scholarly journals Wintertime Cold-Air Pools in the Bavarian Danube Valley Basin: Data Analysis and Idealized Numerical Simulations

2005 ◽  
Vol 44 (12) ◽  
pp. 1950-1971 ◽  
Author(s):  
Günther Zängl
Keyword(s):  
Numerical Simulations ◽  
Observational Data ◽  
Wind Direction ◽  
Vertical Mixing ◽  
Cold Pool ◽  
Cold Air ◽  
Ambient Wind ◽  
Mountain Ranges ◽  
Alpine Foreland ◽  
Basin Area

Abstract This paper investigates wintertime cold-air pools in a basinlike part of the Danube Valley, located in the German state of Bavaria. Specifically, the focus is on cold-pool events restricted to the basin area, that is, not extending to the more elevated parts of the Alpine foreland. An analysis of observational data indicates that the delay of warm-air advection in the basin area relative to the Alpine foreland plays a major role in these events. However, the relationship between warming in the Alpine foreland and a temperature deficit in the northeast–southwest-oriented basin appears to depend sensitively on the ambient wind direction. A statistically significant correlation is found only for westerly and southerly wind directions but not for easterly directions. To examine the dynamical reasons for this phenomenon, idealized numerical simulations have been conducted. They are initialized with a pronounced cold pool in the basin area and examine the response of the cold pool to the dynamical forcing imposed by a geostrophically balanced large-scale wind field of various directions and strengths. Sensitivity tests consider the effects of the surrounding mountain ranges and of turbulent vertical mixing. The model results indicate that the most important dynamical processes capable of dissolving cold-air pools in a large basin are (i) ageostrophic advection of the cold air toward lower ambient pressure and (ii) downstream advection by the ambient flow. The former might also be interpreted as an adjustment of the cold air to the external pressure gradient, which can be balanced by the development of a spatial gradient in cold-pool depth. In principle, both advection processes are most effective in the along-basin direction because the advected air does not have to surmount significant topographic obstacles. However, a combination of several effects induced by the surrounding mountain ranges—for example, upstream flow deceleration and wake formation—modifies the dependence of the cold-pool persistence on the ambient wind direction. In agreement with observational data, the simulations with full topography predict a higher tendency for cold-pool persistence in the Danube basin for westerly and southerly flow than for easterly flow. Turbulent vertical mixing is found to make a significant contribution to the erosion of cold pools, but its effect is smaller than the sensitivity to the ambient wind direction.

scholarly journals Dynamical Aspects of Wintertime Cold-Air Pools in an Alpine Valley System

2005 ◽  
Vol 133 (9) ◽  
pp. 2721-2740 ◽  
Author(s):  
Günther Zängl
Keyword(s):  
Pressure Gradient ◽  
Cold Pool ◽  
Drainage Flow ◽  
Dynamical Mechanism ◽  
Cold Air ◽  
Ambient Wind ◽  
Alpine Valley ◽  
Ambient Flow ◽  
Alpine Foreland

Abstract This study presents high-resolution numerical simulations in order to examine the dynamical mechanisms controlling the persistence of wintertime cold-air pools in an Alpine valley system. First, a case study of a cold-pool episode is conducted, the formation of which was related to the passage of a warm front north of the Alps. While the preexisting cold air was rapidly advected away in the Alpine foreland, a persistent cold pool was maintained in the inner-Alpine part of the valley system, associated with sustained horizontal temperature differences of up to 10 K over a distance of 30 km. The case study is complemented by a series of semi-idealized simulations, combining realistic topography with idealized large-scale flow conditions. These simulations consider a range of different ambient wind directions in order to investigate their impact on the cold-pool persistence. The results indicate that the most important dynamical mechanism controlling the persistence of cold-air pools in deep Alpine valleys is cold-air drainage toward the Alpine foreland. The preferred direction for such a drainage flow is down the pressure gradient imposed by the (geostrophically balanced) ambient flow. Thus, for a given valley geometry and a given strength of the ambient flow, the probability for persistent cold-air pools mainly depends on the ambient wind direction. If the direction of the imposed pressure gradient matches a sufficiently wide connection to the foreland (a valley or a low pass), then a drainage flow will lead to a rapid removal of the cold air. However, the presence of pronounced lateral constrictions in the connecting valley may strongly reduce the drainage efficiency. Cold-pool erosion by turbulent vertical mixing seems to play a comparatively minor role in deep valley systems as considered in this study.

