Buoyant Air Research Articles

The Development of Organized Convection In A Simplified Squall‐Line Model

AbstractSquall lines stand out from ordinary cumulonimbus convection because of a special structure that dramatically extends their duration, precipitation output and area of influence. Time‐dependent models have identified surface outflow boundaries and low‐level environmental wind shear as the crucial elements in squall‐line formation and/or maintenance. Steady‐state models have shown how properties of the downdraught reservoir, or ‘cold pool’, must be matched to the far‐field wind and thermodynamic profiles to sustain the convection. However, the dynamical role of the environmental shear and the nature of the interaction between the positively and negatively buoyant air have remained uncertain. For help with these issues, a time‐dependent numerical model is developed with all moist processes either parametrized or based on ad hoc assumptions. the simplifications make it possible to distinguish more clearly than in the past between the formation and maintenance of squall lines and between the initial and disturbed far‐field environments.The convective updraught forms within an expanding region of disturbed flow separated from the undisturbed environment by ‘storm fronts’. Forced (or neutral) ascent occurs at a surface ‘gust front’ on the upwind side of the cold pool. We distinguish squall lines from ordinary convection by looking at the coherence between the forced and convective updraughts. It is found that subsidence over the cold pool disorganizes the updraughts, whereas a deep overturning circulation downshear from the gust front has an organizing effect. Hence, factors which allow the initial subsidence to occur ahead of the gust front, namely contrasting lower‐and upper‐level winds and large potential buoyancy, favour squall‐line development. It is argued that, in cases with large convective potential energy, the deep mesoscale circulation is more important in the formation and maintenance of the line than either the vertical shear of the air reaching the cold pool or the strength of the forced updraught.

Instrumented Aircraft Observations of the Katabatic Wind Regime Near Terra Nova Bay

Abstract Two aircraft missions to sample the boundary layer dynamics associated with the intense katabatic wind regime at Terra Nova Bay, Antarctica were flown on successive days in early November 1987. Light winds averaging 5 m s−1 were monitored at the 170 m flight level over the interior of the ice sheet. Dramatic acceleration of the airflow and abrupt 5°–7°C cooling were encountered on both days near the head of Reeves Glacier just upslope from where the terrain steepens considerably. These results suggest that much of the airflow convergence which sustains the coastal katabatic winds is forced by localized topographic channeling into Reeves Glacier, and that the descending airstream is negatively buoyant. The horizontally propagating katabatic winds were followed for 250 km directly offshore and for 200 km southward parallel to the Victoria Land coast the airstream momentum gradually decreased along both flight paths. In conjunction with the descent of negatively buoyant air down Reeves Glacier and h...

Surface winds over the Antarctic continent: A review

Surface winds over Antarctica, often classified as “katabatic,” are intimately related to the orientation and steepness of the underlying ice terrain. Such flows result primarily from the strong radiational cooling of the sloping ice surface which creates a downslope‐directed horizontal pressure gradient force. It is possible to diagnose the time‐averaged, near‐surface streamline patterns of cold air drainage currents using recently compiled, detailed Antarctic ice topography maps and appropriate estimates of the strength of the surface temperature inversion. The resulting streamlines agree with available observations and illustrate the high degree of nonuniformity of gravity‐driven flows off the ice sheets. In certain interior sections, negatively buoyant air from a large catchment area drains through a restricted section of coastline. Such confluence zones represent regions of enhanced supplies of cold air and are shown to be related to anomalously strong katabatic winds. Implications for sea ice formation are discussed.

Similarity structure in the convective boundary layer of a lake

Natural convection influences a variety of geophysical phenomena, including daily mixing cycles in the atmosphere and in surface waters, the seasonal overturn of lakes, and global climate variability. Conceptual models of convection in water are primitive and based on very limited data, particularly at small, dissipative scales. Here, I present an analysis of microscale data from the convective boundary layer of a lake, indicating a complex depth-dependence in the rate at which temperature fluctuations are dissipated. Suitably scaled, this vertical structure is remarkably similar to that found in the atmospheric convective boundary layer1,2, stirred by rising 'thermals' of buoyant air heated at the Earth's surface. This comparison, consistent with recent documentation of similarity in the more uniform vertical structure of energy dissipation3–5, suggests another unifying link in the structure and dynamics of convection in boundary layers of lakes, oceans and the atmosphere6.

