Authors: Chris Lunger*, Nathan Hosannah, Jorge Gonzalez
The City College of New York, Department of Mechanical Engineering and NOAA CREST Center, New York, NY 10031
Easterly trade-winds traveling along the tropical belt are modulated by the North Atlantic High Pressure (NAHP), and transport vast amounts of Saharan dust from the African Sahel across the Atlantic. The frequency and intensity of Saharan dust episodes peak during the summer months and create a dry air layer that inhibits convection and precipitation over the Caribbean region. Local island effects such as land surface heating, orographic lifting over high elevations, and convergence over many of the islands in the region modify convection and thereby impact precipitation. The work herein is geared towards determining which conditions produce local rainfall over the west coast of Puerto Rico during light, moderate, and intense dust episodes. Six of 19 storms occurring during the summer phases of the Convection, Aerosol, and Synoptic-Effects in the Tropics (CAST) campaign (22 June 2015 – 11 July 2015 and 27 June 2016 – 12 July 2016) were selected for study. CAST campaign data included radiosondes and three short range X-band Doppler radar systems (Tropinet) to supplement Next Generation Radar (NEXRAD), Aerosol Robotic Network (AERONET) sun-photometer aerosol measurements, Environmental Protection Agency (EPA) air quality data, Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) aerosol backscatter and type, and ground weather station precipitation and wind vector variables. High-resolution Tropinet radar imagery allows for a detailed characterization of horizontal and vertical storm profiles including convection height and rain rate variables that NEXRAD radar cannot due to the earth’s curvature and topographical interference. Results show that the average rainfall in each 2016 case was 0.8 to 35 mm higher than the 2015 case due to greater convective available potential energy (CAPE) and increased available precipitable water (28 to 39 mm higher) despite the presence of moderate to heavy dust indicated by aerosol optical thickness (AOT) values exceeding 0.2. Local island-scale enhancers which include orographic forcing and sea-breeze/trade-wind convergence overcame the precipitation-mitigating effects of the dust and forced intense localized rainfall in western Puerto Rico.