Estimating Downwelling Longwave Radiation for Input to a Land Surface Model

Interaction between the land and atmosphere is an important research theme that is being addressed from both hydrological and atmospheric science perspectives. Significant focus has been placed on coupling detailed models of land surface hydrology to general circulation models (GCMs) and to mesoscale weather models such as the operational Eta model of the National Center for Environmental Prediction (NCEP). NCEP's land surface model (LSM), and derivatives of it, require input information on weather, soil and vegetation for estimating soil moisture and evapotranspiration.

Downwelling longwave radiation is one of several weather variables needed to determine the radiation budget in the LSM, but it is a variable that is rarely measured. Various techniques have been developed to estimate downwelling longwave radiation for daytime conditions. This study was undertaken in order to evaluate the available techniques, to investigate possible improvements and/or simplifications to those techniques, and to incorporate nighttime as well as daytime conditions. Of the five models analyzed, one estimates downwelling longwave radiation as a function of atmospheric emittance and surface temperature only, while the other four models also include the cloud fraction as an influencing factor. Comparisons of model results to measured data in Oklahoma suggest that a relatively simple model can be used to accurately estimate downwelling longwave radiation during both daytime and nighttime periods.