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Jonathan M. Davies

Abstract

Recent studies have suggested that supercell tornado environments are usually associated with large 0–1-km storm-relative helicity (SRH) and relatively low lifting condensation levels (LCL heights). However, occasional tornadoes of significance occur in environments having characteristics that appear less supportive of supercell tornadoes, including small SRH values and/or relatively high LCL heights. Such tornadoes, whether associated with supercell or nonsupercell processes (more precisely termed mesocyclone and nonmesocyclone processes), present a challenge for forecasters. This empirical study uses a database of soundings derived from the Rapid Update Cycle model to examine thermodynamic characteristics of F1 and greater intensity tornado events associated with small SRH and/or high LCL heights. Results strongly suggest that many such tornado events are associated with steep lapse rates in the lowest few kilometers above ground. The low level of free convection heights, small convective inhibition, and sizable convective available potential energy below 3 km were also found to be of possible importance. These thermodynamic characteristics combined would likely reduce resistance to upward accelerations, potentially enhancing ascent for low-level parcels entering thunderstorm updrafts and, hence, low-level stretching. From prior research, if preexisting boundaries were available to provide surface vertical vorticity for stretching, such thermodynamic characteristics could be an important component of tornado events that involve nonmesocyclone processes. These same thermodynamic characteristics may also offer clues for the investigation of mesocyclone tornado events that do not fit well with accepted tornado forecasting parameters from prior studies.

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Jonathan M. Davies

Abstract

Tornadoes that occur in close proximity to midlevel closed lows with a core of cold temperatures aloft are not uncommon, particularly in the central United States. Although several informal studies have shown that severe weather and tornadoes can occur with these midlevel lows, little in the way of formal work has been published documenting features and ingredients of such systems, especially those that produce what are sometimes called cold core tornadoes. Of particular concern is that these tornadoes can be associated with surface and low-level moisture that appears deceptively small or marginal regarding severe weather potential, yet on occasion tornadoes of F2 or greater intensity can develop. In other cases, vertical shear may appear relatively weak at locations close to the midlevel low, suggesting little potential for tornadoes. These “atypical” characteristics can result in poor anticipation by forecasters of tornado events associated with closed 500-mb lows. This note documents some synoptic and mesoscale features commonly associated with tornado events in close proximity to cold core 500-mb lows using four tornadic cases in Kansas as examples, including photographs to show the small nature of storms associated with such systems. Recognition of surface patterns with a particular organization of boundaries and surface heating positioned near midlevel lows, along with the presence of some amount of buoyancy, can help with the operational awareness of the potential for tornadoes in many 500-mb closed low settings.

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Jonathan M. Davies

Abstract

It is generally understood that tornadoes are less likely in environments where surface-based instability is absent and the only convective available potential energy (CAPE) is from elevated parcels originating well above the surface. However, little research has been done to examine tornado occurrence in environments where surface-based CAPE is clearly present, but located above a deep layer of surface-based convective inhibition (CIN) associated with a relatively high level of free convection (LFC) heights. A database of 518 model analysis soundings was collected during 2001–03 from recent versions of the rapid update cycle (RUC) analysis and forecast system. All model sounding profiles were in proximity to selected supercell storms associated primarily with tornado warnings, and were “nonelevated” with surface-based CAPE present. This database is used to examine CIN and LFC to determine environments where instability from mixed-layer and surface-based parcels was associated with significant negative buoyancy in low levels. Findings evaluated for tornadic and nontornadic supercells show that the frequency of F1–F4 intensity tornadoes decreased significantly for increasing CIN and LFC height. The fact that tornado warnings were issued for many nontornadic supercells with large CIN and high LFC heights in the database examined suggests that better awareness of environments having increased CIN and LFC characteristics may be useful operationally in discriminating between some tornadic and nontornadic supercell settings.

