Study On The Characteristics Of Tropical Cyclone Volume And Its Relationship With Intensity In The Northwest Pacific

The tropical cyclone(TC)structure(including shape)is closely related to the TC intensity and intensity change.Such as the famous Dvorak technique,which vividly describes the relationship between TC horizontal shape and intensity.Recently,the relationship between horizontal size and TC intensity has been paid more and more attention,some studies also focus on the effect of the TC vertical structure(shape)features(such as the height of warm core)on intensity.However,the overall shape(such as three-dimensional envelope,volume)of TC and its relationship with TC intensity has not been involved so far,comprehensive analysis of TC horizontal and vertical size are still not enough.The object of this paper is to search the possible relationship between TC volume and TC intensity change.Firstly,based on the ERA-Interim reanalysis data from the European Centre for Medium-Range Weather Forecasts(ECMWF)with a horizontal resolution of 0.25 degrees,the characteristics of horizontal size and “volume” of TC in the Northwest Pacific during2006~2015 and their relationships with TC intensity were statistically analyzed.The results show that the correlation between the horizontal size and TC intensity is generally weak,especially when TC intensity is weak,the trend of horizontal size with TC intensity is not obvious.Only above typhoon intensity,the horizontal size increases significantly with TC intensity.While the correlation between TC “volume” and TC intensity is relatively more significant.With the increase of TC intensity,TC “volume” increase steadily,and the frequency of large(small)“volume” TC steadily increase(decrease).there is almost no small volume for super typhoon.In addition,the TC vertical size(partially described by the extended height of positive vorticity)also has a significant correlation with TC intensity,especially when TC is relatively weak(below typhoon).It suggests that the weak TC enhancement mainly accompanies with increase in vertical size.When TC intensity reaches typhoon,further intensification of TC mainly accompanies with increase in horizontal size.Therefore,neither horizontal size nor vertical size can describe the relationship between TC structure(shape)change and its intensity in all intensity levels.However,the “volume” of TC comprehensively characterizes the horizontal and vertical size features,so it can better describe this relationship.That is,TC intensified with the increase of the TC “volume”.The former statistical relationship between the “volume” and intensity of TC was validated by conducting a series of sensitivity experiments,and the potential vorticity(PV)equation was used to analyze the possible mechanism of the “volume” change on the intensity of TC.The results show that PV in the inner-core region increases significantly as the “volume” of TC becomes larger,which will lead to an increase in TC intensity.The diabatic heating in the lower level and vertical flux divergence of PV in middle to upper troposphere plays a major role in increasing the PV in the inner-core region,while the horizontal flux divergence of PV is relatively small.With the increase of TC “volume”,both the vertical flux divergence of PV and the diabatic heating are significantly increased.The net effect increases PV in the inner-core region,which is the main reason for the increasing of TC intensity.This paper is a preliminary attempt to explore the possible relationship between TC“volume” and TC intensity change.Due to the data limitation and so on,the "volume" in this paper is not the exact TC volume in the literature,the conclusions need more theoretical research and observational data validation.