scholarly journals Katabatically Driven Cold Air Intrusions into a Basin Atmosphere

2018 ◽  
Vol 57 (2) ◽  
pp. 435-455 ◽  
Author(s):  
C. David Whiteman ◽  
Manuela Lehner ◽  
Sebastian W. Hoch ◽  
Bianca Adler ◽  
Norbert Kalthoff ◽  
...  
Keyword(s):  
Hydraulic Jump ◽  
Cold Pool ◽  
Crater Floor ◽  
Stable Layer ◽  
Katabatic Flow ◽  
Cold Air ◽  
Low Wind Speed ◽  
Air Penetration ◽  
Negative Buoyancy ◽  
Penetration Depths

AbstractThe interactions between a katabatic flow on a plain and a circular basin cut into the plain and surrounded by an elevated rim were examined during a 5-h steady-state period during the Second Meteor Crater Experiment (METCRAX II) to explain observed disturbances to the nocturnal basin atmosphere. The approaching katabatic flow split horizontally around Arizona’s Meteor Crater below a dividing streamline while, above the dividing streamline, an ~50-m-deep stable layer on the plain was carried over the 30–50-m rim of the basin. A flow bifurcation occurred over or just upwind of the rim, with the lowest portion of the stable layer having negative buoyancy relative to the air within the crater pouring continuously over the crater’s upwind rim and accelerating down the inner sidewall. The cold air intrusion was deepest and coldest over the direct upwind crater rim. Cold air penetration depths varied around the inner sidewall depending on the temperature deficit of the inflow relative to the ambient environment inside the crater. A shallow but extremely stable cold pool on the crater floor could not generally be penetrated by the inflow and a hydraulic jump–like feature formed on the lower sidewall as the flow approached the cold pool. The upper nonnegatively buoyant portion of the stable layer was carried horizontally over the crater, forming a neutrally stratified, low–wind speed cavity or wake in the lee of the upwind rim that extended downward into the crater over the upwind sidewall.

scholarly journals The Nocturnal Evolution of Atmospheric Structure in a Basin as a Larger-Scale Katabatic Flow Is Lifted over Its Rim

2018 ◽  
Vol 57 (4) ◽  
pp. 969-989 ◽  
Author(s):  
C. David Whiteman ◽  
Manuela Lehner ◽  
Sebastian W. Hoch ◽  
Bianca Adler ◽  
Norbert Kalthoff ◽  
...  
Keyword(s):  
Lower Layer ◽  
Shear Instability ◽  
Cold Pool ◽  
Katabatic Wind ◽  
Katabatic Flow ◽  
Cold Air ◽  
Flow Acceleration ◽  
Initiation Phase ◽  
Atmospheric Structure ◽  
Strong Winds

AbstractThe successive stages of nocturnal atmospheric structure inside a small isolated basin are investigated when a katabatically driven flow on an adjacent tilted plain advects cold air over the basin rim. Data came from Arizona’s Meteor Crater during intensive observing period 4 of the Second Meteor Crater Experiment (METCRAX II) when a mesoscale flow above the plain was superimposed on the katabatic flow leading to a flow acceleration and then deceleration over the course of the night. Following an overflow-initiation phase, the basin atmosphere over the upwind inner sidewall progressed through three stages as the katabatic flow accelerated: 1) a cold-air-intrusion phase in which the overflowing cold air accelerated down the upwind inner sidewall, 2) a bifurcation phase in which the katabatic stable layer lifted over the rim included both a nonnegatively buoyant upper layer that flowed horizontally over the basin and a negatively buoyant lower layer (the cold-air intrusion) that continued on the slope below to create a hydraulic jump at the foot of the sidewall, and 3) a final warm-air-intrusion phase in which shear instability in the upper overflowing layer produced a lee wave that brought warm air from the elevated residual layer downward into the basin. Strong winds during the third phase penetrated to the basin floor, stirring the preexisting, intensely stable, cold pool. Later in the night a wind direction change aloft decelerated the katabatic wind and the atmosphere progressed back through the bifurcation and cold-air-intrusion phases. A conceptual diagram illustrates the first four evolutionary phases.

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 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.

