Dissipation Stage Research Articles

Surface Characteristics of Observed Cold Pools

Abstract Cold pools are a key element in the organization of precipitating convective systems, yet knowledge of their typical surface characteristics is largely anecdotal. To help to alleviate this situation, cold pools from 39 mesoscale convective system (MCS) events are sampled using Oklahoma Mesonet surface observations. In total, 1389 time series of surface observations are used to determine typical rises in surface pressure and decreases in temperature, potential temperature, and equivalent potential temperature associated with the cold pool, and the maximum wind speeds in the cold pool. The data are separated into one of four convective system life cycle stages: first storms, MCS initiation, mature MCS, and MCS dissipation. Results indicate that the mean surface pressure rises associated with cold pools increase from 3.2 hPa for the first storms’ life cycle stage to 4.5 hPa for the mature MCS stage before dropping to 3.3 hPa for the dissipation stage. In contrast, the mean temperature (potential temperature) deficits associated with cold pools decrease from 9.5 (9.8) to 5.4 K (5.6 K) from the first storms to the dissipation stage, with a decrease of approximately 1 K associated with each advance in the life cycle stage. However, the daytime and early evening observations show mean temperature deficits over 11 K. A comparison of these observed cold pool characteristics with results from idealized numerical simulations of MCSs suggests that observed cold pools likely are stronger than those found in model simulations, particularly when ice processes are neglected in the microphysics parameterization. The mean deficits in equivalent potential temperature also decrease with the MCS life cycle stage, starting at 21.6 K for first storms and dropping to 13.9 K for dissipation. Mean wind gusts are above 15 m s−1 for all life cycle stages. These results should help numerical modelers to determine whether the cold pools in high-resolution models are in reasonable agreement with the observed characteristics found herein. Thunderstorm simulations and forecasts with thin model layers near the surface are also needed to obtain better representations of cold pool surface characteristics that can be compared with observations.

Characterization of warm season convective systems over US in terms of Cloud to Ground lightning, cloud kinematics, and precipitation

Convective systems are tracked for two summer months over the continental USA and their properties at different stages of life are investigated in terms of Cloud to Ground (CG) lightning flashes, cloud kinematics, and precipitation. The study employs lightning data from the National Lightning Detection Network (NLDN) along with 1/2-hourly satellite Infrared observations, and hourly rainfall accumulation fields from the nationwide WSR-88D radar network. Results show strong dependence of the amount of CG lightning flashes with the stage of the storm's life cycle. The highest number of CG lightning flashes and highest area expansion rates are also seen to be during the growth stage of the systems. The stronger the lightning density, or cloud top area expansion rate, of the storms at their initiation stage is the longer is shown to be their life duration. The average rainfall yield per flash in the growing, mature and dissipation stages of storms with duration greater than 4 h is 4.7, 12.5 and 8.5 10 7 kg, respectively. The temporal evolution of the storms' area and CG flash rate are shown to correlate well with the evolution of rainfall parameters, such as storm's rainfall yield (m 3), conditional mean rainfall rate, and the fraction of rain area. The consideration of the kinematics and CG lightning flashes of convective systems and the knowledge of their life cycle stage demonstrate a potential for drawing information useful in rain rate retrievals and their variability.

Turing pattern formation in Pr2Fe14B-based alloy and its role in anisotropy inducement at the early stage of disproportionation

At the dissipative disproportionation stage, the hexagonally spotted Turing pattern is formed on the reaction front as expected in a reaction-diffusion system. Turing patterns fill three dimensions resulting in the nanosized PrH2 rods embedded in a Fe matrix. The PrH2 rods are found to be oriented along the [110] direction with the stable {111} facets and a pair of face-to-face {002} facets. Based on both the geometrical configuration and the anisotropic interfacial energy, the mechanism whereby the Fe2B phase finally takes the place of the paired {002} facets as an orientation carrier has been proposed.

