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Lester F. Hubert

Abstract

Cloud motions in the immediate vicinity of a tropical disturbance, measured with geosynchronous satellite data, were compared with independent aircraft wind observations by Smith and Hasler (1976). These data have been further analyzed here to determine whether a component of cloud pattern motion might be associated with vertical shear.

An association was detected between vertical shear and cloud pattern motion only in the region of enhanced convection east of the disturbance center. The shear–related effect is to produce a motion component (probably cloud generation) that can add to or subtract from the speed and direction of the cloud patterns as they are advected by winds in the cloud layer.

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Lester F. Hubert

Abstract

The presence of large-scale westerly currents in the high troposphere over the equatorial western Pacific during the late northern summer is illustrated with 40,000-ft charts for the period 22–27 September 1945. The upper westerlies are shown to be a broad stream subject to a periodic fluctuations. They represent part of the mechanism for the exchange of air and vorticity across the equator.

Interaction of upper currents of both hemispheres is illustrated as well as the relation between upper currents and surface weather, including typhoon formation during the period.

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Lester F. Hubert

Abstract

Terms of the horizontal equations of motion are evaluated from surface analysis of three hurricanes and three nonhurricane cases in order to investigate the components of friction (tangential to and normal to the anemometer wind direction) in the boundary layer. A method of moisture-flux analysis is developed to study the variation of height of the inflow layer which in turn yields a clue concerning the variation of stress profile in time and space.

While the magnitude of friction increases toward the hurricane center, it is clear that it is not a simple function of wind speed or surface roughness. It is also shown that (1) the component of friction normal to the surface wind direction is almost always significant, (2) accelerations are quite small and are not correlated with the down-stream pressure force, (3) accelerations and angle of cross-isobar flow are correlated, and (4) total friction is not correlated with angle of cross-isobar flow.

It is suggested that a compensating mechanism characterizes the boundary layer which changes the angle of cross-isobar flow (thus changing the down-stream pressure force) to compensate a tendency toward acceleration brought about by changes of momentum flux from upper layers as well as changes of surface stress. This compensation in turn provides a variable component of friction normal to the surface wind which is almost always significant.

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LESTER F. HUBERT

Abstract

On April 28, 1980, TIROS I obtained pictures of an exceedingly sharp-edged cloud deck over the Pacific Ocean west of Chile, of cirrus associated with the jet stream on the west coast of South America, and of a mature cyclone in the central South Atlantic. Surface, upper-air, and cross section analyses are presented and compared with pictured cloud features.

Because of the wide spacing of the data, the locations of double jet streams had to be deduced by examining the horizontal temperature gradients associated with Northern Hemisphere jets and assuming that the same general relation holds in the Southern Hemisphere.

An operational surface analysis is shown to be inconsistent with the pictured data in the oceanic region where the analysis was based on a single ship report. A modification of the analysis is suggested to illustrate the potential of meteorological satellite data in data-sparse regions.

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Lester F. Hubert

As a hurricane moves from ocean to land, the surface roughness and the wind stress increase, which in turn causes the surface wind to blow across the isobars at an increased angle.

The resulting increase in mass convergence in the surface friction layer is examined in the light of the Rossby-Montgomery theory on the influence of friction on surface flow, and the results are compared with the analysis of fifty years of hurricanes that moved into the United States.

Those hurricanes are also analyzed to study the rate of pressure gradient decrease in the maximum wind zone. The results of this study indicate that frictional effect alone cannot dissipate the dangerous winds of a mature hurricane within the first eight to ten hours after moving onshore, but that dissipation is controlled essentially by the energy budget.

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Lester F. Hubert

Abstract

Seventeen forecasts of hurricane tracks, each up to 72 hours, were made by numerical methods under operational conditions as a test of Kasahara's [5] prediction model. Although the small size of the sample precludes making firm conclusions, the results here obtained compare unfavorably with the regularly issued subjective forecasts. In general, the forecast motion is too slow and to the right of the actual hurricane track.

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ANDREW TIMCHALK and LESTER F. HUBERT

Abstract

TIROS I satellite pictures for a 3-day period about, a developing “Low system” over the central United States are shown. Cloud distribution about surface fronts and upper-air troughs was examined and compared to the “ideal” (classical) distribution. The pictures and analyses reveal that cloud patterns were significantly changed by strong subsidence and advection of dry air along a low-level wind maximum. The cloud patterns when compared to the 600-mb. vertical motions computed routinely by the Joint Numerical Weather Prediction Unit showed only partial agreement.

A strong relationship between low cloud cover and surface relative humidity was found; this relationship may be useful under certain conditions in determining the stability of the air over areas where upper-air data are sparse.

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LESTER F. HUBERT and ANDREW TIMCHALK

Abstract

The empirical relationship between the maximum wind speed (MWS) of tropical storms and their characteristics pictured from satellites has been employed routinely since 1964. A large number of cases accumulated since then now enables these relationships to be put on a more firm statistical basis and revised MWS curves are presented.

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