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Relative Humidity Correction Method of Microwave Radiometer Combined with Cloud Radar
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摘要: 基于中国气象局大气探测试验基地地基遥感垂直廓线系统中云雷达与微波辐射计同址观测的优势, 使用2021年8月—2022年7月毫米波云雷达、探空数据, 分析云雷达反射率因子与相对湿度特征关系, 提出联合云雷达的微波辐射计相对湿度分段校正方法, 实现云区微波辐射计相对湿度实时校正, 并利用2023年1—8月探空和2023年7—8月ERA5(ECMWF reanalysis version 5)逐小时再分析数据进行误差分析。结果表明: 入云区的相对湿度与反射率因子间呈正相关关系, 云区中段相对湿度近似饱和状态, 出云区与入云区相对湿度随高度变化近似对称; 层状云条件下校正后微波辐射计与探空和ERA5相对湿度的均方根误差比校正前分别减小7.99%和8.91%, 偏差中位数绝对值分别减小12.62%和13.05%, 且连续观测时次经校正后误差均减小, 校正效果较好; 对流云条件下校正效果也较好, 但部分个例存在过度校正。因此, 联合云雷达的相对湿度分段校正方法能够实现微波辐射计相对湿度廓线的连续实时校正, 可提高有云条件下微波辐射计的观测质量。Abstract: The microwave radiometer can detect and retrieve temperature and humidity profiles with high spatial and temporal resolution throughout the day. However, microwave radiometers have few detection frequencies in the middle and upper layers, making them easily affected by clouds. After integrating cloud information into brightness temperature data, the improvement in detection accuracy in the middle and upper layers still remains insufficient, failing to meet accuracy standards required for relative humidity. With the construction of a national ground-based remote sensing vertical profile system, the continuous observation of cloud radar and microwave radiometer at the same site has been achieved, enhancing the spatial and temporal resolution. Combined with the relationship between humidity and cloud formation, a comprehensive quality control method is proposed for relative humidity using cloud radar and microwave radiometer. It plays a crucial role in enhancing the accuracy of humidity profile of microwave radiometer under cloudy conditions.By analyzing the characteristic relationship between the cloud radar reflectivity factor and the radiosonde relative humidity, a piecewise correction method for the microwave radiometer relative humidity of the combined cloud radar is proposed. Error correction results are analyzed using the radiosonde and ERA5 data. It shows that there is a positive linear correlation between the relative humidity and the reflectivity factor, the relative humidity in the middle of the cloud region is approximately saturated, and the relative humidity variation trend with the height of the cloud exit region and the cloud entry region is approximately symmetric about a certain height. Under the condition of stratiform clouds, the root mean square error of relative humidity by microwave radiometer decreases by 7.99% and 8.91% when comparing with radiosonde and ERA5, and the absolute value of median deviation decreases by 12.62% and 13.05%, respectively. The absolute median deviation also decreases. Further investigation indicates the method is also effective under the condition of convection cloud, but the relative humidity in the cloud region after correction is larger than that of sounding and ERA5, and the median deviation changes from negative deviation to positive deviation. Therefore, the relative humidity segment correction method of combing cloud radar can realize the continuous real-time correction of the relative humidity profile of microwave radiometer, which partly improves the observation quality of microwave radiometer under cloud conditions.
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图 1 云区探空相对湿度与云雷达反射率因子关系
Fig. 1 Relationship between relative humidity of radiosonde and reflectivity factor of cloud radar in cloud
图 2 2021年8月7日08:00和10月14日20:00探空相对湿度和云雷达反射率因子廓线
Fig. 2 Relative humidity profiles of radiosonde and reflectivity factor of cloud radar at 0800 BT 7 Aug and 2000 BT 14 Oct in 2021
图 3 入云区探空相对湿度与云雷达反射率因子的关系
Fig. 3 Relationship between relative humidity of radiosonde and reflectivity factor of cloud radar in cloud-entering region
图 4 云区中段探空温度和冰面相对湿度箱线图
Fig. 4 Box plots of temperature and relative humidity on ice surface of radiosonde in middle part of cloud
图 5 探空出云区和入云区相对湿度与归一化高度关系
Fig. 5 Relationship between relative humidity of radiosonde and normalized height in cloud-exiting and cloud-entering regions
图 6 2023年1—8月层状云条件下校正前后微波辐射计与探空相对湿度偏差箱线图
Fig. 6 Box plots for relative humidity deviation of microwave radiometer before and after correction to radiosonde under stratiform cloud from Jan to Aug in 2023
图 7 2023年7月1日—8月10日层状云条件下校正前后微波辐射计与ERA5相对湿度偏差箱线图
Fig. 7 Box plots for relative humidity deviation of microwave radiometer before and after correction to ERA5 under stratiform cloud from 1 Jul to 10 Aug in 2023
图 8 2023年7月28日08:00—29日10:00连续云天条件下云反射率因子(a)和校正前后微波辐射计与ERA5相对湿度的均方根误差(b)
Fig. 8 Reflectivity(a) and relative humidity root mean square error of microwave radiometer before and after correction to ERA5(b) under continuous cloudy sky from 0800 BT 28 Jul to 1000 BT 29 Jul in 2023
图 9 层状云条件下校正前后微波辐射计与探空(a)和ERA5(b)相对湿度对比
Fig. 9 Relative humidity comparison of microwave radiometer before and after correction and radiosonde(a) and ERA5(b) under stratiform cloud
图 10 对流云条件下校正前后微波辐射计相对湿度与探空相对湿度偏差箱线图
Fig. 10 Box plots for relative humidity deviation of microwave radiometer before and after correction to radiosonde under convective cloud
图 11 对流云条件下校正前后微波辐射计相对湿度与ERA5相对湿度偏差箱线图
Fig. 11 Box plots for relative humidity deviation of microwave radiometer before and after correction to ERA5 under convective cloud
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