The Role of Aerosols in Tropical Orographic Convection and Precipitation

Results from the Dominica Experiment field campaign (DOMEX) in April/May 2011 have shown a strong relationship between wind speed and convective type over a small island in the Caribbean. When the wind speed was low, thermally driven convection was present and surface derived tracers were lofted into the cloud. When the wind speed was strong, orographic uplift and plunging flow dominated, and little to no surface derived tracers were found in the convection above the island.

The droplet size is significantly smaller on days with low wind speed (15 μm) than on days with high wind speed (25 μm). Also, low wind days have a much higher aerosol number concentration than high wind days (Smith et al. 2012). Vigorous convection is found over Dominica on both high and low wind days, but little precipitation is found on low wind days (Nugent et al, submitted). We hypothesize that island-derived aerosols on low wind days get into the convective clouds, decreasing the size of the cloud droplets and thereby preventing precipitation.

This hypothesis is tested using Weather Research and Forecasting (WRF) model simulations and an updated version of the Thompson et al (2008) microphysics scheme. The modified microphysics parameterization explicitly predicts the number concentration of available aerosols and activates aerosols as CCN using look-up tables. The look-up tables were created from a detailed parcel model that treated the full Kohler theory for activating aerosols. A continuous source of aerosols is emitted from the surface of an idealized mountain where they are carried with the flow, explicitly involved in cloud microphysics, and depleted by precipitation. The resulting cloud droplet concentration varies in space and time and affects the rate of precipitation formation due to changes in overall mean size and precipitation efficiency. Comparing a high and low wind speed case, the affect of aerosols on convection and precipitation is explored.

References:

Nugent, A. D., R. B. Smith, and J. R. Minder: Wind speed control of tropical orographic convection. J. Atmos. Sci, submitted March 2013.

Smith, R. B., et al., 2012: Orographic precipitation in the tropics: the Dominica experiment. Bull. Amer. Meteor. Soc., 93, 1567–1579.

Thompson, G., P. R. Field, R. M. Rasmussen, W. D. Hall, 2008: Explicit forecasts of winter precipitation using an improved bulk microphysics scheme. part II: implementation of a new snow parameterization. Mon. Wea. Rev., 136, 5095–5115.