The effect of vertical resolution on clear-sky radiation calculations: tests with two schemes

Abstract

The dependence on vertical resolution of the clear-sky results produced by two radiation codes (a slightly modified version of the ECMWF scheme and the scheme used at the DWD) is investigated. Results are presented both for the McClatchey et al. atmospheres and for a set of fully detailed sounding profiles measured at Jokioinen, Finland. The effect of vertical discretization on shortwave calculations is found to be very small, particularly for the DWD scheme. The downward longwave flux at the surface is found to be sensitive to vertical resolution only in the lower troposphere, whereas for the upward flux at the top of the atmosphere, upper tropospheric resolution seems the most important. The major difference found between the smooth McClatchey et al. profiles and the detailed soundings is that discretization errors of longwave cooling rates are larger for the latter data set, particularly in the lower troposphere. Furthermore, in cases with sharp surface inversions, large errors of downward longwave flux at the surface occur if there is poor resolution in the boundary layer. The average errors of the longwave fluxes at the boundaries are, however, fairly similar for the two data sets. Quantitatively, the discretization errors of longwave fluxes are relatively small, especially for the ECMWF scheme. About 20 layers is sufficient for both schemes to keep the maximum value of net flux error in the vertical column within 2 Wm-2 even for most of the detailed sounding profiles. For the calculation of radiative fluxes, it seems preferable to have the majority of the layers in the troposphere. Finally, the profiles of longwave fluxes and cooling rates in surface inversion situations are discussed.