Model predictions of coastal winds in a small scale

Abstract

Details of surface winds across a flat and non-curved coastline have been considered, mostly in the 1–10 km scale, by looking at some published observed cases and simulating with a high-resolution numerical model. Stability conditions were emphasized by having the land and sea at slightly different temperatures, but sea breeze conditions were excluded.

A warm surface generally enhances momentum transfer and wind speed, and reduces the cross-isobar angle. This transforms to rapid acceleration and downstream veering of offshore winds as they enter a warm, smooth sea. Cold sea, on the other hand, strongly inhibits vertical momentum transfer. This leads to offshore winds temporarily backing downstream off the coast, accelerating only slowly and even decreasing in speed in some cases. These observed facts were well simulated by the model.

Onshore flow on to cold land brought along rapid deceleration and strong inland backing of winds. The deceleration was weaker and more gradual over warm land with downstream veering instead. Smaller geostrophic wind speed accentuated these and all other stability effects as the wind shears then were weaker relative to the buoyancy effects (i.e. the absolute values of the local Richardson numbers were larger).

For large-scale winds along the shore with land on the right, the induced coastal convergence and rising motion appears only above about 100 m height in the cold sea simulations (typical of early summer). Below, there is divergence and hence relatively weak coastal wind. The case is the opposite for winds with land on the left (e.g. westerlies along a south coast): then the coastal surface winds are relatively strong along a cold sea. For a warm sea (typical of autumn) these low-level features disappear and coastal 10 m winds tend to be close to the geostrophic wind in speed and direction for all onshore directions.