Abstract

Rainfall collected for the squall line case of 21 August 1972 in the St. Louis area has been analyzed for certain pollutants known to be present in urban air. To aid in the interpretation of these measurements a two-dimensional time-dependent squall line model is utilized to approximate the internal air flow and rainwater distribution within the storm. At a particular time when a typical mature circulation is present, a steady state is assumed and a pollutant plume with dimensions and concentrations characteristic of the St. Louis urban plume is allowed to interact with the model thunderstorm circulation. The study focuses upon the effect of internal storm motion upon precipitation scavenging and treats microphysical processes in a relatively simple manner.

Two extreme situations are considered: 1) the pollutant does not interact with the water substance in the storm, but is merely redistributed by the storm circulation; and 2) all pollutant enters into cloud-water immediately upon entering the cloud boundary. In the second case, scavenging of pollutants by precipitation is calculated along with the deposition of pollutant on the ground in rainfall. The deposited amounts are compared with the limited number of measurements from the same storm. The trend and order of magnitude of deposition arrived at in the model compare favorably with the observations. Areas where additional observations are greatly needed are specified and desirable directions for model development are discussed.

Abstract

Description is given of a technique for determining optimal storm (reference frame) motion based upon application of a dynamic-retrieval method to velocity datasets derived from multiple-Doppler radar observations. The method depends upon the necessary consistency between the steady-state assumption and assumed reference-frame (storm) motion and uses the quantity E, from dynamic-retrieval calculations as a measure by which to judge when this consistency is best achieved. Application of the technique is demonstrated in case examples including an Oklahoma squall line, a Montana hailstorm, and an Oklahoma tornadic storm.

In the squall-line case the question of the dependence of optimal reference-frame motion upon analysis domain (e.g., convective versus stratiform regions of the system) is explored. Similar optimal frame motions for different regions of the system are found. Optimal frame motion corresponds more closely to cell motion than to line motion. In the Montana case the dependence upon analysis domain is again explored, and significant differences between a large domain and subdomain are found. Retrieved pressure compares favorably with independent below-cloud-base measurement of perturbation pressure by aircraft. It is shown that agreement between retrieved and observed pressure patterns is best when optimal reference-frame motion is assumed. In the tornadic-storm case, optimal frame motion is very similar to storm motions derived from reflectivity-core tracking and from numerical simulation of this storm. Investigation of the question of height variation of optimal reference-frame motion is investigated and found to be influenced by, but not completely dependent upon, both the environmental winds and mean in-storm air motion. The notion of using optimal reference-frame motions as a basis for adjustment of nonsimultaneous Doppler radar observations to a common reference time is discussed.

Abstract

The structure and mechanism for maintenance of the Great Plains squall line thunderstorm are studied through formulation of a two-dimensional, time-dependent numerical model. The environmental conditions known to be favorable for squall line development and maintenance include a convectively unstable air mass whose motion is characterized by strong vertical shear of the horizontal wind. These conditions are used to specify an environment in an x-z plane upon which a disturbance is superimposed. The appropriate physical equations are integrated forward in time to study changes in the motion, thermal and moisture fields in and around the squall line thunderstorm.

The vertical shear of the horizontal environmental wind is varied from one experiment to another with the result that broader and longer lasting cloud circulations occur in the stronger shear cases. Specific areas where three-dimensional effects must be important are discussed from an examination of variable fields during periods when the system undergoes a lessening in intensity. It is found that the system, rather than reaching a quasi-steady state, undergoes a series of developments (three or four during a 100-min period) as measured by the time variation of maximum updraft speed, downdraft speed, and rainwater mixing ratio. However, the system structure during the intense stage of each development is basically the same as that during the other developments, and strongly resembles the structure envisaged in qualitative physical models suggested in the past: 1) the updraft and downdraft exist side by side, the updraft possessing an upshear tilt from the vertical through the lower half of the troposphere; 2) rain produced in the updraft falls into the downdraft, strengthening or maintaining the downdraft due to its own weight and through negative buoyancy produced by evaporation; and 3) maintenance of the downdraft results in a strongly convergent region in lower levels downshear from the system and a tendency toward updraft maintenance or redevelopment. The implication is that the squall line thunderstorm, once initiated, maintains itself by interaction with its synoptic environment as long as it remains within an environment containing convectively unstable air whose motion is characterized by moderate-to-strong vertical shear.

