"An attempt is made to examine the credibility and explore the implications of the reports of convective clouds to extreme heights which have been indicated by radar in recent years. The extreme cases have not been verified by independent observations and it is shown that inherent limitations in radar observations of convective cloud heights are such as to raise serious doubts in regard to the accuracy of many of the extreme echo heights which have been reported"--Abstract. / Typescript. / "Scientific report to the U.S. Navy Weather Research Facility, Norfolk, Virginia, under contract N189-50775A." / "15 April 1962." / Includes bibliographical references (leaves 32-34).

This study is mainly concentrated on examining the positive cloud-to-ground lightning activity associated with Mesoscale Convective Systems. Six MCS events which occurred during the O.K. PRE-STORM program in 1985 are studied. Data indicating the location and polarity of the cloud-to-ground lightning flashes from a lightning location network are analyzed in conjunction with the low-level echo patterns as obtained from radar. Spatial and temporal characteristics of positive cloud-to-ground flashes are identified from the data analysis. For all cases examined, positive cloud-to-ground flashes were found most commonly in the stratiform regions of the MCSs examined, and their frequency tended to peak during the later stages of the storm lifecycle. Two mechanisms responsible for the occurrence of positive cloud-to-ground lightning flashes with the above spatial and temporal characteristics are discussed. Based on the laboratory results, a 1-D charge generation model is developed. The model results show that in-situ charging is unlikely to be the dominant mechanism for charge generation in the stratiform region under normal atmospheric conditions. Sensitivity studies show, however, that in-situ charging processes strongly depend upon the liquid water, graupel and snow contents in the cloud. Under favorable atmospheric conditions, in-situ charging may lead to a significant charge generation. Hence, we cannot completely dismiss in-situ charging mechanism. Analysis of wind fields from dual-Doppler radar in combination with vertical profile of electric fields indicates that charge advection from the convective region to the stratiform region of MCSs may be a potential mechanism responsible for the occurrence of positive cloud-to-ground lightning flashes in the stratiform region. / Graduation date: 1989

Thesis (M. Sc.)--University of Canterbury, 2009. / Typescript (photocopy). Includes bibliographical references (p. 109-120). Also available via the World Wide Web.

Thesis (M.S.)--University of Wisconsin--Madison, 1984. / Typescript. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaves 153-162).

Surface heterogeneities cause differential heating that can generate mesoscale convective boundaries, sometimes leading to cloud development and enhanced localized precipitation. A preferred cloud pattern has been identified across Maryland and the Delmarva Peninsula region from 1998-2006 through the detection of cumuliform clouds on days when synoptic-scale forcing is weak. Hourly visible Geostationary Operational Environmental Satellite (GOES) imagery data are used to identify convective cloud masses. This allows quantitative description of the frequency and spatiotemporal extent of the clouds, helping forecasters gain insight into when and where they are likely to develop. Despite the inability to determine the underlying causes of the distinct cloud pattern, primarily due to the complex land cover, results indicate that the land receives significantly higher average total cloud cover than the Chesapeake Bay with Delaware receiving the highest average total cloud cover per state. Average total precipitation amounts follow this same trend on synoptically-weak days.

Thermodynamical and dynamical aspects of tropical cloud clusters are studied using data from the GARP Atlantic Tropical Experiment (GATE). The data set used in this study is a three-dimensional gridded set of upper-air analyses constructed by Ooyama and Chu (Hurricane Research Division, AOML/NOAA and SSEC-University of Wisconsin) for wind data and Esbensen (Oregon State University) for thermodynamic data. The energy and momentum budgets are estimated on the scale of large cloud clusters. A strong upper-tropospheric heat source and middle-tropospheric drying are characteristic features of the mature stage of the observed cloud clusters. The heat source, moisture sink and the virtual heat flux for cloud clusters are larger than the corresponding quantities from GATE easterly-wave composites. The surface precipitation estimates produced from the vertically integrated moisture budget are consistent with direct observations. From the momentum budget study, the following conclusions are drawn concerning the cumulus momentum effects. In the growing stage, the mesoscale and cumulus scale effect tends to: 1) provide a vertically integrated net sink for westerly momentum around the cluster center; 2) induce a convergent circulation in the lower layer. In the mature stage, the effects are to: 1) induce a divergent circulation in the upper layer and maintain a vorticity couplet pattern; 2) maintain a weak convergent circulation in the lower layer; and 3) cause a relatively weak easterly acceleration in the upper layer at the center. A hypothesis is postulated to illustrate the convective dynamical effects. A simple barotropic non-divergent model was constructed to investigate the large-scale response to the hypothesized cumulus momentum forcing similar to that found in the GATE cloud-cluster momentum budget. The numerical results show that the cumulus momentum forcing is a plausible kinetic energy source for the mesoscale wavenumber spectrum. The sporadic nature of the convective mass flux does not have a significant effect on the large-scale dynamical response for physically realistic parameters in a barotropic non-divergent dynamical system. / Graduation date: 1987

