GPS radio occultation with CHAMP: an innovative remote sensing method of the atmosphere

Advances in Space Research 33 (2004) 1036–1040 www.elsevier.com/locate/asr

GPS radio occultation with CHAMP: an innovative remote sensing method of the atmosphere T. Schmidt a

a,*

, J. Wickert a, C. Marquardt b, G. Beyerle a, C. Reigber a, R. Galas a, €nig a R. Ko

GeoForschungsZentrum Potsdam (GFZ), Department 1, Telegrafenberg A17, D-14473 Potsdam, Germany b The Meteorological Office, London Road, Bracknell, Berkshire, RG12 2SZ, UK

Abstract The availability of GPS (Global Positioning System) radio signals has introduced a new promising remote sensing technique for the EarthÕs atmosphere. GPS-based radio occultation exploits GPS signals received onboard a Low Earth Orbiting (LEO) satellite for atmospheric limb sounding. Temperature and water vapour profiles with high accuracy and vertical resolution can be derived from these measurements. The GPS radio occultation technique requires no calibration, is not affected by clouds, aerosols or precipitation, and the occultations are almost uniformly distributed over the globe. The radio occultation experiment aboard the German CHAMP (CHAllenging Minisatellite Payload) satellite provided its first measurements in February 2001. Since then up to 250 daily occultation measurements were performed. About 70% of these were successfully processed to yield atmospheric temperature and water vapour profiles. The CHAMP radio occultation experiment demonstrated the immense potential of this technique for the global monitoring of stratospheric temperature, the tropopause region and the global humidity distribution in the troposphere. All of these are important variables of the climate system. In this paper the GPS radio occultation experiment onboard CHAMP will be described. At GeoForschungsZentrum (GFZ) Potsdam an operational system was established to process CHAMP radio occultation data, orbit data, and GPS ground station observations in an automatic and operational way to derive profiles of atmospheric parameters. The CHAMP temperature bias compared to ECMWF analyses is less than 1.5 K between the tropopause layer and the 30 hPa level, and less than 0.5 K between 12 and 20 km at mid and high latitudes. Further comparisons between CHAMP atmospheric profiles and radiosonde data as well as applications of CHAMP radio occultation data for atmospheric research and climate monitoring are given. Ó 2003 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: GPS; Radio occultation; CHAMP; UTLS; Tropopause

1. Introduction GPS radio occultation offers a valuable new data source for global and continuous monitoring of the EarthÕs atmosphere. Due to the unique combination of high accuracy, global coverage, high vertical resolution, long-term stability and weather independent capability the technique has a wide spectrum of applications in climate monitoring, weather forecast and atmospheric research (Melbourne et al., 1994; Kursinski et al., 1997; Anthes et al., 2000; Hajj et al., 2002). Data assimilation studies using radio occultation data have already *

Corresponding author. Tel.: +49-331-288-1743; fax: +49-331-2881732. E-mail address: [email protected] (T. Schmidt).

shown that this new type of remote sensing improves the accuracy of global and regional weather analyses and predictions (Kuo et al., 2000). The proof-of-concept GPS radio occultation experiment GPS/MET (GPS/Meteorology) performed between 1995 and 1997 has demonstrated for the first time the potential of GPS-based limb sounding from LEO satellites for deriving atmospheric temperature and water vapour profiles (Ware et al., 1996; Kursinski et al., 1997). Further missions with radio occultation experiments followed (Oersted, Sunsat, SAC-C, and CHAMP), but the German geoscience satellite CHAMP (Reigber et al., 2000, 2002a) is the only one measuring continuously atmospheric profiles in an operational manner since mid-2001.

0273-1177/$30 Ó 2003 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/S0273-1177(03)00591-X

T. Schmidt et al. / Advances in Space Research 33 (2004) 1036–1040

CHAMP was launched on July 15, 2000 from Plesetsk (Russia) into an almost circular (eccentricity ¼ 0.004) and near polar (inclination ¼ 87.2°) orbit with an initial altitude of 454 km. The GPS radio occultation experiment was successfully started on February 11, 2001 (Wickert et al., 2001b).

2. Data processing A detailed description for deriving vertical atmospheric profiles from CHAMP occultation measurements is presented in Wickert et al. (2001a,b). A Black-Jack GPS receiver (provided by Jet Propulsion Laboratory, JPL) onboard CHAMP records phase and amplitude variations with high temporal resolution (50 Hz) during an occultation event (see Fig. 1). By using high precision orbit information provided by GFZ from CHAMP and occulting GPS satellites (K€ onig et al., 2002) the atmospheric excess path delay can be extracted which is related to a bending angle profile. A double differencing method is applied to remove clock errors.

