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This dissertation is a collection of studies focusing on improving our understanding of atmospheric aerosols using both observational data and model simulations. EOF analysis of Aerosol Index (AI) product from Total Ozone Mapping Spectrometer (TOMS) and Ozone Monitoring Instrument (OMI) reveals global distribution of absorbing aerosols, with major sources lying in Sahara deserts, the Sahel region, South America and South Africa. Analysis of aerosol Single Scattering Albedo (SSA) data from AErosol RObotic NETwork (AERONET) further indicate trends in SSA over a number of globally distributed stations, which might be associated with changes in aerosol composition and thus their optical properties. More importantly, the changes in SSA alter the radiative forcing of aerosols. They may also potentially impact satellite retrievals of aerosol properties as generally a constant SSA is assumed in the retrieval algorithms. In order to assess satellite retrieved aerosol optical properties, collocated pixel level Aerosol Optical Depth (AOD) and Ångström Exponent (AE) data from MODerate resolution Imaging Spectroradiometer (MODIS) are compared with AERONET measurements over 10 stations representing typical aerosol regimes. The results show that while MODIS AOD well agrees with AERONET in both the magnitude and seasonal variability for all stations, comparatively large discrepancies are found in the AE, especially for over land. Further investigation reveals that the dependence of the AE on AOD for MODIS data are quite different from AERONET data, which suggest problems in the aerosol models used in MODIS retrieval. MODIS ocean data are generally reliable. Focusing on ocean data, a strong correlation between the AE and ENSO index has been found, and the roles of relevant physical mechanisms are discussed. While the exact cause of the correlation is still unclear, the results indicate aerosol properties can be influenced by major climate modes such as ENSO. The sensitivity of aerosol Direct Radiative Forcing (DRF) to perturbations of major aerosol parameters are tested using the GISS GCM. Among the three perturbed parameters, AOD, SSA and asymmetry parameter g, DRF appears to be most sensitive to SSA. Moreover, changing aerosol dry sizes result in larger fluctuation in DRF than the previous three parameters. Based on the sensitivity studies, an optimal fitting technique based on AERONET data is developed to better constrain aerosol dry size parameterization in the GCM. Model results for AOD and SSA are also improved by adjusting the size and applying "uncertainty parameters". The fitting results indicate an overall underestimate in GCM aerosol loading. In particular, aerosol absorption has been underestimated by approximately a factor of 2. The low bias might be attributed to insufficient aerosol mass loading, lack of internal mixing of black carbon with other species, etc. After incorporating the optimized sizes and uncertainty parameters into the GCM, estimated global mean DRF is significantly larger than the original aerosol field. Regionally the changes in DRF are more diverse due to the relative fraction of absorbing and non-absorbing aerosols. The method still has limitations. Further improvements are required including examining the fine/coarse aerosol fraction, better identifying the absorbing species, and using advanced observations with global coverage.

For the first time in the history of the Intergovernmental Panel on Climate Change, the Paleoclimate and Coupled Model Intercomparison Projects Phases 3 and 5 produced an ensemble of forced transient simulations of the last millennium. This wealth of model output, when combined with a growing collection of high spatial and temporal resolution pa- leoclimate estimates of past climate variability, represents an important and unprecedented source of information on climate variability over decades to centuries. This dissertation thus combines paleoclimate evidence with climate modeling to define a physical and statistical paradigm through which to analyze these combined sources of information and subsequently to characterize the features of the North American climate system that cannot be sufficiently understood using instrumental data alone. This includes features that have long timescales of variability or that are rare, and by consequence have few degrees of freedom over the short instrumental interval (1850 C.E. to Present), as well as interannual dynamical relationships that, while potentially well characterized by observations, are non-stationary. An integrative approach to analyzing these features or relationships serves two fundamental purposes: 1) It provides a more comprehensive characterization of past climate variability, albeit with the caveat of model bias, to clarify understanding of the dynamics that produce these features or relationships in the real world; and 2) it assesses whether coupled general circulation models (CGCMs) are able to simulate these features or relationships, which is necessary to determine that state-of-the-art CGCMs can accurately constrain the risk of future climate change. The focus herein will be on multidecadal hydroclimate change, or megadroughts, in the paleoclimate record of the American Southwest to better inform our understanding of the risk of future hydroclimate change over the region. Two fundamental understandings are derived from this work. Firstly, CGCMs are successful at simulating discrete periods of mul- tidecadal hydroclimate change that are characteristic in length, magnitude, and frequency of occurrence of megadroughts in the paleoclimate record. The simulated megadroughts are not tied in any coherent way to exogenous forcing, however, suggesting that CGCMs simulate large-magnitude internal variability on multidecadal timescales. Secondly, the dynamical characteristics of CGCMs are important in determining the atmosphere-ocean variability that drives multidecadal hydroclimate change. The dynamical characteristics of relevance include teleconnection realism and stationarity, the magnitude of ocean variability, and the relative magnitudes of different modes of atmosphere-ocean and purely atmospheric vari- ability. Additionally, a new understanding of real-world megadrought dynamics is derived herein, with the characteristics of some CGGMs providing a better representation of these dynamics.