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 The 6 May 2010 Elevated Supercell during VORTEX2

2017 ◽  
Vol 145 (7) ◽  
pp. 2635-2657 ◽  
Author(s):  
Christopher W. MacIntosh ◽  
Matthew D. Parker
Keyword(s):  
Cold Pool ◽  
Surface Winds ◽  
Stable Layer ◽  
Low Level ◽  
Stable Inversion ◽  
Idealized Modeling ◽  
Weak Surface ◽  
Negatively Buoyant

An elevated supercell from the second Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX2) on 6 May 2010 is investigated. Observations show that the supercell formed over a stable inversion and was likely decoupled from the surface. Quintessential features of a supercell were present, including a hook echo (albeit bent anticyclonically) and midlevel mesocyclone, and the storm was quasi steady during the observing period. A weak surface cold pool formed, but it was apparently devoid of air originating from midlevels. Idealized modeling using near-storm soundings is employed to clarify the structure and maintenance of this supercell. The simulated storm is decoupled from the surface by the stable layer. Additionally, the reflectivity structure of the simulated supercell is strikingly similar to the observed storm, including its peculiar anticyclonic-curving hook echo. Air parcels above 1 km reached their LFCs as a result of the simulated supercell’s own dynamic lifting, which likely maintained the main updraft throughout its life. In contrast, low-level air in the simulation followed an “up–down” trajectory, being lifted dynamically within the stable layer before becoming strongly negatively buoyant and descending back to the surface. Up–down parcels originating in the lowest 100 m are shown to be a potential driver of severe surface winds. The complementary observations and simulations highlight a range of processes that may act in concert to maintain supercells in environments lacking surface-based CAPE.

Warm-Air Intrusions in Arizona’s Meteor Crater

2012 ◽  
Vol 51 (6) ◽  
pp. 1010-1025 ◽  
Author(s):  
Bianca Adler ◽  
C. David Whiteman ◽  
Sebastian W. Hoch ◽  
Manuela Lehner ◽  
Norbert Kalthoff
Keyword(s):  
Surface Layer ◽  
Atmospheric Stability ◽  
Crater Floor ◽  
Drainage Flow ◽  
Cold Air ◽  
Stable Surface ◽  
Isothermal Layer ◽  
Stable Surface Layer ◽  
Strong Winds ◽  
Spatial And Temporal Characteristics

AbstractEpisodic nighttime intrusions of warm air, accompanied by strong winds, enter the enclosed near-circular Meteor Crater basin on clear, synoptically undisturbed nights. Data analysis is used to document these events and to determine their spatial and temporal characteristics, their effects on the atmospheric structure inside the crater, and their relationship to larger-scale flows and atmospheric stability. A conceptual model that is based on hydraulic flow theory is offered to explain warm-air-intrusion events at the crater. The intermittent warm-air-intrusion events were closely related to a stable surface layer and a mesoscale (~50 km) drainage flow on the inclined plain outside the crater and to a continuous shallow cold-air inflow that came over the upstream crater rim. Depending on the upstream conditions, the cold-air inflow at the crater rim deepened temporarily and warmer air from above the stable surface layer on the surrounding plain descended into the crater, as part of the flowing layer. The flow descended up to 140 m into the 170-m-deep crater and did not penetrate the approximately 30-m-deep crater-floor inversion. The intruding air, which was up to 5 K warmer than the crater atmosphere, did not extend into the center of the crater, where the nighttime near-isothermal layer in the ambient crater atmosphere remained largely undisturbed. New investigations are suggested to test the hypothesis that the warm-air intrusions are associated with hydraulic jumps.

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.

scholarly journals Assessment of valley cold pools and clouds in a very high-resolution numerical weather prediction model

2015 ◽  
Vol 8 (10) ◽  
pp. 3105-3117 ◽  
Author(s):  
J. K. Hughes ◽  
A. N. Ross ◽  
S. B. Vosper ◽  
A. P. Lock ◽  
B. C. Jemmett-Smith
Keyword(s):  
High Resolution ◽  
Relative Humidity ◽  
Numerical Weather Prediction ◽  
Weather Prediction ◽  
Wave Radiation ◽  
Cold Pool ◽  
Cold Air ◽  
Numerical Weather ◽  
Pool Formation ◽  
Very High

Abstract. The formation of cold air pools in valleys under stable conditions represents an important challenge for numerical weather prediction (NWP). The challenge is increased when the valleys that dominate cold pool formation are on scales unresolved by NWP models, which can lead to substantial local errors in temperature forecasts. In this study a 2-month simulation is presented using a nested model configuration with a finest horizontal grid spacing of 100 m. The simulation is compared with observations from the recent COLd air Pooling Experiment (COLPEX) project and the model's ability to represent cold pool formation, and the surface energy balance is assessed. The results reveal a bias in the model long-wave radiation that results from the assumptions made about the sub-grid variability in humidity in the cloud parametrization scheme. The cloud scheme assumes relative humidity thresholds below 100 % to diagnose partial cloudiness, an approach common to schemes used in many other models. The biases in radiation, and resulting biases in screen temperature and cold pool properties are shown to be sensitive to the choice of critical relative humidity, suggesting that this is a key area that should be improved for very high-resolution modeling.

scholarly journals Seven-Doppler Radar and In Situ Analysis of the 25–26 June 2015 Kansas MCS during PECAN

