青藏高原探空大气水汽偏差及订正方法研究

Systematic Errors and Their Calibrations for Radiosonde Precipitable Water Vapor on the Tibetan Plateau

水汽是大气的主要成分和降水的主要物质来源。青藏高原大气水汽分布对区域天气和气候有很大影响,为了探讨探空观测的大气水汽总量(R)资料的可靠性,本文以地基GPS遥感的大气水汽总量(G)为参照标准,对拉萨(1999～2010年)和那曲(2003年)的R进行对比分析和偏差(R-G)订正。结果表明:近10多年拉萨站R比G明显偏小,偏小程度随使用不同的探空仪而异。GZZ-2型机械探空仪和GTS-1型电子探空仪多年平均的PW偏差分别为-8.8%和-3.9%,随机误差分别为17.6%和13.6%。近10多年PW偏差变化呈减少趋势,这与探空仪性能改进有关。分析发现,青藏高原PW偏差具有明显季节变化和日变化特征,夏季比冬季明显,1200UTC比0000UTC明显。拉萨站GZZ-2型和GTS-1型探空仪在1200UTC多年平均的PW偏差分别为-15.8%和-7.3%,在0000UTC分别为-1.6%和-0.4%。那曲站GZZ-2型探空仪在1200UTC和0000UTC的PW偏差分别为-12.4%和-0.3%。分析还表明,太阳辐射加热与气温的日变化和季节变化是造成高原PW偏差日变化和季节变化的重要原因。据此,提出了高原PW偏差的订正方法,并以拉萨和那曲站为例进行PW偏差订正,订正后的PW系统偏差显著减少,随机误差也相应得到了改善。

Water vapor is one of the major components of the atmosphere and material resource for rainfall. The spatial distribution and temporal variation of precipitable water vapor on the Tibetan Plateau play an important role in the regional weather and climate. The reliability of precipitable water vapor measurements is greatly of concern. The characteristics of the systematic and random errors of the radiosonde （RS） precipitable water （PW） data by comparison with ground-based GPS measurements are studied at Lhasa during the period from year 1999 to 2010 and at Naqu in year 2003. The results show that the radiosonde PW is significantly drier than GPS PW at Lhasa during a period of more than one decade. Different types of radiosonde humidity sensors show different magnitudes of PW dry biases. GZZ-2 （goldbeater＇s skin hygrometer） and GTS-1 （carbon hygristor） have relative mean dry biases of -8.8% and -3.9% and relative mean random errors of 19.8% and 13.6%, respectively. Due to the introducing of the high performance humidity sensors （GST-1）, the relative PW difference is apparently reduced over the past 10 years. The temporal variation characteristics of the radiosonde PW dry bias are also investigated. The results show that the radiosonde PW dry bias exhibits pronounced diurnal and annual variations. The dry bias of the radiosonde PW is much larger at 1200 UCT than that at 0000 UTC, and larger in summer than that in winter. The relative mean PW biases for GZZ-2 and GTS-1 are respectively -15.8% and -7.3% at 1200 UTC, and -1.6% and -0.4% at 0000 UTC at Lhasa. The relative mean PW bias for GZZ-2 is -12.4% at 1200 UTC, and -0.3% at 0000 UTC at Naqu. Additionally, the causes of diurnal and annual variations of the radiosonde PW dry bias are analyzed. The solar radiative heating to the humidity sensors may have played an important role in the radiosonde PW dry bias diurnal and annual variations. It can be seen that the diurnal variations of the radiosonde PW dry bias are significant partly because the air temperature is higher at 1200 UTC than that at 0000 UTC. The annual variations of the radiosonde PW dry bias are pronounced partly because the air temperature is higher in summer than that in winter. The calibration methods for the radiosonde PW dry bias are developed and applied to the GZZ-2 and GTs-1 sounding PW datasets at Lhasa and Naqu. The corrections greatly improve the accuracy of the radiosonde PW.