Dynamic response of microwave land surface properties to precipitation in Amazon rainforest

The responses of land surface microwave properties to precipitation, including the brightness temperature (Tbs) and emissivities (MLSEs) at 18.7, 23.8 and 37.0 GHz, are studied by combining satellite synergetic retrievals (from AQUA AMSR-E and MODIS) with in-situ measured land surface temperature, soil moisture (SM), and precipitation at two Amazonia tropical rainforest sites Km83 and Km67. In general, Tbs and MLSEs are found to be negatively correlated to SM. Their dependences on the vegetation water content (VWC) are not monotonic: positive correlations at relatively low VWC and negative correlations at high VWC range. Although Amazon dense rainforest is the most stable vegetation regime, there are significant seasonal variations in both MLSE and Tbs, with amplitudes of about 0.025 and 7.0 K, respectively. The day-to-day variation of MLSE and Tbs is larger in the wet seasons than in the dry seasons, although the mean MLSE and Tbs are lower in the wet seasons than in the dry seasons. After precipitation, the SM, VWC and canopy interception water increase and reach their peak values at different transit times. There is an equilibrium time point (about 4 h in this case) at which the vegetation–soil system holds the largest amount of moisture. Consequently, Tbs and MLSEs decrease during the first several hours after the start of precipitation and then increase after the equilibrium time point. Ignoring the “turning point” may introduce significant errors in the estimation of MLSEs (up to 0.04) and Tbs (up to 10 K) under rainy conditions.

► The response of microwave land surface emissivity (MLSE) to precipitation is studied.
► Significant seasonal and daily variations of MLSE are found in Amazon rainforests.
► A conceptual model of MLSE response to precipitation is proposed.
► A joint soil-vegetation effect of precipitation is parameterized for rain retrievals.