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Vernon G. Plank

Abstract

The cumulus cloud populations of the Florida peninsula were photographed periodically and comprehensively on 19 days in August and September 1957, using multiple, high-flying aircraft. Particular sets of these photographs, called population samples, were selected which showed representative examples of the cumulus populations of customary, widespread occurrence of the various days and hours. These were analyzed to determine the size distribution characteristics of the cumuli and the time trends.

The analyses revealed 1) that the number density of the cumuli decreased nearly exponentially with increasing cloud-size, 2) that a definite maximum-size-class of the cumuli existed in the populations, which increased in size diurnally in a regular fashion, 3) that there was considerable variance, both within single populations and with the time of day, in the relationships of the vertical and horizontal dimensions of the clouds, 4) that particular modal classes of the cumuli, of intermediate size, existed in the populations which were the primary contributors to the population sky cover and cloudy volume, and 5) that group structures occurred in the populations which began forming noticeably about mid-morning and which were an important developmental feature of the cumulus convection.

The equations of a cumulus population model were developed on the results of the Florida sampling.These equations were employed to facilitate comparisons of the Florida data with other population data previously reported by Blackmer and Serebreny. They were also applied in other ways, as are pointed out.

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Vernon G. Plank

Abstract

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David Atlas and Vernon G. Plank

Abstract

A sequence of five closely spaced raindrop samples taken during the passage of a shower displayed approximate monodispersity in each sample, and drops decreasing to drizzle size with time. Correlations of median volume diameter and of liquid-water content with rain intensity agree very well with previously established empirical relations, except for the very first drops. However, values of reflectivity (Z = ∑Nd 6) are approximately half those predicted by Z = 200 R 1.6, due primarily to the narrow drop-size spectra. Such variations in spectrum width may be accounted for by varying the coefficient of the Z-R relation.

The smallest particles are shown to originate at the very edge of the shower, having evaporated during their fall. The largest particles originate toward the inner portion of the shower, and probably grow to some extent by coalescence in the cloud before evaporating slightly underneath it. All the drops appear to have originated above the melting level.

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Vernon G. Plank and Alfred A. Spatola

Abstract

Helicopter wake effects on stratus and stratocumulus clouds are described. Holes and troughs were created in such clouds with dimensions several hundred feet across. The velocities and penetration distances of the downwash flow beneath the HH-53B helicopter employed in the experiments were investigated by the release of a “tracing agent” from the helicopter and by hovering at various altitudes above the water surface of a Florida bayou. The downwash velocities were as large as 100 ft sec−1, near the rotor, and the downwash extended some 1300 ft beneath the helicopter (under near dry adiabatic conditions).

A suggestion is made that helicopters might be employed operationally to clear radiation-type ground fog from airfield runways. The cloud clearing capabilities of the HH-53B helicopter are discussed in support of this suggestion, by reference to observational data and to the theory of Hohler.

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Vernon G. Plank, David Atlas, and Wilbur H. Paulsen

Abstract

A survey is made of the clouds and precipitation detected at a wavelength of 1.25 centimeters. The frequencies of various cloud types and their typical echo characteristics are presented. Within 89 periods during which echoes were received, slightly less than 50 per cent of the echoes could be classed as echoes from internationally defined clouds. Detailed correlation of cloud echoes with the visual clouds present show that, provided each cloud type observed is assigned equal weighting, about 47 per cent of the visual clouds were detectable.

The radar characteristics of common cloud types and the conditions favoring radar return from individual types are discussed. With most types, cloud temperature is indicated as being a critical factor governing detectability. This fact and the frequent occurrence of ice-crystal streamers indicate that the presence of ice crystals favors detectability. The detection of certain non-visible clouds is cited, and explanations for their presence are offered.

The detectability of water clouds as a function of drop size and liquid-water content is discussed quantitatively. Radar theory and radar calibrations are used to determine altitude limits to which common clouds as sampled by Diem would be detected. It is found that the 1.25-cm radar used would detect only about 15 per cent. of these clouds at a range of 1 mile or less. “Clouds” detected at ranges greater than 10,000 feet must, therefore, be comprised either of relatively large water particles or ice crystals.

