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Sheffield Colostate 0053N 10856.Pdf (4.527Mb) THESIS ASSESSING THE IMPACTS OF CLOUD CONDENSATION NUCLEI ON CUMULUS CONGESTUS CLOUDS USING A CLOUD RESOLVING MODEL Submitted by Amanda M. Sheffield Department of Atmospheric Science In partial fulfillment of the requirements For the Degree of Master of Science Colorado State University Fort Collins, Colorado Fall 2011 Master’s Committee Advisor: Susan C. van den Heever Sonia Kreidenweis Richard Eykholt Copyright by Amanda M. Sheffield 2011 All Rights Reserved ABSTRACT ASSESSING THE IMPACTS OF CLOUD CONDENSATION NUCLEI ON CUMULUS CONGESTUS CLOUDS USING A CLOUD RESOLVING MODEL Cumulus congestus clouds are mid-level clouds that form part of the trimodal tropical cloud distribution. They act to moisten the atmosphere and may become mixed-phase in their lifetime. Congestus typically surpass the tropical trade wind inversion from where they may either develop into deeper convection, or alternatively remain as terminal congestus. Such growth is dependent on multiple factors, including those which alter the local environment and the microphysical structure of the cloud. This study investigates the impacts of cloud condensation nuclei (CCN) on cumulus congestus clouds through the use of large domain, cloud- resolving model (CRM) simulations in radiative convective equilibrium (RCE). Previous studies have focused on the convective invigoration of congestus and their subsequent growth to deep convection in association with ice processes. This study will focus on the response of congestus clouds to more polluted conditions, with particular emphasis on the development and growth of congestus from the warm phase to beyond the freezing level. It is found that convection is invigorated in the more polluted cases in association with the enhanced latent heat released during the vapor diffusional ii growth of cloud droplets in the warm phase. Such invigoration results in stronger updraft speeds, enhanced vertical lofting of cloud water, and a subsequent increase in the number of clouds growing to above the freezing level. The lofted cloud water is available to form more ice, however the ice water produced is smaller in magnitude compared to cloud water amounts above the freezing level. The low amounts of ice result in relatively insignificant contributions of the latent heat of freezing to the updraft strength. The impacts of enhanced CCN concentrations on various other cloud characteristics and microphysical processes are also investigated. iii ACKNOWLEDGEMENTS I would like to acknowledge those that helped produce this work, including my advisor Dr. Sue van den Heever, the students and research staff of the van den Heever research group and the atmospheric science department, and my husband, Jesse, who is always there to encourage me. This work was supported by two fellowships during my master’s tenure in addition to National Science Foundation Grant ATM-0820557, including the American Meteorological Society First Year Graduate Fellowship (2009-2010) and the Department of Energy (DOE) Office of Science Graduate Fellowship (SCGF) (2010 – present). Lastly, I would like to thank my masters committee, Sue, Dr. Sonia Kreidenweis and Dr. Richard Eykholt, for their review of this thesis and its results. iv TABLE OF CONTENTS ABSTRACT……………………………………………………………………………………..………….ii ACKNOWLEDGEMENTS……………………………………………………….……………………iv 1. INTRODUCTION............................................................................................................................1 2. BACKGROUND...............................................................................................................................4 2.1 THE IMPORTANCE OF CLOUDS......................................................................................4 2.2 CLOUD NUCLEI AND THEIR EFFECTS.........................................................................4 2.3 DUST AS CLOUD NUCLEI...................................................................................................7 2.4 TRIMODEL TROPICAL CLOUD DISTRIBUTION AND CUMULUS CONGESTUS CLOUDS..........................................................................................................8 2.5 RELEVANT STUDIES.........................................................................................................11 2.6 THE W MOMENTUM EQUATION................................................................................12 3. METHOD........................................................................................................................................24 3.1 THE RAMS MODEL............................................................................................................24 3.2 MODEL CONFIGURATION..............................................................................................26 3.3 EXPERIMENT DESIGN......................................................................................................28 3.4 CUMULUS CONGESTUS CLOUD SELECTION..........................................................29 4. RESULTS........................................................................................................................................36 4.1 A SINGLE CUMULUS CONGESTUS CLOUD...............................................................36 4.2 CLOUD TOP DISTRIBUTION..........................................................................................37 4.3 UPDRAFTS.............................................................................................................................39 4.4 CHARACTERISTICS OF CUMULUS CONGESTUS CLOUDS..................................40 4.4.1 CLOUD AND RAIN WATER..............................................................................41 4.4.2 PRISTINE ICE, SNOW, AND AGGREGATES...............................................44 4.4.3 GRAUPEL AND HAIL..........................................................................................45 4.5 MICROPHYSICAL PROCESSES.....................................................................................46 4.6 CONVECTIVE INVIGORATION.....................................................................................51 4.6.1 LATENT HEAT RELEASE - VAPOR...............................................................52 4.6.2 LATENT HEAT RELEASE - FREEZING…………………………………….....54 4.6.3 CONDENSATE LOADING..................................................................................56 4.6.4 COLD POOLS..........................................................................................................57 5. CONCLUSIONS............................................................................................................................77 6. REFERENCES..............................................................................................................................82 v 1. INTRODUCTION The presence of clouds is important to our understanding of the climate system, as they serve as integral parts of the radiative and hydrologic systems (Solomon et al. 2007). Of interest to this study is a convective cloud type that forms part of the tropical cloud regime (Johnson et al. 1999): the cumulus congestus cloud. The formation of convective clouds and precipitation is dependent on ambient environmental characteristics. These large regional to global scale factors affect a cloud’s vertical and horizontal development, size and thickness, updraft speed, and precipitation. However, various microphysical factors may also influence cloud characteristics, including the impact of aerosols on the thermodynamic and dynamic structure of clouds. The goal of this study is to examine the impacts of ambient aerosol concentrations on these cloud characteristics from a large sample of cumulus congestus clouds, including the production of cloud water and ice and contributions to convective invigoration. This goal is to be achieved using a series of idealized radiative convective equilibrium (RCE) simulations of tropical oceanic clouds under the influence of an aerosol layer of varying number concentrations. Cumulus congestus clouds are defined as “a strongly sprouting cumulus species with generally sharp outlines and, sometimes, with a great vertical development; it is characterized by its cauliflower or tower aspect, of large size.” (American Meteorology Society Glossary 2011). These mid-level clouds supply 1 moisture to the middle troposphere as they evolve from trade wind cumulus clouds and may transition into deep convection. They frequently reach the freezing level and above. As part of the trimodal distribution of clouds commonly observed in the tropics (Johnson et al. 1999), understanding these clouds is important to understanding the overall sensitivity of the development of tropical convective clouds to aerosol indirect effects. Evaluation of aerosol impacts on cumulus congestus clouds will allow a better understanding of their mixed-phase growth. These clouds commonly reach the freezing level of the atmosphere and subsequently become mixed-phase systems. Such growth may be enhanced through convective invigoration in association with the development of cloud water and ice (Andreae et al. 2004), but may be reduced by the stable layer formed in association with the freezing level (theorized to be formed by subsidence and melting in the tropics) (Johnson et al. 1999, Posselt et al. 2008). The aerosol-congestus cloud interaction may alter these enhancing and reducing factors. An aspect influential in cloud and ice formation processes is the presence of cloud condensation nuclei (CCN). It has been hypothesized
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