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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

Recent observations indicate that ice pellets and snow pellets are present in most convective clouds in the Central United States by the time these clouds reach top temperatures of −10C. The attendant circumstances raise the question of whether the ice plays an active role in rain development in these clouds or whether its presence is purely incidental. The ice pellets are usually preceded by the development of liquid precipitation particles large enough to produce rain by coalescence with cloud droplets. The pellet concentrations are not related to ground-level ice nuclei concentrations. Apparently the pellets form as a result of freezing of the drops, contrary to most laboratory studies of droplet freezing. Observations can be brought into harmony by invoking the droplet splintering measurements of Mason and Maybank. The presence of numerous small ice particles in these clouds at temperatures warmer than −10C casts doubt upon the value of seeding with ice nuclei for rain inducement.

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Roscoe R. Braham Jr

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A quantitative estimate, based upon data from the Thunderstorm Project, is made of the various water sources and sinks and the energy sources and sinks of an average thunderstorm cell. It is found that a large fraction of the water carried into the storm in vapor form is used to maintain the cold downdraft inside the storm, and that the energy restratification accompanying this downdraft is a major source of energy to the storm.

From the estimates of the water and energy budgets, the following hypothesis is tested by means of radar measurements: the net energy available (energy sources minus energy sinks) is related to the amount of development of convective rainstorms.

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Mark R. Hjelmfelt and Roscoe R. Braham Jr.

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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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Horace R. Byers and Roscoe R. Braham Jr

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Data from Thunderstorm Project observations in Florida, derived mainly from airplane traverses through thunderstorms, are analyzed in order to obtain a description of thunderstorm structure and circulation. Thunderstorms are normally found to consist of several more-or-less independent convective systems or “cells”. Each cell goes through a life cycle represented by three fairly distinct stages-the cumulus stage, the mature stage, and the dissipating stage.

In the cumulus stage the cell is formed from an updraft of air which, as in the other stages, “entrains” air from the environment. In this stage no rain has yet reached the ground. In the mature stage, rain is occurring and a large part of the cell consists of a downdraft which characterizes the rain area. The updraft continues in a portion of the cell in the low and intermediate levels and in all parts of the top levels. In the dissipating stage, downdrafts are present throughout, although weak upward motion is apparent in the upper parts.

The thunderstorm of 9 July 1946 is taken as typical of the structure and vertical currents that have been described. Radar photographs and data from the surface micronetwork are used in substantiation of the airplane findings. Strong horizontal divergence at the surface in the downdraft and rain areas and convergence under the updrafts are shown.

From the abundant upper-air wind and temperature-humidity soundings made with balloons in and around the thunderstorms, data from the various sources are combined to study the thermodynamics involved in the circulation. The entrainment of air into the cells has an important effect which by actual measurement and introduction of reasonable values of entrainment satisfactorily accounts for the observed air currents. With entrainment, the updraft lapse rate approaches that of the environment. It is then demonstrated that it is possible for falling rain to “trigger” a downdraft of cold air that reaches and spreads out over the surface of the earth as the cold core of the rain area.

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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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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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Louis J. Battan and Roscoe R. Braham Jr.

Abstract

Observations of precipitation and cloud-top height in the central United States and in the Caribbean area, obtained from radar-equipped airplanes, have been analyzed in terms of the fraction of clouds of a given height which contains precipitation. These data are compared with observations of a similar type taken by Braham, Reynolds and Harrell in New Mexico. It is concluded that the condensation-coalescence process can account for the formation of precipitation in convective clouds in all three regions but in varying proportions of clouds.

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