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 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 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

A three-dimensional thermodynamic retrieval method has been developed and tested for application to deep convective clouds. To test the accuracy of the method and for sensitivity studies, output from a three-dimensional numerical cloud model has been utilized in place of observations. Input to the method are wind component and liquid water fields and basic output variables within the same volume are the deviation of potential temperature and perturbed pressure from their respective horizontal averages. The derivation of the retrieval equations from the momentum equations and the programming of these equations is shown to be correct by comparison of the retrieved fields with output from the numerical model. Other cases test the sensitivity of the retrieved result to inadequacies potentially present in observed (Doppler radar) wind and water fields. Tests are carried out examining the problem of time resolution in the observed data, possible inadequacies in observation and parameterization of turbulence, and accuracy in the measurement of liquid water fields. In other experiments velocity perturbations are added to the input velocity fields to simulate very crudely the errors which might be present on various scales in observed fields, and the sensitivity of the retrieved fields to these errors is assessed. Filtering the result is shown to be effective if the predominant scale within which input errors occur is known.

Abstract

Through a case study approach the motion of a dryline (on 16 May 1991) within a synoptically active environment in the southern plains, along which severe storms ultimately developed, is examined in detail. Observations from research aircraft, surface mesonetwork stations, mobile ballooning vehicles, radar, wind profilers, and operational surface and upper air networks are examined and combined. Additionally, output from the operational mesoscale Eta Model is examined to compare predictions of dryline motion with observations and to aid in interpretation of observations.

The dryline on this day advanced rapidly eastward and included formation of a bulge; additionally, in at least two instances it exhibited redevelopment (loss of definition at one location and gain at another). Aircraft observations revealed that an eastward redevelopment occurred in the early afternoon and was characterized by a series of four “steps” along the western edge of the boundary layer moisture. The westernmost and easternmost steps coincide with the locations of the dryline before and after redevelopment, respectively. The retreat of the dryline in the central and southern portion of the analysis domain in the late afternoon included both continuous motion and redevelopment toward the west-northwest. This dual-mode retreat of the dryline was accompanied by gradual backing of the winds and moistening in low levels.

The Eta Model forecast initialized at 1200 UTC produced dryline features that were qualitatively similar to observed fields. The eastward motion of a broad area of enhanced moisture gradient agreed well with observations following an initial spinup period. A north–south moisture convergence axis preceded the rapid eastward motion of the dryline by several hours. Lack of subsidence in the air behind the modeled dryline leads to the conclusion that processes other than downward transfer of horizontal momentum by larger-scale motions (that would support eastward advection) produced the rapid dryline motion and observed eastward dryline bulge. Results of diagnosing physical processes affecting model dryline motion point toward boundary layer vertical mixing coupled with advection of dry air aloft as vital components in rapid advance of the dryline eastward in this synoptically active case.

Abstract

Long-lived thunderstorms were initiated during the afternoon of 26 May 1991 ahead of a dryline in northwestern Oklahoma. Various reasons for initiation in this particular along-dryline location are investigated through analysis of observations collected during the Cooperative Oklahoma Profiler Studies—1991 field program. Observing systems included in situ and radar instrumentation aboard a research aircraft, soundings from mobile laboratories, a mesonetwork of surface stations, meteorological satellites, and operational networks of surface and upper-air stations.

Elevated moistening east of the dryline revealed by soundings and aircraft observations in combination with thermal plume activity was apparently insufficient to promote sustained convection on this day without aid from an additional lifting mechanism. Satellite observations reveal scattered convection along the dryline by midafternoon and a convective cloud line intersecting the dryline at an angle in the area of most pronounced storm initiation, extending southwestward into the dry air. Another prominent feature on this day was a mesoscale bulge along the dryline extending northeastward into southwest Kansas. Deep convection was initiated along this bulge, but was in general short-lived.

