The representation of Convectively Coupled Equatorial Waves and upscale energy transfer in models with explicit and parametrized convection
Abstract
Convectively Coupled Equatorial Waves (CCEWs) dominate atmospheric variability on timescales of 2--30 days in the Tropics, bringing episodes of widespread heavy precipitation. This study compares the representation of CCEWs and their connection to upscale energy transfer in two Met Office Unified Model simulations of the full tropical channel with identical km-scale resolution. The principal difference between the simulations is that one parametrizes convection (GAL), while the other (RAL) is convection permitting. This means GAL acts to remove vertical instability without explicitly representing the resolved-scale circulation associated with convective plumes. We present the first quantitative diagnosis of interscale energy transfer and its relation to CCEWs. This diagnosis is important because upscale energy transfer between convection and large-scale waves may influence accurate simulation and predictability of tropical weather systems. The average upper-tropospheric upscale transfer simulated by RAL is approximately 50\% higher than GAL. CCEWs are more coherent in RAL, with an average phase-speed variability 80\% higher than observations, compared with 166\% higher in GAL. RAL also simulates greater upscale energy transfer within waves than GAL, with a stronger correlation between the interscale energy transfer rate and equatorial wave winds. Simulated Kelvin and Rossby waves are associated with upscale energy transfer from scales 2--8 times smaller than the dominant wavelength, related to active deep convection within a particular sector of the wave phase. Our findings show that the explicit representation of convective motions has a significant impact on the simulation of upscale energy transfer, and is therefore very likely to be a significant factor in the faithful simulation of the convective coupling within CCEWs.