Abstract

Aircraft measurements at low- and mid-cloud levels near two isolated and persistent Great Plains thunderstorms concurrently scanned by radar are analyzed to determine the kinematic properties of the near-cloud air flow, the moisture budget, and the dynamical interactions between the cloud and its surroundings. Time variation in velocity divergence, relative vorticity, and moisture flux convergence in the subcloud layer relate well to changes in storm development and translation. Aircraft winds and radar chaff trajectories substantiate the premise in some models that mature thunderstorms, moving more slowly than ambient winds, divert and distort mid-tropospheric air motion in a manner similar to solid obstacles in relative streaming flow. Observed ingestion of mid-tropospheric air motion tracers demonstrates that, at the same time, internal circulations aloft are not entirely insulated from the environment.

A graphical synthesis of three-dimensional air flow within and around a typical Great Plains cumulonimbus is presented, accommodating concepts in earlier models with the resolved circulation features. The involvement of cool, dry middle-level air in the internal circulation and its role in maintaining downdrafts are discussed. In considering factors influencing storm movement, it is concluded that, in addition to propagative mechanisms, hydrodynamical drag and deflection forces acting at cloud boundaries may play a significant role in determining a thunderstorm's preferred path with respect to the mean winds.

Abstract

In typical squall-line situations wherein the wind veers strongly with height, individual convective storms move as much as 60 deg right or 30 deg left of the direction of the mean wind in the cloud layer. It is shown that, on the average, the radar echoes having largest diameters move farthest to right of the wind.

This behavior is consistent with physical considerations and with supply-and-demand requirements of the storm water budget. For a given rainfall intensity, the amount of water precipitated by a storm is proportional to its area or to the diameter squared. The amount of water vapor intercepted is proportional directly to the diameter, and to the velocity of the storm relative to the winds of the lower-tropospheric moist layer. A large storm must intercept more vapor in proportion to its diameter than a small one, requiring a larger migration velocity relative to the moist layer. This requirement is satisfied if (wind veering with height) large storms move toward the right of the mean wind.

Based on these considerations, a simple expression is derived for the direction of storm motion as related to storm diameter. This describes the mean behavior fairly well, but there is considerable residual scatter. With this taken into account, an expression is given for the probability of storm passage over a given point as related to the initial storm location and size.

Some characteristic patterns of development are illustrated. New convective elements tend to form on or amalgamate with the right-hand side or end of an existing storm cluster or squall line, somewhat on the up-wind side relative to the mean wind. This pattern of generation contributes to the movement of large storm clusters strongly toward the right of the winds, and also to make large storms move consistently more slowly than small ones.

Abstract

The evolution of the vertical mass flux in isolated cumulus and cumulus congestus clouds is documented using two King Airs during the Convection and Precipitation/Electrification Experiment (CaPE,), conducted in east-central Florida during the summer of 1991. These clouds develop over the sea-breeze convergence in an environment characterized by low shear and moderate convective available potential energy. The aircraft, separated vertically by 600–1000 m, commence flying near-simultaneous penetrations as the cloud top passes through the altitude of the upper aircraft and continue sampling until the cloud disappears. The net vertical mass flux for each level is estimated; the difference in the mass flux between the two levels leads to a diagnosis of the net entrainment or detrainment that occurs laterally in the intervening cloud layer. This kinematic technique relies on determination of cloud edge based on liquid water measurements, a cloud shape factor, and the vertical velocity. The technique is not limited to the period prior to precipitation fall out like most conserved variable techniques nor does it require accurate measurement of in-cloud total water and temperature.

Results from 12 isolated clouds with radii of 0.5–1.5 km and lifetimes less than 25 min demonstrate that the vertical mass flux evolves in a well-behaved manner with clear growth and decay phases. Significant net lateral entrainment or detrainment is diagnosed, which complements the top entrainment that has been inferred from conserved thermodynamic variable techniques. Net entrainment dominates the growth stage, whereas net detrainment is most often seen during the decay stage of the cloud. An approximate entrainment rate averaged over cloud life is 1 km⁻¹. The continuous nature of the updrafts demonstrates that these small clouds are best described as a single shedding thermal, not a series of bubbles ascending in the wake of each preceding bubble.

