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David B. Johnson

Abstract

This note compares and evaluates the analytical solutions of Squires and Twomey for cloud droplet concentration. Either solution is likely to be fairly accurate (±30%) when the slope parameter (β) of the cloud condensation nuclei distribution is small (β<0.8). For large β errors increase rapidly and both solutions are likely to underestimate the actual drop concentration by a wide margin.

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David B. Johnson

Abstract

Riming growth rates for graupel and frozen raindrops are compared with those of unfrozen drops growing by coalescence. The results indicate that graupel can have a considerable advantage over unfrozen raindrops in regions where the cloud droplets are large (e.g., 20 μm diameter). In this case, the graupel growth rates win also be enhanced by reductions in the graupel bulk densities, at least down to values as low as 0.2 g cm−3 or less. When encountering smaller cloud droplets, however, the apparent advantage of the graupel particles fades, and at times may even be reversed.

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David B. Johnson

Abstract

The importance of multiple cycles of raindrop growth and breakup in the evolution of warm rain is limited by the time required for breakup fragments to grow back to breakup sizes and, if the updraft is not strong enough to suspend the drops involved, the distance the drops fall during the multiplication cycle. This note introduces analytical solutions that allow quantitative examination of these limits.

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David B. Johnson

Abstract

No abstract available.

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David B. Johnson

Abstract

Cloud-base temperature is an important, but often neglected, parameter in the activation of cloud drops. Other factors being equal, clouds with colder bases will activate more condensation nuclei than clouds having warmer bases. In a typical case, a decrease in cloud base temperature from +10°C to −10°C will result in approximately a one-third increase in the drop concentration. Variations in cloud base pressure, on the other hand, are relatively unimportant.

The primary tool used in this evaluation is a detailed model of droplet activation and condensational growth which is significantly more efficient than previously reported models of this type. In addition, Squires” analytical expression for drop concentration is recast to permit direct evaluation of the influence of temperature and pressure.

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David B. Johnson

Abstract

Giant and ultragiant aerosol particles can play an important role in warm rain initiation. Recent aerosol measurements have established that particles as large as 100 μm are a regular part of the atmospheric aerosol. When ingested in growing clouds, these particles will produce a tail of large drops on the upper end of the cloud-droplet distribution. In a series of numerical computations, this tail of large drops was found to be a significant destabilizing factor which can speed precipitation development.

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David B. Johnson

Abstract

Multiple-parameter radar measurements involve the combination of several different radar measurements, such as reflectivity measurements at different frequencies or polarizations, into a single parameter. Prediction of the effects of antenna sidelobes on such measurements can be rather involved since each component measurement is subject independently to sidelobe effects. This paper develops a simple model that can be used to examine sidelobe contributions to multiple-parameter radar measurements and identifies the factors controlling the magnitude of the sidelobe contribution.

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David B. Johnson and Kenneth V. Beard

Abstract

When raindrops collide, some of the kinetic energy involved in the collision will be available to initiate or sustain oscillations in the surviving drops. This paper presents results of a simple model of drop collisions that generates an estimate of the expected distribution of energies in an ensemble of colliding raindrops as a function of drop size and rain intensity. The results indicate that drop collisions can be an effective source of raindrop oscillations and that within any one rain shaft, it tends to produce a range of oscillation energies from intense to imperceptible. In every case, however, the fraction of drops oscillating and the severity of the oscillations increase with increasing drop size and rainfall intensity.

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David B. Johnson and Roy M. Rasmussen

Abstract

The transition between wet and dry growth for graupel and hail is examined, and new figures are presented illustrating the critical water contents necessary for transitions into or out of the wet-growth regime. These figures are extended to smaller sizes and lower bulk densities than considered in previous studies. In addition, the possibility of hysteresis in the transitions is examined.

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Harry T. Ochs III and David B. Johnson

Abstract

The properties of 3 cm radar first echoes were used to study the effects of the St. Louis, Missouri metropolitan area on precipitation initiation during the summer months of Project METROMEX. Good statistical support was found for a ∼150 m lowering of urban first echo tops and a ∼250 m lowering of urban first echo bases when compared to rural echoes observed on the same day. Urban echoes were found to be thicker than their rural counterparts; however, there is no statistical support for this difference. This result is most easily interpreted as suggesting a slight weakening of the updraft in urban clouds.

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