A Case Study of the Formation of an Eastern Pacific Tropical Cyclone by Terence Kung B.S. Atmospheric and Oceanic Sciences (1997) University of Wisconsin at Madison Submitted to the Department of Earth, Atmospheric and Planetary Sciences in Partial Fulfillment of the Requirements for the Degree of MASTER OF SCIENCE IN METEOROLOGY at the MASSACHUSETTES INSTITUTE OF TECHNOLOGY June 1999 © 1999 Massachusetts Institute of Technology. All Rights Reserved. Author ...... ........ ......... ...... ... Department of Earth, Atmospheric and Planetary Sciences May 6, 1999 Certified by .............. ... Kerry A. Emanuel Professor of Meteorology Thesis Supervisor Accepted by........................................ Ronald G. Prinn I MASSACHUSETTS INSTITUTE Department Head OF TECHNOLOGY JUN 0 1 1999 ''~u~p LIBRARIES A Case Study of the Formation of an Eastern Pacific Tropical Cyclone by Terence Kung Submitted to the Department of Earth, Atmospheric and Planetary Sciences on May 7, 1999 in partial fulfillment of the requirements for the Degree of Master of Science in Meteorology Abstract A case study is performed to investigate the nature of tropical cyclogenesis in the eastern Pacific Ocean. Focus is given to the formation and development of the initial circulation which eventually intensified into Hurricane Fefa. Using satellite imagery, the author studies the development of convective activity in the genesis region. Gridded reanalysis data are used to document the synoptic-scale flow, with emphasis on tracing the easterly wave which is associated with the formation of Fefa. The data show that the easterly wave propagated across the Caribbean Sea and the Central American mountains, and the initial circulation developed while the wave had moved into the eastern Pacific. The wave is found to have moved through an unstable basic state while it was in the Caribbean, which is favorble for its growth and maintenance. Two phenomena are observed prior to the formation of the low-level circulation. These include an easterly jet in the eastern Pacific that may have been associated with the blocking effect of the Central American mountains, and a southerly wind surge into the monsoon trough region. In addition, aircraft observations collected during Tropical Experiment in Mexico are used to study the evolution of the mesoscale system. Initially, a circulation in the middle troposphere with a cold core in the boundary layer, and a shear line located to the west were found. One day later, a low-level warm core vortex had developed, and it was displaced from the mid-level vortex. It is suggested that the low-level vortex formed from the spin-up of the monsoon trough, independent of the mid-level vortex. Thesis Supervisor: Kerry A. Emanuel Title: Professor of Meteorology Acknowledgements I would like to thank professor Kerry Emanuel for stimulating my interest in the problem of tropical cyclogenesis, and suggesting this topic to me. I thank him for his guidance, constructive criticisms along the way, and reviewing several drafts of my thesis. I would also like to thank professor Alan Plumb for providing me financial support, and acting as my academic advisor. I value the friendships with fellow graduate students in the department. I appreciate their warmth and concerns with me. I thank Lodovica Illari for supplying me tapes of the NCEP/NCAR reanalysis data. I also thank Luis Farfan of the University of Arizona for sending me the satellite images, and helping me out in some displaying problems. I thank all brothers and sisters in the Boston Chinese Evangelical Church for their love and spiritual support. Most of all, I thank my parents for their love, support and understanding. The research was supported under NSF grant ATM-9528471. Table of Contents 1 1.1 Introduction 11 1.2 Overview of the Tropical Experiment in Mexico (TEXMEX) and Hurricane Fefa 13 1.3 Review of previous work on tropical cyclogenesis 14 2 Data and analysis methods 23 2.1 Doppler radar data 26 2.2 In situ data 29 2.3 Satellite data 30 2.4 NCEP/NCAR reanalysis data 30 3 Observations of the synoptic-scale circulation 31 4 Aircraft data analysis results 51 4.1 Flight 1P 51 Flight 2E 58 Flight 3P 62 4.2 Comparison to the genesis of Guillermo 70 5 Summary and suggestions for Future Work 73 6 Bibliography 76 _~II~~_ -I.