7 Compatibility of High Resolution Terrain with High Resolution Model Grid

Tuesday, 6 August 2013
Holladay-Halsey (DoubleTree by Hilton Portland)
William Y. Y. Cheng, NCAR, Boulder, CO; and Y. Liu, G. Roux, L. Pan, Y. Liu, J. Y. Byon, and Y. J. Choi

As computing power increases, mesoscale models are being run at a higher and higher resolution. Naturally, the user wants to represent the underlying terrain in the mesoscale model as realistically as possible by matching the terrain resolution to that of the model. However, from a theoretical standpoint, this may not be the best option. For example, it has been shown by Skamarock (2004) that the effective resolution of ARW WRF is 7 Δx. Terrain features of 7 Δx or smaller may impose forcings on the WRF model that are unphysical, generating noise or instability. One way to alleviate this problem is to apply smoothing to the terrain. However, there is a drawback in this procedure. Smoothing decreases the realism of the terrain and defeats the purpose of running a high resolution model. In this paper, the flow over complex terrain in the Korean Peninsula will be examined using WRF-RTFDDA with two identical domains of the same horizontal extent with horizontal grid spacing of 3 and 9 km. Sensitivity experiments were conducted with the underlying terrain smoothed using wavelet transforms by removing the 2 Δx, 4 Δx, and 8 Δx features. Sensitivity experiments were compared with each other to show how the model grid spacing and terrain smoothing affected phenomena such as sea/land breeze and flow around/over mountains. Preliminary results indicate that although higher resolution represented the mesoscale flow features much better, the wind speed mean absolute error (MAE) was higher for some stations for the 3-km grid. Smoothing via wavelet transform reduced in wind speed MAE in some of these stations. The ultimate goal of this study is find a balance between smoothing the underlying terrain while maintaining realism of flow over complex terrain. Additional results will be shown at the conference.
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