Carl E. Hane

Application of the Piecewise Parabolic Method (PPM) to Meteorological Modeling

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 PPMs solutions free from Gibbs oscillations. PPMs third attribute is that each zone boundary is treated as a discontinuity. Using the method of characteristic the nonlinear flux of qua...

An Observational Study of the Dryline

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 ...

Severe Thunderstorm Development in Relation to Along-Dryline Variability: A Case Study

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 i...

Investigation of the Dryline and Convective Storms Initiated along the Dryline: Field Experiments during COPS–91

Abstract 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...

A Case Study of Severe Storm Development along a Dryline within a Synoptically Active Environment. Part II: Multiple Boundaries and Convective Initiation

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-cen...

Retrieval of Thermodynamic Variables within Deep Convective Clouds: Experiments in Three Dimensions

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 turbu...

A Case Study of Severe Storm Development along a Dryline within a Synoptically Active Environment. Part I: Dryline Motion and an Eta Model Forecast

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.

Temperature and Pressure Perturbations within Convective Clouds Derived from Detailed Air Motion Information: Preliminary Testing

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.

Dynamic Aspects of a Distinctly Three-Dimensional Mesoscale Convective System

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 b...

The Evolution of Morning Convective Systems over the U.S. Great Plains during the Warm Season. Part II: A Climatology and the Influence of Environmental Factors

Abstract Mesoscale convective systems that affect a limited area within the southern plains of the United States during late morning hours during the warm season are investigated. A climatological study over a 5-yr period documents the initiation locations and times, tracks, associated severe weather, and relation to synoptic features over the lifetimes of 145 systems. An assessment is also made of system evolution in each case during the late morning. For a subset of 48 systems, vertical profiles of basic variables from Rapid Update Cycle (RUC) model analyses are used to characterize the environment of each system. Scatter diagrams and discriminant analyses are used to assess which environmental variables are most promising in helping to determine which of two classes of evolutionary character each system will follow.