Erik N. Rasmussen

A baseline climatology of sounding-derived supercell and tornado forecast parameters

Abstract All of the 0000 UTC soundings from the United States made during the year 1992 that have nonzero convective available potential energy (CAPE) are examined. Soundings are classified as being associated with nonsupercell thunderstorms, supercells without significant tornadoes, and supercells with significant tornadoes. This classification is made by attempting to pair, based on the low-level sounding winds, an upstream sounding with each occurrence of a significant tornado, large hail, and/or 10 or more cloud-to-ground lightning flashes. Severe weather wind parameters (mean shear, 0–6-km shear, storm-relative helicity, and storm-relative anvil-level flow) and CAPE parameters (total CAPE and CAPE in the lowest 3000 m with buoyancy) are shown to discriminate weakly between the environments of the three classified types of storms. Combined parameters (energy–helicity index and vorticity generation parameter) discriminate strongly between the environments. The height of the lifting condensation level a...

The Effect of Neglecting the Virtual Temperature Correction on CAPE Calculations

Abstract A simple theoretical analysis of the impact of neglecting the virtual correction on calculation of CAPE is made. This theory suggests that while ignoring the virtual correction does not introduce much error for large CAPE values, the relative error can become substantial for small CAPE. A test of the theory is done by finding the error made by ignoring the virtual correction to CAPE for all the soundings in 1992 having positive CAPE (when the correction is made). Results of this empirical test confirm that the relative error made in ignoring the correction increases with decreasing CAPE. A number of other “corrections” to CAPE might be considered. In a discussion of the issues associated with the results of the analysis, it is recommended that CAPE calculations should include the virtual correction but that other complications should be avoided for most purposes, especially when making comparisons of CAPE values. A standardized CAPE calculation also is recommended.

Direct Surface Thermodynamic Observations within the Rear-Flank Downdrafts of Nontornadic and Tornadic Supercells

Abstract Despite the long-surmised importance of the hook echo and rear-flank downdraft (RFD) in tornadogenesis, only a paucity of direct observations have been obtained at the surface within hook echoes and RFDs. In this paper, in situ surface observations within hook echoes and RFDs are analyzed. These “mobile mesonet” data have unprecedented horizontal spatial resolution and were obtained from the Verifications of the Origins of Rotation in Tornadoes Experiment (VORTEX) and additional field experiments conducted since the conclusion of VORTEX. The surface thermodynamic characteristics of hook echoes and RFDs associated with tornadic and nontornadic supercells are investigated to address whether certain types of hook echoes and RFDs are favorable (or unfavorable) for tornadogenesis. Tornadogenesis is more likely and tornado intensity and longevity increase as the surface buoyancy, potential buoyancy (as measured by the convective available potential energy), and equivalent potential temperature in the R...

Precipitation Uncertainty Due to Variations in Precipitation Particle Parameters within a Simple Microphysics Scheme

This work reports on the sensitivity of accumulated precipitation to the microphysical parameterization in simulations of deep convective storms using a three-dimensional, nonhydrostatic cloud model with a simple liquid‐ice microphysics scheme. Various intercept parameters from an assumed Marshall‐Palmer exponential size distribution are tested along with two particle densities for the hail/graupel (qh) category. These variations allow testing of unique qh distributions that have been observed and documented in previous literature. Tests are conducted for a single thermodynamic profile and three idealized wind shear profiles. The amount of accumulated precipitation at the ground is very sensitive to the way the qh category is parameterized. Distributions characterized by larger intercepts and/or smaller particle density have a smaller mass-weighted mean terminal fall velocity and produce smaller qh mixing ratios spread over a larger area. For example, for a qh category weighted toward graupel, only a fourth as much precipitation accumulates on the ground over 2 h (and none is hail) compared to a qh category weighted toward large hail (with baseball-sized stones common). The inherent uncertainty within the qh distribution for this simple cloud-scale three-class ice microphysics scheme suggests limited usefulness in the forecasting of ground-accumulated precipitation and damaging hail.

Verification of the Origins of Rotation in Tornadoes Experiment: VORTEX

Abstract This paper describes the Verification of the Origins of Rotation in Tornadoes Experiment planned for 1994 and 1995 to evaluate a set of hypotheses pertaining to tornadogenesis and tornado dynamics. Observations of state variables will be obtained from five mobile mesonet vehicles, four mobile ballooning laboratories, three movie photography teams, portable Doppler radar teams, two in situ tornado instruments deployment teams, and the T-28 and National Atmospheric and Oceanic Administration P-3 aircraft. In addition, extensive use will be made of the new generation of observing systems, including the WSR-88D Doppler radars, demonstration wind profiler network, and National Weather Service rawinsondes.

