Observed Characteristics of Clouds and Precipitating Systems Associated with the Tropical Circulation in Global Models and Reanalyses
Stachnik, Justin Paul
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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 Ni?o 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.