Comparison of modeled and observed cloud-free longwave downward radiation over the Alps

Differences between computed and observed cloud-free longwave downward radiation (LDR) were examined both for Payerne and six further radiation sites of the Alpine Surface Radiation Budget (ASRB) network. LDR was computed by the complex radiative transfer model (RTM) MODTRAN v4.0 and by a simplified RTM used in the numerical weather prediction model "alpine model" (aLMo). MODTRAN computed LDR in Payerne using sounding data and ASRB observed LDR show an average difference of 1.5 W m(-2) ( +/- 3.5 W m(-2)), and -3.2 W m(-2) +/- 3.6 W m(-2)) for night- and daytime respectively from 1996 to 2001. Nighttime LDR is underestimated by the aLMo RTM in the order of -20 W m(-2) compared to ASRB measurements both at Payerne and at six further ASRB sites. However LDR bias is strongly reduced in Payerne during daytime. For comparison MODTRAN computed LDR using aLMo forecasted atmospheric profiles shows mean differences in the order of -3 W m(-2) both for night- and daytime. Hence the comprehensive radiation scheme used in aLMo is mainly responsible for the substantial underestimation of cloud-free LDR.