scholarly journals Low Level Jets in the Pantanal Wetland Nocturnal Boundary Layer – Case Studies

2013 ◽  
Vol 3 (1) ◽  
pp. 32-47 ◽  
Author(s):  
Hardiney S. Martins ◽  
Leonardo D. A. Sá ◽  
Osvaldo L. L. Moraes
Keyword(s):  
Boundary Layer ◽  
Case Studies ◽  
Nocturnal Boundary Layer ◽  
Low Level ◽  
Low Level Jets ◽  
Pantanal Wetland

Pollutant vertical mixing in the nocturnal boundary layer enhanced by density currents and low-level jets: two representative case studies

2019 ◽  
Vol 174 (2) ◽  
pp. 203-230 ◽  
Author(s):  
Mireia Udina ◽  
Maria Rosa Soler ◽  
Miriam Olid ◽  
Bernat Jiménez-Esteve ◽  
Joan Bech
Keyword(s):  
Boundary Layer ◽  
Case Studies ◽  
Vertical Mixing ◽  
Nocturnal Boundary Layer ◽  
Density Currents ◽  
Representative Case ◽  
Low Level ◽  
Low Level Jets

Impact of Low-Level Jets on the Nocturnal Urban Heat Island Intensity in Oklahoma City

2013 ◽  
Vol 52 (8) ◽  
pp. 1779-1802 ◽  
Author(s):  
Xiao-Ming Hu ◽  
Petra M. Klein ◽  
Ming Xue ◽  
Julie K. Lundquist ◽  
Fuqing Zhang ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Critical Role ◽  
Vertical Mixing ◽  
Atmospheric Stability ◽  
Nocturnal Boundary Layer ◽  
Oklahoma City ◽  
Near Surface ◽  
Low Level ◽  
Urban Heat ◽  
Low Level Jets

AbstractPrevious analysis of Oklahoma City (OKC), Oklahoma, temperature data indicated that urban heat islands (UHIs) frequently formed at night and the observed UHI intensity was variable (1°–4°C). The current study focuses on identifying meteorological phenomena that contributed to the variability of nocturnal UHI intensity in OKC during July 2003. Two episodes, one with a strong UHI signature and one with a weak signature, were studied in detail using observations along with simulations with the Weather Research and Forecasting model. Mechanical mixing associated with low-level jets (LLJs) played a critical role in moderating the nocturnal UHI intensity. During nights with weak LLJs or in the absence of LLJs, vertical mixing weakened at night and strong temperature inversions developed in the rural surface layer as a result of radiative cooling. The shallow stable boundary layer (SBL < 200 m) observed under such conditions was strongly altered inside the city because rougher and warmer surface characteristics caused vertical mixing that eroded the near-surface inversion. Accordingly, temperatures measured within the urban canopy layer at night were consistently higher than at nearby rural sites of comparable height (by ~3°–4°C). During nights with strong LLJs, however, the jets facilitated enhanced turbulent mixing in the nocturnal boundary layer. As a consequence, atmospheric stability was much weaker and urban effects played a much less prominent role in altering the SBL structure; therefore, UHI intensities were smaller (<1°C) during strong LLJs. The finding that rural inversion strength can serve as an indicator for UHI intensity highlights that the structure of the nocturnal boundary layer is important for UHI assessments.

scholarly journals Observations of the Nocturnal Boundary Layer Associated with the West African Monsoon

2010 ◽  
Vol 138 (8) ◽  
pp. 3142-3156 ◽  
Author(s):  
Caroline L. Bain ◽  
Douglas J. Parker ◽  
Christopher M. Taylor ◽  
Laurent Kergoat ◽  
Françoise Guichard
Keyword(s):  
Boundary Layer ◽  
Case Studies ◽  
Large Scale ◽  
Surface Wind ◽  
West African Monsoon ◽  
Moisture Flux ◽  
Nocturnal Boundary Layer ◽  
Near Surface ◽  
Low Level ◽  
Strong Inversion

Abstract A set of nighttime tethered balloon and kite measurements from the central Sahel (15.2°N, 1.3°W) in August 2005 were acquired and analyzed. A composite of all the nights’ data was produced using boundary layer height to normalize measured altitudes. The observations showed some typical characteristics of nocturnal boundary layer development, notably a strong inversion after sunset and the formation of a low-level nocturnal jet later in the night. On most nights, the sampled jet did not change direction significantly during the night. The boundary layer thermodynamic structure displayed some variations from one night to the next. This was investigated using two contrasting case studies from the period. In one of these case studies (18 August 2005), the low-level wind direction changed significantly during the night. This change was captured well by two large-scale models, suggesting that the large-scale dynamics had a significant impact on boundary layer winds on this night. For both case studies, the models tended to underestimate near-surface wind speeds during the night, which is a feature that may lead to an underestimation of moisture flux northward by models.

