**2. Numerical model**

A nonhydrostatic model (NHM) has been developed jointly by the Numerical Prediction Division (NPD) and Meteorological Research Institute (MRI) of the Japan Meteorological Agency (JMA) (hereafter JMANHM). The NHM, which has been equipped with two-way triply nested movable functions, was developed at MRI and was coupled with a multilayer ocean model (MRINHM) developed by (Wada, 2009). The physical processes in JMANHM and MRINHM include cloud physics expressed in an explicit three-ice bulk microphysics scheme based on the work of (Lin et al., 1983), a resistance law assumed for momentum and

mesovortices and vortex Rossby waves. Mesovortices and vortex merger events are directly affected by SSC, which slows the formation of an annular potential-vorticity (PV) ring, whereas SST changes have little effect on the radius of maximum wind speed (MWS) at the mature phase of a TC when the annular PV ring is completely formed, even though mature

TC-induced SSC is caused mostly by vertical turbulent mixing in the oceanic mixed layer and upwelling below a seasonal thermocline. In addition, strong wind stresses that accompany TCs cause variations in sea state or surface roughness length, leading to changes in frictional velocity and exchange coefficients for drag and enthalpy. Breaking surface waves are caused by variations of sea state under high winds and the resultant high waves. The breaking surface waves play an essential role in mechanical mixing near the surface (Wada et al., 2010). However, improvement of vertical turbulent mixing schemes and parameterizations in the ocean model is a challenging issue owing to a lack of *in situ*

Changes in exchange coefficients lead to changes in surface wind stresses and turbulent heat fluxes from the ocean to the atmosphere. The change in turbulent heat fluxes, particularly latent heat flux, enhances the secondary circulation of a TC through a planetary-boundarylayer process (Emanuel, 1986; Smith, 2008). In particular, turbulent heat fluxes vertically transferred from the warm ocean affect cloud microphysics and atmospheric radiation in the middle to upper troposphere, causing latent heat release through condensation, thus

These atmospheric dynamics are confined to within the inner core of a TC. We need also to consider the role of atmospheric and oceanic environmental influences, such as vertical shear, synoptic thermal stratification, and warm-core oceanic eddies with a few hundred kilometers, in the evolution of a TC. (Wada & Usui, 2010) reported that changes in preexisting oceanic conditions are synoptically related to the formation of spiral rainbands that accompanied Typhoon Hai-Tang in 2005 and are thus important for predictions of the

This chapter explores the effect of the ocean on TC intensity, intensification and structural change from the viewpoints of internal dynamical and pre-existing external atmospheric and oceanic environmental conditions. Section 2 describes the numerical model used in this study, and section 3 describes the experimental design. Section 4 provides the results of numerical experiments of an idealized TC and simulations of Typhoon Choi-Wan in 2009. The effect of the ocean on TC evolution at the mature phase and associated numerical issues

A nonhydrostatic model (NHM) has been developed jointly by the Numerical Prediction Division (NPD) and Meteorological Research Institute (MRI) of the Japan Meteorological Agency (JMA) (hereafter JMANHM). The NHM, which has been equipped with two-way triply nested movable functions, was developed at MRI and was coupled with a multilayer ocean model (MRINHM) developed by (Wada, 2009). The physical processes in JMANHM and MRINHM include cloud physics expressed in an explicit three-ice bulk microphysics scheme based on the work of (Lin et al., 1983), a resistance law assumed for momentum and

resulting in the formation of a warm core within the inner core of a TC.

are discussed in section 5. Section 6 presents the conclusion of this study.

TCs continue to cool the underlying ocean (Wada, 2009).

observations under high winds.

intensity of Hai-Tang.

**2. Numerical model** 

enthalpy fluxes in the atmospheric surface-boundary layer, exchange coefficients for momentum and enthalpy transfers over the sea based on the bulk formulas of (Kondo, 1975), a turbulent closure model in the atmospheric boundary layer formulated from the work of (Klemp & Wilhelmson, 1978) and (Deardorff, 1980), and an atmospheric radiation scheme based on (Sugi et al., 1990).

The JMANHM used here for numerical simulations of Typhoon Choi-wan in 2009 is an older version of the current nonhydrostatic mesoscale model of (Wada et al., 2010), but it is coupled with not only a multilayer ocean model but also a third-generation ocean-wave model developed for operational use at JMA (Wada et al., 2010). No cumulus parameterization was used in either MRINHM or JMANHM in this study.

A reduced-gravity approximation, a hydrostatic approximation, and Boussinesq fluid are assumed in the multilayer ocean model. The model has three layers and four levels. The uppermost layer represents a mixed layer where density is vertically uniform, the middle layer represents a seasonal thermocline where the vertical temperature gradient is greatest, and the bottom layer is assumed to be undisturbed by entrainment. Entrainment is calculated by using a multi-limit entrainment formulation proposed by (Deardorff, 1983) and modified by (Wada et al., 2009). The model calculates water temperature and salinity at the surface and at the base of the mixed layer, and calculates layer thickness and 2 dimentional flows for all three layers.

The JMA third-generation ocean-wave model is coupled with JMANHM to estimate changes in surface roughness lengths, drag coefficients and enthalpy coefficients. The method of (Taylor & Yelland, 2001) in which surface roughness length over the ocean depends on wave steepness is adopted here. The detailed wave-ocean coupling procedure is as described by (Wada et al., 2010).

The atmosphere-ocean coupling procedure in MRINHM and JMANHM is as follows. Shortwave and long-wave radiation, sensible and latent heat fluxes, wind stresses, and precipitation are provided to the ocean model for every time step of the ocean model. Land and sea distributions extracted from GTOPO30 digital elevation data from the US Geological Survey are provided from the atmosphere model to the ocean model only at the initial time in order to adjust the land and sea distributions between the atmosphere and ocean models. Oceanic topography is provided by ETOPO-5 data from the National Oceanic and Atmospheric Administration (NOAA) National Geophysical Data Center; these elevations are spaced at 5-minute intervals of latitude and longitude. Conversely, SST calculated by the ocean model is provided to MRINHM and JMANHM for every time step of the ocean model.
