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Larissa J. Reames

Abstract

Nocturnal tornadoes are disproportionately dangerous compared with their daytime counterparts; thus, it is imperative to improve the forecasting of these tornadoes. This study uses a large (194 cases) and geographically expansive dataset of Rapid Update Cycle tornado proximity (within 80 km of initial tornado touchdown) soundings from 2003 to 2011 to investigate tornado forecast parameter differences between F1–F2 (weak) and F3+ (strong) nocturnal and daytime tornadoes. The findings suggest that, when considered alone, 0–1- and 0–3-km wind shears show the highest skill in distinguishing environments associated with weak and strong tornadoes, with 0–1-km shear being most effective at night and 0–3-km shear showing the most skill during the day. The results also indicate that combining most unstable CAPE with 0–3-km shear and 0–1-km shear with 3–9-km shear resulted in the most skillful daytime and nighttime forecasts, respectively.

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Larissa J. Reames and David J. Stensrud

Abstract

The world’s population is increasingly concentrated in large urban areas. Many observational and modeling studies have explored how these large, population-dense cities modify local and mesoscale atmospheric phenomena. These modeling studies often use an urban canopy model to parameterize urban surfaces. However, it is unclear whether this approach is appropriate for more suburban cities, such as those found in the Great Plains. Thus, the Weather Research and Forecasting Model was run for a week over Oklahoma City, Oklahoma, and results were compared with observations. Overall, four configurations were examined. Two simulations used the Noah LSM, one with all urban areas removed (CTRL), and the other with urban areas parameterized by a modified Noah land surface model with three urban categories (LSMMOD). Additional simulations utilized a single-layer urban canopy model (SLUCM) either with default urban fraction values (SLUCM1) or with urban fractions taken from the National Land Cover Database (SLUCM2). Results from the three urban runs compared favorably to high-density temperature observations of the urban heat island. The SLUCM1 run was the most realistic, although the urban fractions applied were the least representative of Oklahoma City. All urban runs also produced a drier and deeper planetary boundary layer over the city. The prediction of near-surface winds was most problematic, with the two SLUCM runs unable to correctly reproduce reduced wind speeds over the city. The modified Noah LSM provided best overall agreement with observations and represents a reasonable option for simulating the urban effects of more-suburban cities.

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Larissa J. Reames and David J. Stensrud

Abstract

The effect of urban areas on weakly forced precipitation systems has been studied extensively. However, interactions between urban areas and strongly forced convection, such as supercells, remain relatively unexamined. The present study uses simulations of a supercell to quantify the impacts of a large plains urban area on the evolution and strength of a supercell. An initial ensemble of simulations (CTRLE) of a supercell over homogeneous land use is performed using the WRF-ARW Model. Additionally, 108 simulations are conducted in which the land-use pattern of Dallas–Ft. Worth, Texas, is placed inside the model domain, with the city center shifted to be in or near the path of the supercell. Simulations with urban areas are compared to CTRLE, with the aid of hierarchical clustering analysis to form statistically similar groups of simulations. Clustering analyses identify groups of ensemble members with closely located urban areas that have similar patterns of 0–1-km updraft helicity and near-surface minimum temperature and maximum wind speeds. Comparison of these groups of ensemble members to CTRLE suggests the urban area has a significant impact on storm characteristics, particularly on low-level rotation and mesocyclone track. Simulations where the storm passes to the north of or directly over the city center late in its life cycle deviate most significantly from CTRLE. In these members, low-and midlevel mesocyclone strength increases, and the mesocyclone tracks farther south when compared to CTRLE.

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Jeremy S. Grams, Richard L. Thompson, Darren V. Snively, Jayson A. Prentice, Gina M. Hodges, and Larissa J. Reames

Abstract

A sample of 448 significant tornado events was collected, representing a population of 1072 individual tornadoes across the contiguous United States from 2000 to 2008. Classification of convective mode was assessed from radar mosaics for each event with the majority classified as discrete cells compared to quasi-linear convective systems and clusters. These events were further stratified by season and region and compared with a null-tornado database of 911 significant hail and wind events that occurred without nearby tornadoes. These comparisons involved 1) environmental variables that have been used through the past 25–50 yr as part of the approach to tornado forecasting, 2) recent sounding-based parameter evaluations, and 3) convective mode. The results show that composite and kinematic parameters (whether at standard pressure levels or sounding derived), along with convective mode, provide greater discrimination than thermodynamic parameters between significant tornado versus either significant hail or wind events that occurred in the absence of nearby tornadoes.

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Adam J. Clark, Israel L. Jirak, Burkely T. Gallo, Brett Roberts, Kent. H. Knopfmeier, Robert A. Clark, Jake Vancil, Andrew R. Dean, Kimberly A. Hoogewind, Pamela L. Heinselman, Nathan A. Dahl, Makenzie J. Krocak, Jessica J. Choate, Katie A. Wilson, Patrick S. Skinner, Thomas A. Jones, Yunheng Wang, Gerald J. Creager, Larissa J. Reames, Louis J. Wicker, Scott R. Dembek, and Steven J. Weiss
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