This research dissertation aims to improve our understandings of land surface emissivity using estimates from the Advanced Microwave Scanning Radiometer - Earth Observing System (AMSR-E) and to investigate the effect of penetration depth of the microwave signal and its effect on the retrieval of this variable. Microwave observations at low frequencies exhibit more sensitivity to surface and subsurface properties with little interference from the atmosphere. The AMSR-E sensor has two additional lower frequencies, at 6.9 and 10.65 GHz (C- and X-band, respectively) with respect to its preceding sensors. Observations at these low frequencies penetrate deeper into the soil layer. Ancillary data used in the analysis, such as surface skin temperature and cloud mask, are obtained from International Satellite Cloud Climatology Project (ISCCP). Atmospheric properties are obtained from the TIROS Operational Vertical Sounder (TOVS) observations to determine the small upwelling and downwelling atmospheric emissions as well as the atmospheric transmission.

The difference in depth of originations causes an inconsistency between diurnal variation of infrared and microwave brightness temperatures, which can lead to more than 10% difference between day and night estimates of land emissivity. In this study, the diurnal cycle of microwave brightness temperature is constructed at different channels using a constellation of satellites which comprises AMSR-E and other SSM/I sensors. Differences in phase and amplitude were observed between the microwave and infrared diurnal cycles. These differences seem to be consistent with land cover and soil texture maps. A principal component analysis (PCA) is conducted to evaluate spatial variation of diurnal cycle of brightness temperature at global scale. A lookup table of effective physical temperature representative of the contributing layers of the microwave signal at each channel and month is adopted based on the diurnal cycle of brightness temperature. The implementation of the proposed effective temperature diurnal cycle lookup table showed that it can significantly mitigate the differences between day and night retrieved emissivities from AMSR-E observations. Therefore, a global product of instantaneous land emissivity that accounts for the difference in penetration depth between thermal and microwave temperatures is proposed.