Global mapping of microwave penetration depths in arid areas using GMI observations

Passive microwave observations are used daily in data assimilation scheme but also to provide long term monitoring of the Earth on a global scale. Over arid areas the lack of modelisation of the penetration depth phenomenon causes large uncertainties in the modelisation of brightness temperatures. This also causes large errors in retrievals of land surface temperature from microwaves compared to the ones derived from infrared observations. Using the non sun synchronous observations of the Global Precipitation Mission Microwave Imager, the diurnal cycle of brightness temperature is reconstructed allowing a precise estimation of the microwave effective emitting depth. The emitting depth is estimated to be up to 25 cm at 36~GHz resulting in large differences between the surface temperature and the effective temperature of the observations. A model of the variation of emitting depth with frequency is used to create a map of parameters describing the penetration depth for arid areas between 10 and 90 GHz globally. This can be used to estimate a correction based solely on the provided parameters and the surface temperature to improve simulation of microwave brightness temperatures.