Sensitivity of severe storm over a complex terrain to model resolution for Typhoon SONGDA in 2004

Abstract:

Under a severe storm condition over a complex terrain, enhanced flow due to such a terrain leads to heavy damage on a local field. To understand the variety of flow caused by a complexity of surface geometry, reproduction of the flow using a numerical model is useful, but the simulated flow would vary depending on resolutions of the model. In this study, to clarify a sensitivity of storm over a complex terrain to model resolution, we compared results simulated using a different horizontal resolution over the northern part of Japan. The winds are caused by Typhoon SONGDA in 2004. Numerical experiments to represent airflows over complex terrain were conducted with the use of the WRF model with its nesting capability employed at 9-km, 3-km, 1-km, and 200-m resolutions.

Comparing terrains represented by the model with 1-km resolution and by that with 200-m resolution, the higher altitude area can be seen in the high resolution model and the lower altitude area also are more clearly distinguished. The maximum and minimum angle of slope is 9.5° and 0.4° for the low resolution and 31.6° and 0.04° for the high resolution, respectively. Wind velocity at 10 m height from the surface over an analyzed area at each calculated time highly concentrates at a certain velocity in the lower resolution model. And on the other hand, the velocity widely distributed from a low velocity to a high velocity across the velocity which is most recorded in the lower resolution model. It is because the high resolution model can express the change of wind due to the complex terrain. Mean velocity and the maximum velocity over the period of the typhoon approach become strong in the high resolution model. Relationship between the velocities and slope angle indicates the velocities increase over a steeper terrain regardless of the model resolution. However, the mean velocity shows highly variable for the high resolution. And, the maximum wind during the period shows weak for the high resolution model within a range of slope angle which is represented in both models.