Turbulent Mixing Processes in Atmospheric Bores and Solitary Waves Deduced from Profiling Systems and Numerical Simulation

Abstract Families of solitary waves (“solitons”) associated with two atmospheric bores on the same day were observed by an unprecedented number of ground-based and airborne profiling systems during the International H2O Project (IHOP). In addition, a very high-resolution numerical weather prediction model initialized with real data was used with success to simulate one of the bores and the evolving soliton. The predicted wave amplitude, phase speed, wavelength, and structure compared well to these extraordinarily detailed observations. The observations suggest that during the active phase (when turbulent mixing was active, which was prior to bore collapse), the bores and waves vigorously mixed dry air from above a nocturnal boundary layer down to the surface. Refractivity computed from near-surface radar observations showed pronounced decreases due to sudden drying during the passage of the bores in this phase, but refractivity increases appeared during the period of bore collapse. During both phases, the bores wafted aerosol-laden moist air up to the middle troposphere and weakened the capping inversion, thus reducing inhibition to deep convection development. The model results indicate that the refractivity decreases near the surface were due to drying caused by downward turbulent mixing of air by the wave circulations. Turbulent kinetic energy was generated immediately behind the bore head, then advected rearward and downward by the solitary waves. During the dissipation stage, the lifting by the bore head produced adiabatic cooling aloft and distributed the very moist air near the surface upward through the bore depth, but without any drying due to the absence of vigorous mixing. Thus, this study shows that the moist thermodynamic effects caused by atmospheric bores and solitons strongly depend upon the life cycle of these phenomena.

Analysis of the viscous/molecular subgrid-scale dissipation terms in LES based on transport equations: A priori tests

One trend in large-eddy simulations (LES) involves the use of a transport equation for the subgrid-scale (SGS) kinetic energy. For problems involving active or passive scalar fields a SGS scalar variance transport equation is also used. The terms from these equations involve sub-filter scale quantities that are not accessible during LES and thus require modelling. By far the greatest challenge for modelling in these equations comes from the viscous and the molecular SGS dissipation terms that represent the final (dissipation) stages of the ‘energy cascade mechanism’ whereby the SGS kinetic energy and SGS scalar variance are dissipated through the action of the molecular viscosity and diffusivity, respectively. In this work direct numerical simulations (DNS) of statistically stationary (forced) homogeneous, isotropic turbulence are used to (i) analyse the topology and spatial localisation of the viscous and the molecular SGS dissipation terms, (ii) assess three models currently used for these terms and (iii) present some guidelines to improve or develop future models for these terms. The models analysed here are (a) the classical model used by e.g. Schumann [1] and Yoshizawa [2], (b) the model used in hybrid RANS/LES by Paterson and Peltier [3], and by Hanjalic [4], and (c) the model for the molecular SGS dissipation of SGS scalar variance from Jiménez et al. [5]. The classical models for the molecular SGS dissipation give very good results in terms of topology, spatial localisation (in the physical space), statistical behaviour and spectral characteristics. Moreover, the model constants approach asymptotically the theoretical values as the Reynolds number and filter sizes increase which supports the use of a constant value in engineering and geophysical applications, instead of using a dynamic procedure for their computation as in Ghosal et al. [6]. For the molecular SGS dissipation of SGS scalar variance the model from Jiménez et al. [5] performs even better than the classical model and should be the preferred model for this term when the Schmidt number is close to 1.0. Finally, all the tests showed that the models used in hybrid RANS/LES tested here give very poor results either in terms of their topological, statistical or spectral characteristics. The reason behind this is connected with the deficient spectral representation of the exact molecular SGS dissipation terms.

Modeling post-depositional mixing of archaeological deposits

We develop a series of simple mathematical models that describe vertical mixing of archaeological deposits. The models are based on assigning probabilities that single artifact specimens are moved between discrete stratigraphic layers. A recursion relation is then introduced to describe the time evolution of mixing. Simulations are used to show that there may be important regularities that characterize the mixing of archaeological deposits including stages of dissipation, accumulation and equilibration. We discuss the impact of post-depositional mixing on the apparent occupation intensities at modeled stratified archaeological sites. The models may also help clarify some of the problems inherent in making general inferences about the nature culture change based on mixed archaeological deposits. We demonstrate the modeling approach by developing a post-depositional mixing model for the Barger Gulch Folsom site.