Abstract

The trajectories of hypothetical dropsondes are calculated in thunderstorm circulations and resultant vertical wind speed profiles as a function of height are constructed for each of the sondes. Motion fields are a) calculated by a time-dependent two-dimensional thunderstorm model and b) constructed based upon observed environmental winds. Model-calculated vertical wind speed profiles are compared with observations for the northeastern Colorado storm of 22 July 1972. Agreement is shown between certain basic features; additionally, other calculations point to various potential features of dropsonde trajectories and vertical wind speed profiles. Possible application of similar methods to the hail growth problem is discussed.

Abstract

The dynamics of a mesoscale convective system that matured in central Kansas on 3–4 June 1985 is investigated based upon data from ground-based dual-Doppler radar and other sources. The system was distinctly three-dimensional, evolving to a wavelike shape owing to the intersection of two convective bands. The two bands, one oriented north–south and the other east-north–west-southwest, are compared and contrasted with respect to their velocity, pressure, and buoyancy structure and the consequent attributes of their momentum transports and budgets.

Dynamic retrieval of pressure and buoyancy from the wind fields provides insight into system structure and allows for the calculation of pressure gradients needed for the horizontal momentum budget. Several independent checks are carried out to ensure the quality of the pressure solution. The solution is found to be more accurate if the velocity time derivatives are included in the retrieval process. Dissimilar structure of the two bands is highlighted by a reversal of the low-level pressure gradient in the line-normal direction that can be related to the presence of a baroclinic zone along the more northern band.

In both convective lines momentum fluxes at all levels are negative in the line-normal direction and positive along the line in agreement with the results of past studies. Line-parallel fluxes are comparable in magnitude to line-normal fluxes owing to the strong line-parallel shear. The calculated wind component tendencics resulting from the line-normal momentum budget for the north–south line include increases in rear-to-front momentum, at all altitudes, that is most pronounced at low and high levels, similar to results of past studies of lines oriented normal to the environmental shear. For the northeast–southwest line, increases in front-to-rear momentum are found at all levels except for a thin layer near the system top. This difference stems from a pronounced low-level pressure decrease toward the rear of the line.

In the north–south line, budget calculations show an increase in along-line momentum with time at low and high levels and a decrease at midlevels. The results for the northeast–southwest band are quite different in that a decrease with time in along-line momentum is calculated at all levels, with a pronounced decrease at high levels owing to outward flux through the downwind boundaries. Along-line pressure gradients are quite weak in both convective bands. The significant influence of the horizontal flux divergence is attributed in part to the three-dimensional character of the system. The momentum budget results for each band are discussed in relation to maintenance of trailing stratiform precipitation, provision of favorable environments for severe weather, and their potential effects upon system evolution.

Abstract

This study presents analyses of data collected in the vicinity of a cloud-free dryline that occurred in western Oklahoma on 24 May 1989. Observations reveal sharp contrasts across the quasi-stationary, north-south dryline during midafternoon. Of greatest significance is a pronounced gradient of virtual potential temperature, although horizontal convergence and vorticity also maximize at the dryline.

The environment of the 24 May dryline is dominated by vertical mixing that maintains a convective boundary layer (CBL) on both sides of the dryline. The dryline resembles a “mixing zone” containing varying proportions of hot, dry air to the west side and warm, moist air from the lowest 200 m within 10 km to the east of the dryline. The mixing zone slopes eastward from the surface dryline location, then becomes a quasi-horizontal elevated moist layer above the CBL east of the dryline. Saturation-point analysis indicates that the mixing zone is characterized by a single mixing-line structure defined by the respective quasi-homogeneous air masses on either side of the dryline.

Dynamical analysis reveals that near-surface westerly flow is accelerated upward and over relatively cool air above the surface by an elevated low pressure region at the dryline. Flow accelerations are nonhydrostatic at the dryline, while the flow is in hydrostatic balance both to the west and to the east of the dryline. Magnitudes of the inertial, pressure, and Coriolis accelerations are comparable to the east of the dryline, implying a considerable ageostrophic flow component as well as a quasigeostrophic linkage between the low-level jet and the west-east horizontal pressure gradient.

Abstract

A method for retrieval of pressure and buoyancy distributions in deep convection is applied to Doppler radar data collected at two analysis times during the tornadic Del City (Oklahoma) thunderstorm of 20 May 1977. Change of a previous version of the technique, necessitated by application to real data, include procedures for handling irregularly-bounded volumes and missing data and new assumptions to include reflectivity data and turbulent effects in the equations. Internal consistency cheeks on the quality of retrieved pressure fields imply that the input data are generally of good quality and point out times and heights within the storm at which greater confidence can be placed in the derived fields.

