Increased melting level height impacts surface precipitation phase and intensity

Abstract

The global near-surface temperature increased by ~0.155 K per decade during 1979–2012, which resulted in decreasing snow and increasing rain events, retreating mountain glaciers and more frequent and intense rainfall extremes. Although surface temperature increases are well studied, less attention is given to the associated changes in the tropospheric thermal structure, such as melting level height, which affects cloud microphysics and surface precipitation. Here we use observations and reanalyses to show that the melting level height increased by 32 ± 14 m per decade over global land areas during 1979–2010, consistent with a warming atmosphere. This causes a transition from snow to rain, the enhanced melting of hail and an increased depth of warm cloud layers (cloud base to melting level distance). Warm cloud layers with a depth beyond ~3.5 km result in an intensification of extreme precipitation at twice the rate of the atmospheric moisture increases. Days with such environments increased by 25% per decade in populated regions, such as the eastern United States. Climate change has altered the vertical structure of atmospheric temperature. Global land observations show melting level height (the altitude where falling precipitation begins to melt) has increased, deepening the warm cloud layer and intensifying rainfall extremes at the expense of snow and hail.