Biases in the standard rain rate product by the TRMM microwave radiometer over land

This study investigates the causes of biases in the rain rate estimates by the TMI standard algorithm. Retrieval error is separated from rain/no-rain classification error. As it is well known, the retrieval error is sensitive to storm height. It is interesting that the observed brightness temperature at 85GHz shows clear diurnal variation for given storm height and surface rain rate. It can be explained by the diurnal variation of the land surface brightness temperature (related with the land surface physical temperature) under inhomogeneous rainfall or the diurnal variation of the freezing height (related with the air temperature).