The mechanism of the intraseasonal oscillation in the South Asian summer monsoon region (ISO) is examined with a zonally averaged, atmospheric model (2D model), a three dimensional, atmospheric intermediate model (3D model). In both models an ocean mixed layer model is added to examine the influence of air-sea interactions on the characteristics of the ISO. Without the ocean mixed layer, an interaction between the baroclinic and barotropic modes of atmosphere can produce the ISO in both 2D and 3D models. The propagation of precipitation is caused by the phase relationship between convection and the barotropic divergence in the atmosphere. Most importantly, in the northern hemisphere, the vertical advection of July-mean easterly wind shear in regions of convection induces barotropic divergence (convergence) to the north (south) of convection. The resulting moisture convergence in the boundary layer induces the northward propagation of precipitation. The initiation of convection is also produced by the barotropic divergence in the atmosphere. Especially, the strong July-mean vertical motion at IDS causes convergence in the boundary layer between IDS and the equator. The baroclinic mode, on the other hand, acts to enhance existing convection. The differences between the ISO simulated by the 2D model and 3D models are caused by the zonal variation of winds, and atmospheric waves in the 3D model. The zonal divergence of barotropic winds enhances the westward propagation of convection along 18N, and the barotropic mode of zonal advection drives the continuous northward movement of convection across the equator. The continuous northward propagation across the equator is also enhanced by the atmospheric waves, since the Rossby wave response to the heating source in both hemispheres creates a divergence in the baroclinic mode near the equator. The inclusion of air-sea interactions in the 2D and 3D models improves the continuity in the northward propagation of convection. The meridional variation of SST enhances the boundary layer moisture convergence in front of the convection, thereby facilitating the northward propagation of convection. In addition, the SST gradient induced by the dipole type of Rossby-wave-like convection in the Indian ocean may increase the development of convection near the equator. / Thesis (Ph. D.)--University of Hawaii at Manoa, 2002. / Mode of access: World Wide Web. / Includes bibliographical references (leaves 117-122). / Electronic reproduction. / Also available by subscription via World Wide Web / xxii, 240 leaves ill. 29 cm

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A ground-based near-infrared spectroradiometer was built and used to measure relative spectral reflectance from cumulus congestus and cumulonimbus clouds during the 1985 and 1986 Arizona summer monsoon seasons. Thermodynamic phase was inferred from spectral features in the regions between 1.55-1.75μm and 2.1-2.3μm where there are distinct differences between absorption in liquid water and ice and absorption by water vapor is very weak. Although liquid water and ice are nearly transparent in the visible, they absorb weakly in the near-infrared and that absorption is amplified by multiple scattering in clouds. Reflectance measurements are simple to make, requiring neither high spectral resolution nor absolute detector response. Three distinct aspects of differences between absorption in liquid water and ice were used to infer phase: (a) Ratio of the signal at 1.65 μm to that at 2.2 μm; (b) Wavelength of peak signal in the 1.65 μm water vapor transmission window; (c) Half-bandwidth of the 2.1-2.3 μm feature. Representative spectra are presented and analyzed on the basis of the predicted behavior of liquid water and ice cloud absorption. The results are consistent with young cumuli rapidly glaciating as they reach cooler levels, well before evidence of anvil formation or fibrous structure, contrary to the notion that phase can be inferred from visible cloud features.

