Congestus Clouds Research Articles

Mixing Mechanisms in Cumulus Congestus Clouds. Part I: Observations

Abstract Aircraft measurements are analyzed to determine the mode of entrainment in cumulus congestus clouds. The data were collected in 15 developing cloud towers sampled in southern Africa on three different days. The Paluch method, which is based on a comparison of the total water mixing ratio and wet equivalent potential temperature in the cloudy air and nearby environment, was used to identify the origin of the in-cloud air at the level of observation. The results indicate clearly the occurrence of cloud top mixing, but the presence of lateral entrainment cannot be ruled out. Strong downdrafts were found near the cloud edges, which suggests that the diluted air from the cloud top can penetrate several kilometers downward.

Convective cloud downdraft structure: An interpretive survey

Observational and modeling studies dealing with different aspects of convective clouds are reviewed and interpreted to construct a generalized description of the structure, dynamics, and thermodynamics of convective cloud downdrafts. Observational studies reveal that downdraft speeds and sizes range from typical values of several meters per second and several hundred meters in nonprecipitating cumulus congestus clouds to typical values of 5–10 m s−1 and several kilometers in precipitating cumulonimbi. Maximum measured downdraft speeds appear to be limited to ∼20 m s−1. Different types of downdrafts appear to exist within precipitating convective clouds. Penetrative downdrafts common to nonprecipitating convective clouds and upper regions of precipitating convective clouds exhibit maximum horizontal dimensions of ∼1 km. These downdrafts emerge when subsaturated environmental air is entrained or mixed into the cloud. A second type, cloud edge downdrafts, appear in both observations and in cloud model results. Although their driving mechanisms are not fully understood, such downdrafts may be forced by cloud edge evaporational cooling and localized updraft mass flux compensation. Overshooting downdrafts comprise a third type and are typically associated with intense convection in which updraft air surpasses an equilibrium level of neutral buoyancy, cools upon further ascent, and then descends but remains within a few kilometers of cloud top. Finally, the precipitation‐associated downdraft is one forced at low levels by precipitation loading, evaporation, and melting. This downdraft may attain relatively large scales, of the order of the horizontal dimension of precipitating regions within the lowest several kilometers. Such large scales provide a clear distinction from (penetrative‐type) downdrafts of ∼1 km maximum scale within nonprecipitating convection. There is evidence from both observational and modeling studies that downdraft dynamical and thermodynamical processes are strongly influenced by static stability, wind shear profiles, cloud microphysical processes, and precipitation characteristics. However, the degree to which downdraft structure depends on such interrelated controlling factors has not yet been determined.

Characteristics of Mountain-Induced Thunderstorms and Cumulus Congestus Clouds from Budget Measurements

Apparent mass, moisture and energy sources associated with individual cumulus congestus and thunderstorm clouds are measured using an instrumented aircraft. The results for thunderstorms are similar to those obtained by other investigators. Cumulus congestus clouds differ greatly from thunderstorms and show the important role of mixing in producing downdrafts. The thunderstorm profiles are strongly affected by precipitation-induced downdrafts.

The Role of an Apparent Mesoscale Frontogenetic Circulation in Squall Line Initiation

The present investigation is concerned with the nature and apparent origin of a mesoscale clear zone and associated frontal line convection (a line of cumulus congestus clouds along an intensifying cold front) which appeared just prior to the explosive development of a line of severe thunderstorms at the cold front. It is pointed out that the clear zone formed immediately behind the cold front in southeastern Kansas and northeastern Oklahoma during the afternoon of April 16, 1982. Sequential displays of GOES-East satellite imagery are considered. The mechanism which seems to explain best the observations is a thermally direct frontogenetic circulation resulting from the combined effects of the synoptic-scale wind deformation field and mesoscale differential sensible heating.

HIPLEX-1: Experimental Design and Response Variables

Abstract The design and conduct of HIPLEX-1, a randomized seeding experiment carried out on small cumulus congestus clouds in eastern Montana, are outlined. The seeding agent was dry ice, introduced in an effort to produce microphysical effects, especially the earlier formation of precipitation in the seeded clouds. The earlier formation was expected to increase both the probability and the amount of precipitation from those small clouds with short lifetimes. The experimental unit selection procedure, treatment and randomization procedures, the physical hypothesis, measurement procedures and the response variables defined for the experiment are discussed. Procedures used to calculate the response variables from aircraft and radar measurements are summarized and the values of those variables for the 20 HIPLEX-1 test cases from 1979 and 1980 are tabulated.