Analysis of Thermal Transients in an Asymmetric Silicon-Based Heat Dissipation Stage

Thermal management is crucial for many microsystems and electronics applications (and that of miniaturized spacecraft is particularly demanding). This paper presents thermal modeling and scaling of a generic multiwafer silicon segment for placement in between two devices, or as a stage for a single one, in need of asymmetric thermal management. The unit is autonomous, i.e., it doesn't require any input signals or power. It comprises paraffin acting both as a heat sink, or thermal storage, and a material activating heat switches. The former mitigates heat bursts and accommodates power initially generated in, e.g., attached electronics, whereas the latter facilitates heat dissipation through heat guides during more intensive operation. Its function and physical properties are described in detail. A lumped thermal model has been constructed and implemented in the Simulink environment to investigate effects from: physical scaling of the unit, and change of its boundary temperature and coupling thereto, power generated, its emission and absorption properties and area fractions dedicated for passive devices, infrared (IR) emission, and heat guides on the unit's exterior, as well as fractional cross sections of paraffin, heat guides and other structural material in its interior. Conclusions, based on simulation results, are made and design rules based on the thermal modeling are presented. It was found that a 68 times 68 mm module could handle more than 10 W for 6 min in its heat sink mode alone. Subjected to 15 W for the same time, the module enters its active dissipation mode by closing its heat switches. A lateral increase and simultaneous vertical decrease of the unit's size resulted in overheating, whereas most scaling did not cause depletion of the heat sink. Changing the area fractions of various constituents also indicated operational stability with exception for excessive enlargement of passive heat guide material, exchanging structural material with paraffin, or severely limiting IR emission (by emitter area reduction or using low emission material), or using high absorbance material. Altering the boundary temperature and interface conductance proved to be means of biasing the system to various operating temperatures.

Simulation study on evolution of coalbed methane reservoir in Qinshui basin, China

A dynamic equilibrium model has been proposed to describe the coalbed methane concentration–dissipation evolution process in coal, including its generation, storage and migration. The model, developed with a computational program, has been applied to the Qinshui basin, China, which allows to evaluate features of coal seam of the basin including coal quality and burial, palaeo-geothermal field, and coal organic material maturation evolution history. The simulation study was focused on coalbed methane concentration–dissipation evolution history of the upper and lower main coal seams in the basin. The results indicate that the whole evolution process can be divided into three stages. At the first stage, i.e. the primary concentration stage, gas generation quantity and gas content were lower and only weak diffusion took place. The second stage can be called the active concentration–dissipation stage. During this period, coal organic material was matured at a higher rate, generating a large amount of gas because of abnormal high palaeo-geothermal field in the Middle and Late Yanshan Epoch. As a result, gas content in coal reservoir was higher and gas could be lost in different ways including diffusion, cap outburst and permeation. The third stage can be defined as the absolute dissipation stage at which coal organic material no longer generates gas. Meanwhile much gas was lost mainly by diffusion and partly by permeation. Simulation results further show that, although the high gas generation led to active dissipation in the coal reservoir, there still remains comparatively high gas content on the south and north ends of the Qinshui basin due to the preserving geological conditions. This suggests that these regions are favorable for coalbed methane exploitation which has been confirmed by recent geological exploitations in Qinshui basin.

The electromagnetic model of gamma-ray bursts

The electromagnetic model (EMM) of gamma-ray bursts (GRBs) and a contrast of its main properties and predictions with the hydrodynamic fireball model (FBM) and its magnetohydrodynamical extension are described. The EMM assumes that rotational energy of a relativistic, stellar-mass central source (black hole–accretion disk system or fast rotating neutron star) is converted into magnetic energy through a unipolar dynamo mechanism, propagated to large distances in the form of relativistic, subsonic, Poynting flux-dominated wind and is dissipated directly into emitting particles through current-driven instabilities. Thus, there is no conversion back and forth between internal and bulk energies as in the case of the fireball model. Collimating effects of magnetic hoop stresses lead to strongly non-spherical expansion and formation of jets. Long and short GRBs may develop in a qualitatively similar way, except that in the case of long burst ejecta expansion has a relatively short, non-relativistic, strongly dissipative stage inside the star. EMMs and FBMs (as well as strongly and weakly magnetized fireballs) lead to different early afterglow dynamics, before deceleration time. Finally, the models are discussed in view of latest observational data in the Swift era.