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For this reason a GPS ground station network is operating jointly by JPL and GFZ. Ionospheric effects are corrected by a linear combination of the bending angles derived from the two GPS frequencies. Refractivity profiles are calculated by inverting the bending angle profiles using an Abel inversion. Finally, with the assumption of dry air the hydrostatic equation is used to calculate pressure and temperature profiles by downward integration of the refractivity profile. By using additional temperature information from operational analyses (ECMWF) water vapour can be calculated in the troposphere. Advanced methods such as onedimensional variational retrieval (Healy and Eyre, 2000) for water vapour determination in the troposphere are subject of investigation for CHAMP data (results not yet available). It could also been shown that the single differencing method leads to results comparable to the once determined by the double differencing method (Wickert et al., 2002a). The complete retrieval algorithms are embedded in an automatic and operational working system feeding CHAMP radio occultation products into the Information System and Data Centre (ISDC) at GFZ for user access. Until October 2002 about 60,000 temperature and water vapour profiles were delivered to ISDC. An additional downlink station for CHAMP data in NyAalesund (Spitsbergen, 79°N) and providing of ultra rapid CHAMP orbits (Reigber et al., 2002b) makes atmospheric CHAMP products available with a latency of 3–6 h necessary to use the data for testing of assimilation in numerical weather analysis and forecast models.

3. Validation of CHAMP radio occultation data

Fig. 1. GPS radio occultation principle using double differencing technique (from Wickert et al., 2001a).

Fig. 2 shows the zonal averaged differences between CHAMP dry temperature and ECMWF analyses in the upper troposphere and lower stratosphere for the period

Fig. 2. Temperature bias between CHAMP and ECMWF data based on about 55,000 occultations.

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Fig. 3. Temperature bias between CHAMP and nearby radiosonde data (<300 km and <3 h), the bottom formulas represent the averaged bias and rms deviation, respectively.

May 2001 and September 2002. The analysis data were obtained by interpolating 6-h ECMWF datasets to the location of the occultation for the 60 ECMWF pressure levels. The zonal statistic was calculated for 5° steps. The averaged bias in the upper troposphere and lower stratosphere (250–30 hPa) is less than 1.5 K outside the tropics with rms deviations between 1.5 and 2.0 K (not shown here). The increase of the tropical bias and rms deviation is related to gravity waves which are resolved by CHAMP data but not by ECMWF analyses (Marquardt et al., 2002). The negative temperature bias in the middle and lower troposphere (below 300 hPa) is caused by ignoring humidity. The warm bias above 30 hPa is related to the bending angle smoothing and the interpolation procedure for the downward integration of the refractivity profile. By using ECMWF data instead of the used MSISE-90 climatology as a priori data this bias disappears (Wickert et al., 2002b). Fig. 3 shows statistical comparisons between CHAMP and radiosonde data for selected standard pressure levels confirming the good agreement in the upper troposphere and lower stratosphere. Refractivity comparisons between CHAMP and ECMWF data show a negative bias below 500 hPa (not shown here) in the order of 5% at mid latitudes and up to 10% in the tropics. This bias was already recognized in the GPS/MET data (Rocken et al., 1997) and is caused by multi-path effects due to inhomogenities in the humidity fields (Beyerle et al., 2002). Apart from temperature measurements water vapour profiles can be determined in the troposphere. Due to the ambiguity with respect to the dry and wet part in the refractivity additional temperature information from meteorological analysis or forecasts is necessary. For the CHAMP data the water vapour is calculated downward from 1.5 km below the tropopause by using 6-h ECMWF temperature analysis. From ECMWF data the

temperature information at the occultation point is extracted. As shown by Marquardt et al. (2002) the averaged relative deviations between CHAMP specific humidity and radiosondes are in the range of 30–60%. A one-dimensional variational retrieval (Healy and Eyre, 2000) for the analysis of CHAMP data will be brought in the beginning of 2004. As meteorological background data the 6-h ECMWF analyses are serving. First tests are in progress but results are not yet available.

