On Changes of Global Wet-bulb Temperature and Snowfall Regimes

To properly interpret the observed shrinkage of the Earth's cryosphere it is important to understand global changes of snowfall dominant regimes. To document these changes, three different reanalysis products of wet-bulb temperature together with observationally-based data sets are processed from 1979 to 2017. It is found that over the Northern Hemisphere (NH), the annual mean wet-bulb temperature has increased at a rate of 0.34$^\circ$C per decade (pd) over land and 0.35$^\circ$C pd over ocean, resulting in a reduction of the annual mean potential areas of snowfall dominant regimes by 0.52/0.34 million km$^2$pd over land/ocean. However, the changes in the Southern Hemisphere (SH) are less conclusive and more uncertain. Among the K$\"o$ppen-Geiger climate classes, the highest warming trend is observed over the NH polar climate regimes. Over studied mountain regions, the Alps are warming at a faster rate compared to the Rockies, Andes and High Mountain Asia (HMA). Due to such warming, potential snowfall areas over the Alps is reducing at 3.64% pd followed by Rockies at 2.81 and HMA at 1.85% pd. On average, these mountain ranges have lost 0.02 million km$^2$pd of potential snowfall areas. The NH potential snowfall areas is retracting towards the North pole over the Central Asia and Europe at a rate of 0.45 and 0.7 degree pd. Furthermore, terrestrial regions over the NH including the Great Plains in the United States, Canadian provinces around the Hudson Bay, Central Siberian and Tibetan Plateaus, are losing as much as 4% of the solid proportion of the annual precipitation amount pd.