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Abstract
All available meteorological rocket soundings through the summer of 1966 are harmonically analyzed to obtain the amplitude and phase of the semidiurnal variation of the meridional wind component in summer for stations located near 30° and 37°N and of the zonal wind component in summer for the stations near 30°N. The results support the earlier finding that a phase reversal occurs at a height of 45–50 km rather than at the theoretically predicted height of 25–30 km. It is suggested that the difference between observation and theory may be attributed to the neglect of the basic wind structure in the theoretical calculation.
Abstract
All available meteorological rocket soundings through the summer of 1966 are harmonically analyzed to obtain the amplitude and phase of the semidiurnal variation of the meridional wind component in summer for stations located near 30° and 37°N and of the zonal wind component in summer for the stations near 30°N. The results support the earlier finding that a phase reversal occurs at a height of 45–50 km rather than at the theoretically predicted height of 25–30 km. It is suggested that the difference between observation and theory may be attributed to the neglect of the basic wind structure in the theoretical calculation.
Abstract
This paper examines the meteorological conditions and physical processes associated with the development of strong downslope winds that caused extensive property damage in two areas of western Washington on 28 November 1979. These areas wore located downwind of the two largest and lowest passageways through the Cascade Range: the Columbia River Gorge and the Stampede Pass region. Findings are as follows:
1) The destructive winds, marked by gusts of 25–30 m s−1, appeared in conjunction with the formation of a deep cyclone offshore and the simultaneous development of an unusually powerful anticyclone inland.
2) The pressure gradient was greatly enhanced in the vicinity of the mountain range attaining values as large as 12 mb (100 km)−1.
3) Hydrostatically, the large pressure differences can be attributed to the effect of the barrier in separating cold air on the cast side from warmer air on the West.
4) Trajectory tracing revealed that the temperature difference formed rapidly as a result of the presence of strong subsidence on the Ice side and the absence of low-level subsidence in the confined, inland basin on the windward side.
5) The undisturbed flow normal to the barrier ranged from light easterly at lower levels (5–10 m s−1 at most), to zero in the layer between 600 and 700 mb, to light westerly above.
Calculations are carried out to demonstrate that the wind speeds were consistent with the observed pressure differences. The large-scale pressure gradient was well predicted 36 h in advance by the limited-area fine-mesh model (LFM) of the National Meteorological Center.
Abstract
This paper examines the meteorological conditions and physical processes associated with the development of strong downslope winds that caused extensive property damage in two areas of western Washington on 28 November 1979. These areas wore located downwind of the two largest and lowest passageways through the Cascade Range: the Columbia River Gorge and the Stampede Pass region. Findings are as follows:
1) The destructive winds, marked by gusts of 25–30 m s−1, appeared in conjunction with the formation of a deep cyclone offshore and the simultaneous development of an unusually powerful anticyclone inland.
2) The pressure gradient was greatly enhanced in the vicinity of the mountain range attaining values as large as 12 mb (100 km)−1.
3) Hydrostatically, the large pressure differences can be attributed to the effect of the barrier in separating cold air on the cast side from warmer air on the West.
4) Trajectory tracing revealed that the temperature difference formed rapidly as a result of the presence of strong subsidence on the Ice side and the absence of low-level subsidence in the confined, inland basin on the windward side.
5) The undisturbed flow normal to the barrier ranged from light easterly at lower levels (5–10 m s−1 at most), to zero in the layer between 600 and 700 mb, to light westerly above.
Calculations are carried out to demonstrate that the wind speeds were consistent with the observed pressure differences. The large-scale pressure gradient was well predicted 36 h in advance by the limited-area fine-mesh model (LFM) of the National Meteorological Center.
