Interactions Between Downslope Flows and a Developing Cold-Air Pool

Burns, Paul Y.; Chemel, Charles

doi:10.1007/s10546-014-9958-7

Cited by 20 publications

(16 citation statements)

References 113 publications

(291 reference statements)

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“…However, in such a shallow valley, thermally driven flows are rapidly suppressed after the evening transition (Clements et al, 2003;Vosper et al, 2014). In a deep and narrow valley dynamically decoupled from the atmosphere above, downslope flows can persist for a longer time, reaching a quasi-steady state if a down-valley flow can develop during the night (Burns and Chemel, 2015;Arduini et al, 2016). Under these conditions, downslope flows are the main driver of the sensible heat flux divergence, which enhances the cooling of the atmosphere in the valley with respect to a flat region nearby.…”

Section: Introductionmentioning

confidence: 99%

Local and non‐local controls on a persistent cold‐air pool in the Arve River Valley

Arduini

Chemel

Staquet

2020

Quart J Royal Meteoro Soc

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A numerical model is used to simulate a persistent cold‐air pool (PCAP) event that occurred in the section of the Arve River Valley around Passy in the French Alps. During this period, an upper‐level ridge from the Atlantic moved over Europe, allowing a PCAP to form and persist over time. The impact of the upper‐level ridge on the PCAP and on the dynamics within the valley section is quantified by examining the mass and heat budgets of the valley atmosphere. During the persistent stage, the magnitude of the flow through the tributary valleys is enhanced by the large‐scale flow. Also, the direction of the flow through one of the tributaries is found to be determined by the height of the PCAP with respect to that of the tributary above the valley floor. The tributary flows, together with subsiding motions at the valley top, control by and large the night‐time valley‐scale circulation and the thermal structure of the upper part of the PCAP, whereas thermally driven valley flows control its lower part. When the upper‐level ridge passes over the Arve River Valley, warm air advection through the tributaries continuously erodes the upper part of the PCAP during night‐time, thereby reducing its depth, while down‐valley flows export the air mass out of the valley. As the ridge moves away from the valley, the near‐surface air is found to be trapped within the valley. This trapping results from the advection of warm air in the upper part of the PCAP by the large‐scale flow channelled through one of the tributaries. This reduces the thermally induced pressure difference in the down‐valley direction, thereby suppressing the near‐surface down‐valley flow. The study therefore highlights the interplay between the large‐scale flow, the tributary flows and the thermal structure of the PCAP.

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Section: Introductionmentioning

confidence: 99%

Local and non‐local controls on a persistent cold‐air pool in the Arve River Valley

Arduini

Chemel

Staquet

2020

Quart J Royal Meteoro Soc

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“…In environmental sciences, aerosols, dust or volcanic ashes settling in the lower atmosphere, as well as marine snow sinking in the upper ocean, face a surrounding fluid medium which may locally comprise large density gradients. The corresponding layers are now recognized as having a strong impact on the settling rate and dispersion characteristics of particles, in the case of both atmospheric inversions (Kellogg 1980;Burns & Chemel 2015) and oceanic thermoclines and haloclines (Riebesell 1992;MacIntyre, Alldredge & Gotschalk 1995). This, in turn, makes the underlying hydrodynamical processes relevant to an understanding of several aspects of air pollution, climate variability or oceanic biochemical cycling (Denman & Gargett 1995;Condie & Bormans 1997).…”

Section: Introductionmentioning

confidence: 99%

Inertial settling of a sphere through an interface. Part 1. From sphere flotation to wake fragmentation

