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Peter C. Banacos and David M. Schultz

Abstract

Moisture flux convergence (MFC) is a term in the conservation of water vapor equation and was first calculated in the 1950s and 1960s as a vertically integrated quantity to predict rainfall associated with synoptic-scale systems. Vertically integrated MFC was also incorporated into the Kuo cumulus parameterization scheme for the Tropics. MFC was eventually suggested for use in forecasting convective initiation in the midlatitudes in 1970, but practical MFC usage quickly evolved to include only surface data, owing to the higher spatial and temporal resolution of surface observations. Since then, surface MFC has been widely applied as a short-term (0–3 h) prognostic quantity for forecasting convective initiation, with an emphasis on determining the favorable spatial location(s) for such development.

A scale analysis shows that surface MFC is directly proportional to the horizontal mass convergence field, allowing MFC to be highly effective in highlighting mesoscale boundaries between different air masses near the earth’s surface that can be resolved by surface data and appropriate grid spacing in gridded analyses and numerical models. However, the effectiveness of boundaries in generating deep moist convection is influenced by many factors, including the depth of the vertical circulation along the boundary and the presence of convective available potential energy (CAPE) and convective inhibition (CIN) near the boundary. Moreover, lower- and upper-tropospheric jets, frontogenesis, and other forcing mechanisms may produce horizontal mass convergence above the surface, providing the necessary lift to bring elevated parcels to their level of free convection without connection to the boundary layer. Case examples elucidate these points as a context for applying horizontal mass convergence for convective initiation. Because horizontal mass convergence is a more appropriate diagnostic in an ingredients-based methodology for forecasting convective initiation, its use is recommended over MFC.

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Peter C. Banacos and Howard B. Bluestein

Abstract

Although the relationship between the behavior of convective storms and their environmental vertical wind shear has been examined using proximity soundings and idealized numerical modeling experiments, the manner in which the vertical shear profiles, as visualized by hodographs, is regulated by the larger-scale baroclinic wave structure has not been considered in detail. To examine this synoptic-scale dependence, a relatively simple, analytic model for baroclinic systems in midlatitudes having exact solutions for a frictionless, quasigeostrophic atmosphere is employed. The analytical model consists of a checkerboard of high and low pressure areas at 1000 mb, hydrostatically modulated above by a mean meridional temperature gradient and a checkerboard of warm and cold centers at 1000 mb. Aloft, the model atmosphere consists of a zonally oriented wave train. This approach allows a systematic examination of the dependence of hodographs on the following five synoptic-scale parameters included in the model: 1) mean meridional temperature gradient, 2) system wavelength, 3) phase lag between the height and temperature fields at 1000 mb, 4) magnitude of the temperature perturbation associated with the checkerboard of warm and cold centers at 1000 mb, and 5) magnitude of the 1000-mb height perturbation.

It is seen that the phase lag between the height and temperature fields and the system wavelength have the greatest quantitative influence on the relative contribution of the ageostrophic wind component to the total wind. These two parameters are associated with significant clockwise curvature with height in the hodograph of the total wind, particularly if the deep-layer ageostrophic wind shear is oriented perpendicular and to the right of the geostrophic shear. Hodograph curvature, however, is not ubiquitous in the model, and despite the model's simplicity, likely speaks to the importance of features departing from the model, mesoscale variability, and boundary layer friction in enhancing hodograph curvature.

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Howard B. Bluestein and Peter C. Banacos

Abstract

A climatological analysis, based upon operational surface and upper-air data from 1957 to 1994, of the wind and temperature profiles composited with respect to each quadrant of surface cyclones and anticyclones, is presented for the eastern two-thirds of the United States. The cyclones and anticyclones are located via an objective procedure. Hodographs and soundings are also composited with respect to season, geographic region, time of day, and, for cyclones only, intensity. Vertical profiles of the static-stability parameter are composited with respect to season and quadrant for both cyclones and anticyclones. The structures of mean cyclones and anticyclones are shown and discussed.

A diurnal variation in hodographs (vertical shear) is found, which shows up in both cyclones and anticyclones as a rotation in the counterclockwise direction between 0000 and 1200 UTC above the boundary layer. The effect is greater in cyclones than in anticyclones. This variation is hypothesized to be in part due to a tidal oscillation and in part due to radiative–thermal effects.

In the mean, a well-pronounced equatorward-directed low-level jet is resolved in the northwest quadrant of surface cyclones. Low-level jets do not show up in the mean in other quadrants of cyclones or in anticyclones. The curvature of hodographs near the tropopause is clockwise in cyclones and counterclockwise in anticyclones.

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Peter C. Banacos and Michael L. Ekster

Abstract

The occurrence of rare but significant severe weather events associated with elevated mixed-layer (EML) air in the northeastern United States is investigated herein. A total of 447 convective event days with one or more significant severe weather report [where significant is defined as hail 2 in. (5.1 cm) in diameter or greater, a convective gust of 65 kt (33 m s−1) or greater, and/or a tornado of F2 or greater intensity] were identified from 1970 through 2006 during the warm season (1 May–30 September). Of these, 34 event days (7.6%) were associated with identifiable EML air in regional rawinsondes preceding the event. Taken with two other noteworthy events in 1953 and 1969, a total of 36 significant severe weather events associated with EML air were studied via composite and trajectory analysis. Though a small percentage of the total, these 36 events compose a noteworthy list of historically significant derechos and tornadic events to affect the northeastern United States. It is demonstrated that plumes of EML air emanating from the Intermountain West in subsiding, anticyclonically curved flows can reinforce the capping inversion and maintain the integrity of the EML across the central United States over a few days. The EML plume can ultimately become entrained into a moderately fast westerly to northwesterly midtropospheric flow allowing for the plume’s advection into the northeastern United States. Resultant thermodynamic conditions in the convective storm environment are similar to those more typically observed closer to the EML source region in the Great Plains of the United States. In addition to composite and trajectory analysis, two case studies are employed to demonstrate salient and evolutionary aspects of the EML in such events. A lapse rate tendency equation is explored to put EML advection in context with other processes affecting lapse rate.

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Kimberly L. Elmore, Steven J. Weiss, and Peter C. Banacos

Abstract

From 15 July through 30 September of 2001, an ensemble cloud-scale model was run for the Storm Prediction Center on a daily basis. Each ensemble run consisted of 78 members whose initial conditions were derived from the 20-km Rapid Update Cycle Model, the 22-km operational Eta Model, and a locally run version of the 22-km Eta Model using the Kain–Fritsch convective parameterization. Each ensemble was run over a 160 km × 160 km region and was valid for the 9-h period from 1630 through 0130 UTC. The ensembles were used primarily to provide severe-weather guidance. To that end, model storms with lifetimes greater than 60 min and/or a sustained correlation of at least 0.5 between midlevel updrafts and positive vorticity (the supercell criterion) were considered to be severe-weather indicators. Heidke skill scores, along with the true skill statistic, are between 0.2 and 0.3 when long-lived storms or storms meeting the supercell criteria are used as severe-weather indicators. Equivalent skill scores result when modeled and observed storms are categorized by lifetime and supercell characteristics and compared with expertly interpreted radar data.

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