校正热带测雨卫星轨道抬升对微波成像仪亮温的影响

热带测雨卫星(Tropical Rainfall Measuring Mission,TRMM)在2001年8月轨道高度从350 km升高至402 km,搭载于其上的微波成像仪(TRMM Microwave Imager,TMI)的入射角随之发生变化,进而使得相应探测结果(亮温)发生改变,从而导致由此反演的大气参数出现虚假的突变。为保证轨道抬升前后TMI亮温资料的连致性,以便更好地用于气候研究,本研究首先分析了洋面轨道抬升前后亮温的差异及变化原因,然后结合微波辐射传输模式,分析了不同环境参数对亮温变化的影响,在此基础上用线性变换的方式对轨道抬升后的亮温进行了修正,并从不同角度检验修正效果。结果表明,轨道抬升前后亮温呈线性关系,低频垂直极化通道亮温轨道抬升后升高了0.8-1.6 K,其他通道亮温变化不大。经过修正,轨道抬升前后的亮温趋于一致,月平均亮温偏差明显减小,低频垂直极化通道亮温在轨道抬升期间的突变被消除,亮温变得连续平稳,可用于气候研究。

A correction of TMI brightness temperatures due to TRMM boost

The satellite orbit of the Tropical Rainfall Measuring Mission (TRMM) was boosted from 350 km to 402 km in August 2001. This caused that the incidence angle of the TRMM Microwave Imager(TMI) was changed, in turn the observed brightness temperature(T_B).As a result, the atmospheric parameters retrieved from-TMI T_B have shown a spurious jump from the pre-to the post-boost periods. In oder to insure measurements consistency between these two periods so as to establish a dependable dataset for climate research, in this study changes in T_B between the pre-and post-boost periods over the ocean are investigated, and their mechanism is analyzed. Based on the MWRT model, the research shows how the various environmental parameters affected the change of brightness temperature related to the boost. Afterwards, the post-boost data are adjusted to match the pre-boost data through linear transformation, and the performance of the correction method is evaluated in several way. The results show that post-boost T_B is linearly related to pre-boost T_B where T_B on vertical polarized channels for the lower frequencies increased by 0.8-1.6 K, while T_B on the other channels showed little chang. Through the adjustment, pre- and post-boost T_B become consistent,and monthly mean T_B bias is reduced significantly Additionally the spurious T_B jump in the low frequency polarized channel is removed. This would be helpful to establish reliable and continuous datasets for climate study in the future.