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Alan M. Blyth

Abstract

Entrainment of dry air into cumulus clouds influences the development of the clouds in a major way. The many aspects of the entrainment process are examined in this paper by critically reviewing the literature from the time when investigations began.

It is an interesting time in the evolution of the study of cumulus clouds with the advent of different models and several new instruments. Traditional entraining plume and thermal models that received considerable attention during the early years are being replaced by episodic-type mixing models. Recent observations of the source of entrained air are in part responsible for the new thinking, but the ideas really originate with the suggestion made by Squires more than 40 years age that vertical rather than horizontal mixing causes the dilution of cumuli by the process of penetrative downdrafts.

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Jørgen B. Jensen and Alan M. Blyth

Abstract

No abstract available.

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Alan M. Blyth and David J. Raymond

Abstract

Recent entrainment studies have showed that entrainment occurs at all levels in cumulus clouds irrespective of the thermodynamic conditions of the environment in which the clouds are growing. They have further suggested that cloudy parcels tend to proceed towards their level of neutral buoyancy after mixing. In this note we compare the observations made in CCOPE by the Wyoming King Air with results from a simple model of cumulus convection. The comparisons, which are surprisingly good, lend support to the idea that cumulus clouds contain individual parcels, each seeking their equilibrium level.

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David J. Raymond and Alan M. Blyth

Abstract

Recent work suggests that nonprecipitating cumulus clouds must be considered as aggregates of many parcels always moving toward buoyancy equilibrium, but otherwise subject to a multiplicity of fates. For example, some parcels originating at low levels seem to ascend to their level of undilute neutral buoyancy before mixing with the environment, while others mix at intermediate levels. We present a model for a cumulus cloud incorporating these ideas. In spite of the apparent simplicity of the model, it compares favorably with observations.

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Dingying Wei, Alan M. Blyth, and David J. Raymond

Abstract

The buoyancy of convective clouds in TOGA COARE was calculated from the NCAR Electra in situ measurements of temperature, humidity, liquid water content, and two-dimensional images of raindrops. Most of the measurements were made at 700 mb (10°C) although some were made at 850 (18°C) and 600 mb (2°C). The temperature was measured with the Ophir radiometer, which does not have the wetting problem that has degraded many previous measurements of in-cloud temperature in warm clouds.

On average, the in-cloud virtual temperature excess was found to be less than the adiabatic value by about 2 K, while the negative influence of total water content on buoyancy was less than 0.5 K. Furthermore, the total water content was highly variable and much smaller than the adiabatic value. The authors conclude, therefore, that entrainment and mixing was usually a much larger factor in reducing the buoyancy than water loading.

The average buoyancy in downdrafts was positive and similar to the value in updrafts.

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Richard L. Carpenter Jr., Kelvin K. Droegemeier, and Alan M. Blyth

Abstract

This paper is the first in a three-part series in which a three-dimensional numerical model is run at high resolution to simulate cumulus congestus clouds in three dimensions with the principal goal of understanding the mechanisms associated with entrainment and detrainment. The clouds are contained within a nested grid having a 50-m uniform grid spacing; the model does not allow precipitation or ice formation and achieves saturation through bulk condensation. The prescribed environment is that associated with nonprecipitating New Mexican cumulus clouds observed on 9 and 10 August 1987.

The convection is initiated using continuous surface heating, including a central Gaussian component to represent the effects of an isolated mountain range. Several regions of concentrated surface heating are used during the first hour to condition the environment. The turbulent motion thereby introduced into the boundary layer is crucial for the accurate simulation of the clouds.

The simulated clouds extend vertically up to 4 km, and model results generally agree with aircraft observations in quantities such as cloud base and top height and the presence or absence of pronounced detrainment layers at midlevels. Further, the pulsating nature of the convection, in which the clouds strengthen and decay over periods of several minutes, is also similar to observations. The cloud-top height is generally not correlated with the level of neutral buoyancy for hypothetical parcels ascending undilute.

Spatial resolution at least as fine as that used here appears necessary in order to capture the details of cumulus entrainment, although clouds simulated on a single coarse grid exhibited a substantial degree of similarity to their nested grid counterparts and were at times somewhat more vigorous.

