Abstract
Abstract. The timing of springtime end of snowmelt (snow-off date) in northern Eurasia in version 5.4 of the ECHAM5 atmospheric general circulation model (GCM) is evaluated through comparison with a snow-off date data set based on space-borne microwave radiometer measurements and with Russian snow course data. ECHAM5 reproduces well the observed gross geographical pattern of snow-off dates, with earliest snow-off (in March) in the Baltic region and latest snow-off (in June) in the Taymyr Peninsula and in northeastern parts of the Russian Far East. The primary biases are (1) a delayed snow-off in southeastern Siberia (associated with too low springtime temperature and too high surface albedo, in part due to insufficient shielding by canopy); and (2) an early bias in the western and northern parts of northern Eurasia. Several sensitivity experiments were conducted, where biases in simulated atmospheric circulation were corrected through nudging and/or the treatment of surface albedo was modified. While this alleviated some of the model biases in snow-off dates, 2 m temperature and surface albedo, especially the early bias in snow-off in the western parts of northern Eurasia proved very robust and was actually larger in the nudged runs. A key issue underlying the snow-off biases in ECHAM5 is that snowmelt occurs at too low temperatures. Very likely, this is related to the treatment of the surface energy budget. On one hand, the surface temperature Ts is not computed separately for the snow-covered and snow-free parts of the grid cells, which prevents Ts from rising above 0 °C before all snow has vanished. Consequently, too much of the surface net radiation is consumed in melting snow and too little in heating the air. On the other hand, ECHAM5 does not include a canopy layer. Thus, while the albedo reduction due to canopy is accounted for, the shielding of snow on ground by the overlying canopy is not considered, which leaves too much solar radiation available for melting snow.
Highlights
Snow cover is one of the most important elements in the climate and hydrology of the Northern Hemisphere
We look in detail at the performance of a single model, the ECHAM5 atmospheric general circulation model (GCM) (Roeckner et al, 2003, 2006), in simulating the timing of snowmelt in spring in northern Eurasia, north of latitude 55◦ N
Mountainous areas are problematic to handle in algorithms based on microwave radiometer data (Mialon et al, 2008; Pulvirenti et al, 2008), these features are expected on physical grounds: colder temperatures and orographically enhanced precipitation favour later snowmelt
Summary
Snow cover is one of the most important elements in the climate and hydrology of the Northern Hemisphere. Large areas of the Eurasian and North American continents are covered by seasonal snow. The varying snow cover affects directly the surface energy balance by interfering with the energy storage, net radiation and fluxes of sensible and latent heat. The snow–albedo feedback (SAF) is largest when changes in snow cover area are linked with substantial changes in regional albedo (Brown, 2000). This coincides with the maximum influence of snow cover
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