A Study On The Effects Of Environmental Circulation And Boundary Layer Processes On Tropical Cyclone Structure And Intensity Change Near Coast

High impact weathers are closely related to tropical cyclone (TC) structure and intensity change. Scientific and forecasting challenges arisen from the TC structure and intensity change are most critical. In this connexion, the world tropical cyclone research community attaches more importance to the area of the TC structure and intensity change.Forecasts of TC track have improved steadily over the last three decades, mostly due to a combination of better observations, improvements in data assimilation techniques and dynamical models as well as improved understanding of physical processes and mechanisms that govern the motion of TCs. On the contrary, there has been relatively little progress in prediction of TC structure and intensity change, in spite of the application of sophisticated numerical models and availability of advanced satellite observations. The prevalent forecasting techniques of TC intensity in operational use arc still statistical scheme. Among many reasons (such as rare observations over the ocean, inadequate model resolution and physical process, and poor initial conditions), deficient understanding of the physical processes governing the structure and intensity change is a fundamental one. What kind and what degree of differential physical mechanism affect the TC structure and intensity change? This is the scientific objective of this dissertation.The WRF model and TCM4 model were employed in the numerical simulation in this dissertation. The diagnostic analysis with vorticity equation, filtering techniques and the statistical analysis were adopted in this study.The relevant data what we used are the best track data from JTWC, NHC/TPC, and CMA. The SST data are the Reynolds SST reanalysis provided by the National Oceanic and Atmospheric Administration-Cooperative Institute for Research in Environmental Sciences (NOAA-CIRES) Climate Diagnostics Center (CDC). The Reynolds SST is weekly mean with a horizontal resolution of 1°latitude/longitudc. National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) reanalysis products were utilized to estimate the large-scale environmental parameter. The data are available at 4 times daily and have a horizontal resolution of 2.5°latitudes/longitudes with 17 vertical levels with pressure surfaces. The objective analysis data of Global Model GFS and FNL were used as the initial background fields for the meso-scale model simulation. The data from the Typhoon Year Book and the Tropical Cyclone Year Book published by Chinese Meteorological Press arc used as well. Many interesting results have been found in this study. Some of them are useful for the operational prediction and some of them are valuable for the theoretical study. The major points of them are summarized as follow.At first, the effects of two environmental dynamical factors, namely, the translational speed and vertical wind shear on tropical cyclone intensity change, and the lifetime peak intensity were analyzed based on the observations in both the western North Pacific and the North Atlantic during 1981-2003. In general, both the fast motion and strong vertical shear are negative to both TC intensification and the lifetime peak intensity. Both the very intense TCs and the TCs with rapid intensification rate are found only to occur in a narrow range of moving speeds between 10-25 km h-1, and in relatively weak vertical shear. The results show that few TCs intensified when they moved faster than 15 m s-1, or when their large-scale environmental vertical shear is larger than 20 m s-1.Based on the statistical analysis, a new empirical maximum potential intensity (MPI) has been developed, which includes the combined negative effect of translational speed and vertical shear as the environmental dynamical control in addition to the positive contribution of SST and the outflow temperature as the thermodynamic control. The new empirical MPI can not only provide more accurate estimation of TC maximum intensity but also better explain the observed behavior of the TC maximum intensity and help to understand the thermodynamic and environmental dynamical controls of TC intensity.An attempt has been made to extend the analysis of environmental dynamical control of tropical cyclone intensity recently performed for the western North Pacific to the North Atlantic. The average intensity of total TCs in the Atlantic is a bit smaller than that in the western North Pacific. The SST-detcrmined empirical TC maximum potential intensity (MPI) for 1981-2003 in this study is slightly higher than that found by DeMaria and Kaplan in the Atlantic for 1962-1992, however. To be consistent with the theoretical TC MPI, a new MPIM has been constructed, which includes the effect of thermodynamic efficiency. This new MPIM improves the estimation of real TC maximum intensity marginally due to the fact that the thermodynamic efficiency is largely determined by SST. With this negative effect of the environmental dynamical efficiency, an empirical maximum intensity (EMI) for Atlantic TCs has been constructed. This EMI includes not only the positive contribution by SST but also the effects of both thermodynamic and dynamical efficiencies. Results show that the EMI not only gives more accurate estimation of real TC maximum intensity but also provides an approximate, explicit measure of the environmental dynamical control of TC maximum intensity through an apparent dynamical efficiency.The effect of vertical wind shear (VWS) between different vertical levels and with different directions of shear on TC intensity change for TCs of different intensities, translational speeds, and latitudes were statistically analyzed. The strong, slow moving, and low latitude TCs are strongly affected by VWS in a deep layer, especially in the boundary layer, while the weak, fast moving, and high latitude TCs are subject to strong effect by VWS in the mid-lower troposphere. Overall easterly shear, especially in the mid-lower troposphere, has considerably weaker effects on TC intensity change than westerly shear. It seems that no universal means by which VWS is measured can fully explain the effects of VWS on TC intensity change.Furthermore, a new parameterization scheme of sea surface momentum roughness length for all wind regimes, including high winds, under TC conditions is constructed based on measurements from Global Positioning System (GPS) dropsonde. It reproduces the observed regime transition, namely, an increase of the drag coefficient with an increase in wind speed up to 40 m s-1, followed by a decrease with a further increase in wind speed. The effect of this parameterization on the structure and intensity of TCs is evaluated using a newly developed numerical model, TCM4. The results show that the final intensity is increased by 10.5%(8.9%) in the maximum surface wind speed and by 8.1 hPa (5.9 hPa) in the minimum sea surface pressure drop with (without) dissipative heating. This intensity increase is found to be due mainly to the reduced frictional dissipation in the surface layer and related with either the surface enthalpy flux or latent heat release in the outer cyewall convection. The effect of the new parameterization on the storm structure is found to be insignificant and occurs only in the inner core region with the increase in tangential winds in the eyewall and the increase in temperature anomalies in the eye. This is because the difference in drag coefficient appears only in a small area under the cyewall.The idealized WRF model with a multiply nesting and one-way feedback is used to study of the effect of sea spray on the simulated TC structure and intensity change and to analysis the effect of sea spray on the TC boundary layer structure. The results show that there is a significant increasing in TC intensity with its boundary layer wind structure changes and its total surface wind changes. The contribution of boundary layer momentum is increasing dramatically with a decrease of friction velocity and with an increase of convergence, while radial and tangential wind is increasing significantly with an increase of the horizontal gradient maximum of the radial wind. Diagnostic analysis of the vorticity budget shows that an increase of convergence in TC boundary layer gives rise to increase TC voritcity due to multi-effects including sea spay of the TC.The study of the structure and intensity change of super typhoon Saomci (0608) was performed with a high resolution and non-hydrostatic WRF model. It was found that the final intensity of the model storm is significantly increased by using of the new boundary layer parameterization for the TC, while the physical factors in its boundary layer have dramatically changes, in particular, during the period of the strongest intensity of TC. The average tangential, radial wind, vertical wind, temperature anormaly, eddy kinetic energy, and absolute angular momentum in its inner core region are increasing with the decrease of the sea surface drag coefficient in TC eyewall. There is rather important effect of distributions of all kinds of the physical elements around 20-40 km of the eyewall region of the TC on its structure and intensity change. The results also show that there is the negative effect of large scale environmental vertical wind shear on the TC intensity change when in the process of TC intensifying and weakening.Coupled the high resolution and non-hydrostatic idealized WRF model with a simple oceanic mixing layer model, the effect of the mixing depth on TC structure and intnensity changes is explored. The results show that, with ocean coupling, the surface heat flux is mainly reduced by oceanic upwelling in the rear-right quadrant of the TC core, which leads to increasing the TC asymmetric structure and decreasing the TC intensity.Finally, the conceptual model of the mechanism of the TC structure and intensity change is established and the possible effects of the environmental circulation and the boundary layer processes on tropical cyclone structure and intensity change near coast is summarized, on the basis of the studies above, which would be useful to the theoretical study and operational forecasting.