The Influence of Coastal Orography: The Yakutat Storm

Abstract

An unforecast windstorm in the vicinity of Yakutat, Alaska, on 14 March 1979 illustrates the importance of ageostrophic dynamics within a coastal zone proximal to significant terrain. Large pressure rises [greater than 4 mb (3 h)?1]were observed along the southeastern Alaska coast after passage of a cold front when the low- level geostrophic flow was directed onshore. These pressure rises did not occur simultaneously along the coast, but rather propagated northward along the coast as a coherent pulse or surge. Strong surface winds (approximately 25?30 m s?1) were observed in the region of laid sea level pressure gradient at the leading edge of the surge and occurred after the passage of the synoptic front. Although the sparseness of the observations prevent definite conclusions, this feature resembles a Kelvin wave more than a density current. Omega dropwindsonde observations collected along the coast of Alaska during two other, less dramatic, situations suggest damming and downslope flow structures important to the interpretation of the Yakutat storm. Coastal semigeostrophic dynamics, that is, an ageostrophic momentum balance in the alongshore direction, occurs when the coastal mountains are hydrodynamically steep. The steep regime is defined by the nondimensional slope (hm/lm)N/f>1, where hm is mountain height, lmis mountain half-width, N is the static stability for the incident flow, and f is the Coriolis parameter. For typical values of N?10?2 s?1 the coast is wall-like when hm>0.01. Given a wall-like nature of the coast, trapped isolated mesoscale features, with an offshore length scale given by a Rossby radius of o(100 km), propagate alongshore ageostrophically due to a combination of Kelvin waves, density currents, or forced response. To correctly forecast in the coastal zone, numerical weather prediction models must qualitatively resolve terrain slopes so that the modeled dynamics are in the correct semigeostrophic or quasigeostrophic hydrodynamic regime.