Warm core and eyewall are characteristic structures of a typhoon. Focusing on a structure of the inner region of a typhoon, we performed a numerical simulation with a very high resolution using a cloud-resolving model, CReSS (the Cloud Resolving Storm Simulator). Purpose of the present study is to clarify contribution of the warm core to typhoon development, origin of airmasses which compose the warm core, and the forces which act on airmasses in the inner region.

The result of the simulation shows that the track and the central pressure drop of the simulated typhoon are similar to those of the observed typhoon. Typical structures and dynamical characteristics such as warm core, the eyewall, lower level inflow, updraft in the eyewall, and upper level outflow are successfully simulated. The warm core is formed at an altitude of 15 km. At this height, potential temperature perturbation was larger than 14 K. The increase of thickness between 200 hPa and 100 hPa resulted form the increase of potential temperature perturbation contributes to the increase of thickness between 900 hPa and 50 hPa. Because the height of 50 hPa is almost constant with time, the height of 900 hPa decreases owing to the increase of the thickness between 900 hPa and 50 hPa. This means the decrease of pressure at the lower atmosphere. Since the variation of height of 900 hPa has the same tendency as that of the surface pressure, we consider that the warm core contributes to the decrease of the surface pressure.

We performed a back trajectory analysis to find the origin of airmasses within the warm core. This revealed that airmasses of the warm core is composed of airmasses which come inward in the lower troposphere and ascend in the eyewall, and airmasses descending form the lower stratsphere. When airmasses which come inward in the lower troposphere ascend in the eyewall, the potential temperature is increased by diabatic heating. The warm core is composed of the airmasses with the high potential temperature.

We also performed a forward trajectory analysis of airmasses with origin in the lower troposphere to find the path of airmass. This revealed that most airmasses flow outward at the upper troposphere, and a small amount of airmasses move into the warm core. Since the centrifugal force on the inner side of the eyewall is larger than the pressure-gradient force in the lower troposphere, the radial velocity changes outward on the inner side. This rapid change of the radial velocity produces the low-level convergence and the resulted outward tilting updraft. This mechanism is common to both airmasses which flow outward in the upper troposphere and airmasses which move into the warm core. We found that the airmasses is accelerated outward by the centrifugal force which is larger than the pressure-gradient force in the upper troposphere. On the other hand, some airmasses are drifted into the warm core by the pressure-gradient force which is larger than the centrifugal force.