85th AMS Annual Meeting

Tuesday, 11 January 2005: 11:00 AM
The impacts of Saharan dust on Florida storm characteristics
Susan C. van den Heever, Colorado State University, Fort Collins, CO; and G. G. Carrió, W. R. Cotton, and W. C. Straka
Poster PDF (1.4 MB)
Dust serves as effective cloud condensation nuclei (CCN), giant CCN (GCCN) and ice forming nuclei (IFN). Dust can therefore have significant effects on the nucleation of ice and liquid water, which in turn may affect other microphysical and dynamical characteristics of convective storm systems. It has been estimated that the origin of as much as 60% of dust can be traced to human activities. High concentrations of Saharan dust were observed on 28 and 29 July during the CRYSTAL-FACE (Cirrus Regional Study of Tropical Anvils and Cirrus Layers – Florida Area Cirrus Experiment) field program conducted over the Florida peninsula by NASA during July 2002. The data from this field program provide us with the opportunity to compare the characteristics of storms that developed on “clean” and “dusty” days. The goal of the research to be presented is thus to investigate the impacts that increasing the concentrations of Saharan dust may have on the characteristics of the convective and anvil stages of Florida convection, through the use of a cloud-resolving numerical model.

The Regional Atmospheric Modeling System (RAMS) developed at Colorado State University is being used to achieve our goal. Four nested grids are employed, the horizontal grid spacing of which is 40, 10, 2 and 0.5 km for grids 1 to 4, respectively. A stretched 36-level grid is used in the vertical. RAMS is initialized at 12Z with 40km ETA data, and the simulations are performed for 12 hours. Both hydrometeor mixing ratios and number concentrations are predicted through the use of the two-moment bulk microphysics scheme, and CCN, GCCN and IFN concentrations are all prognosed in RAMS. Generalized vertical profiles of CCN, GCCN and IFN concentrations for “clean” and “dusty” days have been generated using CRYSTAL-FACE data. These profiles are used to initialize the model. The 16 and 28 July 2002 have been chosen as suitable case study days, the latter being a day on which high dust concentrations were observed, while only moderate amounts of dust were observed on 16 July. For the control simulation, the clean vertical profiles are used for all three aerosol species. Sensitivity tests are then conducted in which various combinations of the clean and dusty profiles are used for CCN, GCCN and IFN. The sensitivity tests already conducted for 28 July demonstrate that variations in the concentrations of CCN, GCCN and IFN have significant impacts on the anvil size and thickness, on the number and strength of the updrafts produced, on the liquid water and ice mixing ratios within the updraft core and in the anvil, and on the precipitation produced at the surface. The output from the 16 July case will be compared with those of 28 July to determine whether the results are consistent. Precipitation efficiencies and anvil efficiencies will also be calculated for both case studies in order to determine the effects of dust on these quantities. The efficiencies calculated using the model output will be compared with those determined using radar data obtained during the field program.

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