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Richard J. Reed
and
Kenneth R. Hardy

Abstract

Widespread and persistent clear air turbulence (CAT) occurred over the Eastern Seaboard of the United States between New York and South Carolina on 18 March 1969. The major synoptic features and a qualitative discussion of the factors contributing to the development of the large vertical wind shears associated with the turbulence are presented. The turbulent region in the vicinity of Wallops Island, Va., was probed with a NASA T-33 research aircraft and with sensitive radars. The clear air radar echoes and the most intense turbulence occurred principally within an upper level frontal zone of about 2 km depth which was produced by the confluence of two currents of widely different origin. The smoothed Richardson number was less than 1.0 throughout the zone and reached its lowest value of ∼0.25 in the region of strongest turbulence. Three distinct types of wave structures were evident in the clear air radar echoes. These were: 1) long sinusoidal arches moving at approximately the wind speed which were oriented in the direction of the wind and wind shear and which had wavelengths of 15–30 km and crest-to-trough amplitudes of nearly 2 km; 2) unstable waves or billows of about 1.6 km wavelength which were superposed on a portion of the long arches and were also oriented in the shear direction; and 3) braided wave-like patterns having a wave-length of ∼5 km and a crest-to-trough amplitude of more than 1 km which were oriented in the cross-wind (and cross-shear) direction.

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Richard J. Reed
and
Ernest E. Recker

Abstract

A compositing technique is used to obtain the average structure of 18 disturbances which traversed an area in the equatorial western Pacific during the wet season (July–September) of 1967. Principal emphasis is placed on the wave properties in the triangular area described by Ponape, Kwajalein and Eniwetok within which it was possible to measure divergence and vertical motion and to compute moisture and heat budgets.

Meridional wind maxima of nearly opposite phase occurred in the lower and upper troposphere. Negative temperature deviations were found in the vicinity of the wave trough at low and high levels; positive deviations were observed at intermediate levels. Highest relative humidities occurred in the trough region. This was also the region of strongest upward motion and greatest rainfall and cloud amount. The maximum upward velocity of 2.5 cm sec−1 was found at 300 mb. Convergence was strongest in the sub-cloud layer; divergence was concentrated near 175 mb. The maximum anticyclonic vorticity was also observed at that level.

The wave structure changed in a systematic fashion across the network. The change is attributed to the variation with longitude of the shear of the basic current.

The rainfall computed from the observed wind and moisture fields agreed well with observed amounts which varied from about 2 cm day−1 in the vicinity of the trough axis to about 0.5 cm day−1 near the ridge axis. The diabatic heating difference between trough and ridge regions was largest at 400 mb where it was estimated to be nearly 10C day−1.

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Richard J. Reed
and
Charles L. Vicek

Abstract

The annual temperature variation in the lower tropical stratosphere has its maximum amplitude just above the equatorial tropopause and has nearly the same phase at a particular level throughout both hemispheres. Coldest temperatures occur during the Northern Hemisphere winter (January–February).

It is hypothesized that the observed behavior is caused by a seasonal variation in the intensity of the Hadley cell of the Northern Hemisphere with strongest upwelling and cooling in the ascending branch of the meridional circulation occurring in January–February. Computations are made which suggest that a twofold increase in the upward motion near the equator from July to January is sufficient to account for the magnitude of the observed temperature decrease.

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Robert M. Lewis
and
Richard J. Reed

Abstract

No abstract available.

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Richard J. Reed
and
Frederick Sanders

Abstract

A synoptic example of intense frontogenesis at 500 mb is presented and the underlying mechanism is investigated both from the thermal and dynamic standpoints. It is shown that the basic process in operation is a cross-stream gradient of sinking motion with strongest subsidence at the warm edge of the frontal zone. This condition gives rise, to an intensified horizontal temperature gradient and, because of the generally disregarded “vertical-shear” terms in the vorticity equation, leads to a marked increase in vorticity as well.

The Ertel vorticity theorem is applied in order to confirm the above conclusions and also to show that in certain regions the frontal zone was of stratospheric origin, the lower boundary being initially part of the polar tropopause.

Finally it is shown how the vertical-shear terms may be incorporated in the Sawyer-Bushby numerical prediction model.

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Richard J. Reed
and
Bruce A. Kunkel

Abstract

On the basis of a number of synoptic and synoptical-climatological studies, the following picture of the arctic circulation in summer is presented. A secondary baroclinic zone, distinct from the polar front, develops along the northern shores of Siberia, Alaska and Canada. Cyclones which originate in this zone, and to a lesser extent in the polar frontal zones to the south, frequently invade the central Arctic. The stagnation of these lows near the pole leads to a high frequency of occurrence of low pressure centers and a weak area of low pressure in the mean.

There is no evidence for the often assumed semi-permanent anticyclone near the pole. Anticyclones are most frequent in the belt between 70N and 75N, favored locations being central Greenland and the area about the Beaufort Sea.

The disturbances of the polar region in summer are shown to be similar to typical middle-latitude storm systems. An example of a typical baroclinic wave development is discussed, a cross section through a high-latitude frontal zone and associated jet stream is presented, composite cloud distributions in arctic weather systems are shown, and the results of a series of dynamical predictions for the area north of 60 deg are compared with similar results for middle latitudes.

The question of the representativeness of the present results, which were based entirely on data for the five summers, 1952 to 1956, is considered, and it is concluded that the characteristics of the circulation during these summers were not significantly different than during a number of years, dating back to 1894, for which expedition data were available.

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Richard J. Reed
and
Dale G. Rogers

Abstract

The circulation is described in terms of three components: the long-term mean, the annual cycle and a newly-discovered 26-month cycle, referred to as the equatorial stratosphere wind oscillation.

Mean winds are everywhere easterly, except that very weak westerlies may possibly exist near the equatorial tropopause. The easterlies increase with height and are strongest between latitudes 10N and 15N.

The amplitude of the annual cycle rises from near zero at the equator to a maximum of about 10 m/sec at approximately 25N and undergoes little change with height. Peak easterlies occur in late July or early August.

The amplitude of the 26-month cycle is greatest near the equator, attaining a value in excess of 20 m/sec at the 25-mb level. Below this level the amplitude decreases, and the oscillation fades away in the vicinity of the tropopause. In the poleward direction it is still faintly detectable near 30N. The phase varies with height, each band of easterly or westerly winds appearing first at the highest levels observed (about 30 km) and progressing downward at a speed of slightly greater than 1 km/mo.

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Richard J. Reed
and
Anthony L. Julius

Abstract

An equation suitable for the study of changes in ozone density in the region of the stratosphere below approximately 25 km is derived and applied to the problem of the observed increase in the ozone content of the layer between 10 and 20 km during winter. Under the assumptions that meridional circulation or turbulent mass exchange alone is responsible for the observed changes, the required vertical velocities and austausch coefficients are computed for various elevations and latitudes. It is concluded that the meridional circulation scheme provides a satisfactory explanation of the ozone rise only if the circuit extends over both hemispheres. On the other hand, turbulent mixing is a sufficient explanation if stratospheric austausch coefficients attain values as high as 1 to 20 g cm−1 sec−1 during winter.

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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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