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Richard J. Reed

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Richard J. Reed

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.

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Richard J. Reed

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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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RICHARD J. REED

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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.

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Richard J. Reed

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Useful forecasts may be obtained by graphical integrations of the dynamical prediction equations for a barotropic and a two-level baroclinic atmosphere. Such forecasts may be prepared without the aid of special equipment and are therefore particularly valuable as a means of training forecasters in physical prognosis.

The present paper reviews the physical principles, modeling assumptions, and methods of solution used in graphical prediction and introduces a method of obtaining surface forecasts which is considerably faster and simpler than previous methods. The predicted surface pressure is shown to be the sum of two components: (1) the pressure advected to the spot by one-half the 500-mb. wind and (2) a pressure change reflected down from aloft (actually one-half the 500-mb. height change expressed in equivalent pressure units at 1000 mb.). The movement of surface pressure systems is thus seen to be largely dependent on upper-level steering, while the deepening is found to be related to the vorticity advection at high levels, since this mainly determines the 500-mb. height changes.

Twenty sample surface forecasts prepared by the graphical method during July 1959 are presented and compared with the forecasts for the same dates issued by the National Weather Analysis Center. Little difference in accuracy is apparent.

Typical shortcomings and failures of the graphical prognoses are discussed. It is believed that the most serious errors are due to the use of only the initial 500-mb. charts in advecting the pressure systems. If the 500-mb. forecasts had been available earlier, it appears that a significant increase in accuracy could have been achieved by using both initial and forecast 500-mb. contours in performing the advections.

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RICHARD J. REED

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The annual temperature regime in the tropical stratosphere between 100 mb. and 10 mb. is examined on the basis of five years of data from six stations, ranging in latitude from 9° N. to 34° N. The principal result of interest is the finding of a pronounced semiannual component in the temperature variation above the 30-mb. level (24 km.), especially at stations near the equator. It is suggested that this may be caused by the direct absorption of solar ultraviolet radiation by ozone in a region where the heating cycle is predominantly semiannual.

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Jordan G. Powers
and
Richard J. Reed

Abstract

An observational study, employing spectral methods, is first made to establish a background for a modeling effort of the mesoscale gravity-wave event of 15 December 1987. The waves are found to have wavelengths of 100–160 km, phase speeds of approximately 30 m s−1, and lifetimes of over 6 h. Conditions for their maintenance are evaluated, indicating the presence of a wave duct and a supportive role for wave-CISK. Convection, shearing instability, and geostrophic adjustment are all implicated as possible source mechanisms for the observed waves.

The case is then simulated with the Pennsylvania State University–National Center for Atmospheric Research MM4 mesoscale forecast model, with the following primary objectives: (i) to test the model's ability to simulate a mesoscale gravity-wave event, (ii) to examine in detail the environments of mesoscale gravity-wave development, and (iii) to investigate the mechanisms of mesoscale gravity-wave generation and maintenance. The full-physics control experiment employed a 30-km grid, the Hsie et al. scheme for explicit moist processes, and a modified Arakawa–Schubert cumulus parameterization. From this experiment it is found that the model can successfully simulate mesoscale gravity waves and can capture many aspects of an observed wave event. For this case the model mesoscale gravity waves arose, matured, and decayed in the same regions as those observed and had similar timing and amplitudes. Model wave speeds, however, were 1–1.8 times those observed. The model output showed that although a good wave duct covered the wave activity area, the model waves were maintained and amplified by wave-CISK processes. These waves appeared to be generated by convection of mesoscale extent above a stable duct. This convection moved with the waves and was associated with steering levels.

Model sensitivity experiments showed that (i) the model mesoscale gravity waves do not stern from initial data imbalances, (ii) model mesoscale gravity-wave development does not occur when latent heating is removed, (iii) model mesoscale gravity-wave production is not necessarily limited to the early hours of a simulation, and (iv) model mesoscale gravity waves can be produced using grid sizes up to 45 km. As applied to the actual case, it is concluded from the simulations that both ducting and wave-CISK contributed to the maintenance of the observed waves. Convection is indicated as the primary wave source, although evidence of shearing instability is also found. The model results, however, do not support the idea of generation by geostrophic adjustment.

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Ying-Hwa Kuo
and
Richard J. Reed

Abstract

A series of nine experiments were conducted using a version of the Pennsylvania State University/National Center for Atmospheric Research (PSU/NCAR) mesoscale model. Objectives were: 1) to test the ability of a high resolution limited-area model to simulate an extraordinary cyclogenesis that occurred in the eastern Pacific in November 1982; 2) to examine the effects of various physical processes on the storm development and 3) to determine the reasons for the failure of the operational Limited-area Fine-mesh Model (LFM) to forecast the event. Six of the experiments employed a 40 km grid and three employed an 80 km grid. Initial data for seven of the experiments consist of fields interpolated from the National Meteorological Center (NMC) operational analysis supplemented by subjective soundings created by Reed and Albright. The supplementary data were withheld in two of the experiments. Principal findings are:

1) The control experiment, which utilized the supplementary dataset, a 40 km grid and an explicit moisture scheme, simulated a mejor cyclone with a central pressure of 969 mb and a deepening rate of 31 mb per 24 h (observed values were 950 mb and 48 mb per 24 h). The path of the cyclone was well predicted, as were several features of the storm that could be verified by satellite and aircraft observations.

