Orographic cloud is investigated in a global context using both observations and a global climate model. Climatological cloud amounts from the International Satellite Cloud Climatology Project (ISCCP) are used in conjunction with wind reanalyses to study orographic cirrus amounts over the globe. Significant increases in cirrus are seen over many land areas, with respect to any surrounding oceans. To aid in interpretation of this result special attention is given to the New Zealand region as a case study for orographic cloud formation. Cirrus is found be more prevalent over New Zealand when compared to the adjacent ocean to the west. ISCCP cloud amounts are also compared with a ten year simulation of the UK Meteorological Office's Unified Model. The model is found to be considerably lacking in both cirrus and total high cloud over major mountain ranges. The model is also found to lack trailing cirrus clouds in the lee of orography despite the inclusion of a prognostic ice variable capable of being advected by the model winds. To improve the simulations of orographic cirrus and high cloud in the Unified Model a linear hydrostatic gravity wave scheme that predicts both the amplitude and phase of subgrid orographic gravity waves is introduced. The temperature perturbation caused by these waves in the troposphere is used to modify the amount of both liquid and ice cloud. One important feature of the parameterisation is that the launch amplitude of the gravity waves is predicted by a directional variance function which accounts for anisotropy in the subgrid orography. The parameterisation is explored in the context of an off-line testbed before implementation in the Unified Model. In a ten year simulation the parameterisation is found to increase the high cloud amounts over a number of the world's major mountain ranges. However, this extra cloud is optically thick and unable to remove the deficiency in optically thin cirrus amounts. Suggestions, as part of future work, for improvements to the model and orographic cloud parameterisation are also made.

Orographic cloud is investigated in a global context using both observations and a global climate model. Climatological cloud amounts from the International Satellite Cloud Climatology Project (ISCCP) are used in conjunction with wind reanalyses to study orographic cirrus amounts over the globe. Significant increases in cirrus are seen over many land areas, with respect to any surrounding oceans. To aid in interpretation of this result special attention is given to the New Zealand region as a case study for orographic cloud formation. Cirrus is found be more prevalent over New Zealand when compared to the adjacent ocean to the west. ISCCP cloud amounts are also compared with a ten year simulation of the UK Meteorological Office's Unified Model. The model is found to be considerably lacking in both cirrus and total high cloud over major mountain ranges. The model is also found to lack trailing cirrus clouds in the lee of orography despite the inclusion of a prognostic ice variable capable of being advected by the model winds. To improve the simulations of orographic cirrus and high cloud in the Unified Model a linear hydrostatic gravity wave scheme that predicts both the amplitude and phase of subgrid orographic gravity waves is introduced. The temperature perturbation caused by these waves in the troposphere is used to modify the amount of both liquid and ice cloud. One important feature of the parameterisation is that the launch amplitude of the gravity waves is predicted by a directional variance function which accounts for anisotropy in the subgrid orography. The parameterisation is explored in the context of an off-line testbed before implementation in the Unified Model. In a ten year simulation the parameterisation is found to increase the high cloud amounts over a number of the world's major mountain ranges. However, this extra cloud is optically thick and unable to remove the deficiency in optically thin cirrus amounts. Suggestions, as part of future work, for improvements to the model and orographic cloud parameterisation are also made.

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.

Two aspects of the study of the Earth radiation budget and the effects of clouds on our climate system are considered in this dissertation : instrumentation and data interpretation. Numerical models have been developed to characterize the optical/thermal-radiative behavior, the dynamic electrothermal response and the structural thermal transients of radiometric channels. These models, applied to a satellite-borne scanning radiometer, are used to determine the instrument point spread function and the potential for optical and thermal-radiative contamination of the signal due to out-of-field radiation and emission from the radiometer structure. The capabilities of the model are demonstrated by scanning realistic Earth scenes. In addition, the optical/thermal-radiative model is used for the development of an infrared field radiometer to interpret results from the experimental characterization of the instrument. The model allowed the sensitivity of the instrument response to assembly uncertainties to be determined. Data processing consists of converting radiometric data into estimates of the flux at the top of the atmosphere. Primary error sources are associated with the procedures used to compensate for unsampled data. The time interpolation algorithm applied to a limited number of observations can produce significantly biased estimates of monthly mean fluxes. A diurnal interpolation protocol using correlative ISCCP cloudiness data is developed to compensate for sparse temporal sampling of Earth radiation budget data. The bias is shown to be significantly reduced in regions where the variability of the cloud cover is well accounted for by ISCCP data. / Ph. D.