Relationships Between Radar-Derived Thermodynamic Variables and Tornadogenesis

A methodology for obtaining pressure perturbations and buoyancy in thunderstorms observed with Doppler radar is described and applied to two tornadic thunderstorms. Extracted thermodynamic information is combined with kinematic analyses to study observed severe storm processes. The intensification of tornado parental circulations (mesocyclones) during tornadogenesis is found to be associated with deepening pressure deficits in lower storm levels. Upward directed perturbation pressure forces in the vicinity of the mesocyclone are reduced and can be reversed as the low-level vorticity amplifies. The sudden formation of concentrated rear downdrafts, commonly observed in tornadic thunderstorms, apparently stems from the vertical pressure gradient reversal. Reduced upward pressure forces decrease the storm's ability to lift low-level negatively buoyant air at the base of the updraft. Further, the restructured pressure forces create a flux of air parcels into the mesocyclone from higher levels on the storms' rear. In the final stages, downdrafts fill the mesocyclone, and updrafts in neighboring regions weaken. Qualitative examination of retrieved buoyancy distributions suggests that horizontal vorticity created by buoyancy gradients in inflow regions is not essential for mesocyclone intensification or for tornadogenesis.

Swaying Motion of Buoyant Air Plume above a Horizontal Line Heat Source : Swaying Spectrum and Several Factors of Swaying Motion in Laminar Region

800×800×1000mm3と比較的大きな容器内の水平線熱源からの空気プルームの層流域内の揺動スペクトルおよび揺動諸因子が実験的に研究された.層流域内の温度波形は不規則であって,スペクトル解析の結果,PSD∝f-4.5,PSD∝f-8.0(ここにPSD,fはパワースペクトル,振動数)であること,揺動振動数は加熱量の1/3乗に比例すること,および波形の特徴などの揺動議因子が明らかにされた.

Perturbation Pressure and Cumulus Convection

Abstract A simple model of perturbation pressure in cumulus convection is presented. The results show that the buoyancy- and drag-induced perturbation pressures act against the prescribed forcing. The dynamic pressure is found to be a consequence of the Bernoulli effect and a balance for the centrifugal force which arises from the curved motion of the air. Quantitative calculations reveal that the pressure force is of the same order of magnitude as the buoyancy and drag and offers a plausible explanation for the acceleration of negatively buoyant air near the base of convective updrafts.

A Numerical Investigation of the Severe Thunderstorm Gust Front

The structure of the thunderstorm gust front is investigated by a nonhydrostatic, two-dimensional (x,z) numerical model. In the model, which is dry, the production of negatively buoyant air by evaporation is parameterized via an externally imposed, local-cooling function. This parameterization sustains a steady cold downdraft, which drives the surface outflow and associated gust front. It is shown that two dominant factors influencing gust front structure in the vertical plane are the solenoidal field coincident with the front and surface friction, modeled by means of a simple bulk aerodynamic drag formulation. The circulation theorem is invoked to illustrate how solenoidal accelerations oppose the deceleration by surface friction. After the onset of a downdraft in the model, these opposing tendencies soon reach a balance. Thus, following a brief transient stage, the model gust front exhibits a persistent configuration as it propagates rapidly forward. The essential features of this configuration are examined and compared with both tower observations of gust fronts and laboratory models of gravity currents.

The Effects of Release of Latent Heat on the Vorticity of a Tropical Storm over Land

The effect of the release of latent heat on the thermal vorticity of Hurricane Diane (1955) is examined when it passed over the east coast of the United States as a weakened tropical storm. A form of the thermodynamic energy equation is used as the diagnostic equation for this study. Because of uncertainty in the data, as well as incomplete physical understanding of the interaction, between synoptic and convective scales, several alternative models are used which employ different assumptions on the influence of convection and the vertical distribution of humidity and temperature. In agreement with the results of previous investigations, Diane was found to weaken after landfall as a result of the lifting of negatively buoyant air at the storm center. However, as the storm readjusted to its new environment the stability at the center again decreased. The latent heating, as represented by its horizontal Laplacian, then becomes effective in the maintenance and redevelopment of the storm.

Observational Investigations of Entrainment Within the Weak Echo Region

Abstract Data were obtained for three High Plains thunderstorms in which penetrations were made of the weak echo region by an instrumented aircraft. The data from one of the cases are presented in detail. Two of the storms were steady state, as revealed by chaff analysis and subsequent subcloud passes. The third storm dissipated during the penetrations. The three storms were each characterized by negatively buoyant air at cloud base. Chaff released into the updrafts of the storms did not decelerate below the level of free convection (LFC). A vertical pressure perturbation gradient, therefore, existed below the LFC and within the weak echo region which acted to accelerate the air parcels in the presence of negative buoyancy. The analysis of the equivalent potential temperature fields for the two steady storm cases revealed considerable entrainment of environment air into the weak echo region. The mixing of the entrained parcels probably caused the observed increase of turbulence with height.

