Ensemble Modelling of Moist Convection in Complex Terrain, a Case Study from the CACTI Field Campaign

During the austral summer of 2018, the CACTI and RELAMPAGO joint field campaigns took place over the Argentinian Sierras de Cordoba (SDC) mountains. This north-south oriented ridge is notable for its frequent initiation of thunderstorms that may grow upscale into intense mesoscale convective systems. The SDC also initiates more garden-variety thunderstorms with shorter life cycles under less synoptically disturbed conditions. Although these isolated storms are less impactful than the more intense mesoscale convective systems, they may be more difficult to predict in convective-scale NWP models due to their strong sensitivities to uncertain subgrid processes and partially resolved processes in the deep-convection gray zone. To gain insight into the mechanisms and NWP predictability of such storms, this study performs convective-scale ensemble simulations of an isolated, diurnally forced SDC thunderstorm during CACTI/RELAMPAGO with the WRF model. The rich observational data sets permit a detailed verification of the default ensemble, which uses the default WRF geographic data. A major deficiency in soil moisture is identified, which causes the simulated convection to develop too early and reach much shallower depths than observed. Upon correction of this bias, the ensemble forecast skill is improved, with some members accurately representing the observed storm. However, substantial variability remains within the ensemble, with some members producing more widespread convection than observed and others producing no deep convection at all. Ensemble analysis suggests that this variability is explained by a combination of thermodynamic and dynamic mechanisms, with the strongest trend a positive sensitivity to variations in low-level moisture over the ridge in the pre-convective period. A weaker but significant negative sensitivity is also found to midlevel winds, which tend to produce elevated downdrafts above the surface-based, thermally forced updraft over the mountain crest.