Tropical cyclone size

: climatology and physics

Abstract

This thesis establishes the climatology of tropical cyclone (TC) size, identifies potential factors likely related to TC size, and proposes mechanisms and processes governing the change in TC size. It consists of three main parts: (I) observational, (II) reanalysis, and (III) numerical studies. In Part I, a comprehensive statistical climatology of the size and strength of the TCs occurring over the western North Pacific (WNP, including South China Sea) and the North Atlantic (NA, including Gulf of Mexico and Caribbean Sea) between 1999 and 2009 is constructed based on Quick scatterometer data. The size and strength of a TC are defined respectively as the azimuthally-averaged radius of 17 m s-1 of ocean-surface winds (R17) and the azimuthally-averaged tangential wind within 1° to 2.5° latitude radius from the TC center (outer-core wind strength, OCS). The mean TC size and strength are found to be 2.13° latitude and 19.6 m s-1 respectively in the WNP, and 1.83° latitude and 18.7 m s-1 in the NA. While the correlation between size and strength is strong (r ≈ 0.9), that between intensity and either size or strength is weak. Seasonally, midsummer (July) and late-season (October) TCs are significantly larger in the WNP while the mean size is largest in September in the NA. The percentage frequency of TCs having large size or high strength is also found to vary spatially and seasonally. In addition, the interannual variations of TC size and strength in the WNP correlate significantly with the TC lifetimes and the effect of El Niño over the WNP. TC lifetime and seasonal subtropical ridge activities are shown to be potential factors that affect TC size and strength. In Part II, the mechanisms that are likely responsible for TC size changes are explored through analyses of angular momentum (AM) transports and synoptic flow patterns associated with the TC. Changes in AM transport in the upper and lower troposphere appear to be important factors that affect TC intensity and size, respectively. The change in TC intensity is positively related to the change in the upper-tropospheric AM export while the change in TC size is positively proportional to the change in the lower-tropospheric AM import. An examination of the synoptic flow patterns associated with WNP TCs suggests that changes in the synoptic flow near the TC are important in determining the change in TC size, with developments of the lower-tropospheric anticyclonic flows (one to the east and one to the west) bordering the TC being favorable for TC growth and a weakening of the subtropical high to the southeast for TC size reduction. A recurving TC tends to grow if the lower-tropospheric westerlies to its west increase. Moreover, a northward TC movement is related to the change in TC size. For example, a higher northward moving speed is found for a larger TC, which also agrees well with the AM transport concept. In Part III, the initial vortex intensity is found to be related to the size growth rate in the developing phase of the vortex life cycle. However, when the vortex reaches its mature and/or decaying phases, the initial vortex intensity (ranges between 20 and 40 m s-1 in this study) has not much effect on size. On the other hand, vortex intensification or re-intensification resulting from inner-core dynamics is apparently favorable for size growth in most instances. The numerical modelling study also examines how the initial vortex size and planetary vorticity (f) influence TC size change. With a given initial intensity and on the same f plane, an initially larger TC generally has a larger size at a later stage because it has a larger horizontal wind extent and higher winds outside the inner core. The larger vortex therefore possesses higher AM in the lower troposphere to increase its size in the outer-core region through AM transport. However, an initially small TC may not be "destined" to be small during its lifetime, which agrees with the observation that TC size has a positive relationship with TC lifetime. In addition, a vortex can apparently grow by itself in a resting environment through fluxes of AM. A vortex at a higher latitude is also found to be not necessarily larger. Furthermore, size change is controlled to some extent by the lower-tropospheric inertial stability associated with the vortex. Consistent with observations, TC size appears to have a maximum at some optimum latitudinal region (~25°N in general). Again, all the results agree well with the AM transport concept such that the outer-core area-integrated symmetric relative AM flux and Coriolis torque in the lower troposphere (especially those at the boundary layer) are important factors that govern size change. In addition, the lower-tropospheric outer winds of a vortex (i.e., winds beyond R17; e.g., the environmental flow around the TC) are found to be an important factor governing size change. The outer winds closer to R17 are more effective and can influence the vortex size at an earlier stage, especially if the winds are strong. The size change is much more sensitive to the outer-core, rather than inner-core, dynamics. The higher the AM beyond R17, the more the AM can be brought towards the center and hence favors size growth, and vice versa.

Date of Award	15 Jul 2014
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Chung Leung Johnny CHAN (Supervisor)