2019 ◽  
Vol 148 (1) ◽  
pp. 211-240 ◽  
Author(s):  
Rachel L. Miller ◽  
Conrad L. Ziegler ◽  
Michael I. Biggerstaff
Keyword(s):  
Doppler Radar ◽  
Mesoscale Convective System ◽  
Cold Pool ◽  
Convective System ◽  
Stable Layer ◽  
Kinematic Evolution ◽  
Stratiform Region ◽  
Stationary Front ◽  
Mesoscale Convective ◽  
Convective Cells

Abstract This case study analyzes a nocturnal mesoscale convective system (MCS) that was observed on 25–26 June 2015 in northeastern Kansas during the Plains Elevated Convection At Night (PECAN) project. Over the course of the observational period, a broken line of elevated nocturnal convective cells initiated around 0230 UTC on the cool side of a stationary front and subsequently merged to form a quasi-linear MCS that later developed strong, surface-based outflow and a trailing stratiform region. This study combines radar observations with mobile and fixed mesonet and sounding data taken during PECAN to analyze the kinematics and thermodynamics of the MCS from 0300 to 0630 UTC. This study is unique in that 38 consecutive multi-Doppler wind analyses are examined over the 3.5 h observation period, facilitating a long-duration analysis of the kinematic evolution of the nocturnal MCS. Radar analyses reveal that the initial convective cells and linear MCS are elevated and sustained by an elevated residual layer formed via weak ascent over the stationary front. During upscale growth, individual convective cells develop storm-scale cold pools due to pockets of descending rear-to-front flow that are measured by mobile mesonets. By 0500 UTC, kinematic analysis and mesonet observations show that the MCS has a surface-based cold pool and that convective line updrafts are ingesting parcels from below the stable layer. In this environment, the elevated system has become surface based since the cold pool lifting is sufficient for surface-based parcels to overcome the CIN associated with the frontal stable layer.

Simulating airborne ‘snow walls’ of Antarctica using CRYOWRF v1.0

2021 ◽  
Author(s):  
Varun Sharma ◽  
Franziska Gerber ◽  
Michael Lehning
Keyword(s):  
Snow Cover ◽  
Hydraulic Jump ◽  
Large Scale ◽  
Satellite Images ◽  
Ice Sheet ◽  
Surface Radiation ◽  
Surface Model ◽  
Blowing Snow ◽  
Katabatic Flow ◽  
Surface Snow

<p>When a well-developed, high velocity katabatic flow draining down the ice sheet of Antarctica reaches the coast, it experiences an abrupt and rapid transition due to change in slope resulting in formation of a hydraulic jump. A remarkable manifestation of the hydraulic jump, given the ‘right’ surface conditions, is the large-scale entrainment and convergence of blowing snow particles within the hydraulic jump. This can result in formation of 100-1000 m high, highly localized ‘walls’ of snow in the air in an otherwise cloud-free sky.</p><p>Recent work by Vignon et al. (2020) has described in detail, the mechanisms resulting in the formation of hydraulic jumps and excitation of gravity waves during a particularly notable event at the Dumont d’Urville (DDU) station in August 2017. They used a combination of satellite images, mesoscale simulations with WRF and station measurements (including Micro Rain Radars) in their study, notably relying on the snow wall for diagnosing and quantifying the hydraulic jump in satellite images. On the other hand, relatively less importance was given towards the surface snow processes including the transport of snow particles in the wall.</p><p>In this presentation, we present results from simulations done using the recently developed CRYOWRF v1.0 to recreate the August 2017 episode at DDU and explicitly simulate the formation and the dynamics of the snow wall itself. CRYOWRF enhances the standard WRF model with the state-of-the-art surface snow modelling scheme SNOWPACK as well as a completely new blowing snow scheme. SNOWPACK essentially acts as a land surface model for the WRF atmospheric model, thus making a quantum leap over the existing snow cover models in WRF. Since SNOWPACK is a grain-scale snow model, it allows for the proper formulation of boundary conditions for simulating blowing snow dynamics.</p><p>Results show the formation of the snow wall due to large scale entrainment over a wide area of the ice sheet, the mass balance of the snow wall within the hydraulic jump and finally, the destruction of the snow wall and the ultimate fate of all the entrained snow. We also show results for the influence of the snow wall on the local surface radiation at DDU. Overall, we test the capabilities of CRYOWRF to simulate such a complex phenomenon and highlight possible applications now feasible due the tight coupling of an advanced snow cover model and a multi-scale, non-hydrostatic atmospheric flow solver.</p><p>Reference:</p><p>Vignon, Étienne, Ghislain Picard, Claudio Durán-Alarcón, Simon P. Alexander, Hubert Gallée, and Alexis Berne. " Gravity Wave Excitation during the Coastal Transition of an Extreme Katabatic Flow in Antarctica". <em>Journal of the Atmospheric Sciences</em> 77.4 (2020): 1295-1312. <>.</p>

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