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

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.

Download Full-text

Double-Nosed Low-Level Jets over Complex Terrain: Driving Mechanisms and Analytical Modeling

10.5194/egusphere-egu21-563 ◽

2021 ◽

Author(s):

Luigi Brogno ◽

Francesco Barbano ◽

Laura Sandra Leo ◽

Harindra Joseph Fernando ◽

Silvana Di Sabatino

Keyword(s):

Wind Speed ◽

Complex Terrain ◽

Structural Characteristics ◽

Careful Examination ◽

Simultaneous Occurrence ◽

Inertial Oscillation ◽

Flow Perturbation ◽

Low Level ◽

Driving Mechanisms ◽

Low Level Jets

Low-level jets (LLJs) are a peculiar feature of the nocturnal Planetary Boundary Layer (PBL) and they have been extensively observed both in flat and complex terrain configurations. On the contrary, double-nosed LLJs have been rarely investigated. They essentially consist in the simultaneous occurrence of two noses (i.e. two wind-speed maxima) within the PBL vertical profile of wind speed, but their origin and mechanisms remain rather unclear.Data collected in an open valley during the MATERHORN field experiment are used here first to demonstrate that double-nosed LLJs are frequently observed at the site during stable nocturnal conditions, and second to describe the mechanisms that drive their formation. Structural characteristics of these double-nosed LLJs are originally described using refined criteria proposed in the literature.Two driving mechanisms for double-nosed LLJs are newly proposed in the current study. The first mechanism is wind-driven, in which the two noses are associated with different air masses flowing one on top of the other. The second mechanism is wave-driven, in which a flow perturbation generates an inertial-gravity wave. This wave vertically transports momentum causing the occurrence of a secondary nose, leading to the formation of a double-nosed LLJ. Careful examination of the temporal evolution of these events also revealed the short-lived and transitional nature of the secondary nose in both the mechanisms, as opposite to the primary nose whose evolution appeared instead driven by inertial oscillations. Application of two analytical inertial-oscillation models retrieved from the literature confirms this hypothesis. Indeed, both models satisfactorily reproduce the observed single-nosed LLJs but fail to capture the temporary formation of the secondary nose.

Download Full-text

Simulated Diurnal Cycles and Seasonal Variability of Low-Level Jets in the Boundary Layer over Complex Terrain on the Coast of Southeast China

Journal of Geophysical Research Atmospheres ◽

10.1002/2017jd026685 ◽

2017 ◽

Vol 122 (20) ◽

pp. 10,594-10,611

Author(s):

Min Shao ◽

Qin Geng Wang ◽

Jianjun Xu

Keyword(s):

Boundary Layer ◽

Seasonal Variability ◽

Complex Terrain ◽

Southeast China ◽

Low Level ◽

Diurnal Cycles ◽

Low Level Jets

Download Full-text

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

Agricultural and Forest Meteorology ◽

10.1016/j.agrformet.2005.10.001 ◽

2005 ◽

Vol 135 (1-4) ◽

pp. 35-43 ◽

Cited By ~ 25

Author(s):

N. Mathieu ◽

I.B. Strachan ◽

M.Y. Leclerc ◽

A. Karipot ◽

E. Pattey

Keyword(s):

Boundary Layer ◽

Low Level ◽

Low Level Jets

Download Full-text

Assessing the influence of complex terrain on severe convective environments in northeastern Alabama

Weather and Forecasting ◽

10.1175/waf-d-20-0136.1 ◽

2021 ◽

Author(s):

Branden Katona ◽

Paul Markowski

Keyword(s):

Boundary Layer ◽

Complex Terrain ◽

Wind Shear ◽

Vertical Wind Shear ◽

Vertical Wind ◽

Spatial Distributions ◽

Convective Storm ◽

Self Organizing Maps ◽

Low Level ◽

Wind Regimes

AbstractStorms crossing complex terrain can potentially encounter rapidly changing convective environments. However, our understanding of terrain-induced variability in convective stormenvironments remains limited. HRRR data are used to create climatologies of popular convective storm forecasting parameters for different wind regimes. Self-organizing maps (SOMs) are used to generate six different low-level wind regimes, characterized by different wind directions, for which popular instability and vertical wind shear parameters are averaged. The climatologies show that both instability and vertical wind shear are highly variable in regions of complex terrain, and that the spatial distributions of perturbations relative to the terrain are dependent on the low-level wind direction. Idealized simulations are used to investigate the origins of some of the perturbations seen in the SOM climatologies. The idealized simulations replicate many of the features in the SOM climatologies, which facilitates analysis of their dynamical origins. Terrain influences are greatest when winds are approximately perpendicular to the terrain. In such cases, a standing wave can develop in the lee, leading to an increase in low-level wind speed and a reduction in vertical wind shear with the valley lee of the plateau. Additionally, CAPE tends to be decreased and LCL heights are increased in the lee of the terrain where relative humidity within the boundary layer is locally decreased.

Download Full-text

Baroclinic and orographic low-level jets in the atmospheric boundary layer over the Fram Strait and their influence on the atmosphere-ocean energy exchange

Proceedings of the International Symposium “Mesoscale and Submesoscale Processes in the Hydrosphere and Atmosphere” (MSP-2018), 30 October – 02 November 2018, Moscow / [Ed. by: Zatsepin A.G., Ginzburg A.I., Kostianoy A.G., Sviridov S.A.] ◽

10.29006/978-5-9901449-4-1-2018-106 ◽

2018 ◽

Author(s):

Dmitry Gennadyevich Chechin ◽

Keyword(s):

Boundary Layer ◽

Atmospheric Boundary Layer ◽

Energy Exchange ◽

Fram Strait ◽

Ocean Energy ◽

Low Level ◽

Low Level Jets

Download Full-text

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

Journal of Applied Meteorology and Climatology ◽

10.1175/2007jamc1637.1 ◽

2008 ◽

Vol 47 (6) ◽

pp. 1770-1784 ◽

Cited By ~ 11

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.

Download Full-text