The Evolution and Structure of Composite Meso-Alpha-Scale Convective Complexes
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The Evolution and Structure of Composite Meso-Alpha-Scale Convective Complexes By Ming-sen Lin Department ofAtmospheric Science Colorado State University Fort Collins, Colorado THE EVOLUTION AND STRUCTURE OF COMPOSITE MESO-ALPHA-SCALE CONVECTIVE COMPLEXES by Ming-Sen Lin Research supported by the National Science Foundation under Grants ATM-8113082. ATM-8312077 and ATM-8S12480 Department of Atmospheric Science Colorado State University Fort Collins. Colorado Atmospheric Science Paper No. 404 ABSTRACT OF DISSERTATION THE EVOLUTION AND STRUCTURE OF COMPOSITE MESO-ALPHA-SCALE CONVECTIVE COMPLEXES The CSU Regional Atmospheric Modeling System (RAMS) data assimila- tion and analysis package is implemented to study the evolution and the structure of composite Meso-Q-scale Convective Complexes (MCCs). Seven years of North American MCC cases are sampled and normalized into seven evolutionary sub-periods according to the temporal life-cycles of the storm systems. The composite MCC shows some general features, such as the lower- tropospheric convergence and cyclonic vorticity associated with the upper-tropospheric divergence and anticyclonic vorticity during the MCC's initial to mature stages, the cold-core structure in the upper troposphere, and the characteristic warm core at middle levels. The low-level convergence, aligned with the intensified cyclonic vorticity, lifts and extends to the middle troposphere at the mature stage. A middle-level jet-like inflow is accompanied by a strengthening middle- level convergence which reaches its maximum intensity at the MCC's mature stage. This middle-level jet-like inflow, located slightly above the melting layer and near the lowest level which might reinforce the ee downdraft. The vorticity budget of the composite MCC shows that the residual of grid-scale vorticity exhibits a vorticity sink in the lower tropo- sphere and reaches its maximum intensity at the MCC mature stage. On the other hand, a mid- to upper-level vorticity source exists during the MCC early and dissipation stages. The imbalance can be attributed to 1i the removal of vorticity-rich air from low levels and its upward tran- sport and deposition aloft by convection. The composite MCC exhibits a apparent heat source of 10-200 day-1 at about the 400 mb level and a moisture sink of -9 to -140 day-1 equivalent heating below the 700 mb level throughout most of the MCC life-cycle. The water vapor bUdget of the composite MCC indicates that water vapor is mainly provided by MCC-scale convergence and the atmos- pheric storage term. At the MCC mature stage, the precipitation effi- ciency reaches 113~ such that the moist atmosphere provides an extra 13~ from its storage, which is accumulated from the MCC early stages. A hypothesized MCC evolution obtained from this composite stUdy suggests that for system development to occur, the low-level convergence must be aligned horizontally with the cyclonic vorticity pattern in the presence of larger Convective Available Potential Energy and/or smaller vertical wind shear. Ming-Sen Lin Dept. of Atmospheric Science Colorado State University Fort Collins, Colorado 80523 Fall 1986 iii ACKNOWLEDGEMENTS I would like first of all to express my appreciation to my disserta tion advisor, Professor William R. Cotton, for his continued support, guidance, and encouragement throughout the course of this research. He and the other committee members, Professors William M. Gray, Roger A. Pielke, and Robert N. Meroney, also made valuable suggestions and con structive criticisms which have helped to fulfill the study and to shape the final version of this manuscript. Significant contributions from a number of other individuals helped make a comprehensive study such as this possible. I owe a great deal of thanks to Professors Wayne H. Schubert, Richard H. Johnson and Paul W. Mielke for their stimulating discussions during the research. Mr. Craig J. Tremback is gratefully acknowledged for assistance with and sugges tions concerning data assimilation and objective analysis. My apprecia tion is also extended to Mr. Ray McAnelly for his cooperation in sam pling MCCs, allowing me to use his composite MCC precipitation, and com ments on the dissertation draft. I am indebted to Dr. Robert A. Maddox, Mr. Dennis M. Rodgers, and Mr. John Augustine of NOAA-ERL for kindly allowing me to use their satellite images during the study. I would also like to express my appreciation to Mr. Mike Moran for his consider able help in proof reading and correcting the dissertation draft. Mrs. Brenda Thompson and Ms. Annette Claycomb contributed to the manuscript preparation and editing, also Ms. Judy Sorbie and Mrs. Lucy McCall helped in drafting figures