Validation of Refractivity Profiles Retrieved from FORMOSAT-3/COSMIC Radio Occultation Soundings: Preliminary Results of Statistical Comparisons Utilizing Balloon-Borne Observations

Hiroo Hayashi,Xinan Lin,Yuichi Aoyama,Yoshinori Shoji,Jun-Ichi Furumoto,Yasuhiro Murayama,Toshitaka Tsuda

doi:10.3319/tao.2008.01.21.01(f3c)

Abstract

The GPS radio occultation (RO) soundings by the FORMOSAT-3 COSMIC (Taiwan's Formosa Satellite Misssion #3/Constellation Observing System for Meteorology, Ionosphere and Climate) satellites launched in mid-April 2006 are compared with high-resolution balloon-borne (radiosonde and ozonesonde) observations. This paper presents preliminary results of validation of the COSMIC RO measurements in terms of refractivity through the troposphere and lower stratosphere. With the use of COSMIC RO soundings within 2 hours and 300 km of sonde profiles, statistical comparisons between the collocated refractivity profiles are performed for some tropical regions (Malaysia and Western Pacific islands) where moisture-rich air is expected in the lower troposphere and for both northern and southern polar areas with a very dry troposphere. The results of the comparisons show good agreement between COSMIC RO and sonde refractivity profiles throughout the troposphere (1-1.5% difference at most) with a positive bias generally becoming larger at progressively higher altitudes in the lower stratosphere (1-2% difference around 25 km), and a very small standard deviation (about 0.5% or less) for a few kilometers below the tropopause level. A large standard deviation of fractional differences in the lowermost troposphere, which reaches up to as much as 3.5-5% at 3 km, is seen in the tropics while a much smaller standard deviation (1-2% at most) is evident throughout the polar troposphere.

Highlights

Since the 1960s the radio occultation (RO) method has been used for the study of planetary atmospheres (e.g., Fjeldbo et al 1971)
The mean absolute difference profile demonstrates a good agreement between COSMIC RO and radiosonde soundings through the troposphere with less than a 1% bias
This means that refractivity profiles retrieved from the COSMIC RO soundings have sometimes positive, otherwise negative, differences from collocated sonde data in the lower troposphere probably because of highly variable distribution of water vapor in the Southeast Asian monsoon region

Summary

Introduction

Since the 1960s the radio occultation (RO) method has been used for the study of planetary atmospheres (e.g., Fjeldbo et al 1971). The GPS RO measurement was followed by the German CHAMP (Challenging Minisatellite Payload technique for tracking GPS RO signals on the receivers, the COSMIC mission is capable of providing more data in the lowermost troposphere where the phase locked loop often fails in tracking the rapid fluctuation of RO signals (Rocken et al 2000; Sokolovskiy 2001b; Sokolovskiy et al 2006) Such a large influx of constantly assimilated data, distributed globally and over a single continent and over the world’s oceans, will improve numerical weather prediction and lead to new climate studies in areas where operational observations have not been available so far, e.g., in the troposphere over the open ocean. Kuo et al (2005) showed an interesting result of comparisons in refractivity between CHAMP RO and radiosonde soundings over five geographical areas, each of which uses a different type of radiosonde, and concluded that high accuracy of RO soundings could differentiate between the performance of various types of radiosonde applications

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Results

Conclusion