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An Observational Study of the South Pacific

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An Observational Study of the South Pacific AN OBSERVATIONAL STUDY OF THE SOUTH PACIFIC CONVERGENCE ZONE USING SATELLITE AND MODEL RE-ANALYSIS DATA A Dissertation by STEPHEN B. COCKS Submitted to the Office of Graduate Studies of Texas A&M University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY December 2003 Major Subject: Atmospheric Sciences AN OBSERVATIONAL STUDY OF THE SOUTH PACIFIC CONVERGENCE ZONE USING SATELLITE AND MODEL RE-ANALYSIS DATA A Dissertation by STEPHEN B. COCKS Submitted to the Office of Graduate Studies of Texas A&M University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Approved as to style and content by: ____________________________ __________________________ Richard Lee Panetta Thomas Wilheit (Chair of Committee) (Member) ____________________________ __________________________ Ping Chang Richard Orville (Member) (Member) __________________________ Richard Orville (Head of Department) December 2003 Major Subject: Atmospheric Sciences iii ABSTRACT An Observational Study of the South Pacific Convergence Zone Using Satellite and Model Re-Analysis Data. (December 2003) Stephen B. Cocks, B.S., University of Oklahoma; M.S., Colorado State University Chair of Advisory Committee: Dr. Richard Lee Panetta Satellite derived wind, rain rate and sea surface temperature data combined with NCEP analysis data are used to examine the structure of the South Pacific Convergence Zone (SPCZ) during La Nina conditions. Annual means indicate that the axes of maximum surface convergence and rain rates are essentially co-located in the Northern Hemisphere Inter-Tropical Convergence Zone and over the tropical (TR) portions of the SPCZ. However, over sub- tropical (ST) and middle latitudes (ML), the axis of maximum convergence is eastward and equatorward of the axis of maximum rain rates such that surface divergence predominates over the these portions of the SPCZ rain rate maximum. Analysis of NCEP data also exhibit a similar convergence distribution at 1000 hPa; however, at the 925, 850 and 700 hPa levels convergence replaces divergence and is co-located with the axis of maximum rain rates. Seasonal rain rate patterns indicated that the TR and ST SPCZ are more convectively active during Austral Summer and Fall while the ML SPCZ is more active during the Fall and Winter. Vertical wind shear and the variability of the meridional flow indicate the ML SPCZ is strongly associated with the mid-latitude storm track while the TR SPCZ is associated with low wind shear. The ST SPCZ represents a transition zone between an environment dominated by low shear and one that is more baroclinic. iv Lag correlation analysis using rain rate, 200 hPa divergence and height anomaly data as well as composite analysis of high rain fall events revealed wave like patterns over the ST and ML SPCZ. The estimated wavelengths range from 4700 to 5500 km, values that correspond to wave numbers 7 and 8; individual anomalies tend to move eastward at speeds of 5 to 7 m/s. The vertical structure of the height anomalies suggest baroclinic dynamics are important to the development of the mean rainfall pattern in the ML SPCZ and to a lesser extent the ST SPCZ. During high rain rate events, only a given portion of the SPCZ, approximately 3000 to 4000 km in length, is convectively active. v TABLE OF CONTENTS Page ABSTRACT ................................................................................................................. iii TABLE OF CONTENTS............................................................................................. v LIST OF FIGURES...................................................................................................... vii LIST OF TABLES ....................................................................................................... xi CHAPTER I INTRODUCTION ........................................................................................ 1 The South Pacific Convergence Zone................................................ 1 Previous Observational Work ............................................................ 3 Numerical Model Studies................................................................... 5 Issues Explored .................................................................................. 7 II DATA SETS USED .................................................................................... 9 NCEP Re-Analysis Data .................................................................... 9 Scatterometer Data ............................................................................. 10 Satellite Derived SST Data ................................................................ 14 Satellite Derived Rain Rates .............................................................. 17 Data Set Distributions ........................................................................ 24 III ANNUAL DISTRIBUTIONS OF SOME PHYSICAL PARAMETERS IN THE SPCZ ............................................................................................. 28 Distribution of Rain Rates and SSTs across the Pacific..................... 28 Distribution of Convergence across the Pacific ................................. 30 Scatter Plots of Rain Rates and Divergence....................................... 46 Lightning Characteristics in the SPCZ Region .................................. 48 Summary ............................................................................................ 49 IV SEASONAL DISTRIBUTIONS OF SOME PHYSICAL PARAMETERS IN THE SPCZ.................................................................. 52 Seasonal Distribution of SSTs, Rain Rates and Vertical Wind Shear 52 Seasonal Distribution of Convergence............................................... 60 The Vertical Distribution of Convergence and Heights..................... 67 vi CHAPTER Page Seasonal Scatter Plots of Rain Rates and Divergence......................... 76 Summary ............................................................................................. 82 V LAG CORRELATION ANALYSIS............................................................ 87 Introduction ........................................................................................ 87 Correlation Analysis Methods............................................................ 88 Spectra and Auto/Cross Correlation of Rain, Height and Div. within the SPCZ ......................................................................... 92 Lag Correlation Using Rain Rate Anomalies..................................... 98 Lag Correlation Using Rain Rate and Divergence Anomalies .......... 102 Lag Correlation Using Rain Rates and Height Anomalies ................ 106 Summary ............................................................................................ 110 VI COMPOSITES OF HIGH AND LOW RAIN RATE EVENTS ................ 112 Compositing Methodology................................................................. 112 Seasonal Trends for High and Low Rain Rate Days.......................... 113 HRRD Composites of the SPCZ Test Regions .................................. 117 LRRD Composites of the SPCZ Test Regions................................... 129 Vertical Structure of Height Anomalies for HRRD Composites ....... 129 Summary ............................................................................................ 136 VII SUMMARY AND CONCLUSIONS........................................................ 139 REFERENCES............................................................................................................. 146 APPENDIX .................................................................................................................. 153 VITA ............................................................................................................................ 155 vii LIST OF FIGURES FIGURE Page 1. A three year average (1998-2001) of global annual precipitation (totaled in mm) estimated from Microwave Sounding Unit data.............................................................. 1 2. Schematic illustrating Kodama's model for Sub-Tropical Convergence Zones .............. 5 3. Example of coverage gaps for approximately one day in scatterometer data; the large gap in the Pacific Ocean is the result of data not yet ingested................................ 11 4. Three day average of SSTs calculated without smoothing (top) and with smoothing (bottom) ......................................................................................................................... 16 5. Schematic of what happens to microwave radiation as it passes through the atmosphere, reflects off the ocean and back into space.................................................. 18 6. Schematic of the contribution, in terms of microwave emission, from various atmospheric constituents to the microwave derived brightness temperature (Tb) ......... 20 7. Schematic of the algorithm used to create GPCP rain rate data ..................................... 23 8. Test regions used for quantitative analysis in chapters III through VI........................... 24 9. Histograms of 500 HPa height anomalies for the TR, ST and ML SPCZ regions ......... 25 10. Histograms of rain rate anomalies for the TR, ST and ML SPCZ regions...................... 26 11. Three year average of SSTs (shaded) and rain rates (contoured) over the South Pacific and the tropical North Pacific............................................................................ 28 12. Three year average
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