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David J. Kellenbenz, Thomas J. Grafenauer, and Jonathan M. Davies

Abstract

Edwards and Thompson have made comprehensive and thorough comments concerning the methods, accuracy, and results in a case study by Kellenbenz et al. These comments questioned the representativeness of model-derived soundings and graphics used, as well as the modification methods employed with soundings presented in the 18 July 2004 case study. Other issues included the application of previous database studies to lifting condensation level (LCL) values found in the 18 July 2004 examination, as well as a focus that emphasized a single tornadic storm. In this response, the authors address problems and oversights with the 18 July 2004 case study, using new data to conduct a reanalysis of the environment on that evening. Additionally, a large independent database of Rapid Update Cycle (RUC) analysis soundings associated with supercell tornadoes is used to provide the context from which to carefully evaluate LCL results from the 18 July 2004 case relative to significant and violent tornadoes. Results indicate that although the LCL height associated with the F4 tornadic supercell in the original case study was probably overestimated, the background LCL environment was still unusually high for a violent tornado. New material presented in this response reinforces the conclusion that, when LCL heights are at the far upper end of empirical study distributions associated with significant tornadoes, undue weight should not be given to LCL as a tornado probability reduction factor when CAPE–storm relative helicity (SRH) combinations and deep-layer shear are also strong.

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David J. Kellenbenz, Thomas J. Grafenauer, and Jonathan M. Davies

Abstract

On the evening of 18 July 2004, several tornadoes occurred with two supercell thunderstorms over eastern North Dakota. The second and smaller in diameter of these storms produced an F4 tornado in an environment with lifting condensation level (LCL) heights that were atypically high according to recent statistical studies about supercell tornado environments. Surface dewpoints were also underforecast by computer models. These two issues are examined in this paper, which provides an overview of this event. The synoptic setting and environment characteristics suggest that evapotranspiration (ET) was responsible in part for enhancing surface moisture. It is likely that ET affected instability and convection initiation. This study also found that the presence of steep low-level lapse rates juxtaposed with low-level convective available potential energy along a surface trough may have contributed to tornado development in a high LCL environment where wind and instability characteristics were otherwise favorable for supporting supercell tornadoes.

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Jonathan M. Davies, Charles A. Doswell III, Donald W. Burgess, and John F. Weaver

This paper considers a tornadic storm that struck south-central and eastern Kansas on 13 March 1990. Most of the devastation was associated with the first tornado from the storm as it passed through Hesston, Kansas. From the synoptic-scale and mesoscale viewpoints, the event was part of an outbreak of tornadoes on a day when the tornado threat was synoptically evident. Satellite imagery, combined with conventional data, suggest that the Hesston storm was affected by a preexisting, mesoscale outflow boundary laid down by morning storms. Radar and satellite data give clear indication of the supercellular character of the storm, despite limited radar data coverage.

Because of the considerable photographic coverage, several interesting features of the storm were recorded and are analyzed here. These include the following: 1) the movement and dissipation of a cloud band associated with an apparent rear-flank downdraft; 2) a transition from a rather large funnel through an apparent dissipation to the formation of a narrow funnel, during which the damage on the ground was continuous; and 3) a period of interaction between the first and second tornadoes.

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William E. Togstad, Jonathan M. Davies, Sarah J. Corfidi, David R. Bright, and Andrew R. Dean

Abstract

Recent literature has identified several supercell/tornado forecast parameters in common use that are operationally beneficial in assessing environments supportive of supercell tornadoes. These parameters are utilized in the computation of tornado forecast guidance such as the significant tornado parameter (STP), a dimensionless parameter developed at the Storm Prediction Center (SPC) that applies a subjectively chosen scale. The goal of this research is to determine if useful logistic regression equations can be developed to estimate the conditional probability of supercell tornadoes that are categorized as level 2 or stronger on the enhanced Fujita scale (EF) when a similar set of environmental background parameters is applied as variables. A large database of Rapid Update Cycle (RUC) analysis soundings in proximity to a representative sample of tornadic and nontornadic supercells over the central and eastern United States, a number of which were associated with EF2 or stronger tornadoes, was used to compute supercell tornado forecast parameters similar to those in the original version of STP. Three logistic regression equations were developed from this database, two of which are described and analyzed in detail. Statistical verification for both equations was accomplished using independent data from 2008 in proximity to supercell storms identified by staff at SPC. A recent version of the STP was utilized as a comparison diagnostic to accomplish part of the statistical verification. The results of this research suggest that output from both logistic regression equations can provide valuable guidance in a probabilistic sense, when adjustments are made for the ongoing convective mode. Case studies presented also suggest that this guidance can provide information complementary to STP in severe weather situations with potential for supercell tornadoes.

