This dissertation presents a series of work related to the representation of the Hadley circulation (HC) in atmospheric reanalyses and general circulation models (GCMs), with connections to the underlying tropical and subtropical cloud systems that comprise the mean meridional circulation. An intercomparison of eight atmospheric reanalyses showed that significant variability exists in the mean state for HC intensity, with less variability in HC width. Ensemble trends were broadly consistent with previous work and suggest a strengthening and widening of the tropical circulation over the last 30 years. Composite profiles of the apparent heat source and moisture sink were calculated for the International Satellite Cloud Climatology Project (ISCCP) cloud regimes using sounding observations from 10 field campaigns. Distinct heating profiles were determined for each ISCCP cloud regime, ranging from strong, upper-tropospheric heating for mesoscale convective systems to integrated cooling for populations associated with marine stratus and stratocumulus clouds. The derived profiles were generally similar over land and ocean with the notable exception of the fair-weather cumulus regime, which leads to some uncertainty in the mid- and upper-level reconstruction of subtropical heating. An instrument simulator indicated that low-latitude cloud properties from the NASA MERRA reanalysis qualitatively matched the distributions of cloud-top pressure and optical thickness in the ISCCP data, though the tallest and thickest clouds were missing from the reanalysis. Simulator results were sensitive to the choice of cloud overlap parameterization and the reanalysis consistently underpredicted the observed cloud fractions for all regimes. The vertical velocity, temperature, and moisture for each regime in MERRA largely matched observations from previous studies, suggesting that the dynamic and thermodynamic properties of the cloud regimes are well captured by the reanalysis. Finally, HC interannual variability was examined as a function of the observed frequency of the ISCCP cloud regimes. The strongest HC overturning events were attributed to an El Niño response in the central Pacific Ocean in addition to links between the intensity and position of the Pacific ITCZ. The ISCCP regime describing the most vigorous and organized convection contributed the most towards the total anomalous heating during HC extremes, despite an overall low frequency of occurrence. Idealized GCM simulations forced with the observed three-dimensional diabatic heating from ISCCP data produced too strong a HC with some improvement in other fields. Overall, much progress has been made regarding the links between low-latitude cloud systems and the HC, though future work will continue to address the upscale feedbacks of regional cloud variations upon the tropical circulation.

Dust-climate interaction over the Middle East and North Africa (MENA) has long been studied, as it is the "dustiest" region on earth. However, the quantitative and qualitative understanding of the role of dust direct radiative effect on MENA climate is still rudimentary. The present dissertation investigates dust direct radiative effect on MENA climate during summer with a special emphasis on the sensitivity of climate response to dust shortwave absorption, which is one of the most uncertain components of dust direct radiative effect. Simulations are conducted with and without dust radiative effect, to differentiate the effect of dust on climate. To elucidate the sensitivity of climate response to dust shortwave absorption, simulations with dust assume three different cases of dust shortwave absorption, representing dust as a very efficient, standard and inefficient shortwave absorber. The non-uniformly distributed dust perturb circulations at various scales. Therefore, the present study takes advantage of the high spatial resolution capabilities of an Atmospheric General Circulation Model (AGCM), High Resolution Atmospheric Model (HiRAM), which incorporates global and regional circulations. AMIP-style global high-resolution simulations are conducted at a spatial resolution of 25 km. A significant response in the strength and position of the local Hadley circulation is predicted in response to meridionally asymmetric distribution of dust and the corresponding radiative effects. Significant responses are also found in regional circulation features such as African Easterly Jet and West African Monsoon circulation. Consistent with these dynamic responses at various scales, the tropical rainbelt across MENA strengthens and shifts northward. Similarly, the temperature under rainbelt cools and that over subtropical deserts warms. Inter-comparison of various dust shortwave absorption cases shows that the response of the MENA tropical rainbelt is extremely sensitive to the strength of shortwave absorption. Further analyses reveal that the sensitivity of the rainbelt stems from the sensitivity of the multi-scale circulations that define the rainbelt. Importantly, the summer precipitation over the semi-arid strip south of Sahara, including Sahel, increases in response to dust radiative effect. The maximum response and sensitivity are predicted over this region. The sensitivity of the responses over Sahel, especially that of precipitation, is comparable to the mean state. Locally, the precipitation increase reaches up to 50% of the mean, while dust is assumed to be a very efficient absorber. As the region is characterized by the "Sahel drought", the predicted precipitation sensitivity to the dust loading over this region has a wide-range of socioeconomic implications. The present study, therefore, suggests the importance of reducing uncertainty in dust shortwave absorption for a better simulation and interpretation of the MENA climate in general, and of Sahel in particular.

This thesis provides the measurements of 222Rn exhalation rates, 210Pb deposition rates and excess 210Pb inventories for locations in and around Ranger Uranium Mine and Jabiru located within Kakadu National Park, Australia. Radon-222 is part of the natural 238U series decay chain and the only gas to be found in the series under normal conditions. Part of the natural redistribution of 222Rn in the environment is a portion exhales from the ground and disperses into the atmosphere. Here it decays via a series of short-lived progeny, that attach themselves to aerosol particles, to the long lived isotope 210Pb (T1/2 = 22.3 y). Attached and unattached 210Pb is removed from the atmosphere through wet and dry deposition and deposited on the surface of the earth, the fraction deposited on soils is gradually transported through the soil and can create a depth profile of 210Pb. Here it decays to the stable isotope 206Pb completing the 238U series. Measurements of 222Rn exhalation rates and 210Pb deposition rates were performed over complete seasonal cycles, August 2002 - July 2003 and May 2003 - May 2004 respectively. The area is categorised as wet and dry tropics and it experiences two distinct seasonal patterns, a dry season (May-October) with little or no precipitation events and a wet season (December-March) with almost daily precipitation and monsoonal troughs. November and April are regarded as transitional months. As the natural processes of 222Rn exhalation and 210Pb deposition are heavily influenced by soil moisture and precipitation respectively, seasonal variations in the exhalation and deposition rates were expected. It was observed that 222Rn exhalation rates decreased throughout the wet season when the increase in soil moisture retarded exhalation. Lead-210 deposition peaked throughout the wet season as precipitation is the major scavenging process of this isotope from the atmosphere. Radon-222 is influenced by other parameters such as 226Ra activity concentration and distribution, soil porosity and grain size. With the removal of the influence of soil moisture during the dry season it was possible to examine the effect of these other variables in a more comprehensive manner. This resulted in categorisation of geomorphic landscapes from which the 222Rn exhalation rate to 226Ra activity concentration ratios were similar during the dry season. These results can be extended to estimate dry season 222Rn exhalation rates from tropical locations from a measurement of 226Ra activity concentration. Through modelling the 210Pb budget on local and regional scales it was observed that there is a net loss of 210Pb from the region, the majority of which occurs during the dry season. This has been attributed to the fact that 210Pb attached to aerosols is transported great distance with the prevailing trade winds created by a Hadley Circulation cell predominant during the dry season (winter) months. By including the influence of factors such as water inundation and natural 210Pb redistribution in the soil wet season budgeting of 210Pb on local and regional scales gave very good results.