On the spectral dependency of UV radiation enhancements due to clouds in Valdivia, Chile (39.8°S)

Data gathered with a five‐channel radiometer are used to analyze the spectral composition of cloud enhancements and attenuations of UV (305, 320, 340, and 380 nm) and photosynthetically active radiation (PAR) (400–700 nm) at Valdivia, Chile (39.8°S), during summer months and within 3 hours from solar noon. Variations are referred to clear‐sky days occurring at not too distant dates or obtained as polynomial fit to cloud‐free periods on partially cloudy days. Photographic images of the sky were taken simultaneously with part of the radiometric data. Frequency of occurrence and duration of enhancements were estimated. Most of the enhancements were associated with cumuliform clouds in dissipation stage while the cloud fringes were crossing over the Sun disk. Both attenuations and enhancements present spectral dependence: the first decreasing and the second increasing toward longer wavelengths. Typical magnitude of the enhancement of 1‐min averages for PAR is close to 20%, with highest values exceeding 40%, and decreasing to 6% for 305 nm. Cases with well‐defined cloud side reflections were not observed. A weak relation between cloud cover and enhancement magnitude was found: Larger cloud cover was associated with higher enhancements.

Life Cycle of Numerically Simulated Shallow Cumulus Clouds. Part I: Transport

AbstractThis paper is the first in a two-part series in which the life cycles of numerically simulated shallow cumulus clouds are systematically examined. The life cycle data for six clouds with a range of cloud-top heights are isolated from an equilibrium trade cumulus field generated by a large-eddy simulation (LES) with a uniform resolution of 25 m. A passive subcloud tracer is used to partition the cloud life cycle transport into saturated and unsaturated components; the tracer shows that on average cumulus convection occurs in a region with time-integrated volume roughly 2 to 3 times that of the liquid-water-containing volume. All six clouds exhibit qualitatively similar vertical mass flux profiles with net downward mass transport at upper levels and net upward mass flux at lower levels. This downward mass flux comes primarily from the unsaturated cloud-mixed convective region during the dissipation stage and is evaporatively driven. Unsaturated negatively buoyant cloud mixtures dominate the buoyancy and mass fluxes in the upper portion of all clouds while saturated positively buoyant cloud mixtures dominate the fluxes at lower levels. Small and large clouds have distinct vertical profiles of heating/cooling and drying/moistening, with small clouds cooling and moistening throughout their depth, while larger clouds cool and moisten at upper levels and heat and dry at lower levels. The simulation results are compared to the predictions of conceptual models commonly used in shallow cumulus parameterizations.

Characteristics of Raindrop Size Distribution Dependent on the Life Stage of a Convective Precipitation Cloud in the Baiu Season

A convective cloud varies considerably in physical structure from development to dissipation. This variation results in changes in the Raindrop Size Distribution (RSD) in the precipitation from a convective cloud. The characteristics of RSD in the life stages of a convective cloud are compared, based on the data measured by an optical raindrop spectrometer during the field projects; TREX (1996), X-BAIU-98 (1998), and X-BAIU-99 (1999), performed on the southern and western coasts of Kyushu Island in the Baiu season. The convective clouds that passed over the observational site were classified as being in the mature, or the dissipation stage. The relationship between the shape of the RSD, and the convective activity of the convective clouds, varied according to the stage of development. When clouds are in the mature stage, the raindrop number concentration in every size range increases as the convective activity increases. On the other hand, when clouds are in the dissipation stage, the raindrop number concentration with larger sizes tends to increase as the convective activity in the mature stage increases. On the basis of these new findings obtained from observations, the major precipitation mechanism is expected to change in the life stages of convective cloud.

Analysis of Humidity Halos around Trade Wind Cumulus Clouds

Regions of enhanced humidity in the vicinity of cumulus clouds, so-called cloud halos, reflect features of cloud evolution, exert radiative effects, and may serve as a locus for new particle formation. Reported here are the results of an aircraft sampling campaign carried out near Oahu, Hawaii, from 31 July through 10 August 2001, aimed at characterizing the properties of trade wind cumulus cloud halos. An Aerodyne Research, Inc., fast spectroscopic water vapor sensor, flown for the first time in this campaign, allowed characterization of humidity properties at 10-m spatial resolution. Statistical properties of 60 traverses through cloud halos over the campaign were in general agreement with measurements reported by Perry and Hobbs. One particularly long-lived cloud is analyzed in detail, through both airborne measurement and numerical simulation, to track evolution of the cloud halos over the cloud's lifetime. Results of both observation and the simulation show that cloud halos tend to be broad at lower levels and narrow at upper levels, and broader on the downshear side than on the upshear side, broadening with time particularly in the downshear direction. The high correlation of clear-air turbulence distribution with the halo distribution temporally and spatially suggests that the halo forms, in part, due to turbulent mixing at the cloud boundary. Radiative calculations carried out on the simulated cloud and halo field indicate that the halo radiative effect is largest near cloud top during mature and dissipation stages. The halo-enhanced atmospheric shortwave absorption, averaged over this period, is about 1.3% of total solar absorption in the column.