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Ron McTaggart-Cowan, Emily L. Davies, Jonathan G. Fairman Jr., Thomas J. Galarneau Jr., and David M. Schultz

Abstract

A high sea surface temperature is generally accepted to be one of the necessary ingredients for tropical cyclone development, indicative of the potential for surface heat and moisture fluxes capable of fueling a self-sustaining circulation. Although the minimum 26.5°C threshold for tropical cyclogenesis has become a mainstay in research and education, the fact that a nonnegligible fraction of storm formation events (about 5%) occur over cooler waters casts some doubt on the robustness of this estimate. Tropical cyclogenesis over subthreshold sea surface temperatures is associated with low tropopause heights, indicative of the presence of a cold trough aloft. To focus on this type of development environment, the applicability of the 26.5°C threshold is investigated for tropical transitions from baroclinic precursor disturbances in all basins between 1989 and 2013. Although the threshold performs well in the majority of cases without appreciable environmental baroclinicity, the potential for development is underestimated by up to 27% for systems undergoing tropical transition. An alternative criterion of a maximum 22.5°C difference between the tropopause-level and 850-hPa equivalent potential temperatures (defined as the coupling index) is proposed for this class of development. When combined with the standard 26.5°C sea surface temperature threshold for precursor-free environments, error rates are reduced to 3%–6% for all development types. The addition of this physically relevant representation of the deep-tropospheric state to the ingredients-based conceptual model for tropical cyclogenesis improves the representation of the important tropical transition-based subset of development events.

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Anne M. Thompson, Herman G. J. Smit, Jacquelyn C. Witte, Ryan M. Stauffer, Bryan J. Johnson, Gary Morris, Peter von der Gathen, Roeland Van Malderen, Jonathan Davies, Ankie Piters, Marc Allaart, Françoise Posny, Rigel Kivi, Patrick Cullis, Nguyen Thi Hoang Anh, Ernesto Corrales, Tshidi Machinini, Francisco R. da Silva, George Paiman, Kennedy Thiong’o, Zamuna Zainal, George B. Brothers, Katherine R. Wolff, Tatsumi Nakano, Rene Stübi, Gonzague Romanens, Gert J. R. Coetzee, Jorge A. Diaz, Sukarni Mitro, Maznorizan Mohamad, and Shin-Ya Ogino

Abstract

The ozonesonde is a small balloon-borne instrument that is attached to a standard radiosonde to measure profiles of ozone from the surface to 35 km with ∼100-m vertical resolution. Ozonesonde data constitute a mainstay of satellite calibration and are used for climatologies and analysis of trends, especially in the lower stratosphere where satellites are most uncertain. The electrochemical concentration cell (ECC) ozonesonde has been deployed at ∼100 stations worldwide since the 1960s, with changes over time in manufacture and procedures, including details of the cell chemical solution and data processing. As a consequence, there are biases among different stations and discontinuities in profile time series from individual site records. For 22 years the Jülich (Germany) Ozonesonde Intercomparison Experiment (JOSIE) has periodically tested ozonesondes in a simulation chamber designated the World Calibration Centre for Ozonesondes (WCCOS) by WMO. During October–November 2017 a JOSIE campaign evaluated the sondes and procedures used in Southern Hemisphere Additional Ozonesondes (SHADOZ), a 14-station sonde network operating in the tropics and subtropics. A distinctive feature of the 2017 JOSIE was that the tests were conducted by operators from eight SHADOZ stations. Experimental protocols for the SHADOZ sonde configurations, which represent most of those in use today, are described, along with preliminary results. SHADOZ stations that follow WMO-recommended protocols record total ozone within 3% of the JOSIE reference instrument. These results and prior JOSIEs demonstrate that regular testing is essential to maintain best practices in ozonesonde operations and to ensure high-quality data for the satellite and ozone assessment communities.

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