In the pretornadic stage the pressure distribution includes at each level a high–low couplet across the updraft with the maximum pressure gradient generally oriented along the environmental shear vector at that altitude. These results are in agreement with predictions of linear theory. Locations of vorticity maxima and areas of updraft development are also discussed in relation to pressure distributions. The buoyancy distribution includes a good correspondence between positive buoyancy and updraft areas. An analysis of the individual terms in the buoyancy equation reveals the importance of advective and vertical pressure gradient terms over water-related and turbulence terms.

In the tornadic stage the pressure field includes a pronounced minimum at low levels coincident with the mesocyclone. An analysis of the factors influencing the pressure distribution reveals that strong low-level vertical vorticity produces this minimum. Vorticity, vertical motion, and pressure relationships in the low-level mesocyclone region tend to agree quite well with results of recent fine-scale numerical simulations as well as with the observationally-based finding of others. The low-level buoyancy field, although noisier at this stage, tends to support the line of reasoning which stress the production of horizontal vorticity as a major factor in low-level mesocyclone development.

Abstract

A method is presented for obtaining temperature and pressure perturbations within convective clouds using detailed in-cloud motion data as input. Initial testing of the iterative method indicates that it converges to a solution consistent with the input motion field. Potential applications of the method are discussed.

Abstract

Mechanisms for maintenance of the strong convection along the leading edge of a broad squall line that occurred in Oklahoma on 19 May 1977 are investigated. The findings are based upon analysis of data from a surveillance radar, a surface mesonetwork, Doppler radars, proximity soundings and aircraft data, and upon the results of a two-dimensional, cloud-scale numerical simulation. The detailed results of the multiple Doppler analysis are contained in the Part I paper reporting results of research on this squall line.

It is found that at a preferred location along the squall line, an area of intense convection is maintained over a long time period. A meso-β scale organized structure, which includes an area of low pressure near the southeast edge of the intense convection and an associated area of convergence extending to the east, promotes the formation of small showers in short line segments. These showers, due to their differing motion from elements within the main line, merge with the line to the north of the mesolow, resulting in maintenance of the strong area of convection. The observed meso-β structure on this day is believed to be made possible by a deep low-level layer of weak vertical wind shear and high water-vapor content.

At other locations along the line, the numerical simulation indicates an unsteady behavior in the maintenance of squall line convection by gust frontal convergence. Perturbations in the vertical motion field are periodically initiated by either (i) enhanced convergence at the gust front resulting from diverging downdrafts at locations farther to the west, or (ii) Kelvin-Helmholtz instability produced at the gust front head. These perturbations move westward relative to the gust front above the low-level cold air and periodically invigorate the main region of updrafts located a few tens of kilometers west of the gust front. Low-level updrafts, forced by diverging surface outflow from weak downdrafts, occasionally interact with the translating perturbations to increase their amplitude. The existence of the westward-moving perturbations is tentatively substantiated by the presence of similar structures in the analyzed Doppler wind fields. Greater time resolution in Doppler data, in combination with more comprehensive surface and upper air data ahead of squall lines of this type, would aid in confirming the reported structures.

The dryline is recognized as a major factor in the initiation of severe thunderstorms in the central and southern plains of the United States during the spring. Although severe thunderstorm forecasters often use the strength and position of the dryline to help determine prime areas for convective development, relatively little is known of the exact mechanisms by which thunderstorms form in the dryline environment. In the spring of 1991 experiments were carried out to study the dryline and convective storms near the dryline as part of the Cooperative Oklahoma Profiler Studies program, which was supported by the National Oceanic and Atmospheric Administration, the National Science Foundation, and the National Aeronautics and Space Administration. Observing systems deployed in these experiments included a research aircraft equipped with both in situ instrumentation and a Doppler radar, two mobile laboratories capable of remote release of rawinsondes, a surface mesonetwork, the Profiler Demonstration Network, and several ground-based Doppler radars. Among the episodes intensively observed during the period were several in which tornadic storms formed in the dryline environment. The goals of the dryline experiments are described. The key weather events and observing strategies are summarized for four of the cases. Primary issues that can be addressed in future in-depth studies using these datasets are noted and preliminary findings from analyses done to date are included.