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James E. Overland

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James E. Overland

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James E. Overland

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James E. Overland

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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.

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James E. Overland

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James E. Overland

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The operational NOAA categorical vessel icing algorithm is evaluated with regard to advances in understanding of the icing process and forecasting experience. When sea temperatures are <2–3°C above the saltwater freezing point there is the likelihood of supercooling of the spray during its trajectory and extreme ice accretion on topside structures. The NOAA algorithm shows excellent results when compared to a new cold-water dataset from the Labrador Sea (mean sea temperature of −13°C), even though the algorithm was developed from an Alaskan dataset with a mean sea temperature of 3.6°C. A rederived algorithm from the combined dataset is nearly identical to the operational algorithm. The influence of sea temperature in the NOAA model is consistent with the supercooling hypothesis and an additional icing category of extreme is recommended for the algorithm. Severe icing in the Bering Sea, Gulf of Alaska, and Sea of Japan is primarily caused by extreme cold-air advection, while low sea temperatures contribute to severe icing in the Labrador Sea, Denmark Strait, and Barents Sea. Indirect verification showed that NOAA provided excellent forecasts to over 140 fishing vessels in Alaskan waters during late January 1989, the worst icing episode of the decade. This case suggests that current-generation atmospheric models are capable of providing reliable 36-h forecasts of cold-air advection, and thus indicating regions of heavy icing. A wave height/wind speed threshold for the onset of topside icing is 5 m s−1 for a 15-m vessel, 10 m s−1 for a 50-m trawler and 15 m s−1 for a 100-m vessel, developed from seakeeping theory. These wind speeds are exceeded 83%, 47% and 15%, respectively, during February in the Bering Sea.

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James E. Overland and Muyin Wang

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The last decade shows increased variability in the Arctic Oscillation (AO) index for December. Over eastern North America such increased variability depended on amplification of the climatological longwave atmospheric circulation pattern. Recent negative magnitudes of the AO have increased geopotential thickness west of Greenland and cold weather in the central and eastern United States. Although the increased variance in the AO is statistically significant based on 9-yr running standard deviations from 1950 to 2014, one cannot necessarily robustly attribute the increase to steady changes in external sources (sea temperatures, sea ice) rather than a chaotic view of internal atmospheric variability; this is due to a relatively short record and a review of associated atmospheric dynamics. Although chaotic internal variability dominates the dynamics of atmospheric circulation, Arctic thermodynamic influence can reinforce the regional geopotential height pattern. Such reinforcement suggests a conditional or state dependence on whether an Arctic influence will impact subarctic severe weather, based on different circulation regimes. A key conclusion is the importance of recent variability over potential trends in Arctic and subarctic atmospheric circulation. Continued thermodynamic Arctic changes are suggested as a Bayesian prior leading to a probabilistic approach for potential subarctic weather linkages and the potential for improving seasonal forecasts.

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James E. Overland and Muyin Wang

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There were extensive regions of Arctic temperature extremes in January and February 2016 that continued into April. For January, the Arctic-wide averaged temperature anomaly was 2.0°C above the previous record of 3.0°C based on four reanalysis products. Midlatitude atmospheric circulation played a major role in producing such extreme temperatures. Extensive low geopotential heights at 700 hPa extended over the southeastern United States, across the Atlantic, and well into the Arctic. Low geopotential heights along the Aleutian Islands and a ridge along northwestern North America contributed southerly wind flow. These two regions of low geopotential height were seen as a major split in the tropospheric polar vortex over the Arctic. Warm air advection north of central Eurasia reinforced the ridge that split the flow near the North Pole. Winter 2015 and 2016 geopotential height fields represented an eastward shift in the longwave atmospheric circulation pattern compared to earlier in the decade (2010–13). Certainly Arctic amplification will continue, and 2016 shows that there can be major Arctic contributions from midlatitudes. Whether Arctic amplification feedbacks are accelerated by the combination of recent thinner, more mobile Arctic sea ice and occasional extreme atmospheric circulation events from midlatitudes is an interesting conjecture.

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James E. Overland and Nicholas Bond

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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.

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Gary M. Lackmann and James E. Overland

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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.

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