P 1.1 Effective Bulk Shear in Supercell Thunderstorm Environments (original) (raw)

Effective Storm-Relative Helicity and Bulk Shear in Supercell Thunderstorm Environments

Weather and Forecasting, 2007

A sample of 1185 Rapid Update Cycle (RUC) model analysis (0 h) proximity soundings, within 40 km and 30 min of radar-identified discrete storms, was categorized by several storm types: significantly tornadic supercells (F2 or greater damage), weakly tornadic supercells (F0-F1 damage), nontornadic supercells, elevated right-moving supercells, storms with marginal supercell characteristics, and nonsupercells. These proximity soundings served as the basis for calculations of storm-relative helicity and bulk shear intended to apply across a broad spectrum of thunderstorm types. An effective storm inflow layer was defined in terms of minimum constraints on lifted parcel CAPE and convective inhibition (CIN). Sixteen CAPE and CIN constraint combinations were examined, and the smallest CAPE (25 and 100 J kg Ϫ1 ) and largest CIN (Ϫ250 J kg Ϫ1 ) constraints provided the greatest probability of detecting an effective inflow layer within an 835-supercell subset of the proximity soundings. Effective storm-relative helicity (ESRH) calculations were based on the upper and lower bounds of the effective inflow layer. By confining the SRH calculation to the effective inflow layer, ESRH values can be compared consistently across a wide range of storm environments, including storms rooted above the ground. Similarly, the effective bulk shear (EBS) was defined in terms of the vertical shear through a percentage of the "storm depth," as defined by the vertical distance from the effective inflow base to the equilibrium level associated with the most unstable parcel (maximum e value) in the lowest 300 hPa. ESRH and EBS discriminate strongly between various storm types, and between supercells and nonsupercells, respectively.

Close Proximity Soundings within Supercell Environments Obtained from the Rapid Update Cycle

Weather and Forecasting, 2003

A sample of 413 soundings in close proximity to tornadic and nontornadic supercells is examined. The soundings were obtained from hourly analyses generated by the 40-km Rapid Update Cycle-2 (RUC-2) analysis and forecast system. A comparison of 149 observed soundings and collocated RUC-2 soundings in regional supercell environments reveals that the RUC-2 model analyses were reasonably accurate through much of the troposphere. The largest error tendencies were in temperatures and mixing ratios near the surface, primarily in 1-h forecast soundings immediately prior to the standard rawinsonde launches around 1200 and 0000 UTC. Overall, the RUC-2 analysis soundings appear to be a reasonable proxy for observed soundings in supercell environments.

Characteristics of Vertical Wind Profiles near Supercells Obtained from the Rapid Update Cycle

Weather and Forecasting, 2003

Over 400 vertical wind profiles in close proximity to nontornadic and tornadic supercell thunderstorms are examined. The profiles were obtained from the Rapid Update Cycle (RUC) model/analysis system. Groundrelative wind speeds throughout the lower and middle troposphere are larger, on average, in tornadic supercell environments than in nontornadic supercell environments. The average vertical profiles of storm-relative wind speed, vertical wind shear, hodograph curvature, crosswise and streamwise vorticity, and storm-relative helicity are generally similar above 1 km in the tornadic and nontornadic supercell environments, with differences that are either not statistically significant or not what most would regard as meteorologically significant. On the other hand, considerable differences are found in these average vertical profiles within 1 km of the ground, with environments associated with significantly tornadic supercells (those producing tornadoes of at least F2 intensity) having substantially larger low-level vertical wind shear, streamwise vorticity, and storm-relative helicity compared to environments associated with nontornadic supercells and weakly tornadic supercells (those producing F0 or F1 tornadoes). These findings may partly explain the extraordinary difficulty in discriminating between tornadic and nontornadic supercell environments in a forecasting setting, given the low temporal and spatial frequency of wind observations in the lowest 1 km. It is believed that it would be a worthwhile investment to augment low-level wind profiling capabilities, in addition to taking a closer look at the dynamical sensitivities of supercell storms to near-surface wind shear by way of high-resolution numerical simulations.

Sounding-derived parameters associated with large hail and tornadoes in the Netherlands

Atmospheric Research, 2007

A study is presented focusing on the potential value of parameters derived from radiosonde data or data from numerical atmospheric models for the forecasting of severe weather associated with convective storms. Parameters have been derived from soundings in the proximity of large hail, tornadoes (including tornadoes over water: waterspouts) and thunderstorms in the Netherlands. 66,365 radiosonde soundings from six stations in and around the Netherlands between 1 Dec. 1975 to 31 Aug. 2003 were classified as being associated or not associated with these weather phenomena using observational data from voluntary observers, the Dutch National Meteorological Institute (KNMI) and lightning data from the U.K. Met. Office. It was found that instability as measured by the Lifted Index or CAPE and 0-6 km wind shear independently have considerable skill in distinguishing environments of large hail and of non-hailproducing thunderstorms. It was also found that CAPE released below 3 km above ground level is on average high near waterspouts and weak tornadoes that mostly occur with low shear in the lowest 1 km above the Earth's surface. On the other hand, low-level shear is strong in environments of stronger (F1 and F2) tornadoes and increases with increasing F-scale. This is consistent with the notion that stretching of preexisting vertical vorticity is the most important mechanism for the formation of weak tornadoes while the tilting of vorticity is more important with stronger tornadoes. The presented results may assist forecasters to assess the likelihood of severe hail or tornadoes.