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 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 Escoamento de drenagem sobre terreno ondulado

2020 ◽  
Vol 42 ◽  
pp. e16
Author(s):  
Maicon Fonseca Andrades ◽  
Luis Eduardo Medeiros ◽  
Felipe Denardin Costa ◽  
Luis Fernando Camponogara ◽  
Débora Regina Roberti
Keyword(s):  
Drainage Flow ◽  
Cold Air ◽  
Ambient Flow ◽  
Surface Drainage ◽  
Weather Stations

The present work investigates drainage events of cold air that occur over an undulating terrain, with elevations differences of some tens meters and grassy surface. For this, data collected in the autumn of 2015 from a micrometeorological tower and four weather stations are used. The results show two different cases of drainage, one when the ambient flow is upslope against the near the surface drainage flow, and another when the ambient flow is downslope in favor of the drainage.

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.

scholarly journals Evaluation of Forecasts of a Convectively Generated Bore Using an Intensively Observed Case Study from PECAN

2018 ◽  
Vol 146 (9) ◽  
pp. 3097-3122 ◽  
Author(s):  
Aaron Johnson ◽  
Xuguang Wang ◽  
Kevin R. Haghi ◽  
David B. Parsons
Keyword(s):  
Cross Sections ◽  
Turbulent Mixing ◽  
Weather Research And Forecasting ◽  
Cold Pool ◽  
Model Errors ◽  
Near Surface ◽  
Cold Pools ◽  
Convection Model

Abstract This paper presents a case study from an intensive observing period (IOP) during the Plains Elevated Convection at Night (PECAN) field experiment that was focused on a bore generated by nocturnal convection. Observations from PECAN IOP 25 on 11 July 2015 are used to evaluate the performance of high-resolution Weather Research and Forecasting Model forecasts, initialized using the Gridpoint Statistical Interpolation (GSI)-based ensemble Kalman filter. The focus is on understanding model errors and sensitivities in order to guide forecast improvements for bores associated with nocturnal convection. Model simulations of the bore amplitude are compared against eight retrieved vertical cross sections through the bore during the IOP. Sensitivities of forecasts to microphysics and planetary boundary layer (PBL) parameterizations are also investigated. Forecasts initialized before the bore pulls away from the convection show a more realistic bore than forecasts initialized later from analyses of the bore itself, in part due to the smoothing of the existing bore in the ensemble mean. Experiments show that the different microphysics schemes impact the quality of the simulations with unrealistically weak cold pools and bores with the Thompson and Morrison microphysics schemes, cold pools too strong with the WDM6 and more accurate with the WSM6 schemes. Most PBL schemes produced a realistic bore response to the cold pool, with the exception of the Mellor–Yamada–Nakanishi–Niino (MYNN) scheme, which creates too much turbulent mixing atop the bore. A new method of objectively estimating the depth of the near-surface stable layer corresponding to a simple two-layer model is also introduced, and the impacts of turbulent mixing on this estimate are discussed.

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.

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