Improved Diurnal Cycle of Precipitation in the E3SM Atmosphere Model Version 1 (EAMv1) with a Modified Convective Triggering Mechanism

Monday, December 10, 2018 - 13:40
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General Circulation Models (GCMs) for weather forecasts and climate simulations continue having difficulties in modeling the diurnal precipitation particularly over land. For example, over mid-latitude lands, such as warm seasons at the Southern Great Plains (SGP), most GCMs tend to simulate a diurnal peak of precipitation around noon, much earlier than the observed late-afternoon peak and often fail to capture the observed nocturnal peak. Problems in simulating the diurnal cycle of precipitation are primarily due to shortcomings and deficiencies in representing convection initiation, evolution, and propagation, as well as the interaction between convection and its large-scale atmospheric environment.

In this study, we revise the convective triggering function used in the DOE’s Energy Exascale Earth System Model (E3SM) Atmosphere Model version 1 (EAMv1) by introducing a simple dynamic constraint on the initiation of convection that emulates the collective effects of lower level moistening and upward motion of the large-scale circulation to prevent convection from being triggered too frequently, as well as allowing air parcel launching above boundary layer to capture elevated nocturnal convection which is often decoupled from the surface. The original convection trigger in EAMv1 assumes that convection is triggered whenever the Convective Available Potential Energy (CAPE) is larger than 70 J/kg, which results in too frequent warm season convection over land arising from strong diurnal variation of solar radiation. We examine the impact of the revised trigger on EAMv1 simulations in 10-year AMIP runs and find that the modified triggering mechanism has led to a significant improvement in the simulation of diurnal cycle of precipitation over different climate regimes, including both midlatitude and tropical lands. More details will be presented at the meeting.

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