Sensitivity of a Land Surface Model to the Diurnal Distribution of Downward Longwave Radiation

Abstract

The Global Soil Wetness Project questioned the accuracy of the ISLSCP Initiative-1 downward longwave radiation data. The data appear to be offset by several hours from what is expected, such that the maximum downward longwave radiation consistently occurs late at night, when air temperatures are decreasing. This problem may result from the formula used to calculate the 6-hourly longwave radiation data. The Biosphere-Atmosphere Transfer Scheme (BATS) was used to test the sensitivity of one land-surface model to changes in the diurnal distribution of downward longwave radiation. Three simulations were performed: 1) a control run, in which the unmodified values were used; 2) a run in which all the longwave values were shifted forward six hours; and 3) a run in which the daily average value was used. Comparisons are made of both the large-scale monthly averages and of the monthly average diurnal cycles at six individual points. The two sensitivity simulations, using the altered downward longwave radiation, both increased total incoming radiation during the day and decreased it at night, relative to the control simulation. Consequently, upward longwave radiation increased during the day while decreasing by a greater amount at night, resulting in a net decrease in upward longwave radiation. The changes in upward longwave radiation must be balanced by changes in sensible, latent and soil heat fluxes. As a result of the changes to longwave radiation, simulated sensible heat flux in the sensitivity simulations increases during the day and decreases at night. The daytime increase is larger than the nighttime decrease, resulting in a net increase in sensible heat. The changes in sensible heat flux are usually within ±1W/m² of the changes in upward longwave radiation, with small changes in latent and soil heat fluxes making up the difference. Water budget terms, and hence any of the GSWP soil wetness index calculations, generally experience only negligible changes.