Comparison Of GPS Radio Occultation Soundings With Radiosonde Measurements

GPS radio occultation (GPS/RO) sounding technique as a robust atomosphere dectection method has brought a new opptunity for synoptic meteorology and climatology study. In this paper, we introduced the development and theory of GPS/RO and the method of retrieving atomoshpere profile from GPS/RO detection. The dry retrievals of GPS/RO data, collected from CDAAC, are assessed by the use of radiosonde (RS) data and ECMWF analysis data from2007to2008.Fractional refractivity difference and temperature difference, from1000hPa to lOhPa, between collocated GPS/RO and radiosonde which occur in3hours and300km, are calculated. Results show that the global mean difference is rather small in Upper Troposphere and Lower Stratosphere (UTLS) and large in middle and low troposphere, where GPS/RO has large error for the effects of vapor. Rather small differences of the mean bias calculated form different GPS/RO mission prove the stability of GPS/RO technique. Comparing the fractional refractivity difference and temperature difference of GPS/RO-RS(GPS minus radiosonde) in different latitude, results show that the mean difference and standard deviation is dependent on latitude. We also compared the bias in different seasons in midlatitue(30°N～60°N), which shows that the mean difference and standard deviation vary with seasons. Fractional refractivity difference and temperature difference over different geographical regions are compared which proves that GPS/RO data are accurate enough to differentiate the quality of different radiosonde sensor types. The subsets of GPS/RO measurements in different latitude zone and seasons are also compared with ECMWF to verify the RS results, Which shows that the observed differences in the radiosonde comparisons are caused by the RS data rather than GPS/RO data. Generally, GPS/RO data has the highest accuracy in UTLS area.Collocation conditions are assessed in this paper. Mean difference is found independent of the collocation condition. We calculate the standard deviation’s sensitivity to time and distance mismatch, which can be used to estimate the standard deviation due to RS balloon drift. The perfect match standard deviations of fractional refractivity difference and temperature difference are calculated in300hPa-30hPa.Temperature difference between GPS/RO dry retrievals and China’s RS temperature profiles from200hPa to30hPa is calculated to analyze the RS temperature errors. Compared to GPS/RO data, China’s RS temperature shows a small warm bias in general, and the absolute mean bias and standard deviation in Upper Troposphere are larger than that in Lower Stratosphere. Latitudinal comparison results show that bias distribution characteristic agrees well with the temperature lapse rate, which indicates that GPS/RO data have the ability to differentiate the lag errors of RS measurements. The bias is larger in low latitude than that in high latitude. Comparison of temperature difference in day time and night time reveals that GPS/RO data can differentiate radiative errors of RS caused by solar radiation. In comparison to the small bias between the new L band electronic RS and GPS/RO profiles, the significant temperature bias between59type mechanic RS and GPS/RO data, especially in low latitude area, which may be attributed to large lag errors caused by the large lag index of59type RS and lapse rate in low latitude area, demonstrates the ability of GPS/RO data to differentiate the old and new RS type of China and the improvements of temperature precision of the L band RS measurements. Our preliminary results show that GPS/RO data has great usefulness to calibrate radiosonde data.