Sensitivity of a land surface model to the diurnal distribution of downward longwave radiation

The Global Soil Wetness Project questioned the accuracy of the ISLSCP Initi ative-1 downward longwave radiation data. The data appear to be offset by s everal hours from what is expected, such that the maximum downward longwave radiation consistently occurs late at night, when air temperatures are dec reasing. This problem may result from the formula used to calculate the g-h ourly longwave radiation data. The Biosphere-Atmosphere Transfer Scheme (BA TS) was used to test the sensitivity of one land-surface model to changes i n the diurnal distribution of downward longwave radiation. Three simulation s were performed: 1) a control run, in which the unmodified values were use d; 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 m ade of both the large-scale monthly averages and of the monthly average diu rnal cycles at six individual points. The two sensitivity simulations, usin g the altered downward longwave radiation, both increased total incoming ra diation during the day and decreased it at night, relative to the control s imulation. 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 mus t be balanced by changes in sensible, latent and soil heat fluxes. As a res ult of the changes to longwave radiation, simulated sensible heat flux in t he 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 usua lly within +/- 1 W/m(2) of the changes in upward longwave radiation, with s mall changes in latent and soil heat fluxes making up the difference. Water budget terms, and hence any of the GSWP soil wetness index calculations, g enerally experience Only negligible changes.