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Roscoe R. Braham Jr. and Paul Spyers-Duran

Abstract

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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Thomas E. Hoffer and Roscoe R. Braham Jr.

Abstract

As a means of studying ice nucleation, snow and ice pellets collected from the tops of clouds were melted and refrozen in order to determine their freezing temperatures. In all cases where a definite cloud top temperature could be assigned, the melted ice pellets froze at a temperature colder than that of the cloud top, indicating that these pellets did not originate through the heterogeneous freezing of cloud drops. Essentially no difference was indicated in the freezing temperatures of ice pellets collected on seeded and non-seeded days. A firm statement on this point could not be made as the number of observations is limited, and it is not certain that the seeding agent had been ingested into the cloud being studied.

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Roscoe R. Braham Jr. and Daniel Wilson

Abstract

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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Clayton H. Reitan and Roscoe R. Braham Jr.

Abstract

The concentrations of salt particles over central Illinois are being monitored through the use of a sodium flame-counter and an impactor. The number of large particles as determined by the impactor has been found to average about two per cubic meter, and high concentrations of the magnitude found over oceans have never been detected. It is concluded that these large salt particles are not found over mid-continents in sufficient numbers to initiate precipitation through a process of condensation and coalescence unless the chain-reaction process is also operative. There is no evidence that these large particles are necessarily carried inland from an oceanic source region.

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Roscoe R. Braham Jr. and Paul Spyers-Duran

Abstract

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

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 Patrick Squires
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Edward N. Brown and Roscoe R. Braham Jr.

Abstract

Precipitation particle measurements from the upper levels of cumulus congestus clouds are analyzed with regard to general cloud characteristics, liquid water content, and precipitation water content as related to the theoretical radar reflectivity. Conclusions are: (1) the majority of the cumulus congestus clouds examined, whose tops exceed 14,000 ft, contained precipitation particles (250-microns diameter) in the upper levels sometime during their life cycle, (2) particle concentrations in excess of 1000 per m3 were found in about 20 per cent of the clouds examined, (3) the relationship Z=1.6×10−2M1.46 for radar reflectivity is applicable for cumulus congestus in the early stages of precipitation development.

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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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Edward N. Brown and Roscoe R. Braham Jr.

Abstract

Precipitation particles in the 100- to 6.50-µ-diam range were sampled in a large number of tropical convective clouds. These samples permit one to trace the development of precipitation in these clouds. Liquid-water-content measurements were made simultaneously with some of the particle measurements. From these data, it is shown that the large concentrations of large drops are associated with low liquid-water contents and, conversely, that the large values of liquid water are associated with small numbers of droplets greater than 150 µ in diam. The computed relationship between radar reflectivity, water-content, and median-volume diameter is very similar to that which has been reported for other cloud types.

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