Storm tracks, baroclinic waves propagation and their interannual variability in the northern hemisphere

The goal of this study is to investigate the properties of storm tracks and baroclinic waves propagation in the Northern Hemisphere. The general characteristics and low-frequency (interannual) variability are the central work of this thesis, which are examined in both observational analysis and theoretical interpretation. We use NCEP/NCAR reanalysis data of 12-hourly wind and geopotential height at 300 hpa, isentropic potential vorticity (IPV) at 330K (potential temperature), monthly mean zonal wind and temperature at 700 and 850 hpa in 23 winters (DJF, 1973-1996) and 16 summers(JJA, 1980-1995). First, complex demodulation technique is applied to meridional wind (v', seasonal mean removed) data to get 12-hourly wave amplitude packets (ve). Based on the data of v' and ve, we compute the timelag correlation (for lags of -2 and +2 days). After this, the wave coherence index (WCI) and packet coherence index (PCI) are constructed to indicate the coherence of waves and wave packets propagation. Contrary to previous study by Wallace et al. (1988), baroclinic wave guides revealed in our study has impressively different distribution with storm tracks, probably due to the unfiltered data we used in calculation of timelag correlation. PCI is found to be in good agreement with the relative change of wave packets in propagation. We suggest that the chaotic development in regions of high baroclinicity can be the reason for the decrease of PCI. 12-hourly group velocity and phase velocity are obtained by tracking the most spatially coherent waves and wave packets between 12-hour time interval, respectively. Based on 12-hourly group velocity, mean growth (decay) rate of wave amplitude following group velocity is calculated. Secondly, theoretical analysis of spatial and temporal coherence of wave packets is given from dispersion view of spatial and temporal packets. The concept of temporal coherence, rather than spatial coherence, is shown to be closer to the timelag correlation. Temporal coherence of wave packets depends on the dispersion relation and group velocity, which can partly explain why the wave coherence is somehow associated with the basic state flow. Spatial coherence of wave packets, however, is mainly determined by the dispersion relation. We also theoretically analyze two special conditions of spatial coherence of wave packets in the atmosphere. In order to give quantitative indication of spatial coherence of baroclinic waves and wave packets, we apply a box technique to calculate 12-hourly spatial coherence of v' and ve respectively. The spatial coherence index (SCI, time-mean result) of wave packets is found to be higher in the regions of lower meridional IPV gradient in winter and summer. This observation shows that PV front theory isn't good at describing the spatial coherence of baroclinic wave packets. The mid-latitude SCI of baroclinic waves is a bit higher than that of baroclinic wave packets in winter, which suggests the way to forcast weather by exploring the evolution of wave packets may not work in winter. The interannual variabilities of storm tracks and baroclinic waves propagation are investigated mainly by using the empirical orthogonal function analysis (EOF's) and composite charts. Interannual seesaws of many fields such as westerlies, baroclinicity, RMS(v') are impressive features in midlatitudes. And close relation among these oscillations can be observed. Interannual variability of the first leading EOF mode of RMS(v') is closely associated with the variations of basic state flow and baroclinicity. The relations between other EOF modes and propagations of baroclinic waves and wave packets are also examined. Both WCI and PCI are found to have high relations the interannual variability of storm tracks. We further investigate interannual variations of timelag correlations of v' and ve in different regions along the baroclinic wave guides. Low and high composite charts of basic state flow and intensity of storm tracks, which are constructed according to the seasonal magnitude of of timelag correlation of v', show that higher correlations are always accompanied by intensification of storm tracks, and sometimes by the stronger basic state flow. We also find in general timelag correlation of ve is higher when correlation of v' is higher. Thesis Supervisor: Edmund K. M. Chang Title: Assistant Professor of Meteorology Acknowledgments I would like to express my sincere thanks to my thesis advisor, Dr. Edmund K. M. Chang. I greatly appreciate his guidance, insightful counsel, and patience throughout the whole process of my thesis research. His allowing me considerable independence to develop my own ideas and conduct the research will definitely benefit me much in future study. I also wish to thank Dr. Mario Molina, who broadened my view on other scientific fields, for his kindness and academic support. My special thanks are given to my sisters, Yijuan and Yishi, for their continuous concern and suggestions, both in my personality and career choice. Their experience helps me to make some important decisions. Among the many students and friends I have known here, I express my deep appreciation to Xiaoyun Zang, Yuanlong Hu, Bruce Kuo, Ji-Yong Wang, Stephanie Shaw, Chris Winkler, Jessica Neu, and Adam Sobel, for their inspiration and friendship. I will also acknowledge PAOC (formerly CMPO) support staff for their excellent work and helpfulness during my stay at MIT. Finally, the deepest thanks will go to my parents, Jianhui Chen and Sulian Qiu, who gave me the life and help me to explore it. Their love and encouragement continue to inspire me to do my best. To express my thanks, I will dedicate this thesis to them.