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  • Author or Editor: Roscoe R. Braham Jr. x
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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. and Paul Spyers-Duran

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On the basis of Millipore filter measurements it is concluded that natural ice nuclei were deactivated in passing over St. Louis, Mo., during March 1973. This conclusion is supported by very strong statistical evidence.

To overcome the effects of soluble materials in desensitizing the filter technique, samples were limited to volumes less than 100 liters with the background crystal count measured for every filter; a forced-flow developing chamber was used; and the Huffman-Vali correction factor, based upon concurrently measured CCN, was applied.

The measurements strongly suggest the possibility of local sources of ice nuclei. However, these nuclei also were deactivated in passing over the city.

Simultaneous measurements with an expansion chamber gave results similar to those of the filters, but did not show statistical support.

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Roscoe R. Braham Jr. and Maureen J. Dungey

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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 Paul Spyers-Duran

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During the summer of 1966 from Bemidji, Minn., aircraft collections of cirrus crystals were made with a continuous particle replicator. Actual samples show that cirrus crystal trails with a concentrations of 105–106 m−3 can survive a fall of 20,000 ft in clear air with a temperature/dew point spread of 15C. Computations of evaporation rates for falling crystals suggest that it is somewhat surprising that the crystals could have survived under the observed conditions; however, this may have been because input data for the calculations are inadequate.

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Roscoe R. Braham Jr. and Daniel Wilson

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Heights and locations of the tallest 3 cm radar echoes at each half-hour through the convective parts of 140 summer days were measured during Project METROMEX. Comparison of the area-weighted frequencies of echoes occurring over and downwind of St. Louis, Mo., with those over a large nearby rural area, shows a substantial enhancement in the frequency of tall echoes over the city and near-down-wind areas. This enhancement comes mainly during the late morning and early afternoon and possibly again during the evening hours. The data also show a substantially different echo height distribution over urban and rural areas. Whereas the rural height distribution is distinctly bimodal, the urban height distribution shows no such bimodal character. This means that urban clouds are frequently able to penetrate mid-level arresting levels which limit the growth of rural clouds. These observations suggest an important role for urban-enhanced cloud dynamics in causing the St. Louis rainfall anomaly.

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Roscoe R. Braham Jr. and Maureen J. Dungey

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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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Paul A. Spyers-Duran and Roscoe R. Braham Jr.

Abstract

An instrument for collecting cloud particles from an airplane has been developed. Cloud particles are captured and permanently replicated using the well known Formvar technique. From the continuous record of hydrometeor replicas, the forms, sizes and frequency distributions can be established. Description of the instrument and examples of data collected from natural clouds are presented. Problems of calibration are discussed.

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