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Roscoe R. Braham Jr.

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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.

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Roscoe R. Braham Jr.

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Roscoe R. Braham Jr.

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Roscoe R. Braham Jr.

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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.

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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

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

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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