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Dry Bias in the Beijing Radiosonde Soundings as Revealed by GPS and MWR Measurements

Dry Bias in the Beijing Radiosonde Soundings as Revealed by GPS and MWR Measurements. Chu Yanli 1 , Ying-Hwa Kuo 2 , John Braun 2 , Zhang Chaolin 1 , Wang Yingchun 1 , Zhang Jingjiang 1 , Wang Jingli 1

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Dry Bias in the Beijing Radiosonde Soundings as Revealed by GPS and MWR Measurements

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  1. Dry Bias in the Beijing Radiosonde Soundings as Revealed by GPS and MWR Measurements Chu Yanli1, Ying-Hwa Kuo2, John Braun2, Zhang Chaolin1, Wang Yingchun1,Zhang Jingjiang1, Wang Jingli1 1. Institute of Urban Meteorology (IUM), Beijing Meteorological Bureau (BMB), Beijing, China 2. COSMIC, University Corporation for Atmospheric Research (UCAR), Boulder, CO., USA

  2. Introduction of the ground-based moisture observation network around Beijing Before 01-Aug-2006(15 GPS stations) After 01-Aug-2006(42 GPS stations) ground-based GPS stations 54511 radiosonde (RS) station, ground-based GPS station, microwave water vapor radiometer (MWR) station 1

  3. Introduction of instruments RS at 54511: GTS-1 radiosonde system from ChangWang MeteoTech Co. Ltd, China MWR at 54511: TP/WVP-3000 from Radiometrics Corporation, US GPS antenna & receiver at 54511: TRM29659.00 & TRIMBLE 5700 from Trimble Company., US GPS antenna & receivers at Fang Shan district: TOPCR3_GGD & TPS LEGACY from Topcon Co., Japan 2

  4. GPS_PW: Methodology RS_PW: Mandatory levels: surface, 1000, 925, 850, 700, 500, 400, 300, 250, 200, 150, 100 Specific levels: about 10 levels 3

  5. 30 km Ground-based GPS network at Fang Shan district to the south of Beijing with 5 ~ 10 km station separation ground-based GPS stations 54511 radiosonde (RS) station 4

  6. Temporal variation of GPS PW in the Fang Shan district of Beijing through the 12-month period of September 2003 - August 2004. 5

  7. Difference between GPS PW and RS PW over the 10-month period of January - October 2004. Comparison of PW derived from GPS and RS over the 10-month period of January - October 2004. (a) scatter diagram of GPS PW versus RS PW, (b) scatter diagram of GPS-RS PW differences versus RS PW. 6

  8. Monthly mean values of RS PW (white dots) and its bias differences (red squares) from the GPS PW over the period of January - October 2004. 7

  9. PW retrieved from IGS ZTD reveals the obvious dry bias in the radiosonde soundings at Beijing 54511 station during August 2004. 8

  10. Investigate the systematic difference between GPS PW and RS PW. Is the systematic difference aroused by the climatic deviation? Or which one is more accurate? 9

  11. (a) (b) (a) Mean bias and (b) SD difference between a GPS station and YSDD GPS station as a function of separation distance over the period of January - October 2004. Difference of station height, station separation, PW mean bias and PW standard deviation (SD) at the radiosonde station and a GPS station as compared against YSDD GPS station over the period of January - October 2004. The height of YSDD station is 38 m. 10

  12. Difference of GPS ZTD between IUM and GFZ and CODE solutions at BJFS GPS station over August 2004. ZTD difference (black) is scaled to PW (red) in II = 6.5. Comparison of GPS ZTD between IUM and GFZ and CODE solutions at BJFS GPS station over the month of August 2004. 11

  13. Bias(mm) SD(mm) Samples GPS-MWR 0.2 0.98 583 RS-MWR -2.3 0.69 28 RS-GPS -1.9 1.30 32 12

  14. Bias(mm) SD(mm) Samples GPS-MWR -0.9 2.4 1347 RS-MWR -2.7 2.7 44 RS-GPS -1.4 2.8 50 13

  15. Difference of the moisture observations in GTS-1 minus RS-80A at the Beijing 54511 station over 26 Nov – 3 Dec 2003by Prof. ShuQing Ma, CMA 14

  16. JingMia Liu etc., CMA, Naqu, Tibet, GTS1 June Wang etc., NCAR, RS80 Dry bias in the radiosonde soundings as revealed in the other experiments 15

  17. Summary and discussion: . Nakamura (2004) compared GPS PW with measurements of the Vaisala RS-80A radiosonde, and found similar dry bias in the RS-80A radiosonde soundings. . Furthermore, he showed that relative humidity as measured by the RS-80A was about 5 to 15% smaller than that of the dew-point hygrometer. . Our results showed that the Beijing radiosonde soundings (using a GTS-1 radiosonde system) has a 15% dry bias compared with the GPS PW over a ten-month period. GTS-1 is a radiosonde made by ChangWang MeteoTech Co. Ltd., which was known to have been calibrated against RS-80A. . We suggest additional comparison of humidity measurements by GTS-1 with a dew-point hygrometer to further evaluate the dry bias of the GTS-1 system. . We also suggest performing similar studies for other radiosonde stations in China. 16

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