Convection-Permitting Simulations in Northwestern South America: Parameterization Sensitivity and Representation of Mesoscale Convective Systems

Convection-permitting (CP) simulations have been useful to advance the understanding of precipitation and organized convection in multiple regions worldwide. However, CP modeling studies are limited in regions such as Northwestern South America (NWSA), where severe convection has compromised human life and frequently impacts sectors such as agriculture, infrastructure, and aviation. The development of CP modeling studies would improve the predictability of severe storms and allow for the projection of future changes in their intensity, frequency, and duration, which is also critical for water security in the region. In this work, we evaluate the performance of CP simulations to represent Mesoscale Convective Systems (MCSs) and precipitation using the Weather Research and Forecasting (WRF) model. First, we studied the sensitivity of three microphysics and two boundary layer schemes to the rainfall representation, including an analysis of key processes for moist convection, such as low-level moisture fluxes, cloud cover and vertical distribution, and latent heat release. We found that both boundary layer and microphysics parameterization schemes significantly influence moisture fluxes and low-level convergence. Furthermore, the boundary layer schemes significantly influence shortwave radiation due to the production of low-level clouds, whereas the microphysics schemes significantly affect vertical velocity which is associated with differences in latent heat release from solid hydrometeor formation. Then, we performed four 1-month simulations with the best-performing WSM6 (microphysics) and YSU (boundary layer) parameterization schemes and analyzed the main characteristics of MCSs at different seasons using an algorithm for identifying and tracking MCSs (ATRACKCS). The model was able to reproduce the complex diurnal and seasonal variations in the occurrence of MCSs in the NWSA. In terms of MCS characteristics, the model adequately represents the systems' evolution, initiation time, speed, and propagation direction. However, the model underestimates the area of the storms and noticeably overestimates their intensity, while the total rainfall is comparable with observations due to error compensation. A more detailed analysis of the rainfall discrepancies shows that the overestimation of MCS intensity is especially evident in smaller systems and specifically during the development stage. Our results point out certain factors that can explain model biases in the representation of MCSs, contributing to the improvement of convection-permitting simulations in tropical regions.