A new data set for monitoring snowmelt onset over Arctic sea ice

Snow plays an integral role in defining the Arctic’s surface energy balance by influencing sensible and latent heat exchanges as well as surface albedo. For instance, wet snow, with a relatively low albedo, absorbs approximately 45% more solar radiation than dry snow. The transition from a highly reflective dry snow cover to an absorptive wet snow over also plays an important role in the temperature-albedo feedback mechanism; melting snow decreases surface albedo, leading to additional energy absorption, warmer air temperatures and consequently additional snowmelt. An accurate method of monitoring snowmelt onset could therefore be used to validate and improve climate simulations during the Arctic snowmelt period. An improved understanding of the spatial and temporal variability in snowmelt onset could also be used as a proxy climate indicator. Accordingly, a new data set of snowmelt onset dates over Arctic sea ice, available from 1979-1999, is discussed here. The derivation of snowmelt onset dates with passive microwave data is discussed, and examples of data use are explored. In particular, an examination of spatial and temporal variations in snowmelt onset illustrates that there are significant trends towards earlier snowmelt onset in several Arctic regions, consistent with trends towards warmer surface air temperatures and increased cloud cover noted in other analyses.