__XII~ULII. List of Figures Figure 2.1: Tracks of aircraft-estimated vortex centers of TEXMEX cases that developed into hurricanes (from D. Raymond) 24 Figure 3.1: Winds and relative vorticity from the NCEP/NCAR reanalysis at 700 mb 32 Figure 3.2: Winds and relative vorticity from the NCEP/NCAR reanalysis at 1000 mb 37 Figure 3.3: Infrared images from GOES 41 Figure 3.4: Absolute vorticity from the NCEP/NCAR reanalysis at 700 mb 45 Figure 4.1: Observations in pre-Fefa MCS during flight IP 52 Figure 4.2: Observations in pre-Fefa MCS during flight IP (cont'd) 57 Figure 4.3: Observations in pre-Fefa MCS during flight 2E 59 Figure 4.4: Observations in Tropical Storm Fefa MCS during flight 3P 63 Chapter 1 1.1 Introduction Tropical cyclones are fascinating natural phenomena for many meteorologists. One reason is that over the years these storms have caused enormous loss of human lives and significant economical impact. The second reason is that they are still not fully understood and pose great challenges to the scientific community. An outstanding problem in current tropical cyclone research is their formation, or the genesis problem. It is not only a subject of scientific interest, but also of practical interest. It is because many mariners and coastal or island communites are quite vulnerable to sudden formation of tropical cyclones. If the time and location of their formation can be well predicted, more advanced warnings can be given to these communities and thereby human and economic loss can be minimized. The necessary environmental conditions for tropical cyclones to form have been well established. They include large values of low-level relative vorticity, a coriolis parameter greater than some finite value, weak vertical wind shear, and sea surface temperatures exceeding 260C (eg. Gray 1968). However, the sufficient conditions for their formation are less clear, as evidenced by the fact that only a small fraction of the disturbances observed in the Tropics grow into tropical storms even when the necessary conditions are met. One of the difficulties is that there are limited data over the oceans where tropical cyclones form. A detailed observation of the formation process is needed to further our understanding of this problem. The subtropical eastern Pacific Ocean is the most prolific region in the world for the formation of tropical cyclones. The tropical cyclones in this region are often associated with African easterly waves (Avila and Pasch 1992). While these easterly waves have often been observed to trigger tropical cyclones, the real mechanism has not yet been well understood. It is not dl^ l*ri~~.r.ar---irt*Y6- ~_-~i; clear whether they transform directly into tropical cyclones, or they merely provide a favorable background environment for pre-existing tropical disturbances to develop into tropical cyclones. The purpose of this study is to provide a detailed documentation of the formation of Tropical Cyclone Fefa in the eastern Pacific. Fefa formed during the Tropical Experiment in Mexico (TEXMEX), an intensive research program on tropical cyclone formation in the eastern North Pacific. Aircraft observations provide detailed mesoscale data during the cyclone's formation. Attention is given to the thermodynamics of the genesis process. In addition, NCEP/NCAR reanalysis, received on 2.50 x 2.50 grids, are used to study the characteristics of the large-scale environment. Particular attention is given to the evolution of the easterly wave associated with the formation of Fefa. In section 1.2, an overview of TEXMEX and hurricane Fefa is given. The next section contains a thorough review of previous studies of tropical cyclogenesis. Chapter 2 gives a description of the data sets and methods of analysis. Chapter 3 describes the observations of the large-scale environment. Chapter 4 contains an analysis of the aircraft data. A summary of the overall findings of this study and suggestions for future work are given in the final chapter. 1.2 Overview of the Tropical Experiment in Mexico (TEXMEX) and Hurricane Fefa One of the outstanding problems in tropical cyclone research is why so many disturbances in the tropics fail to develop into tropical cyclones. Rotunno and Emanuel (1987, hereinafter RE) suggested that convective downdrafts in the core of the incipient disturbances bring air of low equivalent potential temperature (0e) into the boundary layer, suppressing further convection and thereby preventing their subsequent development. For the disturbances to further develop, the negative effect of the downdrafts has to be overcome. In principle, this can be accomplished by an increase of the equivalent potential temperature in the middle troposphere, an increase of relative humidity so that evaporation of rain is suppressed, and/or an increase of wind speed in the boundary layer so that the sea surfaces fluxes can keep replenishing the Ge in that layer. The simulations of RE and Emanuel (1989), in which a warm core vortex was used in the initial state, suggested that an increase of Oe in the middle troposphere is necessary for tropical cyclogenesis. The main goal of TEXMEX was to test a hypothesis stated in the TEXMEX Operations Plan (Emanuel 1991): The elevation of ee in the middle troposphere just above a near surface vorticity maximum is a necessary and perhaps sufficient condition for tropical cyclogenesis. It was assumed that the elevation of ee is accomplished by deep convection bringing high 8e to the middle troposphere, as occurred in the models initialized by warm core vortices.