Sensitivity of the Simulated Tropical Cyclone Intensification to the Boundary-Layer Height Based on a K-Profile Boundary-Layer Parameterization Scheme

Abstract

The sensitivity of simulated tropical cyclone intensification to the boundary-layer height is investigated in this study, using the Yonsei University (YSU) scheme to parameterize boundary-layer processes. Results reveal nonlinear effects of the boundary-layer height on the intensification of simulated tropical cyclones. For shallow boundary layer (e.g., below 1,000 m), the storm intensification is sensitive to the depth of the boundary layer; decreasing the boundary-layer height would lead to a progressive increase in the intensification. For deep boundary layer, the storm intensification is weakly affected by the boundary-layer height. The roles of unbalanced processes are primarily responsible for these discrepancies. The frictional dissipation is confined in a relatively thin layer for shallow boundary layer, and decreasing the boundary-layer height causes a larger extent of gradient wind imbalance and stronger radial inflow in the boundary layer. As a consequence, the unbalanced flow effects are more pronounced and contribute to a more rapidly intensifying tropical cyclone. The vertical mixing is intense in a deep boundary layer, resulting in weak unbalanced processes; therefore, further enhancing the boundary-layer height has less appreciable influence on the tropical cyclone intensification than that in a shallow boundary layer. Increasing the boundary-layer height has also consequences of drying and warming the lower part of the boundary layer while moistening and cooling the atmosphere aloft, which may be favorable for encouraging outer-core convection activities. These results underscore the importance of accurately estimating the boundary-layer height in a K-profile boundary-layer scheme for tropical cyclone simulations.