Effects of the changing heating profile associated with melting layers in a climate model

Abstract

The impact of modifying the melting behaviour at the freezing level in the GA2.0 version of the Met Office Unified Model is investigated. By allowing snow to melt over a greater depth, biases in rainfall over the Maritime Continent (MC) are found to be improved, and there is an indication of benefits to the simulation of the Madden–Julian Oscillation.Moistening diagnostics under weak temperature gradient theory are used to explain how and why changes to the treatment of melting influence tropical rainfall biases. The modified melting experiment increases the lower tropospheric diabatic heating rate per unit column‐integrated convective heating in the MC, which helps to increase lower tropospheric vertical moisture advection per unit column convective heating, making conditions more favourable for convection there. Changes of the opposite sense occur in tropical ocean regions of the west Pacific and Indian Ocean. Changes in lower tropospheric radiative heating per unit convection produced by the different treatment of melting are particularly influential in engendering mean precipitation changes between the experiments. Differences in precipitation in the MC region between the control and melting experiments and opposite changes in oceanic regions to the east and west are linked through changes in the Walker circulation, making it unclear which region is most influential for forcing the improvement in the pattern of precipitation biases. Sensitivity experiments that artificially enhance convection in one region through imposition of sea‐surface temperature anomalies produce a negative precipitation response in the other region.