 20 0 0 –100 –50 0 50 –100 –50 0 50 Temperature (°C) Temperature (°C) FY-3C FY-3C Radiosonde Radiosonde (c) (d) Figure 9: Comparison of the temperature profiles obtained from FY-3C GPS radio occultation data and radiosonde data. FY-3C (km) Height (km) Height (km) FY-3C (°C) MSL altitude (km) Height (km) Advances in Meteorology 9 30 30 25 25 20 20 15 15 10 10 –2 0 24 –0.5 00.5 11.5 Temperature (°C) Relative refraction index (%) Diff Diff RMS RMS (a) (b) Figure 10: Statistics of the temperature profiles and the relative refraction index obtained from FY-3C occultation data and radiosonde data. Tropopause height Tropopause temperature corr coef = 0.93 corr coef = 0.95 30 –50 –60 –70 –80 5 –90 5 10 15 20 25 30 –90 –80 –70 –60 –50 Radiosonde (km) Radiosonde (°C) (a) (b) Figure 11: Comparison of the tropopause height (a) and temperature (b) obtained from FY-3C occultation data and radiosonde data. temporal distribution of FY-3C occultation data, the Earth is interception frequency of 0.1–0.5 ×10 MHz for bandpass meshed into a 90 × 90 ×1 grid with a spatial resolution of 2 filtering, and finally, the inverse transformation is performed in latitude, 4 in longitude, and a temporal resolution of to get the background temperature field. 1 month. ,e temperature profile is interpolated with a ,e atmospheric gravity wave is produced by tiny air resolution of 200 m, and the average is taken as the value of mass through reciprocating vibration under the influence the grid node. of gravity and buoyancy. We call this oscillation frequency First, we performed S-transform on each height of every buoyant frequency (Brumt–Vaisala frequency): latitude circle and extracted the components whose wave- number are of 0–6 to eliminate the interferences of some g zT g N � 􏼠 + 􏼡, (12) zonal disturbances, like the planetary wave [26, 27]. ,e T zz c method to separate disturbance temperature field from the background temperature field is to conduct continuous wavelet transform and then use the Butterworth filter with the where N is the buoyant frequency, g is the gravitational FY-3C (km) Height (km ) FY-3C (°C) Height (km ) 10 Advances in Meteorology 60°N 30°N 30°S 60°S 180°W 120°W 60°W 0° 60°E 120°E 180°W Longitude Figure 12: Global distribution of gravity wave potential energy from FY-3C occultation data at the height of 20–30 km in northern hemisphere winter from 2015 to 2016. acceleration, T is the background temperature field, and c is Furthermore, the gravity wave potential energy in the ° ° the heat capacity at constant pressure. ,e gravity wave eastern hemisphere (0 E–180 E) is more obvious than that potential energy can be described as in the western hemisphere, which may be due to the greater land area of the Eurasia in the eastern hemisphere. 2 2 At the same time, we can also find high values of gravity 1 g T (13) E (z) � 􏼠 􏼡 􏼠 􏼡 , p wave potential energy at the western side of the Rocky 2 N T Mountains, the northern side of the Himalayas in Eurasia in northern hemisphere summer, and on the west of the where E is the gravity wave potential energy and T is the Andes in northern hemisphere winter. To sum up, the gravity wave disturbance temperature. As for the processing gravity wave at the height of 15–20 km is basically excited mode of T , we adopted the method of Tsuda et al. and take by the terrain, and there are extreme values of the gravity 2 km as the window and 200 m as the step size to calculate wave potential energy at the topographic shear. the smoothing values in the window [28]: Terrain is one of the main factors that excite gravity wave potential energy [30]. Figure 14 shows the zonal 2 1 2 distribution of gravity wave potential energy at the lati- ′ ′ T � T(z) dz. 􏽚 (14) ° ° z − z z tude of 0 S–30 S in northern hemisphere summer. ,ere 2 1 1 are high values of the gravity wave potential energy below ° ° ° ° 25 km at the longitude of 120 W, 90 W, 50 W, and 10 E. Figure 12 shows the global distributions of gravity wave Examining terrain heights suggests that the high gravity potential energy obtained from FY-3C occultation data at wave potential energy values at longitude 120 W are likely the height of 20–30 km in northern hemisphere winter from caused by the topography of islands in the ocean, which 2015 to 2016. ,e white patches in the map are missing data confirms the conclusion of Alexander et al. [30]. ,e high due to the insufficient number of FY-3C occultation events. values at about 90 W are due to the Andes of South ,e results show that the average of the gravity wave po- America on the east, the high values at 50 W are due to the tential energy near the equator is the maximum with a value continental terrain of the South American continent, and of 10 J/kg due to strong convection near the equator and the high values near 10 E are due to the presence of the decreases toward the two poles, while the potential energy in African continent on the east. ,erefore, most gravity the northern hemisphere is stronger than that in the waves are excited by the landform and then spread up to southern hemisphere. Also, the gravity wave potential en- the height of about 25 km. ergy over the Eurasian continent is relatively higher. ,ese features are consistent with the excitation mechanism of gravity wave, such as the terrain, wind shear, and so on. 4. Summary Moreover, it is also consistent with other results [26, 27]. In In this paper, we analyzed the influence of high-order addition, the filtering methods or the density of the grid also ionospheric delay on atmospheric occultation and pro- affects the accuracy of the gravity wave parameter estimation posed a corresponding correction algorithm to eliminate [29]. the effect. ,e effect of the ionospheric correction residual Figure 13 shows the seasonal distributions of global on the phase delay is less than 20 mm, which has little gravity wave potential energy at the height of 15–20 km. effect on bending angle and about 1 K on average tem- High values of the gravity wave potential energy are basically perature profile. By comparing with COSMIC occultation distributed near the equator and it decreases toward the two data and radiosonde data, the difference of the temper- poles, which may be related to the convection near the ature between FY-3C occultation data and COSMIC equator. Latitude J/kg Advances in Meteorology 11 15 15 80°N 80°N 10 10 40°N 40°N 0° 0° 40°S 5 40°S 5 80°S 80°S 0 0 160°W 80°W 0° 80°E 160°E 160°W 80°W 0° 80°E 160°E Longitude Longitude (a) (b) 15 15 80°N 80°N 10 10 40°N 40°N 0° 0° 40°S 5 40°S 5 80°S 80°S 0 0 160°W 80°W 0° 80°E 160°E 160°W 80°W 0° 80°E 160°E Longitude Longitude (c) (d) Figure 13: Global distributions of gravity wave potential energy at the height of 15–20 km in northern hemisphere. Spring (a), summer (b), autumn (c), and winter (d). 30 3 120°W 90°W 60°W 30°W 0° 30°E Longitude (a) Andes Mount. 5 Africa 120°W 90°W 60°W 30°W 0° 30°E Longitude (b) ° ° Figure 14: Zonal distribution of gravity wave potential energy at the latitude of 0 S–30 S in summer of the northern hemisphere (a) and maximum terrain height (b). occultation data is within 1 C, and the difference between 3C occultation data, which are consistent with those FY-3C occultation data and the radiosonde data is within obtained by COSMIC occultation data. ,e high values of 2 C. ,is proves that the data from Chinese meteoro- gravity wave potential energy are basically distributed logical satellite FY-3C are highly reliable and can be used near the equator and decrease toward the two poles. Most for in-depth study of atmospheric science. Finally, the of the gravity waves below 25 km are mainly excited by the global gravity wave potential energy is obtained from FY- terrain. 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