Frictional convergence at coastlines

The coastline generally represents a marked discontinuity in surface roughness. The resulting mechanical forcing leads to a secondary circulation in the boundary layer, and consequently to a vertical motion field that may have a strong influence on the weather in the coastal zone. In potentially unstable air masses, frictional convergence may cause a more-or-less stationary zone of heavy shower activity, for example.

In this paper, we present a calculation of secondary flow patterns forced at a roughness discontinuity, as a function of the geostrophic wind. A neutral boundary layer is studied, with homogeneous conditions along the coastline (i.e., we study the circulation in a plane perpendicular to the coastline). The closure scheme for the turbulent mixing of momentum is based on a calculation of the eddy kinetic energy density (the I,E closure). The equations are solved on a grid stretched in both the vertical and horizontal directions, with largest resolution at the roughness discontinuity. Various relaxation techniques are combined to give an efficient calculation of the stationary flow fields.

Upward motion turns out to be most pronounced when the geostrophic wind makes a small (~ 20°) angle with the coastline (in a clockwise direction), and not when the geostrophic wind is perpendicular to the coastline, as is sometimes mentioned. The asymmetry relative to the normal to the coastline is due to the Coriolis acceleration, and not a nonlinear effect Nevertheless, nonlinear effects are important, because they create a tendency to frontogenesis in the case mentioned above. This is shown by a comparison of the linear and nonlinear solution.

We finally present an example of heavy shower activity in the coastal zone of Belgium and The Netherlands, which is apparently due to frictional uplift and frontogenesis in a maritime polar air mass hitting the coastline at the critical angle referred to above.