Xuyang Ge

Tropical Cyclogenesis Associated with Rossby Wave Energy Dispersion of a Preexisting Typhoon. Part II: Numerical Simulations*

Abstract The cyclogenesis events associated with the tropical cyclone (TC) energy dispersion are simulated in a 3D model. A new TC with realistic dynamic and thermodynamic structures forms in the wake of a preexisting TC when a large-scale monsoon gyre or a monsoon shear line flow is present. Maximum vorticity generation appears in the planetary boundary layer (PBL) and the vorticity growth exhibits an oscillatory development. This oscillatory growth is also seen in the observed rainfall and cloud-top temperature fields. The diagnosis of the model output shows that the oscillatory development is attributed to the discharge and recharge of the PBL moisture and its interaction with convection and circulation. The moisture–convection feedback regulates the TC development through controlling the atmospheric stratification, raindrop-induced evaporative cooling and downdraft, PBL divergence, and vorticity generation. On one hand, ascending motion associated with deep convection transports moisture upward and le...

Monsoonal Influence on Typhoon Morakot (2009). Part II: Numerical Study

AbstractIn the second part of this study, numerical experiments are conducted to investigate the influences of multi-time-scale monsoonal flows on the track change of Typhoon Morakot (2009). While the control simulation captures the slowing and northward deflections in the vicinity of Taiwan Island, the highly asymmetric rainfall structure, and the associated rainfall pattern, sensitivity experiments suggest that the westward movement prior to the landfall on Taiwan and the subsequent northward shifts in the vicinity of Taiwan were closely associated with the interaction between Morakot and multi-time-scale monsoonal flows.Prior to the landfall on Taiwan, Morakot moved westward directly toward Taiwan because of a synoptic wave train–like pattern, which consisted of Goni over mainland China, Morakot, and a cyclone over the western North Pacific with an anticyclone to the west of Morakot. Numerical simulation suggests that strong northerly winds between Morakot and the anticyclone reduced the northward stee...

Effects of Vertical Shears and Midlevel Dry Air on Tropical Cyclone Developments

AbstractA set of idealized experiments using the Weather Research and Forecasting model (WRF) were designed to investigate the impacts of a midlevel dry air layer, vertical shear, and their combined effects on tropical cyclone (TC) development. Compared with previous studies that focused on the relative radial position of dry air with no mean flow, it is found that the combined effect of dry air and environmental vertical shear can greatly affect TC development. Moreover, this study indicates the importance of dry air and vertical shear orientations in determining the impact. The background vertical shear causes the tilting of an initially vertically aligned vortex. The shear forces a secondary circulation (FSC) with ascent (descent) in the downshear (upshear) flank. Hence, convection tends to be favored on the downshear side. The FSC reinforced by the convection may overcome the shear-induced drifting and “restore” the vertical alignment. When dry air is located in the downshear-right quadrant of the ini...

Tropical Cyclone Energy Dispersion in a Three-Dimensional Primitive Equation Model: Upper-Tropospheric Influence*

Abstract The three-dimensional (3D) Rossby wave energy dispersion of a tropical cyclone (TC) is studied using a baroclinic primitive equation model. The model is initialized with a symmetric vortex on a beta plane in an environment at rest. The vortex intensifies while becoming asymmetric and moving northwestward because of the beta effect. A synoptic-scale wave train forms in its wake a few days later. The energy-dispersion-induced Rossby wave train has a noticeable baroclinic structure with alternating cyclonic–anticyclonic–cyclonic (anticyclonic–cyclonic–anticyclonic) circulations in the lower (upper) troposphere. A key feature associated with the 3D wave train development is a downward propagation of the relative vorticity and kinetic energy. Because of the vertical differential inertial stability, the upper-level wave train develops faster than the lower-level counterpart. The upper anticyclonic circulation rapidly induces an intense asymmetric outflow jet in the southeast quadrant, and then further ...

