Integrated Pan-Arctic Melt Onset Detection From Satellite Active/Passive Microwave Measurements, 2000-2009

An integrated pan-Arctic melt onset dataset is generated for the first time by combining active and passive microwave satellite derived estimates from algorithms developed for northern high latitude land surface, ice caps, large lakes, and sea ice. The dataset yields new insights into the spatial and temporal patterns of mean melt onset date (MMOD) and the associated geographic and topographic controls. For example, in the terrestrial Arctic, tree fraction and latitude explain more than 60% of the variance in MMOD with the former exerting a stronger influence on MMOD than the latter. Elevation is also found to be an important factor controlling MMOD with most of the Arctic exhibiting significant positive relationships between MMOD and elevation, with a mean value of 24.5 m.day-1. Melt onset progresses fastest over land areas of uniform cover and/or elevation (40 - 80 km.day-1), and slows down in mountainous areas, on ice caps, and in the forest-tundra ecotones. Over sea ice, melt onset advances very slowly in the marginal seas, while in the central Arctic the rate of advance can exceed 100 km.day-1. Comparison of the observed MMOD with simulated values from the third version of the Canadian Coupled Global Climate Model showed good agreement over land areas, but weaker agreement over sea ice, particularly in the central Arctic, where simulated MMOD is about 2-3 weeks later than observed due to a cold bias in simulated surface air temperatures over sea ice.