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M. L. Thompson
and
P. Guttorp

Abstract

We consider data on severe cyclonic storms striking the Day of Bengal coast during the period 1877–1977. In the literature these data have been modeled by a homogeneous Poisson process in which case times between storm occurrences are independent of one another, making prediction, and hence advance planning, impossible. We give some evidence against the adequacy of a Poisson process model and suggest a Poisson cluster model that appears to describe the data better. The features of fills model are such as to enable some planning procedures to he developed.

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R. L. Thompson
,
J. M. Lewis
, and
R. A. Maddox

Abstract

The return of tropical air from the Gulf of Mexico is examined in the autumnal cool season. Results from the thermodynamic equilibrium model of Betts and Ridgway are used to calculate the equilibrium equivalent potential temperature (θ e ) over the gulf and the northwestern Caribbean Sea. With a climatological study as a backdrop, a case of severe weather outbreak in mid-November 1988 is analyzed with emphasis on the analysis of low-level θ e that flowed into the storm region from the Gulf of Mexico.

The primary results of the study are the following:

  1. The climatological distribution of equilibrium θ e over the gulf and the Caribbean in November serves as a useful tool for the analysis of the 1988 case study.

  2. Between 5 and 15 November 1988, equilibrium in the marine layer was established over the gulf due to the absence of any deep cold-air penetrations during this period.

  3. The high-valued θ e that streamed into the severe storm region on 15 November 1988 tracked from the Yucatán straits and the northwestern Caribbean over a three-day period.

  4. This air was able to maintain its high-θ e property because of an anomalously warm gulf.

  5. Significant increases in available energy for deep convection could have been anticipated by means of the upper bounds on coastal θ e predicted by the Betts and Ridgway formulation, which was supported by observations along the Texas coast.

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Richard L. Thompson
,
Corey M. Mead
, and
Roger Edwards

Abstract

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−1) and largest CIN (−250 J kg−1) 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 θe value) in the lowest 300 hPa. ESRH and EBS discriminate strongly between various storm types, and between supercells and nonsupercells, respectively.

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John M. Peters
,
Christopher J. Nowotarski
,
Jake P. Mulholland
, and
Richard L. Thompson

Abstract

The relationship between storm-relative helicity (SRH) and streamwise vorticity ω s is frequently invoked to explain the often robust connections between effective inflow layer (EIL) SRH and various supercell updraft properties. However, the definition of SRH also contains storm-relative (SR) flow, and the separate influences of SR flow and ω s on updraft dynamics are therefore convolved when SRH is used as a diagnostic tool. To clarify this issue, proximity soundings and numerical experiments are used to disentangle the separate influences of EIL SR flow and ω s on supercell updraft characteristics. Our results suggest that the magnitude of EIL ω s has little influence on whether supercellular storm mode occurs. Rather, the transition from nonsupercellular to supercellular storm mode is largely modulated by the magnitude of EIL SR flow. Furthermore, many updraft attributes such as updraft width, maximum vertical velocity, vertical mass flux at all levels, and maximum vertical vorticity at all levels are largely determined by EIL SR flow. For a constant EIL SR flow, storms with large EIL ω s have stronger low-level net rotation and vertical velocities, which affirms previously established connections between ω s and tornadogenesis. EIL ω s also influences storms’ precipitation and cold-pool patterns. Vertical nonlinear dynamic pressure acceleration (NLDPA) is larger at low levels when EIL ω s is large, but differences in NLDPA aloft become uncorrelated with EIL ω s because storms’ midlevel dynamic pressure perturbations are substantially influenced by the tilting of midlevel vorticity. Our results emphasize the importance of considering EIL SR flow in addition to EIL SRH in the research and forecasting of supercell properties.

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Jonathan M. Garner
,
William C. Iwasko
,
Tyler D. Jewel
,
Richard L. Thompson
, and
Bryan T. Smith

Abstract

A dataset maintained by the Storm Prediction Center (SPC) of 6300 tornado events from 2009 to 2015, consisting of radar-identified convective modes and near-storm environmental information obtained from Rapid Update Cycle and Rapid Refresh model analysis grids, has been augmented with additional radar information related to the low-level mesocyclones associated with tornado longevity, pathlength, and width. All EF2–EF5 tornadoes [as measured on the enhanced Fujita (EF) scale], in addition to randomly selected EF0–EF1 tornadoes, were extracted from the SPC dataset, which yielded 1268 events for inclusion in the current study. Analysis of those data revealed similar values of the effective-layer significant tornado parameter for the longest-lived (60+ min) tornadic circulations, longest-tracked (≥68 km) tornadoes, and widest tornadoes (≥1.2 km). However, the widest tornadoes occurring west of −94° longitude were associated with larger mean-layer convective available potential energy, storm-top divergence, and low-level rotational velocity. Furthermore, wide tornadoes occurred when low-level winds were out of the southeast, resulting in large low-level hodograph curvature and near-surface horizontal vorticity that was more purely streamwise when compared with long-lived and long-tracked events. On the other hand, tornado pathlength and longevity were maximized with eastward-migrating synoptic-scale cyclones associated with strong southwesterly wind profiles through much of the troposphere, fast storm motions, large values of bulk wind difference and storm-relative helicity, and lower buoyancy.