scholarly journals Transient Cold Air Drainage down a Shallow Valley

2014 ◽  
Vol 71 (7) ◽  
pp. 2534-2544 ◽  
Author(s):  
L. Mahrt ◽  
Jielun Sun ◽  
S. P. Oncley ◽  
T. W. Horst
Keyword(s):  
Surface Wind ◽  
Vertical Mixing ◽  
Cold Pool ◽  
Drainage Flow ◽  
Wind Maximum ◽  
Near Surface ◽  
Cold Air ◽  
Vertical Scale ◽  
Sonic Anemometers ◽  
Small Valley

Abstract Drainage of cold air down a small valley and associated near-surface wind maxima are examined from 20 stations with sonic anemometers at 1 m and from a 20-m tower that includes six sonic anemometers in the lowest 5 m, deployed in the Shallow Cold Pool Experiment (SCP). The small valley is about 270 m wide and 12 m deep with a downvalley slope of 2%–3%. The momentum budget indicates that the flow is driven by the buoyancy deficit of the flow and opposed primarily by the stress divergence while the remaining terms are estimated to be at least an order of magnitude smaller. This analysis also reveals major difficulties in quantifying such a budget due to uncertainties in the measurements, sensitivity to choice of averaging time, and sensitivity to measurement heights. Wind maxima occur as low as 0.5 m in the downvalley drainage flow—the lowest observational level. The downvalley cold air drainage and wind maxima are frequently disrupted by transient modes that sometimes lead to significant vertical mixing. On average, the downvalley drainage of cold air occurs with particularly weak turbulence with stronger turbulence above the drainage flow. The momentum flux profile responds to the shear reversal at the wind maximum on a vertical scale of 1 m or less, suggesting the important role of finescale turbulent diffusion.

scholarly journals Thermal Submeso Motions in the Nocturnal Stable Boundary Layer. Part 2: Generating Mechanisms and Implications

2021 ◽  
Author(s):  
Lena Pfister ◽  
Karl Lapo ◽  
Larry Mahrt ◽  
Christoph K. Thomas
Keyword(s):  
Boundary Layer ◽  
Stable Boundary Layer ◽  
Side Wall ◽  
Cold Pool ◽  
Continuous Data ◽  
Valley Bottom ◽  
Near Surface ◽  
Cold Air ◽  
Stable Boundary ◽  
Surface Temperatures

AbstractIn the stable boundary layer, thermal submesofronts (TSFs) are detected during the Shallow Cold Pool experiment in the Colorado plains, Colorado, USA in 2012. The topography induces TSFs by forming two different air layers converging on the valley-side wall while being stacked vertically above the valley bottom. The warm-air layer is mechanically generated by lee turbulence that consistently elevates near-surface temperatures, while the cold-air layer is thermodynamically driven by radiative cooling and the corresponding cold-air drainage decreases near-surface temperatures. The semi-stationary TSFs can only be detected, tracked, and investigated in detail when using fibre-optic distributed sensing (FODS), as point observations miss TSFs most of the time. Neither the occurrence of TSFs nor the characteristics of each air layer are connected to a specific wind or thermal regime. However, each air layer is characterized by a specific relationship between the wind speed and the friction velocity. Accordingly, a single threshold separating different flow regimes within the boundary layer is an oversimplification, especially during the occurrence of TSFs. No local forcings or their combination could predict the occurrence of TSFs except that they are less likely to occur during stronger near-surface or synoptic-scale flow. While classical conceptualizations and techniques of the boundary layer fail in describing the formation of TSFs, the use of spatially continuous data obtained from FODS provide new insights. Future studies need to incorporate spatially continuous data in the horizontal and vertical planes, in addition to classic sensor networks of sonic anemometry and thermohygrometers to fully characterize and describe boundary-layer phenomena.

Offshore Wind Energy and the Mid-Atlantic Cold Pool: A Review of Potential Interactions

2021 ◽  
Vol 55 (4) ◽  
pp. 72-87
Author(s):  
Travis Miles ◽  
Sarah Murphy ◽  
Josh Kohut ◽  
Sarah Borsetti ◽  
Daphne Munroe
Keyword(s):  
Wind Energy ◽  
Wind Farm ◽  
Vertical Mixing ◽  
Wind Farms ◽  
The United States ◽  
Offshore Wind ◽  
Cold Pool ◽  
Ocean Bottom ◽  
Offshore Wind Farms ◽  
Seasonal Thermocline

