Observations and Scale Analysis of Coastal Wind Jets

Abstract

Blocking of onshore flow by coastal mountains was observed south of Vancouver Island, British Columbia, by the NOAA P-3 aircraft on 1 December 1993. Winds increased from 10 m s?1 offshore to 15 m s?1 nearshore and became more parallel to shore in the blocked region, which had a vertical scale of 500 m and an offshore scale of 40?50 km. These length scale and velocity increases are comparable to theory. The flow was semigeostrophic with the coast being hydrodynamically steep; that is, the coast acts like a wall and the alongshore momentum balance is ageostrophic. This is shown by the nondimensional slope parameter?the Burger number, B = hmN/fLm?being greater than 1, where hm and Lm are the height and half-width of the mountain, N is the stability frequency, and f is the Coriolis parameter. The height scale is given by setting the local Froude number equal to 1?that is, hl = U/N ? 500 m, where U is the onshore component of velocity. This scale is appropriate when hl is less than the mountain height, hm; in this case hl/hm ? 0.4. The offshore scale is given by the Rossby radius LR = (Nhm/f)Fm = U/f ? 50 km for Fm < 1, where the mountain Froude number Fm = hl/hm = U/hmN ? 0.4. The increase in the alongshore wind speed due to blocking, &DeltaV, is equal to the onshore component of the flow, U ≈ 6 m s?1 or in this case about half of the near-coastal alongshore component. A second case on 11 December 1993 had stronger onshore winds and weak stratification and was in a different hydrodynamic regime, with Fm ? 6. When Fm > 1, LR = Nhm/f ? 200 km, and ?V = hmN ? 2 m s?1, a small effect comparable to changes in the synoptic-scale flow. The authors expect a maximum coastal jet response when Fm ? 1.