Browsing by Subject "aerosol-cloud interactions"
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Item Aircraft Observations of Sub-cloud Aerosol and Convective Cloud Physical Properties(2011-02-22) Axisa, DuncanThis research focuses on aircraft observational studies of aerosol-cloud interactions in cumulus clouds. The data were collected in the summer of 2004, the spring of 2007 and the mid-winter and spring of 2008 in Texas, central Saudi Arabia and Istanbul, Turkey, respectively. A set of 24 pairs of sub-cloud aerosol and cloud penetration data are analyzed. Measurements of fine and coarse mode aerosol concentrations from 3 different instruments were combined and fitted with lognormal distributions. The fit parameters of the lognormal distributions are compared with cloud droplet effective radii retrieved from 260 cloud penetrations. Cloud condensation nuclei (CCN) measurements for a subset of 10 cases from the Istanbul region are compared with concentrations predicted from aerosol size distributions. Ammonium sulfate was assumed to represent the soluble component of aerosol with dry sizes smaller than 0.5 mm and sodium chloride for aerosol larger than 0.5 mm. The measured CCN spectrum was used to estimate the soluble fraction. The correlations of the measured CCN concentration with the predicted CCN concentration were strong (R2 > 0.89) for supersaturations of 0.2, 0.3 and 0.6%. The measured concentrations were typically consistent with an aerosol having a soluble fraction between roughly 0.5 and 1.0, suggesting a contribution of sulfate or some other similarly soluble inorganic compound. The predicted CCN were found to vary by +or-3.7% when the soluble fraction was varied by 0.1. Cumulative aerosol concentrations at cutoff dry diameters of 1.1, 0.1 and 0.06 mm were found to be correlated with cloud condensation nuclei concentrations but not with maximum cloud base droplet concentrations. It is also shown that in some cases the predominant mechanisms involved in the formation of precipitation were altered and modified by the aerosol properties. This study suggests that CCN-forced variations in cloud droplet number concentration can change the effective radius profile and the type of precipitation hydrometeors. These differences may have a major impact on the global hydrological cycle and energy budget.Item Simulation of Aerosol-Cloud Interactions in the WRF Model at the Southern Great Plains Site(2012-08-21) Vogel, Jonathan 1988-The aerosol direct and indirect effects were investigated for three specific cases during the March 2000 Cloud IOP at the SGP site by using a modified WRF model. The WRF model was previously altered to include a two-moment bulk microphysical scheme for the aerosol indirect effect and a modified Goddard shortwave radiation scheme for the aerosol direct effect. The three cases studied include a developing low pressure system, a low precipitation event of mainly cirrus clouds, and a cold frontal passage. Three different aerosol profiles were used with surface concentrations ranging from 210 cm-3 to 12,000 cm-3. In addition, each case and each aerosol profile was run both with and without the aerosol direct effect. Regardless of the case, increasing the aerosol concentration generally increased cloud water and droplet values while decreasing rain water and droplet values. Increased aerosols also decreased the surface shortwave radiative flux for every case; which was greatest when the aerosol direct effect was included. For convective periods during polluted model runs, the aerosol direct effect lowered the surface temperature and reduced convection leading to a lower cloud fraction. During most convective periods, the changes to cloud, rain, and ice water mixing ratios and number concentrations produced a nonlinear precipitation trend. A balance between these values was achieved for moderate aerosol profiles, which produced the highest convective precipitation rates. In non-convective cases, due to the presence of ice particles, aerosol concentration and precipitation amounts were positively correlated. The aerosol threshold between precipitation enhancement and suppression should be further studied for specific cloud types as well as for specific synoptic weather patterns to determine its precise values.