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  • Author or Editor: Maureen J. Dungey x
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Roscoe R. Braham Jr.
and
Maureen J. Dungey

Abstract

The properties of 3 cm radar first echoes are used to study the effects of the St. Louis, Mo., metropolitan area on precipitation initiation in summer convective clouds. Based on a sample of 4553 first echoes, obtained on 82 echo-producing days of 1972–75, it is shown that the area-normalized frequency of first echo formation over the city and in the “near” downwind region is approximately a factor of 2 greater than for nearby rural regions. The maximum enhancement in first echo formation occurs over the downtown area and along the Mississippi River, which separates St. Louis from industrial suburbs to the east. The downwind extent of the region of first echo enhancement appears to be limited to about 1 h of wind travel. The enhancement occurs mainly on weekdays.

Temperatures of first echo tops and bases indicate that precipitation initiation is most frequently through drop collection, though there is evidence that ice processes may contribute a small fraction of the first echoes. Urban first echoes have lower and warmer bases and greater vertical thickness than rural first echoes.

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Roscoe R. Braham Jr.
and
Maureen J. Dungey

Abstract

Aircraft measurements of snow particle size spectra from 36 flights on 26 snowy days are used to estimate snow precipitation rates over Lake Michigan. Results show that average rates during 14 wind-parallel-type lake-effect storms increased from the upwind shore to about midlake and then were essentially uniform (1.5–2 mm day−1, liquid water equivalent) to the downwind shore. Snow from midlake bands and shoreline bands maximized over the lake. The position of the maximum during these types of lake-effect storms depends on meteorological conditions. In any given case it may be near either shore or anywhere between them. This study combines 12 cases of midlake and shoreline bands. The resulting cross-lake snow profile shows a broad maximum reaching over 4 mm day−1 near midlake. The single sample maximum snow precipitation rate encountered in this study was 77.7 mm day−1. The average cross-lake profile from combining 26 cases of lake-effect storms shows that snowfall into the lake is considerably greater than one would expect from a linear interpolation between values measured along either shore.

An attempt is made to estimate the average increase in snow over lake Michigan resulting from combined lake-effect and large-scale cyclonic storms. The result is interesting but not considered very reliable because it depends upon the relative frequencies of different types of lake-effect storms as well as overtake snow rates from large-scale cyclonic storms; neither is well known.

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Roscoe R. Braham Jr.
and
Maureen J. Dungey

Abstract

A climatological study of snowfall in the snowbelts of Michigan shows that decade-average amounts varied by a factor of 2 during the period from 1909/10 through 1980/81.

The effect of Lake Michigan on total winter snowfall along its shores has been estimated. A long-term average effect of ∼ +10% is found for the Wisconsin shore south of Sheboygan, and an average of ∼ +60% for the Michigan shore, south of Hart, with a minimum effect in the 1930s and a maximum in the 1960s.

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