Heavy residues and intermediate-mass fragment production in dissipative197Au+86Krcollisions atE/A=35 MeV

Neutrons and charged light and intermediate-mass products from the ${}^{197}\mathrm{Au}{+}^{86}\mathrm{Kr}$ reaction at ${E}_{\mathrm{lab}}/A=35 \mathrm{MeV}$ were measured with $4\ensuremath{\pi}$ angular coverage, in coincidence with projectilelike (PLFs) and targetlike fragments (TLFs). The characteristics of PLF and TLF yields and their correlation with neutron, light-charged particles, and intermediate-mass fragments (IMFs) are consistent with a scenario where essentially all collisions proceed through a dissipative stage. It is found that the damping of the available kinetic energy into thermal excitation is essentially incomplete, with no positive evidence for complete damping. Slow, massive reaction products are observed with significant yields. These products, termed also heavy residues, are identified as TLF evaporation residues and as TLF fission fragments. The TLF fission mode is seen to fade away in favor of TLF evaporation residue production for excitation energies above 3--4 MeV/nucleon. The suppression of the TLF fission can be traced to the dynamical IMF production process which reduces considerably the sizes of the primary reaction fragments and, hence, their fissility.

Microscopic contact charging and dissipation

We investigated elementary processes of microscopic contact charging and charge dissipation on and in thin silicon oxide using a reproducible and controllable contact charging method. As a result, we found that negative charges have three stages of contact charging and charge dissipation, while positive charges have only one stage. By continuing with the contact charging, negative charges became high density, i.e. a solid phase of charges, on the silicon oxide surface at room temperature in air. Furthermore, we succeeded in imaging point charges on n +-GaAs(110) cleaved surfaces with atomic resolution using a modified non-contact atomic force microscope.

Interpretation of incomplete dissipation data from piezocone tests

The piezocone has been widely used as a tool for identifying and profiling soft soils, but it can also be used to determine the coefficient of consolidation of cohesive deposits. Conventional methods of interpretation require, however, that pore pressure measurements during the dissipation stages of a test are carried on until a very large proportion of the excess pore pressure has decayed, and are thus generally time-consuming. This paper reports a faster method, which uses shorter, incomplete dissipation data.

Towards an elementary theory of drainage basin evolution: II. A computational evaluation

Numerical solutions to a recently-introduced family of continuous models provide realistic representations of the evolution of fluvial landscapes. The simplest subfamily of the models offers a characterization of the evolution of “badlands” as a process involving (1) a first, transient stage in which branching valleys emerge from unchanneled surfaces; (2) a second, “equilibrium” stage in which a fully-developed surface with branching valleys and ridges declines in a stable, self-similar mode; and (3) a final, dissipative stage in which regularities in the landscape break down. In the transient stage of development, small perturbations to the surface induce local variations in water flow, differential erosion, and the rapid emergence of a coherent, fine-scale structure of channelized flow patterns. The small-scale features evolve into larger scale features by a process in which small flows intersect and grow. Standard linearized analyses of the equations are inadequate for characterizing this process, which appears to be initially dominated by random effects and non-linear saturation. In the second stage, the numerical solutions converge towards satisfaction of an optimality principle by which the patterns of ridges, valleys, and surface concavities minimize a function of the sediment flux over the surface, subject to two constraints. This stage is in accordance with a theoretical analysis of the model presented in a previous paper, and the numerical solutions are stable in accordance with this analysis. The optimality principle is associated with both the emergence of separable solutions to the conservation equations and a variety of regularities in landscape form and evolution, including self-similar decline of forms and a “law of height-proportional erosion”. The numerical solutions provide detailed insight into the co-evolution of landforms and flows of water and sediment. The family of models provides an elementary theory characterizing the evolution of drainage basin phenomena, and in particular (1) possesses interpretations in terms of various geomorphological concepts and observations; (2) appears capable of explaining variations in geomorphic forms over a wide variety of environments; and (3) unifies certain aspects of the continuous, discrete, and variational approaches to landscape modeling.

