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J. Warner

Abstract

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J. Warner

Abstract

Results are given of detailed measurements of vertical air velocity in cumulus clouds of depth ranging from 0.7–4 km. Both peak updrafts and downdrafts and rms vertical velocity were found to increase with height above cloud base, the latter at a rate of about 0.7 m sec−1 km−1. The rms velocity also increased as the stability of the clear air environment of the cloud decreased, at a rate of about 0.3 m sec−1 per °C km−1. Maximum, upper decile and median peak updrafts were 12.7, 10.0 and 4.9 m sec−1, respectively; corresponding values for downdrafts were 9.5, 7.5 and 3.5 m sec−1. The structure obtained by combining measurements of horizontal and vertical air velocity at short intervals throughout a traverse showed well-defined overturning motions near the cloud top but not elsewhere. Power spectra are presented which show that most of the energy is at long wavelengths; peaks are also present at wavelengths of 500–600 m within the body of the cloud, but at 100–200 m near cloud base or cloud top.

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J. Warner

Abstract

A critical examination is undertaken of current steady-state one-dimensional cloud models which postulate lateral entrainment. It is found that such models cannot simultaneously predict values of liquid water content and cloud depth which are in agreement with observations. If sufficient entrainment is postulated to get agreement with observations of cloud water content, the model cloud does not grow as high as those observed in the given environment, while entrainment appropriate to the observed height yields liquid water contents that are too high. It is suggested that the success claimed for these models is based on a choice of arbitrary constants which may be valid only for a limited range of atmospheric conditions; in other conditions, predictions from the model may be seriously in error. In addition, since the physical basis of these models is invalid, we have no way of knowing what conditions are most important or how to avoid the necessity of selecting fresh constants by trial and error with each fresh use of the model.

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J. Warner

Abstract

Observations of the droplet size distribution at fixed levels in cumulus cloud for periods ranging from 13–44 min indicate that there is little change with time in the shape of the distribution or in the mean droplet diameter. The number concentration increases initially and then decreases, with the greatest relative changes occurring in the upper levels of the cloud.

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J. Warner

Abstract

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J. Warner

Abstract

The concentration of cloud droplets in natural cloud has been found to be fully consistent with existing theory and with observations of cloud nuclei and updraft speed. In general, turbulence in the updraft does not broaden the droplet size distribution markedly beyond that produced by condensation in a steady updraft. However, some updraft structures, particularly those involving a general acceleration, are capable of broadening the spectrum by the repeated activation of fresh nuclei and of producing a persistent, strongly bimodal, size distribution. In order to account for the almost invariable observations of numbers of droplets <10 µm diameter at heights of 100 m or so above cloud base, it appears necessary for the accommodation coefficient to be not greatly in excess of 0.05; and for droplets to grow to greater than this size in a reasonable period the accommodation coefficient cannot be markedly less than 0.02. Within these limitations it has been found possible to account fairly closely for the droplet size distributions actually observed.

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J. Warner

Abstract

On the basis of an examination of sooted sampling slides exposed to a variety of cloud conditions, it is concluded that spatial variation of the droplet concentration in the vicinity of cloud base is unlikely to be important except when the nuclei on which the droplets form have originated close to the cloud and have not had time to become a well-mixed aerosol. Fluctuations in liquid water content in the main body of a convective cloud may occur on all scales, but apart from a scale of the order of 100 m, which is associated with the updraft structure, it appears that the largest fluctuations occur on a scale of the order of 1 m.

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J. Warner

Abstract

Calculations have been made of the effect on the droplet size distribution of mixing between a cloudy parcel and its clear air environment, with attention being concentrated on the first few hundred meters above cloud base where condensation is the dominant process. If the environment is nucleus-free, we conclude that mixing broadens the spectrum only slightly, while the mean droplet size and total concentration are reduced. If the environment contains nuclei which are activated to produce droplets after mixing has occurred, the spectrum is broadened considerably but in a way which 15 not observed in natural clouds. In natural clouds we find that the dispersion of the droplet size distribution is independent of the amount of mixing that has taken place if, as a measure of the mixing, we use the ratio of the observed liquid water content to its adiabatic value at the position of the droplet sample. Thus, from both theory and observation we must conclude that simple mixing between cloud and environment is unimportant in determining the drop size distribution, at least in the early stages of cloud growth.

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J. Warner

Abstract

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J. Warner

Abstract

Observations in small cumuli of depth 1.5–2 km indicate that at any given height above cloud base the average turbulent velocity and liquid water content remain more or less constant for periods of up to 20 min or so during the central part of the cloud lifetime. A region can often be identified near the upshear side of the cloud in which the updraft persists with little change in magnitude for periods of 10–15 min. Outside this region there are large fluctuations in the vertical velocity and when averages are taken over the full visible width of the cloud no consistent pattern is observed.

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