A Cloud Microphysics Scheme to Resolve the Origins of the Caribbean Mid-Summer Drought

The annual precipitation pattern in the Caribbean basin shows a distinct bimodal behavior, where the first mode is called the Early Rainfall Season (ERS, April-July), and the second mode the Late Rainfall Season (LRS, August-November). The brief, relatively dry, period in July is usually referred to as the mid-summer drought (MSD). It has been hypothesized that the migration through the Caribbean basin of the Intertropical Convergence Zone (ITCZ) and increases in aerosols due to the passing of Saharan Dust across the Caribbean in the summer months may result in the observed precipitation pattern. This paper focuses in determining the origins of the Caribbean MSD. A multivariable regression analysis was carried out to determine which climatological variables correlate with the Caribbean MSD, these variables are the ITCZ, the North Atlantic Oscillation (NAO) index, Vertical Wind Shear (VWS) and different aerosol particle concentrations coming from northern Africa. It is shown that the ITCZ and the SST are weakly correlated with the Caribbean bimodal precipitation; however, the VWS and the aerosol particles revealed an important contribution to rainfall during the summer months. Spectral analysis revealed that the NAO has minimum periodicity leaving the VWS and AP as potential controlling variables. Numerical experiments are then performed to quantify individual contribution of the VWS and AP to the Caribbean precipitation bimodal behavior. The numerical approach uses the Regional Atmospheric Modeling System (RAMS) coupled with a new cloud microphysics module that allows discrimination between small and giant AP, as well as CCN/GCCN activation. The numerical experiments were design to evaluate the influence of different VWS scenarios and different AP concentrations on simulated Caribbean precipitation. These numerical experiments support the statistical result that the VWS and the AP influence the rainfall production during the MSD. Results indicate large-scale dynamics play a stronger role in determining the strength and pattern of the bimodal events or behavior, while cloud microphysics is more important in modulating the amounts of rainfall produced during each simulation. Semi-idealized numerical experiments were further performed to analyze in detail the interaction between cloud formation, rain development, accumulated precipitation, different AP concentrations, and their associated microphysical processes.