Use of artificial subice air pockets by wild ringed seals (Phoca hispida).

Ringed seals were observed to inhale exhaust air expelled by SCUBA divers who were diving under 1.9-m thick, 1-year-old fast ice in Lancaster Sound during mid-April, 1973, The underside of the sea ice was imperfectly flat so that some of the bubbles of exhaust air coalesced into large lenticular bubbles 1 to 3 cm in depth, and these, on occasions, were partially inhaled by seals. Between dives, separated in time by as little as 3 h, all the under-ice air bubbles, large and small, vanished. It is assumed that the buoyant air migrated upward into the sea ice by the progressive displacement of brine in drainage channels. Although the exhaust air was readily used by ringed seals, it is unlikely that naturally occurring air pockets can persist beneath 1 -year-old sea ice to enable seals to extend their under-ice travels.

AN OBSERVATIONAL STUDY OF CUMULUS

Abstract An observational study of cumulus is made from time-lapse film records, and laws describing the growth of cloud thermals (discrete masses of buoyant air) are deduced. The diameter of isolated thermals emerging from the summits or on the flanks of a cumulus is found to have a well-defined upper limit which is a simple linear function of height. It is inferred that, in the saturated interior of a cloud, thermals broaden linearly with height incorporating into themselves the residues of earlier (now decaying) thermals. Their rate of broadening with height, 0.40 ± .04, is found insensitive to variations of the stability and humidity of the cloud layer and to the presence or absence of precipitation. From the observed broadening, the buoyancy of cloud thermals can be deduced if a knowledge of the properties of decaying thermal residues is given. A probable upper limit to thermal buoyancy is computed and is found to be some fraction of the adiabatic value. Observations of the rate of rise and the diame...

Experiments on convection of isolated masses of buoyant fluid

Isolated masses of buoyant fluid were released in a water tank. Their width, 2r, and the distance travelled, z, were measured as functions of time and were found to follow roughly the laws where w is the vertical velocity, the mean buoyancy, and n and C are constants. These equations are predicted by dimensional analysis, assuming viscosity to be negligible, and the constants appear to be independent of the Reynolds number. It is found that C [eDot ] 1·2 and n is in the neighbourhood of 4.Since the Froude number relating the buoyancy and inertia forces is the same as for isolated masses of buoyant air in the atmosphere, it is concluded that the constants will have the same value in this latter case. This is confirmed roughly by observation of cumulus cloud towers.Some of the characteristics of the motion observed in the experiments are described and comparison is made with vortex rings.

THE EROSION OF CUMULUS TOWERS

Abstract A differential equation is formulated for the rate of rise of isolated buoyant elements in the atmosphere, based in part on analogy with the work of Davies and Taylor (1950) who studied the ascent of air bubbles in liquid. The equation involves a drag force which is proportional to the ascent velocity squared and which depends upon a linear dimension of the buoyant element, such as the radius of curvature of its upper surface. To eliminate this dimension from the differential equation, a law for the erosion, or rate of decrease in size of these buoyant air bubbles has been proposed. The basic assumptions are that the elements erode sufficiently slowly so that the flow around them is nearly the same as if they were in equilibrium, and that they retain their shape. From combination of the erosion law and the equation of motion, an erosion parameter, E, is derived, which to the extent that the laws are valid should be constant for all isolated cloudy bubbles, regardless of air mass or location. The ...

DIFFUSION, ENTRAINMENT AND FRICTIONAL DRAG ASSOCIATED WITH NON-SATURATED, BUOYANT AIR PARCELS RISING THROUGH A TURBULENT AIR MASS

Abstract The entrainment equations of Houghton and Cramer have been extended to include the effects of diffusion of heat and momentum and the frictional pressure drag. The equations developed from these considerations show that certain combinations of parcel size, temperature difference, vertical velocity, and turbulence permit buoyant air parcels to pass from the bottom to the top of the turbulent ground layer. Other combinations lead to rapid dissipation of the parcel's temperature difference and velocity. The equations have been checked against three sets of observations within unstable, dry air masses. Observed properties of buoyant parcels existing in super-adiabatic atmospheres agree, within observational error, with combinations leading to extended survival of air bubbles as predicted by the equations. One set of data leads to the approximation of the value of the coefficient of pressure drag, 0.1, associated with an air parcel moving through the air.