A GPS retrieval algorithm is developed for obtaining in-cloud vertical profiles of the atmospheric state from COSMIC GPS RO data, with the MODIS (Moderate Resolution Imaging Spectroradiometer) cloud-top pressure and cloud-top temperature as auxiliary information. Moreover, the cloud base heights and cloud thicknesses could also be derived based on the density scale height, temperature lapse rate and relative humidity using GPS wet retrievals. The proposed algorithm is tested upon 31 COSMIC GPS RO profiles, which occurred from May to September 2006 within a domain bounded by 0-50 o N and 80-145 o E. These 31 GPS ROs were located in cloudy regions and were collocated with MODIS observations within a distance of less than 30 km and a temporal interval of less than half hour. It is found that the retrieval temperature is warmer than NCEP-reanalysis in the upper levels of the cloud and colder in the lower levels of and below the cloud. Dropsonde observations for Hurricane Rita confirm this characteristic feature of the NCEP analysis within clouds. The cloud thickness and cloud-base height that are determined by the proposed criteria are validated qualitatively with IR and VIS satellite images. Sensitivity of the GPS in-cloud profile retrieval to the MODIS cloud top pressure is also shown.

Next, CloudSat data, which provides cloud geometrical profile, including cloud layer number, cloud top height and cloud base height, are taken globally for the same time period to study the statistical characteristics of cloud top properties. There are 89 one-layer cloudy GPS ROs found with CloudSat data available within the distance of less than 30 km and the interval of less than half hour. It's noticed that averagely on the cloud top, COSMIC wet retrieval temperature is generally more compatible with CloudSat ECMWF-AUX data ( <1.2oC); COSMIC dry retrieval temperature is as good as COSMIC wet retrieval in the higher level (>8km), but deviates more as the height decreases (it could reach -16oC close to the surface); NCEP reanalysis temperature is mostly warmer than CloudSat ECMWF-AUX, and the deviation is larger in higher level (>1oC above 7.5km) than lower level (<1oC below 7.5km). The temperature profile comparison shows that there are two kinds: one is similar as the 31 soundings found from MODIS observations with systematic positive deviation and negative deviation; the other one is that the in-cloud retrieval temperature profile deviates from CloudSat ECMWF-AUX more than COSMIC dry or wet retrieval.

Furthermore, the validation of proposed cloud-base height determination is quantitatively studied using the cloud base observation from CloudSat. Among those 89 soundings, no less than one possible cloud bases could be determined for 36 soundings, and the best-determined cloud base (the possible cloud base that is closest to the “true” one from CloudSat cloud base) could be in the accuracy of 1oC for 15 of those 36 soundings. It's also found that lower accuracy of the determined cloud base height is related to the soundings whose cloud top height is larger than 6km. Some modifications of the proposed cloud base determination criteria are also discussed.