Spectral Weak Temperature Gradient Simulation of Convection in OTREC

Convection in the Tropical East Pacific and Southwest Caribbean studied in the OTREC2019 project was simulated in a spectral weak temperature gradient convective model. Observed mesoscale mass flux profiles were mostly reproduced in these simulations. Once initiated, convection typically persisted in the simulations, either as repeated convective pulses separated by quiescent conditions or as continuous, statistically steady convection. This corresponds to the showers vs. rains categorization of Ramage (1971) and the monsoon break vs. monsoon trough dichotomy of Williams et al. (1992). From analyzing the mechanisms supporting convection in these two cases, we found that the pulsating behavior of the showers regime resulted from the periodic development of strong convective downdrafts and associated cold pools topped by stable layers (``onion'' soundings) that suppressed further convection until the cold pools dissipated. In the rains cases the downdrafts were not strong enough to suppress continuing convection. Showers tended to occur in cases exhibiting drier, more unstable environments that were more receptive to strong downdraft production. The trajectory of simulated showers in the gross moist stability (GMS) plane shows initial negative GMS as the system intensifies and large positive GMS as the system decays. In contrast, the simulated rains exhibit weak negative GMS at initialization, but then maintain near-zero GMS thereafter. In some sense rains convection is ``weaker'' than showers convection, but it tends to produce more time- and area-averaged rain in the simulations, as observed by Ramage and Williams et al.