OBSERVATIONS AND SCALE ANALYSIS OF COASTAL WIND JETS

Blocking of onshore flow by coastal mountains was observed south of Va ncouver Island, British Columbia, by the NOAA P-3 aircraft on 1 Decemb er 1993. Winds increased from 10 m s(-1) offshore to 15 m s(-1) nearsh ore 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 le ngth scale and velocity increases are comparable to theory. The flow w as semigeostrophic with the coast being hydrodynamically steep; that i s, the coast acts like a wall and the alongshore momentum balance is a geostrophic. This is shown by the nondimensional slope parameter-the B urger number, B = h(m)N/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 fr equency, and fis the Coriolis parameter. The height scale is given by setting the local Froude number equal to 1-that is, h(l) = U/N similar to 500 m, where U is the onshore component of velocity. This scale is appropriate when h(l) is less than the mountain height, h(m); in this case h(l)/h(m) similar to 0.4. The offshore scale is given by the Ros sby radius L(R) = (Nh(m)/f) F-m = U/f similar to 50 km for F-m < 1, wh ere the mountain Froude number F-m = h(l)/h(m) = U/h(m)N similar to 0. 4. The increase in the alongshore wind speed due to blocking. Delta V, is equal to the onshore component of the Row, U approximate to 6 m s( -1) or in this case about half of the near-coastal alongshore componen t. A second case on 11 December 1993 had stronger onshore winds and we ak stratification and was in a different hydrodynamic regime, with F-m similar to 6. When F-m > l, L(R) = Nh(m)/f similar to 200 km, and Del ta V = h(m)N similar to 2 m s(-1), a small effect comparable to change s in the synoptic-scale flow. The authors expect a maximum coastal jet response when F-m similar to 1.