Boundary layer model for moving tropical cyclones

We propose a simple theoretical model for the boundary layer (BL) of moving tropical cyclones (TCs). The model estimates the horizontal and vertical wind velocity fields from a few TC characteristics: the maximum tangential wind speed V _max, the radius of maximum winds R _max, and Holland's B parameter away from the surface boundary where gradient balance is approximately valid, in addition to the storm translation velocity V _t, the surface drag coefficient C _D, and the vertical diffusion coefficient of the horizontal momentum K. The model is based on Smith's (1968) formulation for stationary (axi-symmetric) tropical cyclones. Smith's model is first extended to include storm motion and then solved using the momentum integral method. The scheme is computationally very efficient and is stable also for large B values and fast-moving storms. Results are compared to those from other studies (Shapiro 1983; Kepert 2001) and validated using the Fifth-Generation Pennsylvania State University/NCAR Mesoscale Model (MM5). We find that Kepert's (2001) BL model significantly underestimates the radial and vertical fluxes, whereas Shapiro's (1983) slab-layer formulation produces radial and vertical winds that are a factor of about two higher than those produced by MM5. The velocity fields generated by the present model are consistent with MM5 and with tropical cyclone observations. We use the model to study how the symmetric and asymmetric components of the wind field vary with the storm parameters mentioned above. In accordance with observations, we find that larger values of B and lower values of R _max produce horizontal and vertical wind profiles that are more picked near the radius of maximum winds. We also find that, when cyclones in the northern hemisphere move, the vertical and storm-relative radial winds intensify at the right-front quadrant of the vortex, whereas the storm-relative tangential winds are more intense in the left-front region. The asymmetry is higher for faster moving TCs and for higher surface drag coefficients C _D .