Pierson

Magnaudet

2017

J. Fluid Mech.

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Experiments are performed to better understand the characteristics of the flow induced by the gravity-driven settling of a rigid sphere through a two-layer arrangement of immiscible Newtonian fluids, mostly in inertia-controlled regimes. High-speed video imaging is employed to follow the sphere motion and the deformation of the interface separating the two fluids. The viscosity ratio between the lower and upper fluids is varied by four orders of magnitude, making it possible to observe highly contrasting interface patterns. Depending on the properties of the sphere and the fluids, the sphere may either float steadily at the interface or cross it by pulling a column of the upper fluid into the lower one. This column, which may be axisymmetric or three-dimensional depending on the relative magnitude of inertia effects in the upper fluid, generally pinches off at some position located either close to the initial interface or, more frequently, close to the sphere. Its lower part then recedes towards the sphere, forming a drop which remains attached to its top half. However, when inertia effects in the lower fluid are large enough and the upper fluid is not ‘too’ viscous, the tail quickly undergoes a complete fragmentation, giving birth to a large quantity of filaments and droplets. These various interface configurations are qualitatively analysed using the five independent dimensionless parameters characterizing the system, and regime maps based on the most relevant of them are provided. The influence of several of these parameters on four specific features observed in the course of the experiments, namely the pinch-off position, the floating/sinking transition, the volume of the attached drops and the average size of the droplets formed during the fragmentation process, is examined in detail. A simple model providing qualitative or quantitative predictions is established in each case, and its validity and limitations are assessed against experimental observations.

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“…In the present work the Weather Research and Forecasting (WRF) model [24], version 3.4.1, is used to perform the numerical simulations. The model was run in a large-eddy simulation (LES) mode, as in Catalano and Cenedese (2010) [25], Wagner et al (2014) [26] and Burns and Chemel (2015) [27] for instance. The model uses an Arakawa grid of type C and a terrain following coordinate system based on the dry-hydrostatic pressure.…”

Section: Numerical Modelmentioning

confidence: 99%

“…The left and right panels in each frame of Figure 4 display ∆θ for a section center located at 10 km from the beginning of the valley, namely the exit of U , and at 16 km, namely half way along D, for P1 (frame a)) and P2 (frame b)). Horizontal white lines indicate the top height of the CAP (denoted by CAP top ), defined as in Burns and Chemel (2015) [27] by the local minimum of the vertical gradient of potential temperature above the ground-based inversion layer, averaged over the valley floor. The CAP is shallower in U than in D for P1 and P2, the difference in CAP heights between the two valley sections being more pronounced for P2 than for P1.…”

Section: Along-valley Flowmentioning

confidence: 99%

Impact of Along-Valley Orographic Variations on the Dispersion of Passive Tracers in a Stable Atmosphere

et al. 2019

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A numerical model is used to investigate the transport of passive tracers in an idealized Alpine valley during stable wintertime conditions after the evening transition. The valley is composed of an upstream-valley section, which opens on a narrower downstream valley section, which opens onto a plain. The ratio between the valley-floor widths of the upstream and downstream sections is either 4 (simulation P1) or 11.5 (P2). The change in the thermal structure of the atmosphere in the along-valley direction and over the plain leads to the development of an along-valley flow. This flow is up-valley in the upstream section during the first three hours of the P1 simulation, reversing to the down-valley direction afterwards, but remains up-valley during the six hours of the P2 simulation. The effect of wind dynamics on the dispersion of passive scalars is identified by tracking areas prone to stagnation, recirculation, and ventilation using the methodology developed by Allwine and Whiteman (1994). Zones identified as prone to stagnation are consistent with those of high tracer concentration in both simulations. The narrowing of the valley is found to significantly reduce ventilation in the upstream section, an observation quantified by a ventilation efficiency.

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Interactions Between Downslope Flows and a Developing Cold-Air Pool

Cited by 20 publications

References 113 publications

Local and non‐local controls on a persistent cold‐air pool in the Arve River Valley

Local and non‐local controls on a persistent cold‐air pool in the Arve River Valley

Inertial settling of a sphere through an interface. Part 1. From sphere flotation to wake fragmentation

Impact of Along-Valley Orographic Variations on the Dispersion of Passive Tracers in a Stable Atmosphere

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