Thermodynamical and dynamical aspects of tropical cloud clusters are studied using data from the GARP Atlantic Tropical Experiment (GATE). The data set used in this study is a three-dimensional gridded set of upper-air analyses constructed by Ooyama and Chu (Hurricane Research Division, AOML/NOAA and SSEC-University of Wisconsin) for wind data and Esbensen (Oregon State University) for thermodynamic data. The energy and momentum budgets are estimated on the scale of large cloud clusters. A strong upper-tropospheric heat source and middle-tropospheric drying are characteristic features of the mature stage of the observed cloud clusters. The heat source, moisture sink and the virtual heat flux for cloud clusters are larger than the corresponding quantities from GATE easterly-wave composites. The surface precipitation estimates produced from the vertically integrated moisture budget are consistent with direct observations. From the momentum budget study, the following conclusions are drawn concerning the cumulus momentum effects. In the growing stage, the mesoscale and cumulus scale effect tends to: 1) provide a vertically integrated net sink for westerly momentum around the cluster center; 2) induce a convergent circulation in the lower layer. In the mature stage, the effects are to: 1) induce a divergent circulation in the upper layer and maintain a vorticity couplet pattern; 2) maintain a weak convergent circulation in the lower layer; and 3) cause a relatively weak easterly acceleration in the upper layer at the center. A hypothesis is postulated to illustrate the convective dynamical effects. A simple barotropic non-divergent model was constructed to investigate the large-scale response to the hypothesized cumulus momentum forcing similar to that found in the GATE cloud-cluster momentum budget. The numerical results show that the cumulus momentum forcing is a plausible kinetic energy source for the mesoscale wavenumber spectrum. The sporadic nature of the convective mass flux does not have a significant effect on the large-scale dynamical response for physically realistic parameters in a barotropic non-divergent dynamical system. / Graduation date: 1987

This work investigates the effects of anthropogenic aerosols on deep convective clouds and the associated radiative forcing in the Houston area. The Goddard Cumulus Ensemble model (GCE) coupled with a spectral-bin microphysics is employed to investigate the aerosol effects on clouds and precipitation. First, aerosol indirect effects on clouds are separately investigated under different aerosol compositions, concentrations and size distributions. Then, an updated GCE model coupled with the radiative transfer and land surface processes is employed to investigate the aerosol radiative effects on deep convective clouds. The cloud microphysical and macrophysical properties change considerably with the aerosol properties. With varying the aerosol composition from only (NH4)2SO4, (NH4)2SO4 with soluble organics, to (NH4)2SO4 with slightly soluble organics, the number of activated aerosols decreases gradually, leading to a decrease in the cloud droplet number concentration (CDNC) and an increase in the droplet size. Ice processes are more sensitive to the changes of aerosol chemical properties than the warm rain processes. The most noticeable effect of increasing aerosol number concentrations is an increase of CDNC and cloud water content but a decrease in droplet size. It is indicated that the aerosol indirect effect on deep convection is more pronounced in relatively clean air than in heavily polluted air. The aerosol effects on clouds are strongly dependent on RH: the effect is very significant in humid air. Aerosol radiative effects (ARE) on clouds are very pronounced for mid-visible single-scattering albedo (SSA) of 0.85. Relative to the case without the ARE, cloud fraction and optical depth decrease by about 18% and 20%, respectively. The daytime-mean direct forcing is about 2.2 W m-2 at the TOA and -17.4 W m-2 at the surface. The semi-direct forcing is positive, about 10 and 11.2 W m-2 at the TOA and surface, respectively. Aerosol direct and semi-direct effects are very sensitive to SSA. The cloud fraction, optical depth, convective strength, and precipitation decrease with the increase of absorption, resulting from a more stable atmosphere due to enhanced surface cooling and atmospheric heating.

