IMPACT OF CLOUDS ON SURFACE RADIATIVE FLUXES AND SNOWMELT IN THE ARCTIC AND SUB-ARCTIC

A comprehensive atmospheric radiative transfer model combined with the surface energy balance equation is applied to investigate the impact of clouds on surface radiative fluxes and snowmelt in the Arctic and s ubarctic. Results show that at the surface, the shortwave cloud-radiat ive forcing is negative, while the longwave forcing is positive and ge nerally much larger than the shortwave forcing. Thus, the all-wave sur face cloud-radiative forcing is positive, with clouds warming the lowe r atmosphere and enhancing snowmelt during the melting period in the A rctic and subarctic. These results agree with and explain observations and measurements over the past three decades showing that the onset o f snowmelt starts earlier under cloudy sky conditions than under clear sky conditions in the Arctic. Clouds could change the date of onset o f snowmelt by as much as a month, which is of the order of the observe d interannual variations in the timing of snowmelt in the Arctic and s ubarctic. The all-wave cloud radiative forcing during the period of sn owmelt reaches a maximum at equivalent cloud droplet radius (r(e)) of about 9 mu m, and cloud liquid water path of about 29 g m(-2). For thi n clouds, the impact of changes in liquid water path on all-wave cloud radiative forcing is greater than changes in equivalent cloud droplet size, while for thick clouds, the equivalent cloud droplet size becom es more important. Cloud-base temperature and to a minor extent cloud- base height also influence the surface radiative fluxes and snowmelt. This study indicates that the coupling between clouds and snowmelt cou ld amplify the climate perturbation in the Arctic.