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Shaowu Bao
,
L. Bernardet
,
G. Thompson
,
E. Kalina
,
K. Newman
, and
M. Biswas

Abstract

The impact of different hydrometeor advection schemes on TC structure and intensity forecasts is examined through the evaluation of HWRF’s simulation of tropical cyclones using the operational Ferrier–Aligo (FA) microphysics that uses total condensate advection versus the same scheme but with separate hydrometeor advection (FA-adv). Results showed that FA-adv simulated larger storms. Idealized simulations revealed that the cause of the simulation differences is the characteristics of the vertical profile of cloud water (Qc), which has a sharp gradient near 850 hPa, and rainwater (Qr), which is vertically uniform below the melting layer. In FA, the resultant total condensate profile has a gradient near 850 hPa that is smaller than that of Qc but larger than that of Qr. In FA when the total condensate is advected and partitioned back to Qc and Qr, the advection of Qc is underestimated and that of Qr is overestimated than that in FA-adv. The separate advection of hydrometeors in the FA-adv scheme corrected this problem and caused the difference in microphysics and dynamics fields between the two schemes. The greater vertical advection of Qc in FA-adv represents a continual source of extra diabatic heating that leads to a greater integrated kinetic energy (IKE) in the storm simulated by FA-adv than FA. However, the radial distribution of the azimuthally averaged additional diabatic heating in FA-adv caused a sea level pressure adjustment that leads to a weaker maximum wind speed. The warming in the outer rainbands strengthens wind away from the inner core, which causes the simulated storm size to increase.

Open access
Katherine M. Willingham
,
Elizabeth J. Thompson
,
Kenneth W. Howard
, and
Charles L. Dempsey

Abstract

During the 2008 North American monsoon season, 140 microburst events were identified in Phoenix, Arizona, and the surrounding Sonoran Desert. The Sonoran microbursts were studied and examined for their frequency and characteristics, as observed from data collected from three Doppler radars and electrical power infrastructure damage reports. Sonoran microburst events were wet microbursts and occurred most frequently in the evening hours (1900–2100 local time). Stronger maximum differential velocities (20–25 m s−1) were observed more frequently in Sonoran microbursts than in many previously documented microbursts. Alignment of Doppler radar data to reports of wind-related damage to electrical power infrastructure in Phoenix allowed a comparison of microburst wind damage versus gust-front wind damage. For these damage reports, microburst winds caused more significant damage than gust-front winds.

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John M. Peters
,
Hugh Morrison
,
Christopher J. Nowotarski
,
Jake P. Mulholland
, and
Richard L. Thompson

Abstract

In supercell environments, previous authors have shown strong connections between the vertical wind shear magnitude, updraft width, and entrainment. Based on these results, it is hypothesized that the influences of entrainment-driven dilution on buoyancy and maximum updraft vertical velocity w in supercell environments are a predictable function of the vertical wind shear profile. It is also hypothesized that the influences of pressure perturbation forces on maximum updraft w are small because of a nearly complete offset between upward dynamic pressure forces and downward buoyant pressure forces. To address these hypotheses, we derive a formula for the maximum updraft w that incorporates the effects of entrainment-driven dilution on buoyancy but neglects pressure gradient forces. Solutions to this formula are compared with output from previous numerical simulations. This formula substantially improves predictions of maximum updraft w over past CAPE-derived formulas for maximum updraft w, which supports the first hypothesis. Furthermore, integrated vertical accelerations along trajectories show substantial offsets between dynamic and buoyant pressure forces, supporting the second hypothesis. It is argued that the new formula should be used in addition to CAPE-derived measures for w in forecast and research applications when accurate diagnosis of updraft speed is required.

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EXECUTIVE COMMITTEE
,
Robert J. Serafin
,
Richard D. Rosen
,
James F. Kimpel
,
George L. Frederick Jr.
,
Anne Thompson
,
Mary M. Glackin
,
Kenneth C. Spengler
, and
Ronald D. McPherson
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Matthew C. Brown
,
Christopher J. Nowotarski
,
Andrew R. Dean
,
Bryan T. Smith
,
Richard L. Thompson
, and
John M. Peters

Abstract

The response of severe local storms to environmental evolution across the early evening transition (EET) remains a forecasting challenge, particularly within the context of the Southeast U.S. storm climatology, which includes the increased presence of low-CAPE environments and tornadic nonsupercell modes. To disentangle these complex environmental interactions, Southeast severe convective reports spanning 2003–18 are temporally binned relative to local sunset. Sounding-derived data corresponding to each report are used to characterize how the near-storm environment evolves across the EET, and whether these changes influence the mode, frequency, and tornadic likelihood of their associated storms. High-shear, high-CAPE (HSHC) environments are contrasted with high-shear, low-CAPE (HSLC) environments to highlight physical processes governing storm maintenance and tornadogenesis in the absence of large instability. Last, statistical analysis is performed to determine which aspects of the near-storm environment most effectively discriminate between tornadic (or significantly tornadic) and nontornadic storms toward constructing new sounding-derived forecast guidance parameters for multiple modal and environmental combinations. Results indicate that HSLC environments evolve differently than HSHC environments, particularly for nonsupercell (e.g., quasi-linear convective system) modes. These low-CAPE environments sustain higher values of low-level shear and storm-relative helicity (SRH) and destabilize postsunset—potentially compensating for minimal buoyancy. Furthermore, the existence of HSLC storm environments presunset increases the likelihood of nonsupercellular tornadoes postsunset. Existing forecast guidance metrics such as the significant tornado parameter (STP) remain the most skillful predictors of HSHC tornadoes. However, HSLC tornado prediction can be improved by considering variables like precipitable water, downdraft CAPE, and effective inflow base.

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