With increasing computational power, simulations of regional climate are now becoming possible on convective-resolving grids, thus eliminating the need for a convective parameterization. In the present study, a series of seasonal calculations using the Weather Research and Forecasting (WRF) model are computed at 4-km grid spacing, which reasonably resolves most convective systems. Simulations are computed for both the DJF and MAM seasons as averaged over 2005-2008, with a model domain covering the majority of the Amazon Basin and the adjacent South American coastline. Precipitation statistics are computed and compared to satellite rainfall retrieval data from the 13-year Tropical Rainfall Measuring Mission (TRMM) record. For comparison, a set of companion simulations with 12-km grid spacing are also computed, using the Kain-Fritsch convective parameterization. As compared to the 12-km runs, the 4-km simulations show significant improvement in the overall mean rain rate, the rain rate probability distributions, and the diurnal evolution and timing of precipitation. Both the 4-km and 12-km cases capture the coastal propagating signal and the interior basin-wide diurnal oscillation; however, the 4-km case shows better timing and evolution statistics. Compared to TRMM, the 4-km case rains too infrequently, but is more likely to produce rain events at high rain rates, thus resulting in a similar overall average rain rate. Overall, the present calculations show significant promise for computing regional rainfall patterns on convective-resolving grids.

This work investigates the effects of anthropogenic aerosols on deep convective clouds and the associated radiative forcing in the Houston area. The Goddard Cumulus Ensemble model (GCE) coupled with a spectral-bin microphysics is employed to investigate the aerosol effects on clouds and precipitation. First, aerosol indirect effects on clouds are separately investigated under different aerosol compositions, concentrations and size distributions. Then, an updated GCE model coupled with the radiative transfer and land surface processes is employed to investigate the aerosol radiative effects on deep convective clouds. The cloud microphysical and macrophysical properties change considerably with the aerosol properties. With varying the aerosol composition from only (NH4)2SO4, (NH4)2SO4 with soluble organics, to (NH4)2SO4 with slightly soluble organics, the number of activated aerosols decreases gradually, leading to a decrease in the cloud droplet number concentration (CDNC) and an increase in the droplet size. Ice processes are more sensitive to the changes of aerosol chemical properties than the warm rain processes. The most noticeable effect of increasing aerosol number concentrations is an increase of CDNC and cloud water content but a decrease in droplet size. It is indicated that the aerosol indirect effect on deep convection is more pronounced in relatively clean air than in heavily polluted air. The aerosol effects on clouds are strongly dependent on RH: the effect is very significant in humid air. Aerosol radiative effects (ARE) on clouds are very pronounced for mid-visible single-scattering albedo (SSA) of 0.85. Relative to the case without the ARE, cloud fraction and optical depth decrease by about 18% and 20%, respectively. The daytime-mean direct forcing is about 2.2 W m-2 at the TOA and -17.4 W m-2 at the surface. The semi-direct forcing is positive, about 10 and 11.2 W m-2 at the TOA and surface, respectively. Aerosol direct and semi-direct effects are very sensitive to SSA. The cloud fraction, optical depth, convective strength, and precipitation decrease with the increase of absorption, resulting from a more stable atmosphere due to enhanced surface cooling and atmospheric heating.

The poor representation of cloud processes in general circulation models (GMCs) has been recognized for decades as one of the major sources of uncertainties in weather and climate predictions. Because of the coarse spatial resolution of GCMs, subgrid- scale cloud and convection processes are modelled by parameterization schemes that provide a statistical representation of the subgrid-scale processes in terms of the large- scale, gridbox fields. This thesis focuses on the stochastic multicloud parameterization of Khouider et al. (2010), which is based on the three cloud types (congestus, deep, and stratiform) that are most observed in tropical convective systems. A rigorous parameter estimation model based on the Bayesian paradigm is developed to infer from data a set of seven convective timescales that determine the transition rates from one cloud type to another in the multicloud framework. The Bayesian posterior is given in terms of a costly model likelihood function that must be approximated numerically using high-performance linear algebra routines for parallel distributed computing. The Bayesian procedure is applied to the Giga-LES dataset of Khairout- dinov et al. (2009), a large-eddy simulation of tropical deep convection that covers a physical domain comparable to that of a typical horizontal grid cell in a GCM. The stochastic multicloud model and its deterministic version are then coupled to a zonally iv symmetric atmospheric model to study the meridional Hadley circulation and mon- soon dynamics. The main model is based on the hydrostatic Boussinesq equations on a rotating sphere, and is composed of a deep convective troposphere and a dynamical planetary boundary layer to sustain shallow convection. The resulting equations form a system of nonconservative partial different equations, which is solved numerically using high order non-oscillatory finite volume methods. Results from deterministic and stochastic simulations reveal a mean local Hadley cell structure with some fea- tures of organized convection. In the stochastic case, the Giga-LES parameter regime best captures the Hadley-type circulation and monsoon trough features, compared to a parameter regime used in a different study. / Graduate / mdelachev@gmail.com

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Thesis (M.S.)--Oregon State University, 2010. / Printout. Includes bibliographical references (leaves 118-123). Also available on the World Wide Web.

