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Stacey M. Hitchcock and Russ S. Schumacher

several different methods of classification, but two distinct categories consistently emerge for events over the central Great Plains. In synoptic-type events, a strong midtropospheric trough and slow moving surface front lead to strong forcing for ascent in a region with southerly flow and associated moisture transport. During the warm season, isentropic ascent of warm, moist air transported by the nocturnal low-level jet (LLJ) can lift an air to saturation on the cool side of a stationary or warm

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J. W. Wilson, S. B. Trier, D. W. Reif, R. D. Roberts, and T. M. Weckwerth

storms in the current case. However, when this effect is combined with the large 800–600-hPa vertical shear in the layer near and above cloud base ( Fig. 7 ), which itself is detrimental to the early stages of CI, and the fact that the mesoscale forcing in this case ( Fig. 10b ) was not strong, it is not surprising that relatively few deep convective storms occurred. b. Winds and thermodynamic stability Insight into the winds and stability in the vicinity of CI-H is provided in Fig. 14 . Shown are

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Thomas R. Parish

central United States during the 1993 flood-altered terrain simulations . Mon. Wea. Rev. , 132 , 396 – 403 , doi: 10.1175/1520-0493(2004)132<0396:ROTIFL>2.0.CO;2 . Parish , T. R. , and L. D. Oolman , 2010 : On the role of sloping terrain in the forcing of the Great Plains low-level jet . J. Atmos. Sci. , 67 , 2690 – 2699 , doi: 10.1175/2010JAS3368.1 . Parish , T. R. , and D. Leon , 2013 : Measurement of cloud perturbation pressures using an instrumented aircraft . J. Atmos. Oceanic

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Manda B. Chasteen, Steven E. Koch, and David B. Parsons

simultaneously (e.g., Schumacher 2015 ). Additionally, strong dynamical forcing associated with mature convection may be sufficient to lift conditionally unstable air within the near-surface stable layer to its level of free convection (LFC), enabling nocturnal convection to remain surface based (e.g., Parker 2008 ; Nowotarski et al. 2011 ; Billings and Parker 2012 ). The degree to which nocturnal convection is surface based is often uncertain and thus creates complications for forecasters, who may

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Coltin Grasmick, Bart Geerts, David D. Turner, Zhien Wang, and T. M. Weckwerth

night of 15 July 2015 in western Kansas as part of the Plains Elevated Convection at Night (PECAN) field campaign ( Geerts et al. 2017 ). This MCS propagated forward (to the east) relatively rapidly, faster than the deep-layer mean westerly flow. Such MCS motion commonly occurs from discrete propagation ( Bodine and Rasmussen 2017 ), during which the MCS merges with new convective cells triggered by cold pool or gravity wave forcing ( Crook and Moncrieff 1988 ; Corfidi 2003 ; Fovell et al. 2006

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David M. Loveless, Timothy J. Wagner, David D. Turner, Steven A. Ackerman, and Wayne F. Feltz

( Knupp 2006 ), while Parsons et al. (2019) identified an average of 900 m using 13 cases. Mechanical lifting caused by bore passages frequently results in cloud formation (e.g., Clarke et al. 1981 ; Smith et al. 1982 ; Knupp 2006 ; Coleman et al. 2010 ) and may trigger convective initiation if parcels are lifted to their level of free convection (LFC) ( Karyampudi et al. 1995 ; Locatelli et al. 2002 ; Koch et al. 2008 ). Studies by Knupp (2006) and Koch et al. (2008) have suggested that

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Joshua G. Gebauer, Alan Shapiro, Evgeni Fedorovich, and Petra Klein

smaller than the 13-km grid spacing used by the RAP, this result does suggest that RAP parameterized convection may be used to help indicate the presence of mesoscale forcing likely to be supportive of the CI. However, a finer-scale, convection-allowing model (e.g., HRRR) may be necessary to identify the cloud-scale processes that potentially contribute to the considered CI events. Since the focus of this study is on the role of the LLJ in initiating convection initiation and not the cloud

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Samuel K. Degelia, Xuguang Wang, and David J. Stensrud

June event featured forcing mechanisms across a spectrum of scales (e.g., shortwave trough, LLJ, outflow boundary), a multiscale DA approach is used like that described in Degelia et al. (2018) . Sensitivity tests are performed to determine the best covariance localization radii ( Table 3 ) for each PECAN observation type described in the previous section. These settings are tuned to produce the highest fractions skill score (FSS; discussed in section 4a ) for the nocturnal CI event of interest

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Elizabeth N. Smith, Joshua G. Gebauer, Petra M. Klein, Evgeni Fedorovich, and Jeremy A. Gibbs

1. Introduction Wind maxima called nocturnal low-level jets (NLLJs) often occur during the night in the lowest kilometer of the atmosphere. In the most general sense, the NLLJ is the result of the disruption of the daytime force balance between the Coriolis, pressure gradient, and frictional forces. Once the sun sets, thermally generated turbulence decays, and the stable boundary layer (SBL) forms. The frictional force weakens above the surface, which eliminates the force balance and leads to

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Stanley B. Trier, James W. Wilson, David A. Ahijevych, and Ryan A. Sobash

categories: 1) strong synoptic forcing associated with a cold front and a midtropospheric short wave ( Fig. 1b ), 2) interaction of a nocturnal LLJ with a quasi-stationary lower-tropospheric front ( Figs. 1a,d,e ), and 3) a nocturnal LLJ located immediately downstream from a midtropospheric ridge axis in the absence of a well-defined surface front ( Fig. 1c ). Fig . 1. Mean 500-hPa geopotential height and horizontal winds with 850-hPa wind speed (shaded) from DART ensemble analyses (see section 3c ) at

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