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Alan M. Blyth, William A. Cooper, and Jørgen B. Jensen

Abstract

Data gathered by the University of Wyoming King Air, the Atmospheric Environmental Services Twin otter and an NCAR Queen Air were used in thermodynamic analyses to determine the sources of environmental air entrained into cumulus clouds. The measurements were made in clouds ranging from small cumuli a few kilometers deep to a large supercell system. Previous results have indicated that the source of entrained air in continental cumuli is generally above the flight level, often near cloud top. The results reported here, however, suggest that the source of entrained air is close to, or slightly above, the observation level of the aircraft, even when the aircraft descends through different levels in the cloud. The results are consistent with the idea that cumulus clouds consist of thermal-like elements from which the least buoyant mixed parcels are shed off and the most buoyant mixed parcels may continue with the general ascent. A schematic model of cumulus convection is presented and supported by measurements of air motions in small cumulus clouds.

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Richard L. Carpenter Jr., Kelvin K. Droegemeier, and Alan M. Blyth

Abstract

This paper is the second in a three-part series in which a three-dimensional numerical cloud model is used to simulate cumulus congestus clouds at high resolution in an effort to better understand the mechanisms associated with entrainment and detrainment. The prescribed environment is that associated with nonprecipitating summertime New Mexican cumulus clouds that formed on consecutive days. Using budgets of mass and moisture, the effects of the clouds and their environment are examined here with an emphasis on understanding the life cycle of the clouds and the production of narrow detrainment layers aloft. Results are compared with measurements obtained in similar environments.

The mass flux profiles indicate the presence of a strong, persistent thermally driven circulation within the boundary layer, with the cloud circulation being secondary. Collapsing turrets appear to be responsible for the significant detrainment that occurs at mid levels within one simulated cloud. Transport by downdrafts is significant throughout the cloud and subcloud layers.

The boundary layer and cloud circulations dry the subcloud layer, with significant detrainment of moisture occurring in the upper portion of the boundary layer. Strong apparent moistening in the upper half of one cloud is driven by mean vertical transport of moisture toward the detrainment layer aloft, although detrainment of moisture at midlevels is a comparatively small component of the apparent moistening. Storage of moisture is found to be an important effect.

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Richard L. Carpenter Jr., Kelvin K. Droegemeier, and Alan M. Blyth

Abstract

This paper is the third in a three-part series in which a three-dimensional numerical cloud model is used to simulate cumulus congestus clouds. The authors conduct a detailed parcel trajectory and conserved variable analysis of the modeled clouds, with the principal goal of understanding the mechanisms associated with entrainment and detrainment.

At any point in their lifetime each of the modeled clouds contains multiple thermals that become detached from the boundary layer as they ascend. Undilute regions of subcloud air occur within the simulated clouds at all levels up to the cloud top. In the upper portion of the clouds, such air is found within small (compared with the overall width of the cloud) thermals that are continually eroding yet vigorously ascending. Such thermals are responsible for most of the entrainment and detrainment. Environmental air entrained by ascending thermals is shed in the wake of the thermal, which contains dilute cloud-base air moving at low velocities. There is no evidence for thermals ascending through the remnants of their predecessors as a favored means for new cloud growth. The source of entrained air within both updrafts and downdrafts is typically a few hundred meters above the observation level (although there is a tendency for updrafts at the highest levels to entrain air from just below that level).

Undilute cloud turrets tended to overshoot their level of neutral buoyancy by a considerable distance. Condensate loading triggers the collapse of individual turrets, with additional reductions in buoyancy resulting from the evaporative cooling due to entrainment as well as the transport of entrained environmental air upward. Strong, narrow downdrafts develop along the top and edges of overshooting turrets. These downdrafts are often marginally saturated (which would be the most dense mixture of two air masses) and are composed of a mixture of cloud-base and cloud-top air. They descend to mid levels within the modeled clouds before being detrained laterally.

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Sonia G. Lasher-Trapp, William A. Cooper, and Alan M. Blyth

Abstract

Ultragiant aerosol particles (UGA) are potentially important for warm rain formation because of their ability to initiate coalescence immediately upon entering a cloud, so it is desirable to obtain local estimates during any field campaign that studies warm rain. Estimates of UGA in clear air from a one-dimensional optical array probe averaged over long time periods from the Small Cumulus Microphysics Study have been published in the literature, but further analysis and comparisons to other probes, presented here, show that the data on which these estimates were based were probably contaminated by noise. A possible explanation for the noise in the probe is given, as are new upper limits, based on few or no particles detected by a two-dimensional optical array probe.

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