Three-Dimensional Simulations of Cumulus Congestus Clouds on GATE Day 261

Schlesinger's (1978) three-dimensional cumulus model is applied to showering congestus clouds on day 261 of GATE. Model results are compared with each other and with observations to analyze the effects of varying shear and altered sounding. Relationships between shear, mesovortices and dynamic entrainment are examined, as well as the model clouds' impact on the environment as a function of shear. The simulations appear to resemble reality in many important aspects. Altostratus layers observed on day 261 are found to be a by-product of convection in three-dimensional shear. Rapid erosion of cloud base to 3.6 km is related to the ambient thermal structure, with wind shear and initial perturbation playing a secondary role. Some of the apparent conflict regarding lateral versus cloud-top entrainment is clarified, as well as some factors governing convective downdraft structure and intensity.

Observations of a Cloud Are Triggered by Thunderstorm Outflow

Abstract This paper presents aircraft, radar, satellite and rawinsonde observations which describe a cloud am formed by moist downdraft outflow from a cumulonimbus cluster in northwest Kansas. Aircraft cross sections are presented of four variables at 160 and 320 m AGL, flown in clear air, which show the detailed structure of the mesoscale cold front in its final dissipating stages 3 h after its initiation. These observations indicate that along the mesoscale cold front, strong vertical velocities of 3–5 m s−1 occur. This lifting was examined in a numerical cloud model to determine its triggering cleat on convective cloud development. Model results, which agree with radar echo observations, indicate that lifting produced by the mesocold front is an important trigger mechanism for the observed cumulus congestus cloud growth. Model analyses also show that the vertical profile of lifting is an important factor in determining its not triggering impact on convective development. Seeding simulations indicate th...

Hail Growth Mechanisms in a Colorado Storm: Part II: Hail Formation Processes

Hail growth mechanisms in a Colorado multicellular hailstorm were investigated through use of dual-wavelength radar data, three-dimensional wind field data, in situ aircraft measurements and trajectory calculations. Hail production consisted of a transient, pulsating component and a weak, quasi-steady component. Peak hail production resulted when graupel particles and hailstones which grew within the updraft region of a feeder cell were advected in the wind field and fell into the main updraft region where they continued to grow. A minimum in hail production resulted when hail growth was confined to the main updraft region. Recirculation of particles within the main updraft had minor importance in the hail production mechanism. Aggregates of dendritic ice crystals probably comprised most of the hail embyros. These aggregates grew along the forward portion of the storm in a region of precipitation debris associated with decaying cumulus congestus clouds. They were introduced into developing feeder cells and were rapidly converted into graupel and small hail. Water drops, which resulted from the melting of aggregates, comprised some of the embryos of graupel particles which were growing within the main updraft region.

Ice Initiation in Unmixed Updraft Cores in Northeast Colorado Cumulus Congestus Clouds

Ice particle concentrations have been measured from the NOAA/NCAR Explorer sailplane in unmixed and mixed updraft regions within northeast Colorado cumulus congestus clouds, and compared with the concentrations predicted from measured ice nucleus spectra. The clouds investigated were “cold, continental” cumulus with droplet populations of ∼1000 cm−3 and cloud base temperatures between +7 and −7°C. The concentrations of ice particles within unmixed updraft regions, exclusive of ice particles so large that they almost certainly entered the unmixed region by sedimentation or recycling, are consistent with those expected on the basis of the ice nucleus spectra, suggesting that primary ice nucleation is the dominant mechanism active within these regions. Millimetric size ice particles found within unmixed updraft cores presumably enter through sedimentation or recycling. The data do not indicate an important role for ice multiplication in these clouds.

Observations of Moist Adiabatic Ascent in Northeast Colorado Cumulus Congestus Clouds

The characteristics of entrainment in and below 12 developing cumulus congestus clouds in the north-eastern Colorado area were investigated using measurements obtained with the NCAR/NOAA sailplane, supporting aircraft and rawinsondes. A region of moist adiabatic ascent was found in eight of the most vigorous clouds sampled. A gradual increase was noted in the equivalent potential temperature and the ratio of the liquid water content to the adiabatic value from the edge of the updraft region inward to the moist adiabatic core. Previous measurements and conceptual and theoretical models of entrainment are discussed in the context of the present set of measurements. The moist adiabatic core was positioned off-center with respect to the boundaries of the updraft region. The measurements supported previous conceptual cloud models in which the updraft acts as an obstacle to the horizontal wind thereby causing the environmental air to flow around the upshear portion of the cell, protecting that region from entrainment. A turbulent wake would be expected to occur in the down-shear portion of the cell, producing increased turbulence and mixing in that region.

