Determining the Impact of Boundary Layer Schemes on the Secondary Circulation of Typhoon Faxai Using Radar Observations in the Gray Zone

Using detailed radar observation data for Typhoon Faxai, which made landfall in the Tokyo metropolitan area in 2019, a sensitivity test of the boundary layer (BL) schemes for a numerical weather prediction (NWP) model was conducted for gray-zone numerical simulations with a grid spacing of 250 m. We compared the results of our simulations using an NWP model with radar observations that captured the BL and the secondary circulation structures of Faxai. We used three BL schemes based on a Reynolds-averaged model, the gray-zone model, and a large-eddy simulation (LES) model: the Mellor-Yamada-Nakanishi-Niino level 3 (MYNN3) scheme, the Anisotropic Deardorff Model (ADM) scheme, and the Deardorff (DDF) scheme, respectively. The turbulence kinetic energy was the smallest, and the inflow near Earth's surface the strongest, in the gray-zone simulation with the DDF scheme. This simulation also produced values for BL thickness and secondary circulation that were the closest to observation and reproduced horizontal roll structures whose scale was larger than the observation. Neither experiment using the MYNN3 scheme or the ADM scheme produced rolls, but the parameterized turbulence seemed to estimate the effects of the rolls. However, their BL heights were higher than observed, suggesting that the MYNN3 and ADM schemes are not appropriate for a 250 m grid simulation of the present case. These results are also confirmed against LES with 50 m grid spacing in which the DDF scheme is used. In summary, this study provides insights into the interpretation of the properties of BL schemes in the gray zone.