Convective-scale Downdrafts in the Principal Rainband of Hurricane Katrina (2005) Anthony Carl Didlake, Jr. A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science University of Washington 2009 Program Authorized to Offer Degree: Atmospheric Sciences University of Washington Graduate School This is to certify that I have examined this copy of a master’s thesis by Anthony Carl Didlake, Jr. and have found that it is complete and satisfactory in all respects, and that any and all revisions required by the final examining committee have been made. Committee Members: ______________________________________________________________________ Robert A. Houze, Jr. ______________________________________________________________________ Clifford F. Mass ______________________________________________________________________ John M. Wallace Date: ____________________________________________ In presenting this thesis in partial fulfillment of the requirements for a master’s degree at the University of Washington, I agree that the Library shall make its copies freely available for inspection. I further agree that extensive copying of this thesis is allowable only for scholarly purposes, consistent with “fair use” as prescribed in the U.S. Copyright Law. Any other reproduction for any purposes or by any means shall not be allowed without my written permission. Signature _________________________________ Date _____________________________________ University of Washington Abstract Convective-scale Downdrafts in the Principal Rainband of Hurricane Katrina (2005) Anthony Carl Didlake, Jr. Chair of the Supervisory Committee: Professor Robert A. Houze, Jr. Department of Atmospheric Sciences Airborne Doppler radar data collected during the Hurricane Rainband and Intensity Change Experiment (RAINEX) document downdrafts in the principal rainband of Hurricane Katrina (2005). Inner-edge downdrafts (IEDs) originating at 6-8 km altitude created a sharp reflectivity gradient along the inner boundary of the rainband. Low-level downdrafts (LLDs), evidently driven by precipitation drag, originated at 2-4 km within the heavy rain cells of each convective element. The IED and LLD were spatially separated by but closely associated with the updrafts within the rainband. The IED was forced aloft by pressure perturbations formed in response to the adjacent buoyant updrafts. Once descending, the air attained negative buoyancy due to evaporative cooling from the rainband precipitation. A convective-scale tangential wind maximum tended to occur in the radial inflow at lower levels in association with the IED, which enhanced the inward flux of angular momentum at low levels. Convergence at the base of the downdrafts on the upwind end of the principal rainband contributed to the principal rainband growing in length. New updraft elements triggered by this convergence led to the formation of new IED and LLD pockets, which were subsequently advected downwind around the storm by the vortex winds while additional new cells continued to form on the upwind end of the band. These processes sustained the principal rainband and helped it to remain nearly stationary relative to the storm center, thus maintaining its impact on the hurricane dynamics over an extended period. TABLE OF CONTENTS Page List of Figures................................................................................................................. ii 1. Introduction..................................................................................................................1 2. Data and methodology.................................................................................................4 2.1. Doppler radar data sets .................................................................................4 2.2. Principal rainband of Hurricane Katrina.......................................................5 2.3. Convective/stratiform separation..................................................................6 2.4. Radial cross section analysis ........................................................................8 2.5. Dropsonde analysis.......................................................................................9 3. Analysis of total rainband..........................................................................................14 3.1. Average updrafts and downdrafts...............................................................14 3.2. Frequency distribution of vertical velocity.................................................15 3.3. Temperature and pressure perturbation fields ............................................16 3.4. Horizontal distribution of vertical velocity.................................................19 4. Forcing of the downdrafts..........................................................................................28 5. Immediate effects and possible impacts of the downdrafts.......................................34 5.1. Sharp inner-edge reflectivity gradient ........................................................34 5.2. Low-level wind maximum (LLWM)..........................................................35 5.3. Increasing the angular momentum of the hurricane vortex........................37 5.4. Growth and sustenance of the principal rainband.......................................38 6. Conceptual model of convection within the principal rainband................................44 7. Conclusions and future work.....................................................................................47 References......................................................................................................................49 Appendix A: Convective/stratiform separation algorithm ............................................53 Appendix B: Thermodynamic retrieval.........................................................................55 i LIST OF FIGURES Figure Number Page 1. Conceptual model of principal rainband (from Hence and Houze 2008) ................3 2. Best track of Hurricane Katrina (2005)..................................................................10 3. Plan view of ELDORA radar data..........................................................................11 4. Convective/stratiform separation and vertical velocity data ..................................12 5. CFADs of radar data...............................................................................................13 6. Average vertical velocity field in cross sections ....................................................22 7. Frequency count of vertical velocities in cross sections ........................................23 8. Mass-transport-weighted CFAD ............................................................................24 9. Conditional probability histograms of downdraft speeds.......................................24 10. Average temperature and pressure perturbations ...................................................25 11. Average buoyancy and dynamic pressure perturbations and vertical gradients.....26 12. Cross section autocorrelation results......................................................................27 13. Buoyancy pressure gradient acceleration field.......................................................31 14. Data from cross section A ......................................................................................32 15. Data from cross section B.......................................................................................33 16. Composite tangential velocity field (from Barnes et al. 1983) ..............................41 17. Radar scans of Tropical Storm Ophelia (2005)......................................................42 18. Schematic of upwind end of principal rainband.....................................................43 19. Schematic of downdrafts in cross section of the principal rainband......................46 ii ACKNOWLEDGEMENTS I thank my advisor, Bob Houze, for his continued advice and support throughout all of the research and writing. I also thank the entire Mesoscale Group, including Stacy Brodzik, Jasmine Cetrone, Deanna Hence, Tomislav Maric, Socorro Medina, Bradley Smull and Jian Yuan. My many discussions with them have been very valuable. Michael Bell and Sandra Yuter were very gracious in providing me with materials necessary for my research. Beth Tully did an amazing job with the figures and also helped with editing the thesis. My research was supported by the National Science Foundation under grants ATM-0432623 and ATM-0743180, and the Department of Defense (DoD) through the National Defense Science and Engineering Graduate Fellowship (NDSEG) Program. Lastly, I thank my fellow Atmospheric Sciences classmates, and all of my family in Michigan and Connecticut. iii 1 1. INTRODUCTION The principal rainband is a prominent feature of tropical cyclones (Willoughby et al. 1984). It consists of convective cells embedded in stratiform precipitation and spirals inward toward the storm center (Fig. 1a). It is larger than numerous other transient rainbands populating the storm, and it is persistent and nearly stationary relative to the storm. Despite its prominent appearance, the dynamic role and origin of the principal rainband in the larger tropical cyclone remains uncertain. Barnes et al. (1983) and Powell (1990a) showed that the air spiraling inward toward the storm center is subjected to an overturning circulation when it encounters the principal rainband. Hence and Houze (2008) used high-resolution