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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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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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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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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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Roscoe R. Braham Jr.
and
Milton Draginis

Abstract

In an effort to disentangle the “orographic barrier” and “high-level heat source” effects, as they may combine to lead to the development of summer convective clouds over mountains, a series of measurements was undertaken in Arizona. With use of an instrumented airplane, measurements of temperature and dew point were obtained for a series of passes up- and downwind across a 9000-ft mountain range. Passes ranged in altitude from 10,000 to 14,000 ft. Data obtained at sunrise show very clearly the barrier effect which forced air to ascend about 1000 ft in crossing the ridge. After the mountain slopes became heated by insolation, a convection core formed over and slightly downwind from the ridge. This core served as the root of several small cumulus clouds which developed during the time of measurement.

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Sam S. Chang
and
Roscoe R. Braham Jr.

Abstract

Using aircraft data collected during the University of Chicago Lake-Effect Snow Storm project, the results of a case study of the convective thermal internal boundary layer (TIBL) over Lake Michigan are presented. An intense cold air outbreak on 20 January 1984 featured a rapid growth of the convective TIBL thickness and the concurrent development of cloud and snow. The average slope of the TIBL top over a fetch of 123.7 km was 1.0%. Microphysical characteristics of cloud and snow along with the TIBL development are also presented. Results of the TIBL integrated budgets of heat and total water (including cloud and snow water) are given in detail. Over the surface of Lake Michigan the average downward snow flux (snow precipitation rate) was 0.79 mm (water) per day. The average sensible and latent heat fluxes at the water surface were 323 and 248 W m−2, respectively. About 13 percent of the total warming of this cloud-topped TIBL was due to radiation.

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Roscoe R. Braham Jr.
and
David A. R. Kristovich

Abstract

Aircraft measurements of vertical air motions are used in a process of conditional sampling to select updraft and downdraft cores during a period of strong lake-effect convection. Corresponding measurements of temperature and moisture are used to calculate the buoyancies of the cores and to evaluate the dependence of the calculated buoyancy on the horizontal extent of core environment used in the calculations. Results suggest that calculated buoyancies are relatively insensitive to the definition of core environment.

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