Performance of a Dynamic Initialization Scheme in the Coupled Ocean–Atmosphere Mesoscale Prediction System for Tropical Cyclones (COAMPS-TC)

AbstractA dynamic initialization scheme for tropical cyclone structure and intensity in numerical prediction systems is described and tested. The procedure involves the removal of the analyzed vortex and, then, insertion of a new vortex that is dynamically initialized to the observed surface pressure into the numerical model initial conditions. This new vortex has the potential to be more balanced, and to have a more realistic boundary layer structure than by adding synthetic data in the data assimilation procedure to initialize the tropical cyclone in a model. The dynamic initialization scheme was tested on multiple tropical cyclones during 2008 and 2009 in the North Atlantic and western North Pacific Ocean basins using the Naval Research Laboratory’s tropical cyclone version of the Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS-TC). The use of this initialization procedure yielded significant improvements in intensity forecasts, with no degradation in track performance. Mean absolute error...

Cyclogenesis Simulation of Typhoon Prapiroon (2000) Associated with Rossby Wave Energy Dispersion

The genesis of Typhoon Prapiroon (2000), in the western North Pacific, is simulated to understand the role of Rossby wave energy dispersion of a preexisting tropical cyclone (TC) in the subsequent genesis event. Two experiments are conducted. In the control experiment (CTL), the authors retain both the previous typhoon, Typhoon Bilis, and its wave train in the initial condition. In the sensitivity experiment (EXP), the circulation of Typhoon Bilis was removed based on a spatial filtering technique of Kurihara et al., while the wave train in the wake is kept. The comparison between these two numerical simulations demonstrates that the preexisting TC impacts the subsequent TC genesis through both a direct and an indirect process. The direct process is through the conventional barotropic Rossby wave energy dispersion, which enhances the low-level wave train, the boundary layer convergence, and the convection‐circulation feedback. The indirect process is through the upper-level outflow jet. The asymmetric outflow jet induces a secondary circulation with a strong divergence tendency to the left-exit side of the outflow jet. The upper-level divergence boosts large-scale ascending motion and promotes favorable environmental conditions for a TC-scale vortex development. In addition, the outflow jet induces a well-organized cyclonic eddy angular momentum flux, which acts as a momentum forcing that enhances the upper-level outflow and low-level inflow and favors the growth of the new TC.

A 3DVAR-Based Dynamical Initialization Scheme for Tropical Cyclone Predictions*

AbstractA combined tropical cyclone dynamic initialization–three-dimensional variational data assimilation scheme (TCDI–3DVAR) is proposed. The specific procedure for the new initialization scheme is described as follows. First, a first-guess vortex field derived from a global analysis will be spun up in a full-physics mesoscale regional model in a quiescent environment. During the spinup period, the weak vortex is forced toward the observed central minimum sea level pressure (MSLP). The so-generated balanced TC vortex with realistic MSLP and a warm core is then merged into the environmental field and used in the subsequent 3DVAR data assimilation. The observation system simulation experiments (OSSEs) demonstrate that this new TC initialization scheme leads to much improved initial MSLP, warm core, and asymmetric temperature patterns compared to those from the conventional 3DVAR scheme. Forecasts of TC intensity with the new initialization scheme are made, and the results show that the new scheme is able ...

Impact of Secondary Eyewall Heating on Tropical Cyclone Intensity Change

Abstract The primary goal of this study is to explore the factors that might influence the intensity change of tropical cyclones (TCs) associated with secondary eyewall replacement. Concentric eyewall structures in TCs with and without large intensity weakening are compared using the Tropical Rainfall Measuring Mission (TRMM) 2A12 and 2A25 data. It is found that the secondary eyewalls with a stratiform-type heating profile show a marked weakening, while those TCs with a convective-type heating weaken insignificantly or even intensify. This observed feature is supported by a set of sensitivity numerical experiments performed with the Weather Research and Forecasting model. With more active convection, the latent heat released in the outer eyewall and moat region can better sustain storm intensity. The prevailing stratiform precipitation results in low equivalent potential temperature air in the moat and reduces the entropy of the boundary layer inflow to the inner eyewall through persistent downdrafts, lea...