Monday, 10 January 2005
Variations of land surface microwave emissivity with vegetation developments
This study tries to use satellite and surface measurements at visible (VIS), infrared (IR) and microwave (MW) wavelengths to evaluate vegetation developments during growing seasons. The land surface microwave emissivity (LSME) over the heavily forested area, Harvard Forest, during the growing season of 1998 has been estimated from SSM/I satellite measurements. The increase in plant water amount due to leave amount and/or area coverage increases over Harvard Forest during the growing season may contribute to the decrease in surface albedos in the near-IR and total shortwave wavelengths. Because of high attenuations to soil emissions at MW wavelengths of heavy canopies, properties of vegetation canopy dominate MW emissions above canopies. Compared to the surface albedo retrievals at VIS and near-IR wavelengths, LSME estimates exhibit certain advantages in representations of land surface vegetation developments. The LSME has an inversed trend to that of canopy leave coverage after the leaf emergence. Diurnal variations of LSME values also decrease significantly as the growing season proceeds due to the acceleration of the rewetting in morning hours after recharging vegetation water during nighttimes and of the dehydration during afternoons in warm days.