Modeling the effects of enhanced aerosol concentrations on urban convection

The relationships that account for the observed anomalies in precipitation and severe weather over and downwind of St Louis, Missouri are still not well understood, in spite of good field studies and data analyses. Similar uncertainties exist for the city of Houston, Texas and other urban areas. One of the leading hypotheses explaining these anomalies is the so-called “glaciation” mechanism. This mechanism is related to air pollutants emanating from an urban area which are generally rich in cloud condensation nuclei (CCN). It is well known that enhanced CCN concentrations can result in narrower droplet spectra and thereby suppress warm rain processes. On the other hand, some urban areas, like St. Louis can also be sources of giant CCN (GCCN) or ultra-giant particles which can enhance warm rain processes. The presence of supercooled raindrops greatly enhances the rapidity of glaciation of cumuli by virtue of the fact that supercooled droplets readily collect ice crystals and freeze and by enhancing the Hallett-Mossop, rime-splintering process.

A series of numerical model simulations, using the Regional Atmospheric Modeling System developed at Colorado State University (RAMS@CSU), has been designed to investigate the hypothesized glaciation mechanism on convective storm evolution and precipitation over and downwind of St. Louis, MO. The sophisticated "Town Energy Budget"(TEB) model of the urban land surface, and the two-moment bulk microphysics which allows for the prognosis of CCN and GCCN, will both be utilized. Forty vertical levels and three two-way interactive nested model grids centered over St. Louis, with horizontal grid spacing of 37.5 km, 7.5 km and 1.5 km for grids 1 through 3 respectively, will be employed. The fine grid spacing on grid 3 is intended to resolve convection explicitly. A control simulation will be conducted in which the model is initialized with “clean air” vertical CCN and GCCN profiles. Once the control simulation has been performed, several sensitivity tests will be conducted in which the initial vertical profiles of CCN, GCCN and both CCN and GCCN will be progressively varied over the urban complex. The initialization profiles will be based on the data acquired during METROMEX. The impacts of varying the initial aerosol concentrations on precipitation and other atmospheric variables will be compared with those of the control simulation. The results of these comparisons will be presented.