The Characteristics of Graupel Particles in Northeastern Colorado Cumulus Congestus Clouds

Abstract The physical characteristics of graupel particles were investigated from in in situ collections of ice particles sampled in first-echo summertime cumulus congestus clouds in northeastern Colorado with the NCAR/NOAA sailplane operating during the National Hail Research Experiment. Ice particles were collected in vials containing silicone oil. Each particle was photographed at several different orientations and then melted to determine its equivalent diameter. This permitted the mass and axial dimensions to be determined and the density, axial ratios and terminal velocity to be estimated for each of 125 particles. The mass and terminal velocity of graupel particles were found to be considerably lower than those of equivalent diameter ice spheres. Best-fit equations to the mass-diameter and terminal velocity-diameter data were computed. Graupel densities were typically lower than 0.5 g cm−3 systematic differences were noted between the densities of lump and conical graupel. Calculations indicated th...

The ionic conductivity of electrified clouds

AbstractCalculations have been made, on the basis of the ionic balance equations derived by Phillips (1967), of the electrical conductivity of clouds in the presence, and in the absence, of secondary ion production resulting from corona.In weakly electrified clouds ‐ no corona ‐ the immobilization of ions on cloud droplets resulting from diffusional capture results in a reduced ionic concentration and conductivity. The normalized conductivity λ the ratio of the in‐cloud value to the clear‐air value at the same altitude Z was calculated for three different cloud types (cumulus congestus, strato‐cumulus and fog) as a function of Z, liquid water content L, droplet charge Q and field strength E. λ was found to be very sensitive to variations in L and E, but only slightly affected by changes in Z, Q and the manner in which the charge is distributed over the size‐spectrum. For example, when E = O λ possessed the values 0.034 and 0.17 in a cumulus congestus cloud for L = 0.5 gm/m3 and 0.1 gm/m3 respectively, whereas when E = 90 kV/m λ was 1.5 times; 10−3 and 7.6 times; 10−3 respectively for the same two values of L.The experimental measurements of corona onset fields and currents from ice hydrometeors, Ec and ic respectively, made by Griffiths and Latham (1974) were used to calculate the normalized conductivity within strongly electrified clouds. Corona from ice provides a copious supply of positive and negative ions which causes λ to increase from between 10−3 and 10−4 to greater than 1 when Ec is achieved. The value of λ once corona is initiated depends on L and the concentration of corona sites, J. In this situation altitude Z plays an important role because Ec decreases quite rapidly with decreasing pressure. For example, with a typical distribution of temperature and pressure within a thundercloud, corona will be initiated from hailstones at Ec = 720 kV/m if Z = 2.5 km (0° C) and 540 kV/m at 5.5 km (‐18° C); the corresponding values of Ec for snowflakes are 540 and 390 kV/m respectively. At 2.5 km is slightly higher than and lower than 10 respectively, as L is increased from 1 to 5 gm/m3 with J = 1 m−3.It is concluded that the introduction of an ionic leakage current into calculations of field growth within thunderclouds is unnecessary, and that the distribution of electric field is crucial in determining whether corona, once initiated, will trigger lightning or merely increase the ionic conductivity.

A Measurement of the Velocity Field of a Cumulus Cloud

A 3-hr flight of a constant-volume balloon which is entrained into a cumulus congestus cloud and lifted to an altitude of nearly 3 km provides an opportunity to determine the trajectory and vertical velocity of the balloon and, in turn, relate these to the circulation of the cloud. Simultaneous supplementary observations by radar, cameras, satellite, and other ground based instruments provide the meteorological context for the event. Computations show that the vertical velocities of the cloud may approach 10 m sec−1, and sustained upward velocities of 3–5 m sec−1 are maintained for considerable periods of time and height. Comparison with a “jet” model shows remarkable agreement, while the “bubble” model would require unreasonably large values of the radius to generate the observed vertical velocities.

