Representing subgrid convective initiation in the Community Earth System Model

A widely recognized issue found in most Earth System Models is that of convective precipitation being triggered too early during the day over land, often resulting in persistent light rainfall. In an effort to address this issue, a model-consistent subgrid convective trigger is demonstrated in the Community Earth System Model (CESM). Subgrid convective initiation is achieved by first calculating a convective threshold within the Community Atmosphere Model (version 5.3). The convective threshold is then passed to each subgrid land cover type within the Community Land Model (version 4.0) and convection is considered to be active if the convective threshold is exceeded and the convective available potential energy (CAPE) exceeds 70 J kg(-1) (the default trigger). In total, three simulations are performed: (1) a Control (CTL) with the default CAPE trigger, (2) a simulation with the default CAPE trigger as well as the subgrid trigger (HCF trigger) applied over land and ocean, and (3) a simulation the same as the HCF trigger but only applied over land (HCF land). By including this subgrid trigger, convective onset time is delayed by 1-4 h; however, the time of peak precipitation is unchanged. This diurnal shift improves the number of hours the deep convective scheme is invoked from more than 75% to less than 50%, alleviating the overactivity problem. The delayed onset time of deep convection allows for greater CAPE accumulation and a greater proportion of precipitation generated outside the convective scheme. More CAPE consequently produces an overall increase in intense precipitation events.