Abstract The U.S. East Coast has 1.7 million acres of federal bottom under lease for the development of wind energy installations, with plans for more than 1,500 foundations to be placed. The scale of these wind farms has the potential to alter the unique and delicate oceanographic conditions along the expansive Atlantic continental shelf, a region characterized by a strong seasonal thermocline that overlies cold bottom water, known as the “Cold Pool.” Strong seasonal stratification traps cold (typically less than 10°C) water above the ocean bottom sustaining a boreal fauna that represents vast fisheries, including the most lucrative shellfish fisheries in the United States. This paper reviews the existing literature and research pertaining to the ways in which offshore wind farms may alter processes that establish, maintain, and degrade stratification associated with the Cold Pool through vertical mixing in this seasonally dynamic system. Changes in stratification could have important consequences in Cold Pool setup and degradation, processes fundamental to high fishery productivity of the region. The potential for these multiple wind energy arrays to alter oceanographic processes and the biological systems that rely on them is possible; however, a great deal of uncertainty remains about the nature and scale of these interactions. Research should be prioritized that identifies stratification thresholds of influence, below which turbines and wind farm arrays may alter oceanographic processes. These should be examined within context of spatial and seasonal dynamics of the Cold Pool and offshore wind lease areas to identify potential areas of further study.

Observed mesoscale patterns in the irrigated Eastern Ebro basin

2021 ◽  
Author(s):  
Antoni Grau Ferrer ◽  
Mª Antònia Jiménez Cortés ◽  
Daniel Martínez Villagrasa ◽  
Joan Cuxart Rodamilans
Keyword(s):  
Sea Breeze ◽  
Research Data ◽  
Ebro Basin ◽  
The West ◽  
Cold Air ◽  
Late Afternoon ◽  
Mountain Ranges ◽  
The North ◽  
Marine Air ◽  
Filtering Procedure

<p>The Eastern Ebro basin is composed of an extensive lower irrigated area, surrounded by dry-fed slopes and wooden mountain ranges to the North, East and South, while to the West is open to the agricultural Western Ebro basin. Previous studies, based on research data or on statistics for one station, indicate that these features determine the local circulations in the area. A network of stations is used here to analyze a period of 15 years, taking representative data for the different units of landscape. A filtering procedure is developed which selects the events with predominance of local circulations, based on detecting stably stratified nights.</p><p>The analysis of the filtered data indicates the presence of a valley circulation between the lower plain and the slopes and mountains that reverses its sense of circulation between day and night, which intensity varies in summer due to an increasing thermal contrast between irrigated and rain-fed areas. The presence of sea-breeze in the late afternoon in the warm months is common, bringing cooler and wetter marine air to the area after crossing the mountain range at the South. At night in the centre of the basin, cold air pools are formed, which evolve to persistent fog events in winter, causing the statistics to be very different in that season compared to the rest of the year.</p>

Bow Echo Sensitivity to Ambient Moisture and Cold Pool Strength

2006 ◽  
Vol 134 (3) ◽  
pp. 950-964 ◽  
Author(s):  
Richard P. James ◽  
Paul M. Markowski ◽  
J. Michael Fritsch
Keyword(s):  
Convective Available Potential Energy ◽  
Three Dimensional ◽  
Cold Pool ◽  
Convective System ◽  
Cold Air ◽  
Convective Systems ◽  
Cold Pools ◽  
Bow Echo ◽  
Dry Conditions ◽  
Water Vapor Mixing Ratio

Abstract Bow echo development within quasi-linear convective systems is investigated using a storm-scale numerical model. A strong sensitivity to the ambient water vapor mixing ratio is demonstrated. Relatively dry conditions at low and midlevels favor intense cold-air production and strong cold pool development, leading to upshear-tilted, “slab-like” convection for various magnitudes of convective available potential energy (CAPE) and low-level shear. High relative humidity in the environment tends to reduce the rate of production of cold air, leading to weak cold pools and downshear-tilted convective systems, with primarily cell-scale three-dimensionality in the convective region. At intermediate moisture contents, long-lived, coherent bowing segments are generated within the convective line. In general, the scale of the coherent three-dimensional structures increases with increasing cold pool strength. The bowing lines are characterized in their developing and mature stages by segments of the convective line measuring 15–40 km in length over which the cold pool is much stronger than at other locations along the line. The growth of bow echo structures within a linear convective system appears to depend critically on the local strengthening of the cold pool to the extent that the convection becomes locally upshear tilted. A positive feedback process is thereby initiated, allowing the intensification of the bow echo. If the environment favors an excessively strong cold pool, however, the entire line becomes uniformly upshear tilted relatively quickly, and the along-line heterogeneity of the bowing line is lost.

scholarly journals 6.2. Efficiency in nuclear fueling

1998 ◽  
Vol 184 ◽  
pp. 265-266
Author(s):  
M. Noguchi
Keyword(s):  
Star Formation ◽  
Interstellar Medium ◽  
Numerical Simulations ◽  
Observational Data ◽  
Formation Process ◽  
Large Body ◽  
Merging Galaxies ◽  
Numerical Studies ◽  
Galactic Astronomy ◽  
Theoretical Side

