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Roscoe R. Braham Jr.

Abstract

In situ snow particle size spectra measured by Particle Measuring Systems probes near the downwind shore of Lake Michigan during lake-effect snow storms are presented and discussed. Ice water contents ranged from 0.002 to 0.264 g m−3. Concentrations of sizes larger than 1 mm were generally exponentially distributed; however, concentrations of smaller particles usually were greater than suggested by the exponential fitted to concentrations of sizes larger than 1 mm. Exponential distribution parameters (N 0 and λ) are consistent with previously reported values. There is evidence of particle aggregation at −25°C.

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Roscoe R. Braham Jr.

Abstract

A series of twelve releases of phloroglucinol were made into stratus clouds at temperatures of −7C to −17C.

Showers produced by dry ice seeding were used to identify particular spots in the layer clouds from which the exact locations of the phloroglucinol releases could be obtained by simple navigation. Visual observations of the cloud behavior and Formvar replicas of cloud and precipitation particles provided a means for judging the effects of the phloroglucinol.

It is concluded that phloroglucinol will induce the formation of ice in undercooled clouds. However, in these experiments, it was not nearly as effective as the dry ice in causing shower formation.

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Roscoe R. Braham Jr.

Abstract

In-cloud collections of snow and ice pellets in summer cumulus clouds have been made on Project White-top. These collections provided an opportunity for measuring the bulk densities of 129 snow pellets and ice pellets. Results show that their densities ranged from about 0.87 gm per cc to 0.91 gm per cc.

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Roscoe R. Braham Jr.

Abstract

An AN/TPS-10 radar, located at the Institute of Atmospheric Physics, University of Arizona, has been used to make extensive measurements of radar returns from cumulus clouds in the vicinity of Tucson. Data from ten days in the summer of 1955 have been analyzed with a view toward establishing the level of first formation of precipitation, day-to-day variation, average dimensions of first echo, average duration, and fraction reaching ground. Strong day-to-day variations and mountain effects are revealed. Although echoes form much more frequently over mountains than over nearby valleys, these echoes individually are less likely to produce rain at the ground.

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Roscoe R. Braham Jr.

Abstract

Conventional surface and upper-air aerological data are combined with radar and aircraft measurements to give a description of a major winter storm that deposited over 69 cm of new snow at Muskegon, Michigan, between 8 and 11 December 1977. It is shown that most of this snow occurred during four distinct episodes, three of which were related to air-water temperature contrasts as Arctic air flowed over the relatively warm surface of Lake Michigan. These four episodes have been identified as 1) pre-cold frontal, 2) post-cold frontal, 3) secondary trough, and 4) mesolow phases.

During the pre-cold front phase heavy snow was associated with an advancing synoptic-scale trough. Little direct effect of the lakes was detected.

Following passage of the cold front, strong northwesterly winds across Lake Michigan resulted in strong air-mass transformation with light steady snow along the downwind shoreline. The evidence suggests the presence of horizontal-roll convection as the dominant organizational mode of convection over the lake during this phase.

The third phase of this storm was associated with the movement of a secondary trough which itself was a direct result of air-mass transformation over the upper-lakes region. In its initial development this trough was oriented approximately east–west, with the warmest and most moist air to the north. This warm air drifted slowly southward and collided with the eastward moving Arctic air. This produced a frontal-like mesostructure with a line of clouds and snow. At Muskegon the heaviest snow of the entire storm occurred during a five-hour period when this mesostructure was overhead.

During the final phase of this storm at Muskegon, a closed mesolow pressure center developed over Lake Michigan as a result of intense air-mass transformation. This set up easterly winds along the Michigan shoreline and focused the convection into a narrow band, parallel to the downwind shore. Very heavy snow occurred in this band, both over the lake and over land at the southern end of the line where it curved eastward in response to winds around the bottom of the secondary trough. This positive feedback nature of shore-parallel bands, i.e., a mesolow resulting from air–mass transformation which in turn focuses the convection into a line producing the maximum residence time for the air over the lake, can explain the protracted periods of heavy snow which often occur along the downwind shores of the Great Lakes.

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Roscoe R. Braham Jr.