Role of low-level jets and boundary-layer properties on the NBL budget technique

2005 ◽  
Vol 135 (1-4) ◽  
pp. 35-43 ◽  
Author(s):  
N. Mathieu ◽  
I.B. Strachan ◽  
M.Y. Leclerc ◽  
A. Karipot ◽  
E. Pattey
Keyword(s):  
Boundary Layer ◽  
Low Level ◽  
Low Level Jets

scholarly journals Summertime Low-Level Jets over the High-Latitude Arctic Ocean

2008 ◽  
Vol 47 (6) ◽  
pp. 1770-1784 ◽  
Author(s):  
Douglas O. ReVelle ◽  
E. Douglas Nilsson
Keyword(s):  
Boundary Layer ◽  
Eddy Viscosity ◽  
Low Level ◽  
Rayleigh Friction ◽  
Low Level Jets ◽  
Low Level Jet ◽  
Horizontal Momentum ◽  
Ocean Observations ◽  
Measured Wind Speed ◽  
Good Agreement

Abstract The application of a simple analytic boundary layer model developed by Thorpe and Guymer did not produce good agreement with observational data for oceanic low-level jet observations even though this model has worked well for the predictions of low-level jets over continental surfaces. This failure to properly predict the boundary layer wind maxima was very puzzling because more detailed numerical boundary layer models have properly predicted these low-level oceanic wind maxima. To understand the reasons for its failure to explain the ocean observations, the authors modified the frictional terms in the horizontal linear momentum equations of Thorpe and Guymer, using a standard eddy viscosity closure technique instead of the Rayleigh friction parameterization originally used. This improvement in the modeling of the dissipation terms, which has resulted in the use of an enhanced Rayleigh friction parameterization in the horizontal momentum equations, modified the boundary layer winds such that the continental predictions remained nearly identical to those predicted previously using the Thorpe and Guymer model while the oceanic predictions have now become more representative of the measured wind speed from recent Arctic expeditions.

scholarly journals Impact of the vertical mixing induced by low-level jets on boundary layer ozone concentration

2013 ◽  
Vol 70 ◽  
pp. 123-130 ◽  
Author(s):  
Xiao-Ming Hu ◽  
Petra M. Klein ◽  
Ming Xue ◽  
Fuqing Zhang ◽  
David C. Doughty ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Ozone Concentration ◽  
Vertical Mixing ◽  
Low Level ◽  
Low Level Jets ◽  
Boundary Layer Ozone

Linkages Between Boundary-Layer Structure and the Development of Nocturnal Low-Level Jets in Central Oklahoma

2015 ◽  
Vol 158 (3) ◽  
pp. 383-408 ◽  
Author(s):  
Petra M. Klein ◽  
Xiao-Ming Hu ◽  
Alan Shapiro ◽  
Ming Xue
Keyword(s):  
Boundary Layer ◽  
Layer Structure ◽  
Boundary Layer Structure ◽  
Low Level ◽  
Low Level Jets

scholarly journals Driving Mechanisms of Double-Nosed Low-Level Jets during MATERHORN Experiment

2021 ◽  
Author(s):  
Luigi Brogno ◽  
Francesco Barbano ◽  
Laura Sandra Leo ◽  
Harindra J.S. Fernando ◽  
Silvana Di Sabatino
Keyword(s):  
Boundary Layer ◽  
Complex Terrain ◽  
Boundary Layer Flows ◽  
Shape Modification ◽  
Air Masses ◽  
Flow Perturbation ◽  
Low Level ◽  
Driving Mechanisms ◽  
Low Level Jets ◽  
Refined Version

AbstractIn 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.

scholarly journals Numerical Study of Nocturnal Low-Level Jets over Gently Sloping Terrain

2017 ◽  
Vol 74 (9) ◽  
pp. 2813-2834 ◽  
Author(s):  
Evgeni Fedorovich ◽  
Jeremy A. Gibbs ◽  
Alan Shapiro
Keyword(s):  
Boundary Layer ◽  
Numerical Study ◽  
Potential Temperature ◽  
Slope Angle ◽  
Vertical Position ◽  
Low Level ◽  
Evening Transition ◽  
Low Level Jets ◽  
Sloping Terrain ◽  
Surface Buoyancy

Abstract Nocturnal low-level jets (LLJs) over gently sloping terrain typical of the U.S. Great Plains are investigated by means of direct numerical simulation. Such LLJs develop in a tilted atmospheric boundary layer as a result of inertia–gravity oscillations initiated by a change of the surface thermal forcing during the evening transition. External parameters are the free-atmospheric geostrophic wind, ambient atmospheric stratification, surface buoyancy forcing, and slope angle. The governing momentum and buoyancy balance equations are written in slope-following coordinates, and solved numerically in the Boussinesq approximation. The surface forcing is prescribed in a form of surface buoyancy or buoyancy flux, both of which are slope-uniform but change in time. LLJs over slopes are contrasted with LLJs over flat terrain. Slope-induced effects essentially modify the entire structure of nocturnal LLJs. The shape of the LLJ wind profile over a slope is characterized by a sharper and larger-magnitude maximum. The presence of the slope causes the along-slope advection of environmental potential temperature during the night. This advection can reignite static instability in the LLJ flow developing after the evening transition. The resulting turbulence leads to a complete or partial remix of the boundary layer flow and drastically changes the appearance of the LLJ in terms of its shape and vertical position. A pronounced nighttime jet can also develop from the daytime convective boundary layer in the absence of any free-atmospheric geostrophic forcing. The daytime flow preconditioning, an important precursor of the nocturnal LLJ development, plays an especially important role in LLJs over a slope.

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