全球变化知识资源中心

( )To facilitate the construction of a satellite-derived 2m air temperature (T-2m) product for the snow- and ice-covered regions in the Arctic, observations from weather stations are used to quantify the relationship between the T-2m and skin temperature (T-skin). Multiyear data records of simultaneous T-skin and T-2m from 29 different in situ sites have been analysed for five regions, covering the lower and upper ablation zone and the accumulation zone of the Greenland Ice Sheet (GrIS), sea ice in the Arctic Ocean, and seasonal snow-covered land in northern Alaska. The diurnal and seasonal temperature variabilities and the impacts from clouds and wind on the T-2m-T-skin differences are quantified. T-skin is often (85 % of the time, all sites weighted equally) lower than T-2m, with the largest differences occurring when the temperatures are well below 0 degrees C or when the surface is melting. Considering all regions, T-2m is on average 0.65-2.65 degrees C higher than T-skin, with the largest differences for the lower ablation area and smallest differences for the seasonal snow-covered sites. A negative net surface radiation balance generally cools the surface with respect to the atmosphere, resulting in a surface-driven surface air temperature inversion. However, T-skin and T-2m are often highly correlated, and the two temperatures can be almost identical (< 0.5 degrees C difference), with the smallest T-2-T-skin differences around noon and early afternoon during spring, autumn and summer during non-melting conditions. In general, the inversion strength increases with decreasing wind speeds, but for the sites on the GrIS the maximum inversion occurs at wind speeds of about 5 m s(-1) due to the katabatic winds. Clouds tend to reduce the vertical temperature gradient, by warming the surface, resulting in a mean overcast T-2m-T-skin difference ranging from -0.08 to 1.63 degrees C, with the largest differences for the sites in the low-ablation zone and the smallest differences for the seasonal snow-covered sites. To assess the effect of using cloud-limited infrared satellite observations, the influence of clouds on temporally averaged T-skin has been studied by comparing averaged clear-sky T-skin with averaged all-sky T-skin.To this end, we test three different temporal averaging windows: 24 h, 72 h and 1 month. The largest clear-sky biases are generally found when 1-month averages are used and the smallest clear-sky biases are found for 24 h. In most cases, all-sky averages are warmer than clear-sky averages, with the smallest bias during summer when the T-skin range is smallest.