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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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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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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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