The effects of atmospheric particle concentration on cloud microphysics over Arecibo

A new cloud microphysics module incorporated to a regional atmospheric model, and atmospheric particle (AP) observations performed at the Arecibo Observatory were used to simulate a short precipitation event observed in the area of the observatory, and to investigate the possible effects of AP on cloud formation and rain development. First, model runs were performed with and without the new cloud module, initialized with the new AP dataset and the previous cloud spectrum available. The combination of the new cloud module and the Arecibo observations produced the most satisfactory results and significant improvements in total precipitation modeled; 70mm vs. 80mm observed. The improvement results in 15% more precipitation predicted when compared with the old cloud information, and more than 50% with respect to simulations without cloud condensation nuclei activation. Then, a set of idealized runs showed that cloud droplet production is significantly larger in polluted air than in clear skies and that rainwater in polluted air is less than that in unpolluted air. This may be because existing droplets will compete more vigorously for the available water vapor, will not reach the necessary radius to fall, and therefore growth by collision and coalescence is subdued.