Diurnal and Seasonal Variations of Passive and Active Microwave Satellite Observations Over Tropical Forests

Documenting the large scale variability of tropical forest structure and function is needed for improved understanding of the carbon and water cycles. The seasonal and diurnal cycles of passive and active microwave satellite observations are jointly analyzed for the first time, using the Global Precipitation Mission (GPM). Collocated backscattering coefficients from the GPM Dual‐frequency Precipitation Radar (DPR) and emissivities derived from the GPM Microwave Imager (GMI) are studied for 4 yr over the Tropics, at the available multiple frequencies, angles, and polarizations. Our analysis confirms the diurnal patterns already observed with radars, with a maximum backscatter in the early morning attributed to the water xylem refill at night. At the same time, early in the morning, we evidence a minimum emissivity over most of tropical forests: the backscatter and emissivity diurnal cycles tend to be in phase opposition, as well as their seasonal cycles. However, during the dry season in the south eastern Amazonian forest where precipitation can be low for several months, the emissivity diurnal cycle changes, implying a different behavior of the forest in this area, related to the state of the canopy. The tropical forest plays an important role in the global water and carbon cycles, and it can be analyzed with satellite observations. The seasonal and diurnal cycles of passive and active microwave satellite observations are studied here simultaneously for the first time, thanks to the radar and radiometer on board the Global Precipitation Mission. Four years of observations are collected and analyzed over the Tropics. Our analysis shows that the passive and active observations vary both seasonally and diurnally in tropical forests, with similar amplitude within a day than over the year. The radar and radiometer signals tend to vary in phase opposition, seasonally and diurnally, over most of the tropical forests. However, during the dry season in the south eastern Amazonian forest where precipitation can be low for several months in a row, changes in the seasonal and diurnal cycles of the radiometer observations are evidenced, implying a different behavior of the forest in this area, related to the state of the canopy. Strong diurnal cycles over tropical forests, for both microwave backscatter and emissivity, with amplitude similar to seasonal variationsSeasonal and diurnal cycles of backscattering and emissivity temporally anti‐correlated over most parts of the tropical forestsOver Amazonia, seasonal cycle of backscatter and emissivity in phase in dry regions and diurnal cycle also in phase there during dry season Strong diurnal cycles over tropical forests, for both microwave backscatter and emissivity, with amplitude similar to seasonal variations Seasonal and diurnal cycles of backscattering and emissivity temporally anti‐correlated over most parts of the tropical forests Over Amazonia, seasonal cycle of backscatter and emissivity in phase in dry regions and diurnal cycle also in phase there during dry season