FY 1996

Observations and scale analysis of coastal wind jets

Overland, J.E., and N.A. Bond

Mon. Weather Rev., 123(10), 2934–2941, doi: 10.1175/1520-0493(1995)123<2934:OASAOC>2 (1995)

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 semi-geostrophic 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 = h_mN/fL_m being greater than 1, where h_m and L_m 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, h_I = U/N ~ 500 m, where U is the onshore component of velocity. This scale is appropriate when h_I is less than the mountain height, h_m; in our case h_I/h_m ~ 0.4. The offshore scale is given by the Rossby radius L_R = (Nh_m/f)F_m = U/f ~ 50 km for F_m < 1, where the mountain Froude number F_m = h_I/h_m = U/h_mN ~ 0.4. The increase in the alongshore wind speed due to blocking, ΔV, 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 F_m ~ 6. When F_m > 1, L_R = Nh_m/f ~ 200 km, and ΔV = h_mN ~ 2 m s^–1, is a small effect comparable to changes in the synoptic-scale flow. The authors expect a maximum coastal jet response when F_m ~ 1.