Abstract:
This paper describes a method of rainfall estimates in midlatitude based on Moderate Resolution Imaging Spectroradiometer (MODIS) 11 μm infrared temperature TB11μm and 0.65μm visible reflectance R 0.65μm. Generally, rain cloud is higher than non rain cloud in altitude and its temperature is lower than that of non rain cloud. It means 11 μm infrared channel carries information of rain cloud and non rain cloud, which can be used to detect rain and estimate rainfall. Moreover, rain cloud has a big cloud optic thickness related with a big R 0.65μm Thus, T B11μm and R 0.65μm should be combined to estimate rainfall.
The method, denoted MODIS multi-channel technique, detects rain cloud according to thresholds of MODIS R0.65μm >0.8 and T B11μm<270 K, in which high visible threshold of R 0.65μm is able to screen out cirrus due to its transparent feature in visible channel. Then a nonlinear function of R 0.65μm and TB11μmcompletes the rain rate estimation. More specifically, different function of R0.65μm for different T B11μm is further used for rainfall estimation upon calibration with AMSR-E rain rate products.
The technique was tested for one Asia case and validated with AMSR-E passive microwave rainfall estimates and rain gauge data from May to July 2005. The result shows that rain estimates of MODIS agrees well with AMSR-E microwave estimates with a correlation coefficient R 2 of 0.8 at a spatial scale of 2°. The correlation coefficient R 2 of MODIS estimates and rain gauge data is 0.32 at a spatial scale of 2°, which coincides with other researches' result.
Physically, AMSR-E passive microwave estimates overland rainfall according to the scattering effect of ice particle in the column of air and thus it is a more direct method of rainfall estimation. The validation of MODIS method indicates that clouds with a big rain intensity observed by AMSR-E often have a high R 0.65μmand low T B11μm. In other words, clouds with a high R0.65μm and low T B11μm often comprise a lot of ice particle in the air. Aqua satellite first simultaneously supplies passive microwave and infrared/visible radiative information of rain, which exposes a promising direction for rainfall research.