On the Investigation of Mesoscale Dynamic and Aerosols Effects on Precipitation over a Urban Tropical Sea-Breeze Environment: Observations and Modeling

Rainfall over the island of Puerto Rico is heavily influenced by local dynamical systems associated with orographic cloud formation over the highest central mountains and convective clouds formed by coastal convergence. These effects may largely contribute to the total annual precipitation during the dry and early rainfall seasons (Dec-July). The west coast of the island is subjected to convergence episodes between the easterly trade winds and a westerly sea-breeze which frequently initiate strong afternoon showers when coupled with surface heating. Variation in the concentration of aerosols due to the Saharan Dust (SD) intrusion, which occurs between the months of May and July has also influenced rainfall across the island and the Caribbean. Records for Caribbean particle size distributions (PSDs) and precipitation indicate strong correlation between high aerosol concentrations and precipitation suppression during the drier summer season (Comarazamy et al.). This finding further validates the hypothesis of aerosol induced precipitation suppression due to cloud condensation nuclei (CCN) competing with each other for available water vapor content during condensation growth. The aim of this study was to analyze the influence of PSD in local dynamical precipitation systems via investigation of PSD data from the NASA Aerosol Robotic Network (AERONET), and mesoscale atmospheric modeling with a cloud resolving model. PSDs were correlated against precipitation totals for the city of Mayaguez (18.20N, -67.14W) on the western side of the island. To further investigate local influences of PSD on dynamical systems, a localized precipitation event which produced over 50 mm according to radar imagery near Mayaguez on 17 June 2013, was chosen as a case study. Before and during this event, an SD intrusion was recorded over the island. Horizontal PSD from the AERONET site located at the University of Puerto Rico at Mayaguez (UPRM) was coupled with vertical PSD attained from a ceilometer to obtain the three-dimensional (3D) aerosol structure in the local atmosphere. This 3D PSD was ingested into the Regional Atmospheric Modeling System (RAMS) and configured using a two-moment microphysics scheme to simulate the event at 1km horizontal resolution with lateral boundary conditions driven by NCEP. A second idealized simulation was conducted using climatological PSDs to investigate the effects of high concentrations brought about by the SD episode. Results show that precipitation rates, accumulated totals, and spatial extent of the precipitation event were impacted by the presence of large concentrations of aerosols, clearly demonstrating that dynamical effects are modulated by microphysical processes of clouds.