A three-dimensional limited-domain Cloud-Resolving Model (CRM) is used in idealized settings to study the interaction between tropical convection and the large scale dynamics. The model domain is doubly periodic and the large-scale circulation is parameterized using the Weak Temperature Gradient (WTG) Approximation and Damped Gravity Wave (DGW) methods. The model simulations fall into two main categories: simulations with a prescribed radiative cooling profile, and others in which radiative cooling profile interacts with clouds and water vapor. For experiments with a prescribed radiative cooling profile, radiative heating is taken constant in the vertical in the troposphere. First, the effect of turbulent surface fluxes and radiative cooling on tropical deep convection is studied. In the precipitating equilibria, an increment in surface fluxes produces a greater increase in precipitation than an equal increment in column-integrated radiative heating. The gross moist stability remains close to constant over a wide range of forcings. With dry initial conditions, the system exhibits hysteresis, and maintains a dry state with for a wide range of net energy inputs to the atmospheric column under WTG. However, for the same forcings the system admits a rainy state when initialized with moist conditions, and thus multiple equilibria exist under WTG. When the net forcing is increased enough that simulations, which begin dry, eventually develop precipitation. DGW, on the other hand, does not have the tendency to develop multiple equilibria under the same conditions. The effect of vertical wind shear on tropical deep convection is also studied. The strength and depth of the shear layer are varied as control parameters. Surface fluxes are prescribed. For weak wind shear, time-averaged rainfall decreases with shear and convection remains disorganized. For larger wind shear, rainfall increases with shear, as convection becomes organized into linear mesoscale systems. This non-monotonic dependence of rainfall on shear is observed when the imposed surface fluxes are moderate. For larger surface fluxes, convection in the unsheared basic state is already strongly organized, but increasing wind shear still leads to increasing rainfall. In addition to surface rainfall, the impacts of shear on the parameterized large-scale vertical velocity, convective mass fluxes, cloud fraction, and momentum transport are also discussed. For experiments with interactive radiative cooling profile, the effect of cloud- radiation interaction on cumulus ensemble is examined in sheared and unsheared environments with both fixed and interactive sea surface temperature (SST). For fixed SST, interactive radiation, when compared to simulations in which radiative profile has the same magnitude and vertical shape but does not interact with clouds or water vapor, is found to suppress mean precipitation by inducing strong descent in the lower troposphere, increasing the gross moist stability. For interactive SST, using a slab ocean mixed layer, there exists a shear strength above which the system becomes unstable and develops oscillatory behavior. Oscillations have periods of wet precipitating states followed by periods of dry non-precipitating states. The frequencies of oscillations are intraseasonal to subseasonal, depending on the mixed layer depth. Finally, the model is coupled to a land surface model with fully interactive radiation and surface fluxes to study the diurnal and seasonal radiation and water cycles in the Amazon basin. The model successfully captures the afternoon precipitation and cloud cover peak and the greater latent heat flux in the dry season for the first time; two major biases in GCMs with implications for correct estimates of evaporation and gross primary production in the Amazon. One of the key findings is that the fog layer near the surface in the west season is crucial for determining the surface energy budget and precipitation. This suggests that features on the diurnal time scale can significantly impact climate on the seasonal time scale.

Thesis (Ph.D.)--City University of Hong Kong, 2006. / "Submitted to Department of Biology and Chemistry in partial fulfillment of the requirements for the degree of Doctor of Philosophy" Includes bibliographical references.

No description available.

Thesis (Ph. D.)--University of Wisconsin--Madison, 1981. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaves 99-104).

Thesis (M.S.)--University of Wisconsin--Madison, 1983. / Typescript. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaves 69-72).

Thesis (Ph. D.)--University of Wisconsin--Madison, 1983. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaves 265-273).

Global and regional-scale climatic changes caused by volcanic eruptions are difficult to discern conclusively based on limited 20th century climate records. Analyses of paleoclimate records and global climate model (GCM) simulations indicate that a significantly increased volcanic signal occurs in East Asia in response to historical eruptions, many of which were much larger than those experienced in the 20th century. Records of historical floods vs. drought in eastern China suggest that major eruptions over the past millennium typically led to a relatively wet north and a dry south. The GCM simulates a 10% reduction in the strength of tropical Hadley circulation and significantly decreased precipitation throughout the tropics under Tambora-like volcanic forcing conditions. The volcanic-induced weakening of the West Pacific sub-tropical high apparently contributes to a decrease in modeled precipitation throughout northeastern China. Meanwhile a general decrease in tropical precipitation resulting from reduced incoming solar radiation and lower evaporation is hypothesized to have caused observed (and modeled) decreases in summertime precipitation in southeastern China.