appearing in this dissertation. Finally, I wish to thank my wife Sue and son Ben for their continuous encouragement and forbearance during my doctoral program. iv The Central Weather Bureau and the National Science Council of the Republic of China initially supported me in this study. Primary support for this research was obtained by the U.S. National Science Foundation under Grants ATM-8II3082, ATM-83I2077 and ATM-85I2480. All computations were performed on the National Center for Atmospheric Research (NCAR) Cray-IA computers. NCAR is supported by the National Science Founda tion. v TABLE OF CONTENTS Section ABSTRACT •••• •• ·.. ·. ·. ·.. .. ii ACKNOWLEDGEMENTS • ·. .. · .. ·. ·. ·.. · . ... iv TABLE OF CONTENTS. •••• · . ·. · .. •• · . ·.. vi LIST OF TABLES • · .. ••••••••• · . .. · . ix LIST OF FIGURES. ••• ·. ·. .. •• · . · . x LIST OF SYMBOLS AND ACRONYMS • · .. .. ..... •• xviii 1.0 INTRODUCTION. • · ••• · • •• • • • · •• • • · • · • • 1 2.0 BACKGROUND • · • • ·· • •• · • · · • • •• · · • • · • • 4 2.1 MCC climatology. • • • • • • • • • 4 2.2 Similarities and differences· · · between· · MCCs· · ·and· other MCSs . • • • • • • • • • • • • • • • • • S 2.3 Synoptic-· ·to· meso-p-scale· features during ·MCC · evolution. ••• · •• • • • · • • •• • • · • • · • 7 2.3.1 Moisture content. • • •• • • • 7 2.3.2 Conditional and potential· · instability· · · ·• · 9 2.3.3 Lifting mechanisms. • · · · 10 2.3.4 Up-scale development of· neighboring· · · · ·· meso-p-· ·· · · scale elements. •• • • • • • •• • • 13 2.3.5 Variations of moisture content.· · · conditional · and potential instability. and lifting mechanisms during the MCC life-cycle. • • • · 15 2.4 MCC structure. ••• • • • · • · · • • · •• · · · · 17 2.5 MCC-related precipitation pattern. • • · • · • • • · 19 2.6 MCC energy and moisture budgets. · · • · • ·· · 20 2.7 Problems to be solved. · · • · • • · · • ·· · • · • 21 2.8 Summary. • •• · · • •• · • • • · · • · · • · · 22 3.0 PROCEDURE . • • • ·· • · · • · • · • • · ··· • • • · • 24 3.1 Objective definition of an MeC · · • ••• · · · · · 24 3.2 Objective analysis and data assimilation · • · · • · 24 3.2.1 Objective analysis selection. • · •• · • · • 25 3.2.2 Barnes objective analysis • • • • · • · • • · 26 3.2.3 Data assimilation and analysis. · ••• • • • 29 3.3 MCC composite analysis • • • • •• • • •• • •• · • 30 vi 3.3.1 The philosophy of composite analysis. •• •• 30 3.3.2 Composite analysis of temporal behavior of MCCs. •••• 31 3.4 Summary. ••••••••••••• 38 4.0 MCC CLIMATOLOGY. KINEMATIC STRUCTURE. DYNAMICS, AND THERMOD YNAMI CS ••••••••••••••••• · . 42 4.1 MCC climatology derived from the composite study 42 4.2 Evolutionary variations of MCC moisture content. potential instability. lifting mechanisms. and the up-scale development of neighboring meso-p- scale convective elements. •••••••••• 47 Moisture content evolution. •••••• · . 47 Conditional and potential instability evolution ••••••••••• 58 4.2.3 Lifting mechanisms evolution. ·.. 66 4.3 Kinematic structure of MCC evolution • • ·. •• • 75 4.3.1 System-relative flow fields ••• · . 75 4.4 Dynamics and thermodynamics of MCC evolution •• .. ·81 4.4.1 Vertical profiles of geopotential height. temperature. and horizontal wind field. • •• 82 4.4.1 Divergence field at various levels. •• 90 4.4.3 Vorticity field at various levels ••• 101 4.4.4 Vertical motion field •••••••••• •• 109 4.5 Comparison of the impact of geography on MCCs. 113 4.5.1 Comparison between orogenic HCCs and Plains MCCs. •••••••••••• •• 113 4.6 Summary••• . · .. · .. 113 5.0 MCC BUDGET EVOLUTION. .. · . .. 118 5.1 MCC vorticity budget. ·. 118 5.1.1 Vorticity bUdget formulation •••• •• 118 5.1.1 Vorticity budget of the composite MCC •• •• 119 5.1.3 Comparison with other vorticity budget studies •••••••••••• 137 5.2 MCC heat bUdget •••••••••••• · . 137 5.2.1 Heat bUdget formulation •••••• •• 139 5.2.1 Heat budget of the composite MCC. •••• 139 5.2.3 Comparison with other heat budget studies.. 149 vii 5.3 MCC moisture budget. ••••••••••••• 152 5.3.1 Moisture budget formulation. ••••• 152 5.3.2 Moisture bUdget of the composite MCC. • 153 5.3.3 Comparison with other moisture bUdget studies 160 5.4 Summary. •••••••••••••••••••• •• 160 6.0 THE INFLUENCE OF ENVIRONMENTAL CONDITIONS ON MCC FORMATION AND DEVELOPMENT ••••••••• 163 6.1 Geostrophic adjustment process related to MCC genesis and development •••••••• 163 6.1.1 Geostrophic adjustment process. •••• 163 6.1.2 Calculation of Rossby radius of deformation. 169 6.2 Static stability and wind shear threshold•• . .. 176 6.2.1 Static stability assooiated with MCC evolution •••••••••••••••• 176 6.2.2 Wind shear assooiated with MCC evolution. 178 6.3 Tropopause temperature threshold • 185 6 .4 Summary. .•.•.•••••.•••• 188 7.0 CONCEPTUAL MODEL OF MCC EVOLUTION. .. 192 7.1 The methodology of constructing a conceptual model.