2) A vertical cross section taken immediately ahead of the storm center at the time of rapid deepening revealed a symmetrically neutral or slightly unstable state in and near the warm frontal zone and a narrow, sloping sheet of rapidly ascending air (w > 50 cm s−1) at the frontal boundary. Low-level convergence exceeded 1.0×10−4 s−1 as the air approached the zone. Vorticity grew from near zero to 6–7 f in only a few hours.

3) Moist processes were essential to the rapid development. Dry simulations produced deepenings of only 13–15 mb in the 24 hour period, implying that roughly half the intensification in the control experiment can be ascribed to dry baroclinicity and half to latent beat release and its interactions with baroclinicity.

4) Surface energy fluxes had no significant impact on the development during the 24 hour period of rapid deepening.

5) An experiment with parameterized convective and nonconvective precipitation yielded essentially the same final pressure as the control experiment. However, the time of most rapid deepening was delayed in the simulation with parameterized convection. The delay was related to differences in the vertical heating profile in the two experiments.

6) Reduction of the grid size from 80 km to 40 km had only a minor effect on the central pressure, suggesting that further reduction would not eliminate the 19 mb error in the predicted central pressure.

7) A considerably weaker cyclone (982 mb vs 971 mb central pressure) resulted when the supplementary data were withheld in an experiment conducted on the 80 km grid.

8) An experiment designed to match most closely the conditions of the LFM forecast yielded the weakest development of all. It is speculated that the absence of development in the LFM forecast stemmed from short-comings of the initial analysis.

9) A possible cause of the failure of the present experiments to fully capture the storm intensity is the deficiency of middle and upper-level observations, and the attendant uncertainties in the upper-level analyses, in the prestorm period.

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Richard J. Reed
and
Warren Blier

Abstract

A case study is presented of the development within a polar air stream of a comma-shaped cloud pattern (comma cloud) and associated small surface cyclone. The disturbance is traced from the time of its development over the eastern Pacific Ocean until it moves inland over California as a mature system. The first sign of the development was the appearance of a region of enhanced convection in the northwesterly flow behind an amplifying large-scale trough and ahead of an embedded short-wave trough. As the development progressed, the cloud field expanded and assumed the comma shape. The head, located on the cold, cyclonic-shear side of the jet stream, was composed of large, deep convective elements that merged sequentially. The tail, located on the warm, anticyclonic-shear side, was composed of shallower, more stratiform clouds. The surface low center formed within the comma head during the phase of rapid organization and strengthening of the convection.

Detailed surface and upper air analyses of the system over California during the mature stage revealed that the disturbance was at this stage associated with a weakly baroclinic region in the lower and middle troposphere and was capped by a strong upper-tropospheric frontal zone. Although no change of air mass accompanied the passage of the comma cloud, the disturbance did exhibit frontal characteristics at the surface as it passed stations near the southern California Coast. Precipitation within the mature cloud band tended to be stratiform, with some rather large precipitation totals reported.

The quasi-geostrophic omega equation is employed to elucidate the processes involved in the development of the comma cloud. A qualitative analysis indicates the likely importance of small static stabilities in enhancing the effect of the relatively modest positive vorticity advection. The possible importance of latent heat release on the development of the system is also discussed. Finally, the main results of this paper and a companion paper are summarized in the form of a schematic diagram.

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Richard J. Reed
and
Warren Blier

Abstract

In a companion paper the authors presented a case study of the development of a comma-shaped cloud pattern and associated small cyclone that formed in a cold air mass over the eastern Pacific. This paper confirms the reproducibility of the previous analysis by documenting a second, similar case. A schematic model of comma cloud development appearing in the companion paper is based in part on this second example.

An added feature of the present paper is a detailed examination of the physical processes responsible for the enhanced convective activity that occurred during the intensifying stage of the disturbance. It is found that the lifted index decreased markedly in the vicinity of the developing comma and that the decrease was associated with warming and moistening of the surface air and cooling of the air at 500 mb. Sensible and latent heat fluxes from the surface of approximately 100 and 300 W m−2, respectively were essential to the low-level warming and moistening. Amplification of an upper-level long-wave trough contributed to the cooling aloft.

As the system came ashore in southern California, convective activity was much more severe in this case than in the previous one. A likely important factor in this difference was the greater warmth of the coastal waters in this (November) case than in the earlier (March) case.

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