This work investigates the effects of anthropogenic aerosols on deep convective clouds and the associated radiative forcing in the Houston area. The Goddard Cumulus Ensemble model (GCE) coupled with a spectral-bin microphysics is employed to investigate the aerosol effects on clouds and precipitation. First, aerosol indirect effects on clouds are separately investigated under different aerosol compositions, concentrations and size distributions. Then, an updated GCE model coupled with the radiative transfer and land surface processes is employed to investigate the aerosol radiative effects on deep convective clouds. The cloud microphysical and macrophysical properties change considerably with the aerosol properties. With varying the aerosol composition from only (NH4)2SO4, (NH4)2SO4 with soluble organics, to (NH4)2SO4 with slightly soluble organics, the number of activated aerosols decreases gradually, leading to a decrease in the cloud droplet number concentration (CDNC) and an increase in the droplet size. Ice processes are more sensitive to the changes of aerosol chemical properties than the warm rain processes. The most noticeable effect of increasing aerosol number concentrations is an increase of CDNC and cloud water content but a decrease in droplet size. It is indicated that the aerosol indirect effect on deep convection is more pronounced in relatively clean air than in heavily polluted air. The aerosol effects on clouds are strongly dependent on RH: the effect is very significant in humid air. Aerosol radiative effects (ARE) on clouds are very pronounced for mid-visible single-scattering albedo (SSA) of 0.85. Relative to the case without the ARE, cloud fraction and optical depth decrease by about 18% and 20%, respectively. The daytime-mean direct forcing is about 2.2 W m-2 at the TOA and -17.4 W m-2 at the surface. The semi-direct forcing is positive, about 10 and 11.2 W m-2 at the TOA and surface, respectively. Aerosol direct and semi-direct effects are very sensitive to SSA. The cloud fraction, optical depth, convective strength, and precipitation decrease with the increase of absorption, resulting from a more stable atmosphere due to enhanced surface cooling and atmospheric heating.

This work investigates the effects of anthropogenic aerosols on deep convective clouds and the associated radiative forcing in the Houston area. The Goddard Cumulus Ensemble model (GCE) coupled with a spectral-bin microphysics is employed to investigate the aerosol effects on clouds and precipitation. First, aerosol indirect effects on clouds are separately investigated under different aerosol compositions, concentrations and size distributions. Then, an updated GCE model coupled with the radiative transfer and land surface processes is employed to investigate the aerosol radiative effects on deep convective clouds. The cloud microphysical and macrophysical properties change considerably with the aerosol properties. With varying the aerosol composition from only (NH4)2SO4, (NH4)2SO4 with soluble organics, to (NH4)2SO4 with slightly soluble organics, the number of activated aerosols decreases gradually, leading to a decrease in the cloud droplet number concentration (CDNC) and an increase in the droplet size. Ice processes are more sensitive to the changes of aerosol chemical properties than the warm rain processes. The most noticeable effect of increasing aerosol number concentrations is an increase of CDNC and cloud water content but a decrease in droplet size. It is indicated that the aerosol indirect effect on deep convection is more pronounced in relatively clean air than in heavily polluted air. The aerosol effects on clouds are strongly dependent on RH: the effect is very significant in humid air. Aerosol radiative effects (ARE) on clouds are very pronounced for mid-visible single-scattering albedo (SSA) of 0.85. Relative to the case without the ARE, cloud fraction and optical depth decrease by about 18% and 20%, respectively. The daytime-mean direct forcing is about 2.2 W m-2 at the TOA and -17.4 W m-2 at the surface. The semi-direct forcing is positive, about 10 and 11.2 W m-2 at the TOA and surface, respectively. Aerosol direct and semi-direct effects are very sensitive to SSA. The cloud fraction, optical depth, convective strength, and precipitation decrease with the increase of absorption, resulting from a more stable atmosphere due to enhanced surface cooling and atmospheric heating.

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No description available.