Tropical Squall Lines of the Arizona Monsoon

Smith, Wally; Gall, Robert

doi:10.1175/1520-0493(1989)117<1553:tslota>2.0.co;2

Cited by 36 publications

(26 citation statements)

References 54 publications

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“…Significant 24-h changes in the midlevel wind field and shear vectors (most notably from 12 to 13 July over Sonora and Sinaloa) are accompanied by significant shifts in the location of MCS activity. These results support the positive relationship between shear and MCS activity over the NAM region found in other studies (Smith and Gall 1989;Farfán and Zehnder 1994;Lang et al 2007). …”

Section: Mechanisms For Convective Organization On Leading Flank Osupporting

confidence: 92%

“…Disturbed days, on the other hand, are characterized by a tendency for precipitating systems to grow upscale, organize into mesoscale convective systems (MCSs), and coherently propagate toward the coastal lowlands. Past studies indicate that increases in midlevel easterlies and vertical wind shear can promote MCS development over the NAM region (Smith and Gall 1989;Farfá n and Zehnder 1994;Lang et al 2007). …”

Section: Introductionmentioning

confidence: 99%

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Observational Analysis of an Upper-Level Inverted Trough during the 2004 North American Monsoon Experiment

Finch

Johnson

2010

Monthly Weather Review

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Upper-level inverted troughs (IVs) associated with midlatitude breaking Rossby waves or tropical uppertroposphere troughs (TUTTs) have been identified as important contributors to the variability of rainfall in the North American monsoon (NAM) region. However, little attention has been given to the dynamics of these systems owing to the sparse observational network over the NAM region. High temporal and spatial observations taken during the 2004 North American Monsoon Experiment (NAME) are utilized to analyze a significant IV that passed over northwestern Mexico from 10 to 13 July 2004. The Colorado State University gridded dataset, which is independent of model analysis over land, is the primary data source used in this study.Results show that the 10-13 July IV disturbance was characterized by a warm anomaly around 100 hPa and a cold anomaly that extended from 200 to 700 hPa. The strongest cyclonic circulation was in the upper levels around 200 hPa. Quasigeostrophic (QG) diagnostics indicate that the upper-level low forced weak subsidence (weak rising motion) to the west (east) of its center. Net downward motion to the west was a result of the Laplacian of thermal advection (forcing subsidence) outweighing differential vorticity advection (forcing weak upward motion). Despite the QG forcing of downward motion west of the upper-level IV, enhanced convection occurred west of the IV center along the western slopes of the Sierra Madre Occidental (SMO). This seemingly contradictory behavior can be explained by noting that the upper-level IV induced a midlevel cyclonic circulation, with northeasterly (southeasterly) midlevel flow to the west (east) of its center. Increased mesoscale organization of convection along the SMO foothills was found to be collocated with IV-enhanced northeasterly midlevel flow and anomalous northeasterly shear on the western (leading) flank of the system. It is proposed that the upper-level IV increased the SMO-perpendicular midlevel flow as well as the wind shear, thereby creating an environment favorable for convective storms to grow upscale as they moved off the high terrain.

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Section: Mechanisms For Convective Organization On Leading Flank Osupporting

confidence: 92%

Section: Introductionmentioning

confidence: 99%

Observational Analysis of an Upper-Level Inverted Trough during the 2004 North American Monsoon Experiment

Finch

Johnson

2010

Monthly Weather Review

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“…The environmental regime investigated here is often observed in pre-squall line environments where high convective available potential energy (CAPE) and a convective inhibition (CINH) layer exist (Schmidt and Cotton 1989;Smith and Gall 1989;Bryan and Parker 2010). An example of this can be seen in Fig.…”

Section: Introductionmentioning

confidence: 98%

Simulated Density Currents beneath Embedded Stratified Layers

Seigel

Heever

2012

Journal of the Atmospheric Sciences

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The goal of this research is to investigate the impacts of a stably stratified layer embedded within a neutrally stratified environment on the behavior of density currents in an effort to extend the environmental regimes examined by Liu and Moncrieff. Such environments frequently support severe weather events. To accomplish this goal, nonhydrostatic numerical model experiments are performed in which the strength and height of the embedded stably stratified layer within a neutrally stratified environment are varied. The 1-km-deep stable layer base is varied between 1, 2, and 3 km AGL. Additionally, the strength of the stable layer is systematically varied between Brunt-Vä isä lä frequencies of 0.006, 0.012, and 0.018 s 21 , following the methodology of Liu and Moncrieff. The model and grid setup are also similar to that of Liu and Moncrieff, utilizing the Arakawa C grid, leapfrog advection, a Robert-Asselin filter, and grid spacing of 100 and 50 m in the horizontal and vertical directions, respectively. Results show that the height of the density current decreases and the propagation speed increases with stronger and lower stable layers, provided that the stable layer is sufficiently thin so as to not act as a gravity wave ducting layer. As the strength of the stable layer increases and the height of this layer decreases, the horizontal pressure gradient driving the density current increases, resulting in faster propagation speeds. Such results have implications for cold pool propagation into more stable environments.

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“…Severe weather in the semiarid-arid climate of the southwestern United States is principally the result of organized convective events that take place during the late summer months and particularly at lower elevations (e.g., Schmidli 1986;McCollum et al 1995;Smith and Gall 1989;Wall et al 2012). These more severe convective events, as well as warm-season rainfall in general in northwestern Mexico and much of the southwestern United States, occur during the North American monsoon (NAM; Adams and Comrie 1997).…”

Section: Introductionmentioning

confidence: 99%

Objective Climatological Analysis of Extreme Weather Events in Arizona during the North American Monsoon

Mazon

Castro

Adams

et al. 2016

Journal of Applied Meteorology and Climatology

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Almost one-half of the annual precipitation in the southwestern United States occurs during the North American monsoon (NAM). Given favorable synoptic-scale conditions, organized monsoon thunderstorms may affect relatively large geographic areas. Through an objective analysis of atmospheric reanalysis and observational data, the dominant synoptic patterns associated with NAM extreme events are determined for the period from 1993 to 2010. Thermodynamically favorable extreme-weather-event days are selected on the basis of atmospheric instability and precipitable water vapor from Tucson, Arizona, rawinsonde data. The atmospheric circulation patterns at 500 hPa associated with the extreme events are objectively characterized using principal component analysis. The first two dominant modes of 500-hPa geopotential-height anomalies of the severe-weather-event days correspond to type-I and type-II severe-weather-event patterns previously subjectively identified by Maddox et al. These patterns reflect a positioning of the monsoon ridge to the north and east or north and west, respectively, from its position in the ''Four Corners'' region during the period of the climatological maximum of monsoon precipitation from mid-July to mid-August. An hourly radar-gauge precipitation product shows evidence of organized, westward-propagating convection in Arizona during the type-I and type-II severe weather events. This new methodological approach for objectively identifying severe weather events may be easily adapted to inform operational forecasting or analysis of gridded climate data.

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Tropical Squall Lines of the Arizona Monsoon

Cited by 36 publications

References 54 publications

Observational Analysis of an Upper-Level Inverted Trough during the 2004 North American Monsoon Experiment

Observational Analysis of an Upper-Level Inverted Trough during the 2004 North American Monsoon Experiment

Simulated Density Currents beneath Embedded Stratified Layers

Objective Climatological Analysis of Extreme Weather Events in Arizona during the North American Monsoon

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