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John H. Hirsch and Carol Lundy Schock

Abstract

Meteorological information from aircraft penetrations of cumulus clouds over western South Dakota has been analyzed to yield distributions of liquid water content, vertical motion and temperature. Ice particles are encountered in only 26% of the penetrations. Medium size clouds (diameters between 0.5 and 2 mi) tend to have higher mean liquid water contents than either small or large clouds; however, large clouds have the highest maximum liquid water contents. Peak values of both up- and downdrafts occur in the larger clouds but mean vertical motions tend to decrease with increasing cloud size. Cross correlations among various parameters show that the locations of wet and dry areas within clouds are not necessarily correlated with vertical air movement.

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John H. Hirsch and D. Ray Booker

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Harold D. Orville, Richard D. Farley, and John H. Hirsch

Abstract

Studies have been conducted to determine the cloud seeding potential of stratiform type clouds using a two-dimensional, time-dependent cloud model. An atmospheric sounding from Villanubla, Spain, in February 1980, was used to initialize the model. The model is designed to allow mesoscale convergence in the lower levels and divergence in the upper levels, which results in a stratiform-type cloud in this Spanish situation.

The seeding of clouds using either dry ice or silver iodide has been tested and rather surprising results are indicated. The silver iodide seeding simulations produce strong dynamic responses in the model clouds, even with small amounts of supercooled liquid available and a few natural ice crystals per liter in the cloud. These effects occur in a nearly moist adiabatic layer as well as in a convectively unstable layer.

The effects appear to be due to the heat released as the liquid freezes and the cloudy environment switches from liquid saturation to ice saturation. Cloud vertical motions of a few to several m s−1 are produced in the seeded cloud region. Vertical motions of 10 to 20 cm s−1 exist in comparable regions of the unseeded cloud. Precipitation is strongly affected. Consequently, this heat release is much more significant in terms of the overall energetics of the cloud than has been evident in our seeding simulation conducted in pure convective situations with much stronger updrafts.

The tests of the dry ice seeding indicate small effects, but this is largely due to the rapid fall of the dry ice pellets through the cloud and to the short time period available for the seeding to take effect.

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Harold D. Orville, John H. Hirsch, and Laurence E. May

Abstract

A steady-state, one-dimensional cloud model has been modified to simulate the growth of plumes (both wet and dry) and clouds from natural and forced draft cooling towers. The modifications to the cloud model are discussed and comparisons are made between predicted height and length of plumes and observed values. A correlation coefficient of 0.78 is achieved for model predictions of plume height and a correlation coefficient of 0.49 for predicted plume length. Comparisons with Benning Road data showed 78% of the model-predicted plume heights were within 50% of the observed height, while 93% of the predicted plume lengths were within 50&percnt of the observed length.

Analysis of the model predicted plumes for a year's morning and evening atmospheric soundings is presented. Comparison of plumes during winter and summer showed dramatic changes, with the longest plumes occurring during the winter. Summer plumes were much shorter with relatively small visible plume heights and tall dry extensions above the visible plume.

A case of wet plume/dry plume/cloud formation is presented to illustrate output from the model.

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Richard D. Farley, Pamela E. Price, Harold D. Orville, and John H. Hirsch

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No abstract available.

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Fred J. Kopp, Harold D. Orville, Richard D. Farley, and John H. Hirsch

Abstract

The application of a two-dimensional, time-dependent cloud model to describe the effects of dry ice cloud seeding is demonstrated. A conservation equation and associated auxiliary equations for the mixing ratio of dry ice (CO2) are presented. The importance of identical time steps in both seeded and unseeded cases is discussed.

Small convective clouds are seeded at about the −10°C level and the seeding agent (CO2) traced as it falls through the cloud creating a mass of cloud ice in its trail. The cloud ice transforms to snow and the snow to graupel/hail which then melts into rain as it falls below the zero degree isotherm level. Precipitation starts about 6 min earlier in the seeded cloud and the timing of the rain fallout affects the interaction with a second cell. Approximately 20% more rain falls from the seeded cell in a very light shower.

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Bruce A. Boe, Jeffrey L. Stith, Paul L. Smith, John H. Hirsch, John H. Helsdon Jr., Andrew G. Detwiler, Harold D. Orville, Brooks E. Mariner, Roger F. Reinking, Rebecca J. Meitín, and Rodger A. Brown

The North Dakota Thunderstorm Project was conducted in the Bismarck, North Dakota, area from 12 June through 22 July 1989. The project deployed Doppler radars, cloud physics aircraft, and supporting instrumentation to study a variety of aspects of convective clouds. These included transport and dispersion; entrainment; cloud-ice initiation and evolution; storm structure, dynamics, and kinematics; atmospheric chemistry; and electrification.

Of primary interest were tracer experiments that identified and tracked specific regions within evolving clouds as a means of investigating the transport, dispersion, and activation of ice-nucleating agents as well as studying basic transport and entrainment processes. Tracers included sulfur hexafluoride (SF6), carbon monoxide, ozone, radar chaff, and silver iodide.

Doppler radars were used to perform studies of all scales of convection, from first-echo cases to a mesoscale convective system. An especially interesting dual-Doppler study of two splitting thunderstorms has resulted.

The objectives of the various project experiments and the specific facilities employed are described. Project highlights and some preliminary results are also presented.

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