Abstract

The complicated wind regimes in straits which develop in response to difFerent large-scale pressure fields are investigated by scale analysis of the equations of motion. Adjustment of the mass and motion fields in straits O(lOs km) in width is governed by four nondimensional numbers: separate along- and cross-strait Rossby numbers, a strait drag coefficient, and a stratification parameter, which relates the internal Rossby radius of deformation to the width of the strait. The wind field is in approximate geostrophic balance with an imposed cross-channel pressure gradient. An along-channel pressure gradient is primarily balanced by ageostrophic acceleration of the wind field down the axis of the strait (the gap wind). Vertical motion and the accompanying horizontal divergence in the near-surface wind field can be large even for moderately stable stratification; as a consequence, there may be particularly abrupt transitions of the surface wind field at the exits of straits, where there is a rapid change of the scaling parameters to match coastal conditions.

The scale analysis also applies to open coasts with the Rossby radius of deformation replacing the width of the strait as the offshore length scale. For the mountainous coasts along Alaska, Canada and Norway, a typical Rossby radius is 0(80 km); within this distance an alongshore pressure gradient Will be principally balanced by the ageostrophic terms in the momentum equation. Since the coastal Rossby radius is smaller than the grid size of present numerical weather prediction models, geostrophic adjustment is not correctly modeled for landfalling storms along mountainous coasts.

Abstract

Gap winds occur in topographically restricted channels when a component of the pressure gradient is parallel to the channel axis. Aircraft flight-level data are used to examine atmospheric structure and momentum balance during an early spring gap-wind event in Shelikof Strait, Alaska. Alongshore sea level pressure ridging was observed. Vertical cross sections show that across-strait gradients of boundary-layer temperature and depth accounted for the pressure distribution. Geostrophic adjustment of the mass field to the along-strait wind component contributed to development of the observed pressure pattern. Boundary-layer structure and force balance during this event was similar to that often observed along isolated barriers. However, the Rossby radius was lager than the strait width, and atmospheric structure in the strait exit region indicates transition of the flow to open coastline conditions. Two across-strait momentum budgets show that the Coriolis force and across-strait pressure gradient were an order of magnitude larger than other terms. Largest terms in the along-strait balance were the pressure gradient force, acceleration, entrainment, and friction. Boundary-layer acceleration in the along-strait direction was 55% of the potential Emit determined by the along-strait pressure gradient. Entrainment of air into the boundary layer was the largest retarding force and contributed to the along-strait profile of boundary-layer depth. Large horizontal divergence was observed within the strait, yet boundary-layer depth increased slightly following the flow. Entrainment at the inversion and sea surface fluxes accounted for along-strait variation of boundary-layer equivalent potential temperature.

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 (h_m /l_m )N/f>1, where h_m is mountain height, l_m is 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 h_m >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.

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 = 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_l = U/N ∼ 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 ∼ 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 _l /h _m = U/h _m N ∼ 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 F _m ∼ 6. When F _m > 1, L _R = Nh _m /f ∼ 200 km, and ΔV = h _m N ∼ 2 m s⁻¹, a small effect comparable to changes in the synoptic-scale flow. The authors expect a maximum coastal jet response when F _m ∼ 1.

Abstract

A technique is presented for selection of principal components for which the geophysical signal is greater than the level of noise. The level of noise is simulated by repeated sampling of principal components computed from a spatially and temporally uncorrected random process. By contrasting the application of principal components based upon the covariance matrix and correlation matrix for a given data set of cyclone frequencies, it is shown that the former is more suitable to fitting data and locating the individual variables that represent large variance in the record, while the latter is more suitable for resolving spatial oscillations such as the movement of primary storm tracks.

Abstract

A monthly storm-track climatology is derived from monthly maps of cyclone tracks for the winter season, October through March, averaged over 23 years, 1957/58–1979/80, for a 2° latitude×4° longitude grid bounded by 51°N, 65°N, 157°W and 171°E. There is a decrease in the number of cyclones with latitude in all months and division into two storm tracks, one propagating north-northeast along the Siberian peninsula and one entering the southern Bering Sea on a northeasterly course and either curving northward into the central Bering Sea or continuing parallel to the Aleutian Island chain.

Monthly average ice extents are established for February and March 1958–80 along a line from Norton Sound southwest toward the ice edge, perpendicular to the average maximum extent. Comparison of composite cyclone charts summed over the winter season and over the five heaviest and five lightest ice years shows a shift in cyclone centers toward the west in light ice years. The correlation between maximum seasonal ice extent and the difference between the number of cyclone centers in the eastern minus the western part of the basin over each winter season is 0.71. The relation of sea ice extent and the location of cyclone tracks is consistent with previous observations that advance of the ice edge in the Bering Sea is dominated by wind-driven advection and that southerly winds associated with cyclone tracks to the west inhibit this advance. These results indicate that the interannual variability in seasonal sea-ice extent in the Bering Sea is controlled by an externally determined variation in storm-track position related to large-scale differences in the general circulation. A skewed distribution of ice extents toward heavy ice years, however, suggests the possibility of an oceanographic constraint on the magnitude of extreme seasonal ice extents, such as the inability of melting ice to cool the mixed layer beyond the continental shelf to the freezing point or the increased influence of the northwestward flowing, continental slope current.