The characteristic features of stratiform precipitation are discussed, and a preliminary correlation is made to determine the conditions necessary and sufficient to the release of such precipitation. Cloud echo-top temperatures between −10 and −20C and cloud echo-thicknesses in excess of 10,000 ft are especially favorable to the onset of precipitation. The ice phase is also found to be critical to the formation of strati-form precipitation (other than drizzle) in New England (although indications are that some showers may form entirely in the water phase). The ice crystals generally occur in the form of streamer-like trails, which may either fall into lower decks acting as natural seeding elements, or may themselves constitute the major portion of the precipitation. The streamers originate in cumuliform cells which are frequently of limited vertical extent.

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Vernon G. Plank, Robert O. Berthel, and Arnold A. Barnes Jr.

Abstract

Uncertainty analyses and operational experience at the Air Force Geophysics Laboratory (AFGL) have shown that for single samples of ice hydrometeors, the factor κ = M/(??), where M (spectral liquid water content) and Z (spectral radar reflectivity factor) are computed from aircraft particle size information, is less subject to the uncertainties of converting the physical size of the particles into equivalent melted diameter than are M and Z themselves. Consequently, in correlating multiple-sample aircraft data with simultaneously acquired, independent radar measurements of ZR, there is an appreciable accuracy enhancement that can be attained in the derivation of M vs Z equations by regression means, if κ is used as the correlation parameter, versus ZR, rather than if spectral M is used as the correlation parameter.

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Vernon G. Plank, Alfred A. Spatola, and David M. Johnson

Abstract

Values of diffusion coefficients determined from the observed closing times of nine conical-shaped clearings in fog produced by hovering helicopters at Lewisburg, WV, in September 1969 are presented. The values were established following the method of Elliott (1970) assuming that the geometric and diffusive properties of the clearings and surroundings could be approximated by theoretical equations of the type governing the diffusion of heat and water substance in a bounded, circular cylinder of infinite length, with appropriate specification of the condensation conditions. The coefficient values for the nine experiments were found to range from 0.7 × 105 to 1.9 × 105 cm2 s−1.

The physical, thermal and kinematic characteristics of the Lewisburg fog were investigated that might explain the variation of the coefficient values across this range. These efforts revealed that the values varied primarily, and inversely, with 1) the depth of the fog and 2) the Richardson number within the fog layer. Secondary correlations were also indicated which are noted.

The coefficient values determined by the described method are appreciably larger than those reported previously in the literature for radiation fog of the Lewisburg type. Possible reasons for this include the uncertainties of specifying the initial and “closed-in” conditions of the theory, the residual, turbulence effects of the helicopters, and the fact that the method emphasizes the horizontal components of diffusion, whereas previous methods emphasized the vertical component.

Theory predictions are illustrated which relate the initial temperature-humidity conditions of artificially produced clearings to the closing-in times of the clearings.

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Vernon G. Plank, Alfred A. Spatola, and James R. Hicks

Abstract

Results of helicopter clearing experiments conducted at the Greenbrier Valley Airport, Lewisburg, W. Va., during the period 7–29 September 1969, are presented and discussed. Thirty-five hover experiments and 18 runway-clearing experiments were performed on 10 separate days with fog layers from 125 to 525 ft in depth. The hover experiments, which were successful in virtually all cases, yielded clearings that varied from 400 to 2800 ft in length. The largest clearings occurred with the shallowest fog during tests conducted within ∼1 hr of the natural dissipation time of the fog. The runway-clearing experiments were successful in clearing the full 6000 ft extent of the runway on two occasions, were partially successful on four occasions, and were unsuccessful on 12 occasions. Six helicopter landings were accomplished through artificially created clearings.

Particular, quantitative results of the hover experiments are described. The wake penetration distance of the helicopters ranged from ∼700–1000 ft. The steady-state clearing times varied from ∼150–260 sec. The total entrainment (mixing) of environmental air into the wake air during the steady-state period was between 350 and 1000%. The clearing ratios, the ratios of cleared volume to down-transported volume, had values of 1.8 to 8.7. The clearing persistence, following helicopter departure from the test sites, varied with log depth and with the convective state of the fog from as little as 1–2 min to as much as 25–30 min. The cleared zones at the surface level were characterized by temperatures 0.3–3.0C warmer than ambient, by relative humidities near 100%, by specific humidities that were 0.1–1.2 gm kg−1 larger than the saturated specific humidities of the fog surroundings, and by downwash-groundwash wind velocities of 6–20 mph with peak gust speeds of as much as 50 mph.

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