*3.2. Snow Albedo Radiative Forcing*

Annual mean snow albedo radiative forcing during 2003–2016 is shown in Figure 4. Areas with strong snow albedo radiative forcing also appear over the NEL, excluding Antarctica. However, areas with large fractional snow cover do not always represent strong snow albedo radiative forcing, and the peak value appears in the Tibetan Plateau, rather than Greenland. This is partly because the Tibetan Plateau is located at the relatively low latitude with much more insolation than the higher latitudes, and snow there can persist until relatively late spring. In addition, areas in western part of the U.S., northern part of Canada, mid-high latitudes of Russia, and the Andes Mountains along the west coast of South America also exhibit strong snow albedo radiative forcing.

Climatological monthly mean snow albedo radiative forcing over the NEL during 2003–2016 is displayed in Figure 5a. Standard deviations are shown as whiskers, which represent interannual variability of each month during the study period. Autumn months from September to November exhibit small snow albedo radiative forcing. Because snow cover over the NEL starts to accumulate in autumn (Figure 3b), there is neither much snow cover nor strong insolation. Winter experiences the largest expansion of snow cover extent and the smallest insolation among the four seasons. Snow albedo radiative forcing is much larger than that in autumn, and it increases from December to February. Snow albedo radiative forcing is the largest in spring months and peaks in April, as a result of both large snow cover extent and strong solar radiation. Spring also experiences the largest decrease of snow cover extent throughout the year. This leaves summer with very little snow cover amount. As a result, in spite of its largest insolation, snow albedo radiative forcing in summer is much smaller than that in spring.

April exhibits the largest snow albedo radiative forcing throughout the year, and its interannual variability is displayed in Figure 5b. There is an overall insignificant (at *p* = 0.05 level) decline in April snow albedo radiative forcing over the NEL during 2003–2016. Large interannual variability with a continuous decline during the last 4 years are experienced during the study period. In general, April snow albedo radiative forcing decreased about 6.10% during 2003–2016.

**Figure 4.** Spatial distribution of annual mean snow albedo radiative forcing during 2003–2016.

**Figure 5.** Snow albedo radiative forcing over the North Hemisphere Extratropical Land during 2003–2016: (**a**) climatological monthly mean; and (**b**) April interannual variability.
