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African Easterly Waves and Their Relationship to Rainfall on a Daily Timescale Jeffrey D Florida State University Libraries Electronic Theses, Treatises and Dissertations The Graduate School 2006 African Easterly Waves and Their Relationship to Rainfall on a Daily Timescale Jeffrey D. Baum Follow this and additional works at the FSU Digital Library. For more information, please contact [email protected] THE FLORIDA STATE UNIVERSITY COLLEGE OF ARTS AND SCIENCE AFRICAN EASTERLY WAVES AND THEIR RELATIONSHIP TO RAINFALL ON A DAILY TIMESCALE By JEFFREY D. BAUM A Thesis submitted to the Department of Meteorology in partial fulfillment of the requirements for the degree of Master of Science Degree Awarded: Spring Semester, 2006 i The members of the Committee approve the thesis defended by Jeffrey D. Baum on December 8th 2005. ----------------------------------------- Sharon E. Nicholson Professor Directing Thesis ----------------------------------------- T. N. Krishnamurti Committee Member ----------------------------------------- Kwang-Yul Kim Committee Member The Office of Graduate Studies has verified and approved the above named committee members. ii This thesis is dedicated to my parents DuWayne and Diane Baum for all there support throughout this entire venture. iii ACKNOWLEDGMENTS I would like to thank my major professor, Dr. Sharon E. Nicholson for her patience and guidance with this work. I would also like to thank my committee members; Dr. K. Kim, Dr. T.N. Krishnamurti. Also I would like to thank Dr. Andreas Fink for discussions about African Easterly Waves and the methodology for identifying African Easterly Waves. Finally I would like to thank my family, especially my lovely wife Samantha and my two kids Connor and Jillian for all their love and support. iv TABLE OF CONTENTS List of Tables ……………………………………………………………………...vii List of Figures …………………………………………………………………..…viii Abstract ………………………………………………………………………........xv 1. INTRODUCTION ………………………………………………………. 1 2. CHARACTERISTICS OF THE REGION …………………………………... 5 2.1 Topography of region. …………………………………………………... 5 2.2 General Circulation over Sahel …………………………………………. 5 2.3 Characteristics of African Easterly Waves ……………………………… 9 2.3.1 Factors influencing AEWs (AEJ, TEJ, ITCZ)…………………….. 13 2.3.2 Previous work on rainfall and AEW connection………………….. 15 3. DATA ………………………………………………………………………… 18 4. METHODOLOGY…………………………………………………………….. 22 4.1 Identifying AEWs…………………………………………………………. 22 4.1.1 The Andreas Fink Method for AEW Tracking…………………….. 23 4.1.2 Vertical Profile Method……………………………………………. 26 4.2 Description of Daily Rainfall Plots………………………………………... 31 5. PRELIMINARY RESULTS RELATED TO METHODOLGICAL ISSUES ………………………………………………………………………… 33 5.1 Validation of Wave Category Determination…..…………………………. 33 5.2 Evaluation of Methods……………………………………………………. 35 5.3 Daily Rainfall and VPM Relationship……………………………………. 39 6. SCIENTIFIC ANALYSIS RESULTS………………………………………… 41 6.1 Comparison between Wet and Dry Years………………………………… 41 6.1.1 Number of Waves, Period and Amplitude………………………… 41 6.1.2 Location of Rainfall within Wave Event………………………….. 45 6.1.3 Migration of Northern and Southern AEW Regimes……………... 48 6.2 Comparison between Early Season (JJ) Waves and Late Season (AS) Waves……………………………………………………………. 51 6.2.1 Number of Waves, Period and Amplitude………………………… 51 v 6.2.2 Location of Rainfall within Wave Event………………………….. 57 6.3 EOF Analysis of Rainfall 1958 and 1983………………………………… 59 6.4 Barotropic and Baroclinic Nature of AEWs……………………...………. 68 7. SUMMARY AND CONCLUSIONS………………………...……………….. 70 APPENDIX A. Rainfall Station Network Plots ....................………………………. 72 APPENDIX B. Vertical Profile Method Plots………………...…………………….. 81 REFERENCES…………………………………………………………………..…... 146 BIOGRAPHICAL SKETCH…………………………………………………..……. 152 vi LIST OF TABLES 1 Definition of the 8 different categories used to describe the features of AEWs………...……………………………………………………………......…31 2 Values of meridional wind component (m/s) at 700mb averaged over the (a) 4 dry years and (b) 4 wet years…………………………………………...34 3 Total number of waves tracked for each year analyzed in this study…………....41 4 Total number of AEWs occurring during the June-July season and August- September season for the 4 wet years (1958-1961)……………………….…….53 5 Total number of AEWs occurring during the June-July season and August- September season for the 4 dry years (1982-1985)……………………………..53 6 Average wave period over the first two and last two months for each year used in this study. Only the VPM waves are used here…………………………….…..55 7 Average wave period over the first two and last two months for each year used in this study. Only the AFM waves are used here……………………...…………55 vii LIST OF FIGURES 1 West African rainfall index based on Nicholson 131 rainfall data………………..2 2 Map of Africa with the shaded region representing the Sahel………………….....6 3 Mean meridional cross section of zonal wind (m/s) at 5E for August 1957-64. From Radiosonde data (from Burpee, 1972)……………………………………...7 4 Cross section of mean August meridional flow zonally averaged between 20W and 10E. (From Grist, 2001)..