Les presses hydrauliques à compression sont utilisées afin de mettre en oeuvre des thermoplastiques. La maîtrise des champs thermiques à la surface des plateaux de presse est essentielle pour garantir une transformation de qualité des matériaux. La mise en forme des composites dits « haute performance » met en jeu d’importants transferts thermiques au niveau du plateau : montée en température jusqu’à 450 °C par résistances électriques, puis refroidissement par écoulement d’eau au sein de canaux cylindriques. L’apparition de phénomènes d’ébullition convective liés aux niveaux de température mis en jeu rend délicate la maîtrise des vitesses de refroidissement lors du procédé. L’objectif des travaux présentés dans ce manuscrit est d’étudier la phase de refroidissement à l’eau de plateaux de presse, dans des conditions représentatives du procédé industriel. Deux configurations expérimentales sont retenues. Dans un premier temps, l’étude se concentre sur l’analyse de l’écoulement diphasique eau-vapeur dans un canal horizontal. Une caméra rapide permet d’observer les régimes d’écoulements apparaissant dans un canal en quartz. Puis, l’évolution des champs thermiques le long d’un canal en acier inoxydable est menée, à l’aide d’une instrumentation thermique fine, dont notamment un capteur intrusif mobile spécialement conçu pour mesurer la température au coeur de l’écoulement. Dans un deuxième temps, l’étude est menée sur un élément de plateau de presse, système plus représentatif de la réalité industrielle. Plusieurs essais expérimentaux sont menés afin de quantifier l’influence du débit d’eau et de la température initiale sur la vitesse de refroidissement. Un modèle numérique vient en appui de l’analyse des résultats expérimentaux pour aider à la compréhension des phénomènes. Enfin, le montage expérimental est modifié, afin d’optimiser le refroidissement du plateau et respecter une vitesse de descente en surface de 25 °C.min-1 et ouvrir des perspectives en terme de régulation du procédé. / Hydraulic press are used to develop thermoplastic materials. The control of the thermal fields at the surface of the press platens are essential to ensure a good-quality transformation. The forming of “high performance” composites is realized within high temperature levels : heating to 450°C, with electrical cartridge ; cooling, with a water flow inside cylindrical channels. The incipience of flow boiling, due to high temperature, makes the control of cooling rate difficult during the process. The goal of the works presented here is to study the cooling process of the platen, in industrial conditions. Two experimental setups are tested. On the one hand, the study focuses on the analysis of the two-phase flow boiling. A high-speed camera enables the different flow regimes to be observed in a quartz channel. Then, the evolution of the thermal fields along a stainless steel channel is led, thanks to a fine thermal instrumentation, including an intrusive mobile sensor which has been especially made to measure the flow temperature. On the other hand, the study focuses on a single part of a platen, which is more representative of the industrial conditions. Several tests are led in order to quantify the influence of the water flowrate and the initial temperature level on the cooling rate. In parallel, a numerical model is developed to comprehensively understand the phenomenon. Finally, the experimental test bench is modified to improve the cooling of the platen and to control a 25°C.min-1 cooling rate, in the purpose of finding some perspectives concerning the process regulation.

The squall line is generally accepted to be the most commonly observed mesoscale convective system (MCS). Squall lines in the Southeastern United States account for a considerable amount of the total severe weather observed in this region (48%), including the production of localized severe wind events (known as Bow Echoes and Downbursts), long-lived severe wind events (known as Derechoes), as well as tornadoes. This study is the beginning of a baseline severe squall line climatology for the county warning areas (CWAs) of the National Weather Service Forecast Offices (NWSFO) located in Jackson, MS and Memphis, TN. This climatology will focus on the seasonal distribution of severe weather events associated with squall lines, as well as the thermodynamic forcing associated with the initiation and life spans of severe squall lines in Mississippi and West Tennessee.