WATERSPOUTS AND TORNADOES OVER SOUTH FLORIDA1

Abstract An analysis of the Lower Matecumbe Key 1967 waterspout data is presented. It was found that the flow field synthesized across the spray vortex of the second, larger waterspout is closely approximated by a Rankine-combined vortex with solid rotation over a circle 24 m in diameter. Five major tornadoes were documented in the Greater Miami area during 1968, and this anomalous number is ascribed to the development of strong localized zones of convergence on the mesoscale along or slightly inland from the southeast coast where the prevailing southwesterly tropospheric flow interacts with the sea breeze induced by the Florida Peninsula. On the other hand, the large number of waterspouts documented in the Lower Keys during the summer of 1968 were spawned by cumulus congestus cloud lines embedded in a very warm undisturbed trade-wind flow. Extensive documentation of close-range observations was obtained for an unusually large “tornadic waterspout” that passed through a crowded coastal marina in Miami. Ev...

Conditions Inside Cumulus Clouds Preceding the Onset of Precipitation

Conditions within 16 cumulus congestus clouds were measured on aircraft penetrations near the −5C level and precipitation developments during the subsequent 45 min were noted. Significant correlations exist between the in-cloud conditions and the time of onset and intensity of precipitation. The degree of spatial correlation among the fields of temperature, vertical motion and water content within a given cloud proved more important to subsequent developments than did the mean values of these variables.

An Empirical Examination of Cumulus Entrainment

Abstract Most analytic theories and laboratory models of cumulus entrainment predict that the rate of influx of ambient fluid into a convective element depends inversely on the diameter of the updraft zone. A study of 15 continental cumulus congestus clouds containing apparent bubble-like rising elements failed to demonstrate the predicted radius dependence. Application of a standard one-dimensional entrainment computation to the measured cloud and environment conditions revealed no significant correlation between cloud dimensions and entrainment.

Precipitation Particle Measurements in Cumulus Congestus

Abstract Precipitation particle measurements from the upper levels of cumulus congestus clouds are analyzed with regard to general cloud characteristics, liquid water content, and precipitation water content as related to the theoretical radar reflectivity. Conclusions are: (1) the majority of the cumulus congestus clouds examined, whose tops exceed 14,000 ft, contained precipitation particles (250-microns diameter) in the upper levels sometime during their life cycle, (2) particle concentrations in excess of 1000 per m3 were found in about 20 per cent of the clouds examined, (3) the relationship Z=1.6×10−2M1.46 for radar reflectivity is applicable for cumulus congestus in the early stages of precipitation development.

PRESENT STATE OF RESEARCH ON ATMOSPHERIC ELECTRICITY

Introduction 292 1. Basic Information 294 2. Two Theories 297 3. Vertical Structure of Field, Conductivity, and Current in 299 4. Distribution of Space Charge in the Atmosphere 302 5. Time Variation of Field and Potential at High Altitudes 303 6. Description of Electric Processes in Regions of Good Weather 305 7. Electricity of Stratified Clouds 307 8. Electricity of Cumulus and Cumulus Congestus Clouds 311 9. Structure of Shower and Thunderstorm Clouds 313 10. Accumulation of Charges in Thunderstorm Clouds 316 References 320

Size Distribution of Large Droplets in Precipitating Clouds

Measurements have been made of the size distribution of large droplets in various clouds at a station situated on Mount Kuro. The results show that most nimbostratus, cumulus congestus and cumulonimbus clouds contain large droplets of radii above 50 ?. The spectral distributions fit fairly well the relation N= N0r ?? where ? has values between 4 and 11. The concentration of large droplets varies widely and values above 10 5 per m ?3 are frequently observed in precipitating clouds. DOI: 10.1111/j.2153-3490.1961.tb00113.x

Showers from cumulus congestus clouds

Quarterly Journal of the Royal Meteorological SocietyVolume 80, Issue 343 p. 105-105 Correspondence Showers from cumulus congestus clouds L. Alpert, L. Alpert 4120 34th Street, North Arlington, Virginia, U.S.A.Search for more papers by this author L. Alpert, L. Alpert 4120 34th Street, North Arlington, Virginia, U.S.A.Search for more papers by this author First published: January 1954 https://doi.org/10.1002/qj.49708034317AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume80, Issue343January 1954Pages 105-105 RelatedInformation