4. Application of CHAMP radio occultation data In the following applications of CHAMP radio occultation data will be discussed with respect to monitoring of global atmospheric parameters of the climate system. The focus is concentrated on the upper troposphere and lower stratosphere (UTLS) region. Dynamical, chemical, and radiative coupling between the stratosphere and troposphere are among the many important processes that must be understood for prediction of global change (Holton et al., 1995; Hoinka, 1998; Seidel et al., 2001). The exchange of mass, water, and chemical matter between the troposphere and stratosphere takes place across the tropopause that is characterized by an abrupt change in the temperature lapse rate. The continuous identification and monitoring of the tropopause on a global scale is an important goal in atmospheric and climate research. This can be performed by the radio occultation technique due to its high vertical resolution and global coverage. 4.1. Monitoring the UTLS region Fig. 4 shows the CHAMP lapse rate tropopause temperature and altitude for the tropical region during winter 2001/2002. For the identification of the lapse rate tropopause the WMO definition for the thermal

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Fig. 4. CHAMP lapse rate tropopause temperature and altitude between December 2001 and February 2002 (based on about 9500 occultations).

Fig. 5. Temperature for the 50 hPa level during winter 2001/2002 based on about 9500 occultation measurements from CHAMP.

tropopause was used (WMO, 1957). Next the data were sorted and averaged by a 15° grid. Increasing the density of radio occultation data by future satellite missions with radio occultation experiments and long-term measurements, e.g. from CHAMP, will allow more detailed pictures of the state and changes of meteorological parameters in the tropopause region and the stratosphere. Due to the high precision and resolution in temperature measurements in the upper troposphere and lower stratosphere (see Fig. 2) the radio occultation technique is suitable for observing temperature and detecting temperature changes in that part of the atmosphere (see Fig. 4). Fig. 5 shows as an example the temperature for the 50 hPa level measured from CHAMP during winter 2001/2002. An other application of the GPS radio occultation measurements is the monitoring of water vapour in the middle and lower troposphere (not shown here).

5. Conclusions and outlook Since February 2001 the GPS radio occultation experiment onboard CHAMP was activated on 510 days (October 2002). During continuous operation since May 2001 about 105,000 occultations were recorded. About 70% of the recorded occultations have passed the different quality control levels of the data processing system to derive profiles of atmospheric temperature and water vapour. An operational data processing system was established at GFZ Potsdam delivering CHAMP atmospheric products with a latency of 1–2 days to the Information System and Data Centre at GFZ in the standard processing mode. The near real time processing with a product delivering latency of 3–6 h enables the testing of CHAMP products by weather service centres for assimilation in weather analysis and forecast models.

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The implementation of advanced retrieval methods as single differencing for extracting the atmospheric excess path were tested. A one-dimensional variational retrieval for the analysis of CHAMP data with the use of 6-h analysis data from ECMWF as background will be available in the beginning of 2004. A good agreement between CHAMP temperature data and ECMWF as well as radiosonde data was found in the upper troposphere and lower stratosphere (250–30 hPa). The temperature bias is less than 1.5 K. CHAMP is in excellent condition. The mission is expected to last until 2007. The GPS radio occultation experiment onboard GRACE (Gravity and Climate Experiment) is expected to start in the beginning of 2004. Acknowledgements The CHAMP mission and the GPS radio occultation experiment involve many colleagues not only at GFZ Potsdam but also at JPL, DLR, and other national and international institutions. We thank all of them. The German Weather Service (DWD) enables us the access to the ECMWF data. Thank to the ECMWF for support and maintain of the data flow. We thank K. Schoellhammer at FU Berlin for radiosonde data preparation. This study was carried out under the grant of the Ministry of Education and Research (01SF9922/2). References Anthes, R.A., Rocken, C., Kuo, Y.-H. Applications of COSMIC to meteorology and climate. Terr. Atmos. Ocean. Sci. 11, 115–156, 2000. Beyerle, G., Hocke, K., Wickert, J., Schmidt, T., Marquardt, C., Reigber, C. GPS radio occultations with CHAMP: a radio holographic analysis of GPS signal propagation in the troposphere and surface reflections. J. Geophys. Res. D 24 (107), 2002, 10.1029/ 2001JD001402. Hajj, G., Kursinski, E., Romans, L., Bertiger, W., Leroy, S. A technical description of atmosphere sounding by GPS occultation. J. Atmos. Sol. Terr. Phys. 64, 451–469, 2002. Healy, S.B., Eyre, J.R. Retrieving temperature, water vapour and surface pressure information from refractive-index profiles derived by radio occultation: a simulation study. Q. J. R. Meteorol. Soc. 126, 1661–1683, 2000. Hoinka, P.H. Statistics of the global tropopause pressure. Mon. Weather Rev. 126, 3303–3325, 1998. Holton, J.R., Haynes, P.H., McIntyre, M.E., Douglass, A.R., Rood, R.B., Pfister, L. Stratosphere–troposphere exchange. Rev. Geophys. 33, 403–439, 1995.

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