Abstract
Two case studies of cyclogenesis that occurred in polar air streams behind or poleward of major frontal bands are presented. Based on the results of the studies, and on other evidence, characteristics of the type of disturbance in question are described. The cyclones in polar air masses are generally of small dimension, being spaced at intervals of 1000–1500 km when they occur in multiple form. They form most often over the oceans in winter, originating in regions of low-level heating and enhanced convection and acquiring a comma-shaped cloud pattern as they mature. They are associated with well-developed baroclinity throughout the troposphere and are located on the poleward side of the jet stream in a region marked by strong cyclonic wind shear and by conditional instability through a substantial depth of the troposphere.
Instability mechanisms for their formation are discussed. It is concluded that they are primarily a baroclinic phenomenon that owe their below average size to the effect that small static stabilities at low levels have in reducing the wavelength of maximum instability and to the fact that they develop on already perturbed large-scale states rather than on uniform zonal flows. Conditional instability of the second kind and barotropic instability cannot be ruled out as possible important additional influences in their formation.
Abstract
Two case studies of cyclogenesis that occurred in polar air streams behind or poleward of major frontal bands are presented. Based on the results of the studies, and on other evidence, characteristics of the type of disturbance in question are described. The cyclones in polar air masses are generally of small dimension, being spaced at intervals of 1000–1500 km when they occur in multiple form. They form most often over the oceans in winter, originating in regions of low-level heating and enhanced convection and acquiring a comma-shaped cloud pattern as they mature. They are associated with well-developed baroclinity throughout the troposphere and are located on the poleward side of the jet stream in a region marked by strong cyclonic wind shear and by conditional instability through a substantial depth of the troposphere.
Instability mechanisms for their formation are discussed. It is concluded that they are primarily a baroclinic phenomenon that owe their below average size to the effect that small static stabilities at low levels have in reducing the wavelength of maximum instability and to the fact that they develop on already perturbed large-scale states rather than on uniform zonal flows. Conditional instability of the second kind and barotropic instability cannot be ruled out as possible important additional influences in their formation.
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The potential vorticity on isentropic surfaces is used to study a characteristic type of upper-level frontogenesis — the development of a sloping stable layer marked by strong vertical wind shear and rapid upward decrease in humidity. In the case studied it was found that the intense portion of the frontal zone consisted of a thin wedge of stratospheric air which had descended to very low levels (700 to 800 millibars), the frontal boundaries being a folded portion of the original tropopause.
The circulation within the frontal zone was indirect solenoidal, and surface cyclogenesis accompanied or slightly preceded the strengthening of the upper-level front. Although the frontal zone formed entirely within a polar air-mass, the strong adiabatic heating at and near the warm boundary could give the false impression that tropical air was present there at the end stage.
Abstract
The potential vorticity on isentropic surfaces is used to study a characteristic type of upper-level frontogenesis — the development of a sloping stable layer marked by strong vertical wind shear and rapid upward decrease in humidity. In the case studied it was found that the intense portion of the frontal zone consisted of a thin wedge of stratospheric air which had descended to very low levels (700 to 800 millibars), the frontal boundaries being a folded portion of the original tropopause.
The circulation within the frontal zone was indirect solenoidal, and surface cyclogenesis accompanied or slightly preceded the strengthening of the upper-level front. Although the frontal zone formed entirely within a polar air-mass, the strong adiabatic heating at and near the warm boundary could give the false impression that tropical air was present there at the end stage.
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A two-level graphical prediction model is extended so as to include the effects of heating of cold air by relatively warm water. Orographic effects are also included in the model.
The model is applied to a case of a major storm development in the Gulf of Alaska attended by a strong outbreak of Arctic air from the Alaskan mainland. In this case the effects of nonadiabatic heating and orography appeared to be significant and in a direction which tended to improve the forecast.
Abstract
A two-level graphical prediction model is extended so as to include the effects of heating of cold air by relatively warm water. Orographic effects are also included in the model.
The model is applied to a case of a major storm development in the Gulf of Alaska attended by a strong outbreak of Arctic air from the Alaskan mainland. In this case the effects of nonadiabatic heating and orography appeared to be significant and in a direction which tended to improve the forecast.