Graduation date: 1989

Cloud reflectivity is a function of cloud liquid water content and droplet number concentration. Since cloud droplets form around pre-existing aerosol particles, cloud droplet number concentration depends on the availability of particles that can serve as cloud condensation nuclei. Given constant liquid water amount, increased availability of cloud condensation nuclei leads to clouds with a greater droplet number concentration, greater total droplet surface area and consequently, greater reflectivity. The change in cloud reflectivity resulting from the increased availability of condensation nuclei is known as the aerosol indirect effect. The aerosol indirect effect ranks as one of the largest sources of uncertainty in current estimates of global climate change, largely due to difficulties in measurement. Changes in cloud reflectivity resulting from the aerosol indirect effect are typically much smaller than the natural background variability observed in clouds. As a result, the modification signal is very difficult to detect against the background noise. Additionally, since atmospheric aerosols are ubiquitous, it is difficult to find polluted and nonpolluted clouds that are sufficiently alike for reasonable comparison. However, ship tracks seen in satellite images present one opportunity to study the aerosol indirect effect in relative isolation. Ship tracks are regions of enhanced reflectivity in marine stratus, resulting from the addition of aerosols from ship exhaust plumes to preexisting clouds. Ship tracks are a common feature of satellite images of the North Pacific. Since the marine atmosphere has comparatively low background aerosol concentrations, the addition of ship exhaust particles can lead to distinct increases in cloud reflectivity. Ship tracks allow for sampling of polluted and nonpolluted clouds from adjacent regions with similar solar and viewing geometry, cloud temperatures and surface properties, and consequently provide a unique opportunity to study the effects of aerosol modification of cloud reflectivity. Using satellite images of the North Pacific in July 1999, over 1000 ship tracks were identified, logged and analyzed, yielding 504 sets of radiance data matching polluted clouds with nearby nonpolluted clouds. It was expected that increasing the size of the region for selection of nonpolluted clouds would increase the variability in observed reflectivity, and make detection of the modification signal more difficult. In order to study this potential effect of domain size for selection of nonpolluted clouds on measurements of the aerosol indirect effect, three data sets were collected, using domain sizes for selection of nonpolluted clouds of 15, 50 and 100 km. Analysis of retrieved optical depth and droplet effective radius for modified and control pixels shows evidence of a 1-5% increase in visible optical depth of marine stratus following modification by addition of ship exhaust particles, but unexpectedly, shows only slight increases in uncertainty with increasing domain size. A subsequent study revealed that autocorrelation lengths of radiances and retrieved cloud properties were only 8-15 km. This indicates that even the 15 km control domain captured much of the background variability present. Domain sizes smaller than 15 km are difficult to sample automatically while avoiding the inclusion of polluted clouds in the nonpolluted cloud sample. As a result, it remains necessary to analyze large numbers of ship tracks to separate the aerosol modification signal from the background variability. / Graduation date: 2003

The Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) mission provides unique measurements of vertical profiles of aerosols and clouds and their properties during day and night-time over all types of surfaces. This information has the potential to significantly improve our understanding of the properties and effects of aerosol and clouds. This dissertation presents the results of a comprehensive analysis of CALIPSO lidar (version 2 and version 3.01) data in conjunction with A-Train satellite and ground-based observations aimed at characterizing mineral aerosol in East Asia and other major dust sources. The specific objectives were to characterize the spatial distribution and properties of atmospheric dust in the dust source regions using new CALIOP (version 3.01) data in conjunction with satellite MODIS, OMI, and CloudSat data and ground-based meteorological and lidar data; investigate changes in the vertical distribution and properties of dust during mid- and long-range transport; perform a modeling of the optical properties of nonspherical dust particles, and assess the radiative forcing and heating/cooling rates of atmospheric dust by performing radiative transfer modeling constrained by satellite data in major dust source regions.

Anthropogenic atmospheric aerosols (suspended particulate matter) can modify the radiative balance (and climate) of the Earth by altering the properties and global distribution of clouds. Current climate models however cannot adequately account for many important aspects of these aerosol-cloud interactions, ultimately leading to a large uncertainty in the estimation of the magnitude of the effect of aerosols on climate. This thesis focuses on the development of physically-based descriptions of aerosol-cloud processes in climate models that help to address some of such predictive uncertainty. It includes the formulation of a new analytical parameterization for the formation of ice clouds, and the inclusion of the effects of mixing and kinetic limitations in existing liquid cloud parameterizations. The parameterizations are analytical solutions to the cloud ice and water particle nucleation problem, developed within a framework that considers the mass and energy balances associated with the freezing and droplet activation of aerosol particles. The new frameworks explicitly account for the impact of cloud formation dynamics, the aerosol size and composition, and the dominant freezing mechanism (homogeneous vs. heterogeneous) on the ice crystal and droplet concentration and size distribution. Application of the new parameterizations is demonstrated in the NASA Global Modeling Initiative atmospheric and chemical and transport model to study the effect of aerosol emissions on the global distribution of ice crystal concentration, and, the effect of entrainment during cloud droplet activation on the global cloud radiative properties. The ice cloud formation framework is also used within a parcel ensemble model to understand the microphysical structure of cirrus clouds at very low temperature. The frameworks developed in this work provide an efficient, yet rigorous, representation of cloud formation processes from precursor aerosol. They are suitable for the study of the effect of anthropogenic aerosol emissions on cloud formation, and can contribute to the improvement of the predictive ability of atmospheric models and to the understanding of the impact of human activities on climate.