Search Results

You are looking at 1 - 10 of 36 items for :

  • Plains Elevated Convection At Night (PECAN) x
  • Refine by Access: All Content x
Clear All
Thomas R. Parish

). It is appropriate to acknowledge the wide variety of local wind maxima described in the literature that have been identified by the term low-level jet (e.g., Stensrud 1996 ). In the discussion that follows, the focus is on the summertime Great Plains nocturnal maximum. Jet profiles linked to transient synoptic disturbances and those tied to the lower branch of a transverse circulation associated with an upper-level jet stream ( Uccellini 1980 ) are not considered. Forcing mechanisms for the

Full access
Yun Lin, Jiwen Fan, Jong-Hoon Jeong, Yuwei Zhang, Cameron R. Homeyer, and Jingyu Wang

forcing and a low background aerosol concentration, we found that the urban aerosol effects on storm intensity and precipitation are much larger than the urban land effect ( Fan et al. 2020 ). Therefore, the relative importance of the urban land effect on storm properties to the urban aerosol effects depends on storm types and associated environmental conditions. We understand the urban land-cover change includes both the surface heat flux change and the surface roughness change. Since the BEP + BEM

Restricted access
Guo Lin, Coltin Grasmick, Bart Geerts, Zhien Wang, and Min Deng

transect of θ υ shows the MCS outflow as a nearly 600-m-deep surface-based cold pool, with slightly (~2 K) lower θ υ than the postfrontal air: the average CRL-retrieved θ υ below 1.10 km MSL was 307 K in the cold-frontal density current (0215–0219 UTC, Fig. 7c ), compared to 305 K in the MCS outflow (0221–0225 UTC). This gradient provides sufficient solenoidal forcing for the lofting of the less dense air mass (e.g., Miao and Geerts 2007 ). The preexisting BL air between the cold front and MCS

Restricted access
Evgeni Fedorovich, Jeremy A. Gibbs, and Alan Shapiro

). Suggested theories of the LLJ point to the jet being a result of the force imbalance in the atmospheric boundary layer induced by the sudden release of the frictional constraint near sunset ( Blackadar 1957 ). In the case when the synoptic-scale pressure gradient is the dominant forcing, the atmospheric flow response to the force imbalance happens in a form of inertial oscillation. The frictional stress, which was not explicitly considered in the Blackadar analysis, was included in the follow-up study

Full access
Thomas R. Parish

results from model simulations (e.g., Zhong et al. 1996 ) show that the summertime LLJ is centered geographically over the southern Great Plains from Texas northward to Nebraska with a maximum over northern Oklahoma and southern Kansas. Two paradigms continue to be espoused in describing the forcing of the LLJ (e.g., Jiang et al. 2007 ; Du and Rotunno 2014 ). The first theory, proposed by Blackadar (1957) , considered the LLJ to be supergeostrophic, the result of an inertial oscillation of the

Full access
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

Full access
Tammy M. Weckwerth, John Hanesiak, James W. Wilson, Stanley B. Trier, Samuel K. Degelia, William A. Gallus Jr., Rita D. Roberts, and Xuguang Wang

systems do not observe temperature, moisture, or wind conditions at this height with adequate temporal and spatial resolutions. To improve the understanding and forecast skill of NCI, the Plains Elevated Convection at Night (PECAN; Geerts et al. 2017 ) field campaign included NCI as a primary scientific objective. This manuscript brings together past NCI research with data from PECAN to document frequencies and forcing mechanisms of different NCI categories. While the impact of NCI in the U.S. Great

Full access
Thomas R. Parish and Richard D. Clark

after sunset. Maximum wind speeds often exceed 20 m s −1 . Wind directions typically shift from southerly early in the evening to southwesterly later on at night. Observations from soundings and profilers indicate that the LLJ is geographically centered over the Great Plains states (e.g., Bonner 1968 ; Mitchell et al. 1995 ; Whiteman et al. 1997 ). Bonner (1968) depicts the center of LLJ frequency over central Oklahoma. The forcing of the LLJ has received considerable attention during the past

Full access
Sean Stelten and William A. Gallus Jr.

challenging. Although previous studies had shown that quantitative precipitation forecast skill increased as the strength of the large-scale forcing increased ( Jankov and Gallus 2004 ; Szoke et al. 2004 ), Duda and Gallus (2013) found that CI forecast skill in 3-km horizontal grid spacing versions of the Weather Research and Forecasting (WRF) Model did not follow such trends. Wilson and Roberts (2006) , however, found that CI tended to be better predicted in a 10-km version of the Rapid Update Cycle

Full access
Alan Shapiro, Evgeni Fedorovich, and Joshua G. Gebauer

LLJ), or along or north of the intersection of an LLJ with a cold front, are more or less clear, but the mechanisms that force ascent on a lateral flank of an LLJ are still not well understood. In this regard, we believe that the recent Pu and Dickinson (2014) explanation for such a mechanism is not wholly satisfactory. In a study of vertical motions in Great Plains LLJs using a North American Regional Reanalysis (NARR) June–July climatology, Pu and Dickinson (2014) suggest that after midnight

Full access