Case-Study Modeling Urban Aerosol-Precipitation Interactions in Southeastern Louisiana

Although aerosol-precipitation interactions is a burgeoning area of study, particulate effects on thunderstorm morphology, especially over urban areas, have received less attention. Higher particulate concentrations near large cities, superimposed upon the urban rainfall effect, may yield competing influence on near-urban thunderstorm rainfall. For instance, while cities can promote thunderstorm development via the urban heat island, urban-enriched particulates simultaneously act as cloud condensation nuclei (CCN) concentrations, leading to diminutive, non-precipitating cloud droplets. The sensitivity of urban rainfall to these superimposed thermodynamic and microphysical effects is not well understood.

This study performs a case study examining the changes in convective hydrometeorology over a 24-hour period from 1200 UTC 7 July — 1200 UTC 8 July 2007 near the New Orleans metropolitan area. Four NASA-Unified Weather Research and Forecasting (NU-WRF), version 4.4, simulations assess the sensitivity of near-urban convective morphology and precipitation to aerosol effects and the city landscape. NU-WRF is forced using the North American Mesoscale (NAM) model with both atmospheric chemistry and urban land cover iteratively turned on and off.

Compared to satellite-based precipitation during the study period, all simulations provide reasonable spatial accuracy of thunderstorm rainfall. The two urban-on simulations are more spatially accurate to radar observations. However, only the aerosol-on + urban-on simulation accurately produces the observed precipitation over the study area. The remaining simulations overproduce precipitation ranging from 7%-150%. Convective initiation began 1-2 hours sooner in the aerosol-on simulations and extended farther west in the domain where the particulate concentrations were greater.