………………………………………………….....7 5 Migration of the ITCZ averaged from 1989 to 2003 over the study region. The solid line represents averaged values from 15W to 5W and the dotted line is averaged from 5W to 5E. (Data from CPC African Desk)………………………..8 6 Streamline analysis of band-pass filtered 700mb wind for 1200 GMT 9/4/74. Full wind barb is 5 m/s, half barb 2.5 m/s. The African Easterly Wave is evident. Dashed lines show wave phase categories. (From Houze, 1977).…….10 7 Composite of AEW Tracks from Aug to Sept. 1985. (From Reed et al., 1988)...11 8 Meridional wind variance (m2 s-2) for waves of 2.9 – 4.3-day period at 9oW. (From Reed et al. 1988b) ………………………………………………………..12 9 Hovmoller plot of rainfall averaged over grid boxes 1 and 2 (7.5oN to 15oN) defined in this study for 1958 based on gauge data. Contours start at 8 mm/day and have a 2 mm/day interval. The dark contours correspond to values of 20mm/day or more……………………………………………………………….16 10 Number of stations available after the rainfall network dataset was completed. Grid boxes are 2.5o x 2.5o and shaded boxes represent 3 or more stations present in the grid box. Each group contains the number of stations for the month of June (top) 1960, (bottom) 1985……………………………………………………......21 11 Power Spectrum of Meridional Wind component from NCEP reanalysis data for June 1 to Sept. 301960 at [10N, 0E]……………………………………………..24 viii 12 Two varying filter techniques applied to the NCEP Meridional wind data. The dark line represents the Butterworth filtered time series. The dotted line represents the Hanning filtered time series………………………………………25 13 (a) Hovmoeller diagram of the meridional wind component for June 1960 at 700mb. The shaded contours are negative and have the units (m/s). The solid line represents the zero gradient or trough location. The dashed line plots where the vorticity should be located on the streamline plot…………………………...27 13 (b) 2-6 day filtered streamline plot at 850mb for June 19, 1960. The shaded areas display the vorticity centers determined from the Hovmoeller diagram…………………………………………………………………………..28 14 Vertical Profile Method for June 1960. The solid lines represent two different AEWs between the 15th and 25th of the month. The text beneath each plot corresponds to the section of the AEW that is present on that day for each location of 2.5oE, 2.5oW, 7.5oW, and 15oW………………………………………………29 15 The vertical profile plot of the meridional wind component (m s-1) at Dakar. The values on the x-axis represent the 8 different categories the AEW can be broken down into (From Burpee, 1974)…………………………………………………30 16 Daily rainfall for June 19, 1960. Rainfall is averaged for each grid box and has the units of mm/day. The shading represents the intensity of rainfall within that grid box. White (0 < rainfall < 10); Light Gray (10 < Rainfall < 20); Med. Gray (20 < Rainfall < 30); Dark Gray (30 < Rainfall < 40); Red (Rainfall > 40)……32 17 Plot of mean values of meridional wind speed (m/s) for Regions 1&2 and Regions 3&4 combined over the 4 wet years (1958-1961) and 4 dry years(1982-1985). The top boxes represent Regions 1&2 for the wet years (top left) and dry years (top right). The bottom boxes represent Region 3&4 for wet years (bottom left) and dry years (bottom right). The green mark represents the mean value, the black bar indictates the spread +/- 1 standard deviation, and the red line is the best fit line (4 degree polynomial)…………………………………………………...36 18 Comparison of the number of waves for each month during the 4 wet years (a) and 4 dry years (b). The green color denotes AFM (15oN to 22.5oN), pink represents VPM (15oN to 22.5oN), yellow signifies AFM (7.5oN to 15oN) and blue represents VPM (7.5oN to 15oN)………………………………………37-38 19 Example of combining the different longitude plots of the VPM process onto a single table in order to track AEW propagation (June 1960)…………………...40 ix 20 Example of wave separation and one to one relation to daily rainfall. This is wave number 4 for the month of June 1960. The upper chart refers to region 1 and 2 and the lower chart refers to region 3 and 4. Numbers to the left of the wave segment are daily rainfall values summed at that longitude……………….……40 21 Comparison of mean period values for wet and dry years. The vertical axis represents period in units of day………………………………………………...43 22 Comparison of wave amplitude between wet (58-61) and dry (82-85) years. The solid line is dry years. The dashed line is wet years………………………...….44 23 Distribution of maximum daily rainfall into the 8 wave categories for each of the 4 wet years …………………………………………..………………………….46 24 Distribution of maximum daily rainfall into the 8 wave categories for each of the 4 dry years……………………………………………………………….….…..48 25 The latitudinal location and speed of the AEJ core averaged daily for the 4 wet years (1958-1961) and the 4 dry years (1982-1985). (Grist, 2001)…….……...50 26 (a) Plot of cyclonic centers of circulation tracks associated with AEWs from June- Sept.
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