Design and Deployment of a Portable, Pencil-Beam, Pulsed, 3-cm Doppler Radar

Abstract A portable, pencil-beam, pulsed, Doppler, 3-cm wavelength radar has been constructed to study a wide variety of meteorological phenomena including tornadoes, severe storms, and boundary layer processes. The new radar, the Doppler on Wheels (DOW), has full scanning capability, a real-time display and archiving, and is mounted on a truck for easy portability and full mobility. This portability allows the radar to be brought to within a kilometer of rare meteorological phenomena. At this range, the pencil beam of the radar is very narrow, permitting significantly higher-resolution measurements (at 3-km range, 64 m × 64 m × 75 m) than are usually possible with stationary or airborne systems. The radar employs a new high-powered, PC-based, digital intermediate frequency (IF) data acquisition scheme called the PIRAQ. The radar has successfully collected data in several tornadoes and tornadic storms and has been used to detect dust devils and other boundary layer structures. The sensitivity and mobility...

THE SEVERE THUNDERSTORM ELECTRIFICATION AND PRECIPITATION STUDY

Abstract During May–July 2000, the Severe Thunderstorm Electrification and Precipitation Study (STEPS) occurred in the High Plains, near the Colorado–Kansas border. STEPS aimed to achieve a better understanding of the interactions between kinematics, precipitation, and electrification in severe thunderstorms. Specific scientific objectives included 1) understanding the apparent major differences in precipitation output from super-cells that have led to them being classified as low precipitation (LP), classic or medium precipitation, and high precipitation; 2) understanding lightning formation and behavior in storms, and how lightning differs among storm types, particularly to better understand the mechanisms by which storms produce predominantly positive cloud-to-ground (CG) lightning; and 3) verifying and improving microphysical interpretations from polarimetric radar. The project involved the use of a multiple-Doppler polarimetric radar network, as well as a time-of-arrival very high frequency (VHF) lig...

The Occurrence of Tornadoes in Supercells Interacting with Boundaries during VORTEX-95

Abstract During the Verifications of the Origins of Rotation in Tornadoes Experiment, nearly 70% of the significant tornadoes occurred near low-level boundaries not associated with the forward or rear flank downdrafts of supercells. In general, these were preexisting boundaries readily identified using conventional data sources. Most of the tornadoes occurred on the cool side of these low-level boundaries and generally within 30 km of the boundaries. It is likely that the low-level boundaries augmented the “ambient” horizontal vorticity, which, upon further generation in the forward-flank region, became sufficient to be associated with tornadic low-level mesocyclones. Some implications for forecasting and further research are discussed.

Precipitation and Evolution Sensitivity in Simulated Deep Convective Storms: Comparisons between Liquid-Only and Simple Ice and Liquid Phase Microphysics*

Abstract Weisman and Klemp suggested that their liquid-only, deep convective storm experiments should be repeated with a liquid-ice microphysics scheme to determine if the solutions are qualitatively the same. Using a three-dimensional, nonhydrostatic cloud model, such results are compared between three microphysics schemes: the “Kessler” liquid-only scheme (used by Weisman and Klemp), a Lin–Farley–Orville-like scheme with liquid and ice parameterization (Li), and the same Lin–Farley–Orville-like microphysics scheme but with only liquid processes turned on (Lr). Convection is simulated using a single thermodynamic profile and a variety of shear profiles. The shear profiles are represented by five idealized half-circle wind hodographs with arc lengths (Us) of 20, 25, 30, 40, and 50 m s−1. The precipitation, cold pool characteristics, and storm evolution produced by the different schemes are compared. The Kessler scheme produces similar accumulated precipitation over 2 h compared to Lr for all shear regimes...

Refined Supercell and Tornado Forecast Parameters

Abstract This note updates a previous study that utilized a baseline climatology of soundings associated with large hail, significant tornadoes, and 10 or more cloud-to-ground lightning flashes from 1992. Expanding on the earlier analysis, it is shown that three modified forecast parameters have more value in distinguishing between environments that favor significant tornadoes and those that favor large hail but no significant tornadoes, in the climatological data. These parameters are storm-relative helicity in the 0–1-km layer adjacent to the ground, energy–helicity index computed from this measure of helicity, and the convective available potential energy that accrues from the surface to 3 km above ground level. In addition, this note provides caveats regarding the interpretation of the climatological findings.