Impacts of Snow-Darkening Effects By Light Absorbing Aerosols on Hydro-Climate and Heat Waves of Eurasia during Boreal Spring and Summer

In this study, we used the NASA GEOS-5 climate model to investigate the impact of snow darkening by light-absorbing aerosols on hydro-climate over Eurasia during boreal spring and summer. Two sets of 10 member ensemble model integrations with prescribed sea surface temperature were carried out for 10 simulated years (2002-2011), one includes snow-darkening effects (SDE) by light-absorbing aerosols, and one without them (NSDE). Differences in the hydro-climates of the two experiments were then evaluated. Results show that SDE causes an increase in net annual mean surface solar radiation of 5-10 Wm^-2, leading to increased surface skin temperature of 1-2.5, with a 5-10% reduction in soil wetness over vast regions of Eurasia north of 40. The SDE-induced warming is most pronounced during the melting season due to strong snow-albedo feedback in the vicinity of the retreating seasonal snowline. In response to the SDE forcing, the snow-land-atmosphere system undergoes a Wet-First-Dry-Later (WFDL) seasonal re-equilibration of the surface energy and water balance, resulting in initial wetting of the land during the melting season and a subsequent pronounced drying of the land during the summer season. The prolonged warming and drying of the land in summer is associated with the development of atmospheric blocking conditions, i.e., higher mid-to-upper-tropospheric geopotential height, lower relative humidity, reduced cloudiness and prevailing atmospheric subsidence. Overall, SDE induces a warmer and drier boreal summer hydro-climate, increasing the frequency of the top 5% and 1% extreme hot days (as defined by NSDE statistics) over western and northern Eurasia by approximately three-fold and tenfold, respectively.