An Investigation of Convection and Aerosol-Effects on Storms in the Tropics (CAST)

The Convection and Aerosol-Effects on Storms in the Tropics (CAST) campaign was conducted in the summer of 2015 to continue work that began in the summer of 2014. It is geared towards improving understanding of surface convective processes and of cloud-aerosol interactions in the Caribbean region with a focus on Puerto Rico. The Caribbean is a highly convective and aerosol rich environment, with intense episodic Saharan dust intrusion peaking during the summer months. The work herein is concentrated on Puerto Rico because it is home to a variety of atmospheric data and instruments useful for CAST, including a three-channel LIDAR system, a ceilometer, multiple sunphotometers in the Aerosol Robotic Network (AERONET), soil moisture sensors, radiosondes, a disdrometer, and high resolution radar via TropiNet. The 2015 midsummer drought was extremely dry over Puerto Rico between 22 June - 10 July. Measurements indicate that the available precipitable water, precipitation frequency, and totals were well below climatological averages. A notable difference between the 2014 and 2015 observation periods was that in 2014, only 13% of campaign days had aerosol optical thicknesses (AOTs) above 0.6 and 73% of campaign days had rainfall, while there were 27% of campaign days with AOT above 0.6 and precipitation occurred on 21% of campaign days in 2015. These observations lead to the following questions that were investigated and will be reported on: a) Did the presence of large scale forcing such as a regional high pressure system and the presence of Saharan Dust (SD) have a causal relation with the summer dry period? b) Was vertical wind shear (VWS), which suppresses convection or sea-breeze trade-wind convergence with easterly winds which enhances convection more dominant during low SD periods? Initial results indeed indicate that SD aerosols may have played an important role in inhibiting regional summer precipitation and that convective storms due to the convergence of westerly sea breeze with easterly trade winds which transport moisture westward are enhanced in its absence, with further lifting due to western-island orographic processes. Analysis of remote sensing and in-situ data show that the presence of a high pressure system , high concentrations of SD, and lower available precipitable water than the seasonal average led to this dry period over the region.