Starburst phenomena in interacting and merging galaxies have been one of the most widely investigated subjects in today's galactic astronomy. On the theoretical side, a large body of numerical studies have been performed in order to interpret available observational data. Numerical simulations have been advanced to the point where they can include interstellar medium (ISM) and star formation process.

scholarly journals Solar-Like Oscillations of Procyon A: Stellar Models and Time Series Simulations versus Observations

2000 ◽  
Vol 176 ◽  
pp. 461-462
Author(s):  
C. Barban ◽  
E. Michel ◽  
M. Martic ◽  
J. Schmitt ◽  
J. C. Lebrun ◽  
...  
Keyword(s):  
Time Series ◽  
Power Spectrum ◽  
Numerical Simulations ◽  
Observational Data ◽  
Theoretical Predictions ◽  
Stellar Models ◽  
Excess Power ◽  
New Evidence

AbstractThe aim of this paper (further developed in Barban et al. 1999) is to present new evidence of the possible stellar origin of the observed excess power in the power spectrum of Procyon A presented in Martic et al. (1999) by comparing these observational data with theoretical predictions and numerical simulations.

scholarly journals Numerical Simulations of Bow Echo Formation Following a Squall Line–Supercell Merger

2014 ◽  
Vol 142 (12) ◽  
pp. 4791-4822 ◽  
Author(s):  
Adam J. French ◽  
Matthew D. Parker
Keyword(s):  
Numerical Simulations ◽  
Vertical Wind Shear ◽  
Leading Edge ◽  
Direct Result ◽  
Squall Line ◽  
Cold Pool ◽  
Surface Winds ◽  
Sensitivity Tests ◽  
Bow Echo ◽  
Inflow Jet

Abstract Output from idealized numerical simulations is used to investigate the storm-scale processes responsible for squall-line evolution following a merger with an isolated supercell. A simulation including a squall line–supercell merger is compared to one using the same initial squall line and background environment without the merger. These simulations reveal that while bow echo formation is favored by the strongly sheared background environment, the merger produces a more compact bowing structure owing to a locally enhanced rear-inflow jet. The merger also represents a favored location for severe weather production relative to other portions of the squall line, with surface winds, vertical vorticity, and rainfall all being maximized in the vicinity of the merger. An analysis of storm-scale processes reveals that the premerger squall line weakens as it encounters outflow from the preline supercell, and the supercell becomes the leading edge of the merged system. Subsequent localized strengthening of the cold pool and rear-inflow jet produce a compact, intense bow echo local to the merger, with a descending rear-inflow jet creating a broad swath of damaging surface winds. These features, common to severe bow echoes, are shown to be a direct result of the merger in the present simulations, and are diminished or absent in the no-merger simulation. Sensitivity tests reveal that mergers in a weaker vertical wind shear environment do not produce an enhanced bow echo structure, and only produce a localized region of marginally enhanced surface winds. Additional tests demonstrate that the details of postmerger evolution vary with merger location along the line.

The Interaction of Simulated Squall Lines with Idealized Mountain Ridges

2006 ◽  
Vol 134 (7) ◽  
pp. 1919-1941 ◽  
Author(s):  
Jeffrey Frame ◽  
Paul Markowski
Keyword(s):  
Squall Line ◽  
Cold Pool ◽  
Cold Air ◽  
Advanced Regional Prediction System ◽  
Topographic Features ◽  
Squall Lines ◽  
Regional Prediction ◽  
Cloud Resolving Model ◽  
Orographic Enhancement ◽  
Flow Transitions

Abstract Numerical simulations of squall lines traversing sinusoidal mountain ridges are performed using the Advanced Regional Prediction System cloud-resolving model. Precipitation and updraft strength are enhanced through orographic ascent as a squall line approaches a ridge. The simulated squall line then weakens as it descends the ridge because some of the cold pool is blocked by the terrain, resulting in less lift along the gust front and weaker convective cells. The flow within the cold pool accelerates slightly and the depth of the cold air decreases owing to upstream blocking, transitioning the flow in the cold pool head from subcritical to supercritical, then back to subcritical at the bottom of the ridge. A hydraulic jump forms when the flow transitions the second time, enabling the development of a new convective line downwind of the mountain. These new updrafts grow and eventually replace the older updrafts that weakened during descent. This process results in the discrete propagation of a squall line just downstream of a ridge, resulting in the formation of rain shadows downstream from topographic features. Discrete propagation only occurs if a ridge is of sufficient height, however. This replacement process repeats itself if a squall line encounters multiple ridges. The risk of damaging winds from a squall line is greater on the lee side of ridges and on the top of high ridges. These terrain-forced intensity fluctuations increase with mountain height, because the higher terrain permits even less cold air to flow over it. A wider ridge results in a more gradual orographic enhancement and downslope-induced weakening.

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