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Erik Rasmussen

Abstract

A synoptic case study where a small-scale low, a so-called polar low, can be followed as a closed circulation on the surface maps around 2O00 km from Iceland to the North Sea is presented. Satellite images show that the polar low develops a cloud pattern of convective clouds very much like that of a tropical cyclone. The 1000–500 mb thickness field shows that the polar low has a warm core during the time when it is best developed.

It is concluded that the polar low discussed in this work probably is a phenomenon different from a comma cloud and also different from small-scale vortices associated with strong mid-tropospheric positive vorticity advection.

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Erik A. Rasmussen

Abstract

No Abstract available.

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Erik N. Rasmussen

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.

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Erik N. Rasmussen and Jerry M. Straka

Abstract

The life cycle of the 2 June 1995 Dimmitt, Texas, tornado cyclone, observed during the Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX), is described. The tornado cyclone here is defined as a significantly axisymmetric flow larger than the visible tornado and characterized by increasing angular momentum with increasing radius. Its life cycle included three phases with somewhat differing evolution of angular momentum, herein called intensifying, transition, and weakening. During the intensifying stage, the funnel and debris cloud gradually increased in size. The azimuthally averaged secondary circulation of the larger-scale tornado cyclone, as determined using high-resolution single-Doppler data obtained by a mobile radar, was primarily inward and upward, consistent with the presence of a wall cloud outside the tornado. The azimuthally averaged angular momentum increased monotonically away from the tornado, so inward advection allowed the angular momentum to increase slowly with time in part of the tornado cyclone. During the transition phase, downdrafts began to occur within the tornado cyclone. The transport of angular momentum by the secondary circulation nearly was offset by eddy flux convergence of angular momentum so that the azimuthally averaged angular momentum tendency was only weakly negative at most radii. The tornado was visually impressive during this stage, featuring a 400-m diameter debris cloud extending to cloud base, while the surrounding wall cloud shrank and eroded. During the weakening phase, the funnel and debris cloud gradually shrank, and the funnel went through a rope stage prior to disappearing. The weakening phase was characterized by extensive downdrafts at all radii outside the tornado, and large-scale near-ground outflow as observed by mobile mesonet systems in a portion of the tornado cyclone. The secondary circulation acted to transport smaller angular momentum downward from aloft, and outward along the ground. All terms of the angular momentum budget became negative throughout most of the low-level (0–800-m AGL) tornado cyclone during the weakening phase. Several hypotheses for this evolution are evaluated, including changes in water loading in the tornado cyclone, cooling of the near-ground air, and the distribution of tangential velocity with height with its concomitant influence on the nonhydrostatic vertical pressure gradient force.

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Jerry M. Straka and Erik N. Rasmussen

Abstract

Prognostic equations are proposed for use in gridpoint models for the purpose of providing Lagrangian information without the need for computing Lagrangian trajectories. The information provided by the proposed methods might lead to improved representations of microphysical conversion processes. For example, the proposed methods could help improve the timing and location of the onset of precipitation in cloud models.

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Erik N. Rasmussen and David O. Blanchard

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 also appears to be generally lower for supercells with significant tornadoes than those without. The causes for the very large false alarm rates in the tornadic/nontornadic supercell forecast, even with the best discriminators, are discussed.

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Albert E. Pietrycha and Erik N. Rasmussen

Abstract

Mobile mesonet line normal, time-to-space converted data analysis on three meridional drylines that occurred in west Texas on 10 June 1999 and 5 May 2000 are presented herein; two occurred in a quiescent environment on 5 May 2000. Based on the data, the mixing zone across the dryline was composed of a series of large horizontal moisture differentials that were highly variable in width, ranging from 5 km down to several hundred meters. The largest dewpoint differential sampled was 10.0°C over 185 m.

Concurrent with a deceleration of dryline movement to nearly stationary, and while moisture differentials strengthened, surface-based mesoscale vertical circulations with horizontal diameters of 2 km down to less than 300 m were resolved in the data, and visual observations were made of numerous, strongly rotating dust devils. The estimated diameters of the largest dust devils were ∼80–100 m and ∼1 km deep, and these persisted for tens of minutes. All vortices were found to move along or adjacent to the zones of moisture differential. Additionally, when the circulations were observed, spatially isolated cumulus clouds located along the dryline exhibited rapid vertical development. It is plausible that the vortices protect an ascending air parcel by inhibiting mixing, thus allowing the parcel to reach its local lifting condensation level and level of free convection with relatively greater buoyancy than parcels not contained in vortices.

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Erik N. Rasmussen and Steven A. Rutledge

Abstract

Doppler radar observations that establish common patterns in the evolution of the reflectivity and flow structures of squall lines are described. A number of squall lines have been analyzed with unprecedented time resolution in order to identity these patterns. All of the squall lines appeared to be approximately two-dimensional and featured a solid leading edge at some time during their life cycle, instead of being composed of discrete cells separated by echo-free regions. A large variety of intensities and evolution lime scales was observed.

It is shown that squall lines of this type evolve through identifiable stages of reflectivity structure. This evolution appears to be strongly related to changes that occur in the kinematic structure. As a typical system evolves, a rearward-sloping zone of horizontal vorticity, predominantly associated with vertical shear, develops on the scale of the system, presumably driven by the horizontal buoyancy gradients across the system. The vorticity that is generated allows further generation to take place by causing the superposition of a saturated, precipitating anvil cloud aloft over potentially cooler air below in the trailing region. The rearward-sloping vorticity zone gradually tilts toward the horizontal. The rate at which this zone tilts seems to be the primary difference between the systems studied. To a first approximation, the inflow streamlines parallel the sloping vorticity zone, so as it approaches a horizontal slope, vertical motion becomes smaller. Eventually, convective-scale ascent ceases, giving the impression that the gust front has surged out ahead of the precipitation.

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Matthew D. Flournoy and Erik N. Rasmussen

Abstract

Recent studies have highlighted the importance of near-ground storm-relative helicity (SRH) in supercell and tornado processes and how surface friction can play a role. In this study, we use an analytical approach to examine how uniform changes to the ground-relative wind profile above the near-ground layer influence SRH within the near-ground layer. We show how the ground-relative influence of surface friction alters the near-ground shear profile. For idealized semicircular and straight shear profiles, increasing preexisting ground-relative flow above the near-ground layer yields increasing SRH. The magnitude of the SRH increase is sensitive to storm motion, with more deviant motion yielding greater SRH increases given the same increase in ground-relative flow. Supercells may be more susceptible to storm-induced SRH enhancements given their deviant motion and ability to increase ground-relative flow in the background environment.

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Charles A. Doswell III and Erik N. Rasmussen

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.

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