Potential causes of the lifting associated with the cloud line that was apparently key to the preferred location for storm development in northwest Oklahoma were investigated: (a) a mesoscale circulation resulting from horizontal differences in radiative (temperature) properties of the underlying surface and (b) upward motion induced by an upper-level mesoscale disturbance. Analysis of vegetative and surface temperature distributions from satellite observations suggests a potential (more research is needed) link between surface characteristics and the development of the dryline bulge and observed cloud line through horizontal differences in vertical momentum transport. A run of the currently operational eta model indicates some skill in predicting dryline location and motion and predicts upward motion in the northern part of the region that was generally more convectively active, but shows no indication of upper-level support in the vicinity of the observed cloud line.

Abstract

A dryline that occurred on 16 May 1991 within a synoptically active environment is examined in detail using research aircraft, radar, surface, satellite, and upper air observations. The work focuses on multiple boundaries in the dryline environment and initiation of tornadic storms in two along-line areas.

Aircraft measurements in the boundary layer reveal that both the east–west extent of moisture gradients and the number of regions containing large moisture gradients vary in the along-dryline direction. Aircraft penetrations of thinlines observed in clear air return from radar reveal that all thinlines are associated with convergence and a moisture gradient, and that more distinct thinlines are associated with stronger convergence. However, significant moisture gradients are not always associated with either thinlines or convergent signatures.

Convective clouds on this day formed at the dryline rather than significantly east of the dryline. The three thunderstorm cells that occurred in east-central Oklahoma developed along a 20-km section of the dryline south of a dryline bulge and within a 30-min period. The storms appear to have developed in this location owing to enhanced convergence resulting from backed winds in the moist air in response to lowered pressure in the warm air to the northwest. Aircraft measurements in the boundary layer and satellite-sensed surface temperature both indicate localized warming in this area to the northwest.

Farther north there was a 70–100-km segment along the dryline where few convective clouds formed during the afternoon. This coincided with a swath of cooler boundary layer air that resulted from reduced surface heating over an area that received significant thunderstorm rainfall during the previous night.

A severe thunderstorm complex that developed along the Kansas–Oklahoma border was initiated at the intersection of the dryline and a cloud line that extended into the dry air. An aircraft penetration of the cloud line about 12 km from its intersection with the dryline shows convergence and deepened low-level moisture at the cloud line. The cloud field that evolved into the cloud line over a period of several hours developed over the area that had received the heaviest rainfall during the previous night.

Abstract

The Piecewise Parabolic Method (PPM), a numerical technique developed in astrophysics for modeling fluid flows with strong shocks and discontinuities is adapted for treating sharp gradients in small-scale meteorological flows. PPM differs substantially from conventional gridpoint techniques in three ways. First, PPM is a finite volume scheme, and thus represents physical variables as averages over a grid zone rather than single values at discrete points. Second, a unique, monotonic parabola is fit to the zone average of each dependent variable using information from neighboring zone averages. As shown in a series of one- and two-dimensional linear advection experiments, the use of parabolas provides for extremely accurate advection, particularly of sharp gradients. Furthermore, the monotonicity constraint renders PPM's solutions free from Gibbs oscillations. PPM's third attribute is that each zone boundary is treated as a discontinuity. Using the method of characteristic the nonlinear flux of quantities between zones is obtained by solving a Riemann problem at each zone boundary in alternating one-dimensional sweeps through the grid. This methodology provides a highly accurate, physically based solution both in the vicinity of sharp gradients and in regions where the flow is smooth.

The ability of PPM to accurately depict the evolution of sharp gradients in small-scale, nonlinear flows is examined by simulating a buoyant thermal and a density current in two dimensions. Comparisons made against Midpoint cloud models reveal that PPM provides superior solutions at equivalent spatial resolution, particularly with regard to resolving shear lines that subsequently become unstable. The PPM model has excellent mass and energy conservation properties, and exhibits virtually no numerical dissipation of resolvable modes. Although PPM is not yet as economical as a conventional gridpoint model, we anticipate that its efficiency can be greatly improved by modifying the treatment of acoustic modes.