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Effects of Radiosonde System Changing to L-band Radar Digital Radiosonde on Humidity Measurements in China
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摘要: 2002—2010年我国高空探测系统逐步完成了由原来的59-701型探空系统升级为L波段雷达-电子探空仪系统的工作，湿度传感器由原来的肠衣更换为碳湿敏电阻。该文对全国98个高空站相对湿度探测值在换型前后的差异进行了统计分析。结果表明：探空系统换型后，相对湿度探测值较换型前显著降低，表现为明显的干偏差，且随着高度的增加而增大，200，500 hPa和850 hPa相对湿度分别偏低14.6%，8.3%和5.3%。受太阳辐射的影响，这种干偏差在白天甚于夜间；换型前后相对湿度的概率分布也发生了明显变化，整个对流层相对湿度低于20%的低值出现频率明显高于换型前，200 hPa相对湿度小于20%的出现频率由换型前的10%增加到换型后的53%。最明显的变化是相对湿度为3%以下的出现频率，换型前各高度层出现频率均接近于0，但换型后200，500 hPa和850 hPa出现频率分别达到16.2%，9.9%和2.2%。Abstract: The radiosonde sounding is a major tool for measuring the vertical structure of atmospheric variables. Accurate monitoring of water vapor is vital for numerical weather prediction and climatic change assessment. The radiosonde sounding system in China begins upgrading to L-band radar digital radiosonde system from 59-701 system since 2002 and completes in 2011, and the sensor for humidity measurement is changed from goldbeater's skin to a carbon hygristor. These changes may result in discontinuity of radiosonde observation series.In order to determine the observational bias brought by the upgrade, comparative analysis of relative humidity (RH) measurement between pre-change and post-change of the system is made using radiosonde humidity observations. 98 radiosonde stations out of 120 stations in China are selected to analyze differences between 3 pre-change years and 3 post-change years. Results show that RH after changing the sensor has a significant dry bias. There are statistically significant dry bias in almost all selected stations at 200 hPa, and in 75% and 54% stations at 500 hPa and 850 hPa, respectively. RH dry biases increase with height in troposphere, with values of 14.6%, 8.3% and 5.3% at 200 hPa, 500 hPa and 850 hPa, respectively. There are more dry biases during daytime than nighttime due to impacts of solar radiation. The probability distribution of relative humidity after the system upgrading has a significant shift comparing to pre-upgrading. The occurrence frequency of RH with the value less than 20% after system change is much higher than that during pre-change, which are 53% vs 10% at 200 hPa. The frequency of RH with the value less than or equal to 3% are 16.2%, 9.9% and 2.2% on 200 hPa, 500 hPa and 850 hPa after system upgrading, but that is nearly 0 at three levels before upgrading. Causes of these dry biases and biases between actual value and the observation will be further studied. Also, methods for correcting the bias should be developed.
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Key words:
- radiosonde;
- sensor change;
- relative humidity
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图 1 2002年1月以来全国探空仪换型时间及站数
Fig. 1 Dates and number of stations for radiosonde system changing since January of 2002
图 2 换型前后200 hPa, 500 hPa, 850 hPa平均相对湿度差值 (单位：%)
(实心圆表示差值达到0.1显著性水平，空心圆圈表示未达到0.1显著性水平)
Fig. 2 Differences of relative humidity between pre-change and post-change of sensor at 200 hPa, 500 hPa and 850 hPa (uint: %)
(the solid circle denotes passing the test of 0.1 level, hollow circle denotes no significance)
图 3 北京站和武汉站换型前后相对湿度的平均差值随高度变化
Fig. 3 Variation with heights for relative humidity difference between pre-change and post-change of sensor at Beijing Station and Wuhan Station
图 4 北京站各高度年平均相对湿度和年平均温度
Fig. 4 Time series of annual mean relative humidity and annual mean temperature at Beijing Station
图 5 换型前后不同高度层00：00和12：00相对湿度差值散点分布
Fig. 5 Scatter diagram for relative humidity difference of different heights between pre-change and post-change of sensor at 0000 UTC and 1200 UTC
图 6 换型前后不同高度相对湿度出现频率分布
Fig. 6 The probability distribution of relative humidity between pre-change and post-change of sensor
表 1 各层换型前后相对湿度差异的全国平均值 (单位：%)
Table 1 Mean differences of relative humidity between pre-change and past-change of sensor (uint: %)
高度 1月 4月 7月 10月 年平均 200 hPa 11.62 12.61 15.74 16.08 14.63 500 hPa 12.33 7.64 5.40 8.41 8.31 850 hPa 5.84 5.87 1.42 5.34 5.32 
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