On Understanding Multi-Scale Precipitation Processes in Tropical Islands Via the Convection, Aerosol, and Synoptic-Effects in the Tropics (CAST) Campaign

The work herein describes a comprehensive effort to improve understanding of the multi-scale atmospheric interactions which modulate local island precipitation in the Caribbean. The island of Puerto Rico is ideal for study as it is subject to variations in convective dynamics due to topography, surface heating, and sea-breeze trade-wind convergence near the west coast, and has a wide network of observational instruments including ground stations, soil moisture sensors, a Next Generation Radar (NEXRAD) site, twice-daily radiosonde launches, and Aerosol Robotic Network (AERONET) sunphotometers. The Convection, Aerosol, and Synoptic-Effects in the Tropics (CAST) campaign sought to improve convective storm monitoring via additional radiosonde launches, high resolution radars (three), a ceilometer, and air sampling in western Puerto Rico. During the CAST phases (22 June 2015 – 10 July 2015, 6 – 22 February 2016, 24 April 2016 – 7 May 2016, and 27 June 2016 - 12 July 2016), a series of intense storms and the most extreme drought in recent history (summer 2015) were observed. CAST data reflects seasonal variations in aerosol optical thickness (AOT) with average values of 0.293, 0.063, 0.068, and 0.251 during phases I through IV respectively, and in the spatial distribution and intensity of storms, with Saharan dust inhibiting island-wide precipitation during the midsummer drought (MSD). Evidence suggests that a strong El Niño and intense dust episodes were key drivers of the extreme drought of MSD 2015. Precipitation events with totals exceeding 50 mm for areas over 200 km² occurred under high dust presence (AOT > 0.4) when local convective available potential energy (CAPE) exceeded 2 kJ kg^-1 and available precipitable water was above 40 mm. DS and ERS rain events tended to be island-wide storms with low aerosol intensity (AOT < 0.1).