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Luigi Brogno, Francesco Barbano, Laura Sandra Leo, Harindra J. S. Fernando, and Silvana Di Sabatino

1. Introduction The low-level jet (LLJ) is a strong and narrow airstream typically observed within the planetary boundary layer (PBL) ( Stull 1988 ). The LLJ wind speed profile usually takes the shape of a nose, namely, a maximum with a fast decay of the wind speed with height both below and above it. Deviations from this “canonical” shape have been observed in the literature, consisting of wind speed profiles with two noses simultaneously observed along the vertical. These “atypical” LLJs will

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Luigi Brogno, Francesco Barbano, Laura Sandra Leo, Harindra J.S. Fernando, and Silvana Di Sabatino

Abstract

In the realm of boundary-layer flows in complex terrain, low-level jets (LLJs) have received considerable attention, although little literature is available for double-nosed LLJs that remain not well understood. To this end, we use the MATERHORN dataset to demonstrate that double-nosed LLJs developing within the planetary boundary layer (PBL) are common during stable nocturnal conditions and present two possible mechanisms responsible for their formation. It is observed that the onset of a double-nosed LLJ is associated with a temporary shape modification of an already-established LLJ. The characteristics of these double-nosed LLJs are described using a refined version of identification criteria proposed in the literature, and their formation is classified in terms of two driving mechanisms. The wind-driven mechanism encompasses cases where the two noses are associated with different air masses flowing one on top of the other. The wave-driven mechanism involves the vertical momentum transport by an inertial-gravity wave to generate the second nose. The wave-driven mechanism is corroborated by the analysis of nocturnal double-nosed LLJs, where inertial-gravity waves are generated close to the ground by a sudden flow perturbation.

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Manuela Lehner, C. David Whiteman, Sebastian W. Hoch, Derek Jensen, Eric R. Pardyjak, Laura S. Leo, Silvana Di Sabatino, and Harindra J. S. Fernando

and 1700 MST 11 May soundings. As the jet region north of Utah moved farther to the south during the second half of the night, northern Utah came under the influence of a southwesterly flow ( Fig. 3 ). Fig . 3. ERA-Interim reanalysis ( Dee et al. 2011 ) of 850-hPa geopotential height (black solid lines; 50-m contour interval), temperature (white dashed lines; 2-K contour interval), wind (arrows), and relative humidity (color contours). The red symbol indicates the location of Granite Mountain, and

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Hailing Zhang, Zhaoxia Pu, and Xuebo Zhang

Forecasting Model (WRF). In particular, version 3.3 of an Advanced Research version of the WRF (ARW; Skamarock et al. 2008 ) is used for three typical severe weather events (i.e., a low-level jet, a cold front, and a wintertime persistent inversion) over the southern Great Plains (SGP) and the Intermountain West of the United States. Our purposes are not only to examine the ability of the ARW to predict near-surface atmospheric conditions, but also to compare the predictability of near-surface conditions

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H. J. S. Fernando, E. R. Pardyjak, S. Di Sabatino, F. K. Chow, S. F. J. De Wekker, S. W. Hoch, J. Hacker, J. C. Pace, T. Pratt, Z. Pu, W. J. Steenburgh, C. D. Whiteman, Y. Wang, D. Zajic, B. Balsley, R. Dimitrova, G. D. Emmitt, C. W. Higgins, J. C. R. Hunt, J. C. Knievel, D. Lawrence, Y. Liu, D. F. Nadeau, E. Kit, B. W. Blomquist, P. Conry, R. S. Coppersmith, E. Creegan, M. Felton, A. Grachev, N. Gunawardena, C. Hang, C. M. Hocut, G. Huynh, M. E. Jeglum, D. Jensen, V. Kulandaivelu, M. Lehner, L. S. Leo, D. Liberzon, J. D. Massey, K. McEnerney, S. Pal, T. Price, M. Sghiatti, Z. Silver, M. Thompson, H. Zhang, and T. Zsedrovits

(big and small) gaps. Figure 8 shows the measured vertical structure of fully established nocturnal downvalley flow in the basins, where a low-level jet is evident. The IOS-Sagebrush exhibits much cooler surface temperatures and a strong low-level elevated capping inversion that prevents the surface jet from mixing vertically. The larger ground heat flux at IOS-Playa leads to warmer nighttime surface temperatures than at Sagebrush, allowing the nocturnal jet to mix deeper aloft. F ig . 8. (bottom

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Matthew E. Jeglum, Sebastian W. Hoch, Derek D. Jensen, Reneta Dimitrova, and Zachariah Silver

gradient over the top of the tower (orange) and the bottom of the tower (blue). Sample size for all variables is 43 LTFs. Prior to the initiation of the LTF, the flow field ( Fig. 5b ) and stability ( Fig. 5c ) at ES2 indicate the presence of a katabatic flow. The wind is out of the west (downslope), with a jet structure with a peak observed wind speed of 4 m. Relatively strong near-surface stratification of 1°C m −1 is also observed ( Fig. 5c ). These flow characteristics are indicative of katabatic

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Raquel Lorente-Plazas and Joshua P. Hacker

, 2008 : A description of the Advanced Research WRF version 3. NCAR Tech. Note NCAR/TN-475+STR, 113 pp., doi: 10.5065/D68S4MVH . 10.5065/D68S4MVH Tenenbaum , J. , 1996 : Jet stream winds: Comparisons of aircraft observations with analyses . Wea. Forecasting , 11 , 188 – 197 , doi: 10.1175/1520-0434(1996)011<0188:JSWCOA>2.0.CO;2 . 10.1175/1520-0434(1996)011<0188:JSWCOA>2.0.CO;2 Wang , J. , H. L. Cole , D. J. Carlson , E. R. Miller , K. Beierle , A. Paukkunen , and T. K. Laine

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