Understanding the Effect of African Dust Aerosols on Cloud Properties and Precipitation in Puerto Rico using the WRF model

Every year African dust is carried across the Tropical Atlantic into the Eastern Caribbean. In summertime African dust becomes a major component of aerosol in the Caribbean due to the long-range transport of African dust. To determine the impacts of long-range transported African dust (LRTAD) on the Caribbean climate and ecosystems and, hence, contribute to the improvement of regional climate models, the Puerto Rico African Dust and Clouds Study (PRADACS) sought to learn how the physico-chemical properties of LRTAD aerosols change typical Caribbean cloud characteristics and how these factors may affect precipitation in a tropical montane cloud forest in northeastern Puerto Rico. A combination of field and modeling work has been done to answer the PRADACS questions. Here, we focus on the modeling results generated in various test cases (July 2011) using the Weather Research and Forecasting (WRF) model version 3.7. The Thompson and Eidhammer “aerosol-aware” physics scheme has been used to determine the effect of LRTAD on precipitation levels in Puerto Rico. The aforementioned scheme incorporates the activation of cloud condensation nuclei (CCN) and ice nuclei (IN) to cloud drops and ice crystals using two aerosol categories, a “water friendly” aerosol (wfa) and an “ice-friendly” (ifa). The water-friendly aerosol category refers to the combination of sulfates, sea salts, and organic matter, whereas the ice-friendly aerosol category refers primarily to mineral dust. Black carbon is not included in these two aerosol variables. By default, the aerosol number concentrations can be taken from the climatology produced by the global model, GOCART (Goddard Chemistry Aerosol Radiation and Transport), but aerosols from other global model simulations can also be used. For our study, we use 6-hourly aerosol concentrations produced by the CAM-Chem (Community Atmosphere Model coupled with Chemistry) model simulation for the July 2011 period. Since LRTAD aerosols tend to be more hygroscopic once they reach the Caribbean, we modified the water-friendly aerosol category by adding dust and varied the hygroscopicity parameter and number concentration as a series of sensitivity studies to examine the effect of LRTAD on precipitation in typical convection over Puerto Rico. Further details of the methodology and results generated will be presented at the meeting.