Abstract

Aircraft distance-measuring-equipment (DME) data are used to update position, velocity, and wind measurements from inertial navigation systems (INS) measurements. Data from conventional single-channel DME sets, suitably calibrated, are shown to be adequate to resolve the Schuler oscillation and correct INS positions to better than 1-km accuracy. The satellite-based NAVSTAR global position system (GPS) is rapidly superseding other systems for external position reference. However, DME is reliable and very accurate and has been recorded on many research datasets. The principal limitation of the DME is that it is restricted to land-based navigation. The regression technique used does not necessitate multiple DME receivers or station switching and involves few restrictions on the collection of the data. However, the results improve when more than one station is used. Comparisons with other navigation systems (interferometer and loran) demonstrate the method's skill in resolving INS errors. Intercomparisons among several research aircraft flying in close formation support the method's usefulness in correcting biases in INS data.

Abstract

Perturbation pressure fields are measured by aircraft around the cloud base updrafts of seven clouds ranging in size from weak cumulus congestus to intense cumulonimbus during CCOPE (1981). The fields are characterized by a high-low pressure couplet of similar size to the updraft, but a quarter-wavelength out of Phase, with the minimum pressure downshear of the updraft maximum. An estimate of the terms in the Poisson equation for pressure show that the pressure perturbation results chiefly from the interaction of the updraft with the vertical shear of the environmental horizontal wind. The behavior of the pressure oscillation is well predicted by inserting sinusoidal functions in the corresponding terms in the Poisson equation. The amplitude of the pressure oscillation is proportional to the wavelengths of the pressure and vertical-velocity fields, the amplitude of the vertical-velocity oscillation, and the vertical shear of the horizontal environmental wind through cloud base, measured in the direction of the maximum pressure gradient.

Abstract

The initiation of thunderstorms is examined through a combined observational and modeling case study. The study is based on Doppler radar, aircraft, mesonet, balloon sounding, and profiler and photographic data from the Convection Initiation and Downburst Experiment (CINDE) conducted near Denver, Colorado. The study examines the initiation of a line of thunderstorms that developed along a preexisting, quasi-stationary boundary-layer convergence line on 17 July 1987. The storms were triggered at the intersection of the convergence line with horizontal rolls where enhanced updrafts were present. The primary effect of the convergence line was to deepen the moist layer locally and provide a region potentially favorable to deep convection. The critical factor governing the time of storm development was apparently related to the attainment of a balance between horizontal vorticity in the opposing flows on either side of the convergence line. The effect was to cause the updrafts in the convergence line to become more erect and the convergence zone deeper, as discussed theoretically by Rotunno et al. Modeling results for this case also indicated that storm initiation was very sensitive to the depth of the convergence-line circulation. Storm initiation also frequently coincided with the location of misocyclones along the convergence line. Model results suggested this was because both events were caused by strong updrafts. The misocyclones resulted from stretching of existing vorticity associated with the convergence line. They tended to form where a convective roll intersected the convergence line leading to a local maximum in convergence and vertical motion. Some misocyclones suddenly deepened and strengthened when they became collocated with the deep, intense updraft of a convective storm. The updraft was responsible for advection and stretching of the vertical component of vorticity, leading in the most intense cases to the development of nonsupercell tornadoes, as discussed previously by Wakimoto and Wilson.

Abstract

An analysis of the seeding operations during the National Hail Research Experiment 1972–74 randomized seeding program is carried out for the purpose of critiquing the seeding procedures and establishing the actual rates at which seeding material was dispensed as opposed to the prescribed rates. The seeding coverage, a parameter defined in the paper, is found to be only about 50% on the average. The reasons for the low seeding coverage are discussed in terms of seeding logistics and storm evolution, and three case studies are presented to illustrate the problems that can arise. Some results on the rate at which storm cells can develop and on the duration of convective activity over a fixed target area are presented. It is concluded that seeding convective clouds using aircraft flying near cloud base is more difficult than is widely acknowledged.

Since the seeding operations were more thorough on some days than on others, one might reasonably expect that seeding effects, if they exist, would be more marked on the days with the higher coverage. Post hoc analyses that stratify the surface hail and rain data according to seeding coverage are presented. The results do not allow one to reject the hypothesis that seeding had no effect on surface precipitation.