Abstract

Comparison is made between wind velocity measurements at two NOAA buoys, EB34 and EB41, located in the New York Bight, and winds extrapolated from nearby coastal stations and inferred from sea level pressure analysis at the National Meteorological Center. The comparison covers 0000 and 1200 GMT observations for November 1975 through March 1976. Surface winds are obtained from gradient winds by means of the analytic single-point boundary layer model proposed by Cardone (1969) and simple empirical relations.

Buoy wind speeds in excess of 10 m s⁻¹ accounted for 28% of the observations. For these strong winds, pressure-gradient based estimates provided adequate specifications of surface winds for 81% of the cases, defined by vector error <5 m s⁻¹, and were in general superior to estimates extrapolated from single coastal stations.

Rapid changes in wind speed and direction recorded in hourly buoy data indicate that resolution of winter storms requires pressure analyses on at least a 6 h cycle. The presence of moving storm systems also suggests that the use of coastal station reports can be improved by extrapolation in time as well as space.

Abstract

Aircraft and satellite data an used to study the structure of longitudinal roll vortices in a nearly neutral (z_i/L=-1.2, where z_i is the inversion height and L is the Monin-Obukhov length) boundary layer over the ice-covered Bering Sea during February. Steam fog, formed over cracks and leads in the ice, was used as a tracer to delineate the various scales of roll motion seen in satellite images. The satellite information combined with aircraft data collected by the NOAA P-3 indicated the presence of a hierarchy of roll vortex motions. It is suggested that interactions of the various scales of motion resulted in certain scales dominating in one area and other scales dominating in another. Two-kilometer wavelength variations an attributed to the inflection point instability mechanism while 12–15 km variations seen to have been reinforced by the upstream topography on the Chukotka Peninsula. Organization of the fog banks on scales of 30 km was also present and may be attributable to resonant subharmonics of the basic boundary layer instability or to a mesoscale entrainment instability.

Abstract

Gap winds can be defined as a flow of air in a sea level channel which accelerates under the influence of a pressure gradient parallel to the axis of the channel. In February 1980 two distinct cases of gap winds were observed in the Strait of Juan de Fuca between western Washington State and British Columbia during a study that measured spatial variation of low-level marine winds and other parameters from the NOAA P-3 research aircraft and a dense network of surface stations which included eight meteorological buoys. These two cases were a high-pressure region over central British Columbia and a low-pressure system propagating northward, seaward of the Washington coast. Both cases produced strong easterly winds of 13–15 m s⁻¹ at the western end of the Strait of Juan de Fuea. The high-pressure region provided a drainage air mass from the interior of British Columbia which flowed through the Straits of Georgia and Juan de Fuca and eventually into the Pacific Ocean. This air mass remained nearly homogeneous and was capped by a well-defined inversion. For the offshore low-pressure center, the lower atmosphere was stably stratified throughout the region, and weak winds were observed at the eastern end of the Strait of Juan de Fuca with strong winds at the western end. Although the features of the flow fields were complex, major characteristics of the wind fields can be accounted for by the combined effect of topography and the synoptic pressure field. Local winds were in approximate ageostrophic equilibrium between the inertia term and the imposed sea level pressure gradient.

Abstract

The behavior of stratified air flowing around an isolated mountain is dependent on an internal Froude number (F), which indicates the relative importance of upstream velocity and vertical stratification. Three cases of the flow in the lee of the Olympic Mountains in the State of Washington are studied where the measured F was in the range 1.0–1.4 but apparently dominated by stable stratification. This study combined measurements of spatial variation of low-level winds and other parameters from a NOAA P-3 research aircraft with a dense network of surface stations including eight meteorological buoys and six upper-air stations. Results from these cases show the presence of an area of light winds in the lee of the Olympic Mountains. The characteristics of the flow are shown to be similar to laboratory results for low Froude number flow around an isolated obstacle where the flow is confined to quasi-horizontal planes. These cases are contrasted with a situation which led to the formation of a mesoscale low-pressure area and high surface winds in the lee of the mountains. The latter case was the Hood Canal Bridge storm on 13 February 1979 where local winds in the lee of the Olympic Mountains were in excess of 50 m s⁻¹. The flow at the surface was produced by down-pressure-gradient acceleration in the confined channels of Puget Sound toward the orographically produced low-pressure center. The measured internal Froude number in this situation was 4.6, and the pressure fields are shown to agree with the linear hydrostatic model developed by Smith (1980) for F > 1. It is suggested that the Froude number calculated from routine, upper-air sounding data is an index that forecasters can use to determine the potential for severe wind conditions over the inland waters of Puget Sound.