Abstract

Recent observations indicate that ice pellets and snow pellets are present in most convective clouds in the Central United States by the time these clouds reach top temperatures of −10C. The attendant circumstances raise the question of whether the ice plays an active role in rain development in these clouds or whether its presence is purely incidental. The ice pellets are usually preceded by the development of liquid precipitation particles large enough to produce rain by coalescence with cloud droplets. The pellet concentrations are not related to ground-level ice nuclei concentrations. Apparently the pellets form as a result of freezing of the drops, contrary to most laboratory studies of droplet freezing. Observations can be brought into harmony by invoking the droplet splintering measurements of Mason and Maybank. The presence of numerous small ice particles in these clouds at temperatures warmer than −10C casts doubt upon the value of seeding with ice nuclei for rain inducement.

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Roscoe R. Braham Jr

Abstract

A quantitative estimate, based upon data from the Thunderstorm Project, is made of the various water sources and sinks and the energy sources and sinks of an average thunderstorm cell. It is found that a large fraction of the water carried into the storm in vapor form is used to maintain the cold downdraft inside the storm, and that the energy restratification accompanying this downdraft is a major source of energy to the storm.

From the estimates of the water and energy budgets, the following hypothesis is tested by means of radar measurements: the net energy available (energy sources minus energy sinks) is related to the amount of development of convective rainstorms.

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Mark R. Hjelmfelt and Roscoe R. Braham Jr.

Abstract

A mesoscale model is used to simulate the airflow over Lake Michigan for the major lake-effect snowstorm of 10 December 1977. This storm was characterized by a land breeze circulation and a narrow shore-parallel radar reflectivity band. The model successfully simulated the major atmospheric circulation features including a mesoscale low pressure center and a land breeze front. The model also captured the general character of the observed precipitation pattern which was typified by a narrow band of heavy precipitation along the eastern shore of Lake Michigan.

Further simulations were made to examine the effects of latent heat release, lake surface temperature distribution and model grid resolution upon the simulation. Latent heat release was found to have an important effect in strengthening convection. However, the basic land-breeze circulation was found to develop for the simulated conditions even without latent beating. For a given mean lake-land temperature difference, details of the lake surface temperature distribution were found to have a small effect. Simulations with varying model grid resolution suggest that a horizontal grid scale ≳ 20 km is insufficient to resolve the observed precipitation and airflow patterns for this storm.

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Horace R. Byers and Roscoe R. Braham Jr

Abstract

Data from Thunderstorm Project observations in Florida, derived mainly from airplane traverses through thunderstorms, are analyzed in order to obtain a description of thunderstorm structure and circulation. Thunderstorms are normally found to consist of several more-or-less independent convective systems or “cells”. Each cell goes through a life cycle represented by three fairly distinct stages-the cumulus stage, the mature stage, and the dissipating stage.

In the cumulus stage the cell is formed from an updraft of air which, as in the other stages, “entrains” air from the environment. In this stage no rain has yet reached the ground. In the mature stage, rain is occurring and a large part of the cell consists of a downdraft which characterizes the rain area. The updraft continues in a portion of the cell in the low and intermediate levels and in all parts of the top levels. In the dissipating stage, downdrafts are present throughout, although weak upward motion is apparent in the upper parts.

The thunderstorm of 9 July 1946 is taken as typical of the structure and vertical currents that have been described. Radar photographs and data from the surface micronetwork are used in substantiation of the airplane findings. Strong horizontal divergence at the surface in the downdraft and rain areas and convergence under the updrafts are shown.

From the abundant upper-air wind and temperature-humidity soundings made with balloons in and around the thunderstorms, data from the various sources are combined to study the thermodynamics involved in the circulation. The entrainment of air into the cells has an important effect which by actual measurement and introduction of reasonable values of entrainment satisfactorily accounts for the observed air currents. With entrainment, the updraft lapse rate approaches that of the environment. It is then demonstrated that it is possible for falling rain to “trigger” a downdraft of cold air that reaches and spreads out over the surface of the earth as the cold core of the rain area.

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PAUL SPYERS-DURAN and ROSCOE R. BRAHAM JR.

Abstract

An intense rainstorm at Fremont, Mo., on July 28, 1964, yielded over 3 in. of rain in 30 min. and a total of 9.5 in. in 5 hr. The synoptic weather situation which was responsible for producing such an intense rain is discussed. Mass rainfall curves, a total storm isohyetal map, an area-depth curve, and a graph of rainfall rates, are presented. Computed updrafts in the clouds versus observed updrafts from radar data are discussed; the maximum cloud penetration height is compared with observed radar echo heights.

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