The Nature and Evolutionary Status of GRO J1744–28

AbstractGRO J1744–28 is the first known X-ray source to display both bursts and periodic pulsations. This source may thus provide crucial clues that will lead to an understanding of the differences in the nature of the X-ray variability among accreting neutron stars. We deduce that the magnetic field of the neutron star is relatively weak (~ 8 × 1010G) but, nevertheless, sufficiently strong to funnel the accretion flow onto the magnetic polar caps and suppress the thermonuclear flashes that would otherwise give rise to type I X-ray bursts. We also present a series of interrelated arguments which demonstrate that the observed bursts are of type II and probably result from an instability associated with the interaction of the neutron-star magnetic field with the inner edge of the accretion disk. From these results, we infer that X-ray pulsars, GRO J1744–28, the Rapid Burster, and the type I X-ray bursters may form a sequence of possible behaviors among accreting neutron stars, with the strength of the magnetic field serving as the crucial parameter that determines the mode of X-ray variability. The companion star in the GRO J1744–28 binary system is probably a very low-mass (~ 0.2M⊙) giant that is in the final stages of losing its hydrogen-rich envelope. We have carried out binary evolution calculations which show that (1) if the mass of the giant was ~ 1M⊙when mass transfer onto the neutron star commenced, then the orbital period and the core mass of the giant have increased from ~ 1 to ~ 11.8 days and from ~ 0.15 to ~ 0.21M⊙, respectively, during the mass-transfer epoch, which has lasted for ~ 8 × 108yr, (2) the present long-term average X-ray luminosity is ~ 4 × 1036ergs s−1, which is at least two orders of magnitude lower than the luminosity at the peak of the transient outburst, and (3) the predicted long-term equilibrium rotation rate of the neutron star is remarkably close to the observed pulse rate. The transient nature of GRO J1744–28 may well be related to the final stages of dissipation of the envelope of the giant companion.

Asymptotic states of decaying turbulence in two-dimensional incompressible flows.

We investigate the relaxation of a strongly turbulent fluid to metastable states, in times much shorter than the dissipation time scale. Several simulations of decaying two-dimensional Navier-Stokes flows were performed, which show the relaxation to organized states dominated by coherent vortex structures of length scales comparable to the size of the system. In the case of periodic boundary conditions, the organized state is characterized by a strong correlation between vorticity and stream function, the second of which satisfies a sinh-Poisson equation quite accurately. However, in the case of free-slip boundary conditions the relaxed state does not display any significant correlation between its vorticity and its stream function. Notwithstanding, in both cases the role of nonlinearities is found to be essential even at these late stages of the evolution. However, we show that even severe truncations of a few (short wave number) nonlinearly coupled Fourier modes provide an accurate description of the long-term dynamics of the fluid. Therefore the dynamics of the flow in these metastable states is somewhere in between a strong turbulent regime and a (mostly linear) dissipative relaxation stage. \textcopyright{} 1996 The American Physical Society.

Updraft Evolution: A Perspective from Cloud Base

A Queen Air, instrumented to make 1-Hz measurements of the kinematic, dynamic, and thermodynamic fields, and radar, mesonet, and soundings from the Cooperative Convective Precipitation Experiment 1981 is used to monitor the evolution of the updraft at cloud base of a large cumulus congestus over the High Plains. The environment is characterized by modest instability and strong horizontal wind shear. Twelve passes completed by the Queen Air just below cloud base from the late growth to the dissipation stage reveal that the main updraft splits into two with the south updraft rotating cyclonically and moving to the right of the mean winds. This cell is associated with a pressure perturbation in excess of 1 mb that is most likely caused by the interaction of the updraft with the shear of the horizontal wind. Saturation-point analyses of the updraft and the subcloud layer demonstrate that in the early stages air from near the surface ascended into the cloud, but as the cloud ages, air from the upper subcloud and transition layers contributes to the updraft. This air has little or no buoyancy, which loads to cloud collapse. Mass flux and saturation-point analyses predict the cloud's demise adequately, in contrast to the trends of vertical velocity, virtual potential temperature, or moisture at cloud base. A pressure perturbation caused by updraft–shear interaction is an important mechanism for cloud intensification, but it must act in concert with another forcing mechanism, typically a gust front, to tap the most unstable air found in the lower subcloud layer in the High Plains. The observations support the numerical simulations of cumulonimbi in the presence of strong shear, albeit for a much smaller cloud. Congestus clouds merit attention as they are suitable targets for a variety of platforms and will lead to a more complete understanding of the convective cloud spectrum.