Effects of land-sea roughness contrast on tropical cyclone winds

The MM5 model is used to simulate the tropical cyclone (TC) wind distribution near landfall under the idealized conditions of a dry f-plane atmosphere and time-invariant axisymmetric mass fields. The winds are allowed to adjust towards a balance state under the influence of the different surface roughness between the land and sea, separated by a north-south-oriented coast. It is shown that the surface-wind asymmetry near the TC core is related to the acceleration of the offshore (onshore) flow prior to (after) landfall.

For pre-landfall cases, the strong surface inflow to the left (or left front) also produces a tangential (total) wind maxima associated with the offshore flow. The tangential wind could reach the gradient wind due to the larger work done by the pressure gradient force. At levels above the surface and near the top of the planetary boundary layer (PBL), the wind is also asymmetric and a supergradient tangential wind is mainly maintained by vertical advection of the radial wind.

Asymmetric surface and PBL convergences are studied in 3 radial bands of 0-50, 50-100 and 100-500 km. Convergence is stronger to the left for 0-50 km core region due to the radial inflow, but is to the right for the 100-500 km outer region due to the tangential wind convergence along the coastline.

Estimation of the gusts gives reasonable results, with gusts originating from higher altitude over land then over the sea, but the maximum gusts speeds are slighter smaller over land.

These results are found to be similar to those from the full-physics version of the MM5, which suggests that in landfall situations, the different roughness between land and sea leads to strong wind asymmetries that can be largely explained by the net acceleration associated with a deviation from gradient balance.