scholarly journals The influence of convective momentum transport and vertical wind shear on the evolution of a cold air outbreak

2020 ◽  
Author(s):  
Beatrice Saggiorato ◽  
Louise Nuijens ◽  
A. Pier Siebesma ◽  
Stephan de Roode ◽  
Irina Sandu ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Wind Shear ◽  
Surface Wind ◽  
Cloud Layer ◽  
Momentum Transport ◽  
Small Scale ◽  
Near Surface ◽  
Cold Air ◽  
Cold Air Outbreak ◽  
Convective Momentum Transport

<p>To study the influence of convective momentum transport (CMT) on wind, boundary layer and cloud evolution in a marine cold air outbreak (CAO) we use Large-Eddy Simulations subjected to different baroclinicity (wind shear) but similar surface forcing. The simulated domain is large enough ( ≈100 × 100 km<sup>2</sup>) to develop typical mesoscale cellular convective structures.  We find that a maximum friction induced by momentum transport (MT) locates in the cloud layer for an increase of geostrophic wind with height (forward shear, FW) and near the surface for a decrease of wind with height (backward shear, BW). Although the total MT always acts as a friction, the interaction of friction-induced cross-isobaric flow with the Coriolis force can develop super-geostrophic winds near the surface (FW) or in the cloud layer (BW). The contribution of convection to MT is evaluated by decomposing the momentum flux by column water vapor and eddy size, revealing that CMT acts to accelerate sub-cloud layer winds under FW shear and that mesoscale circulations contribute significantly to MT for this horizontal resolution (250 m), even if small scale eddies are non-negligible and likely more important as resolution increases. Under FW shear, a deeper boundary layer and faster cloud transition are simulated, because MT acts to increase surface fluxes and wind shear enhances turbulent mixing across cloud tops. Our results show that the coupling between winds and convection is crucial for a range of problems, from CAO lifetime and cloud transitions to ocean heat loss and near-surface wind variability.</p>

scholarly journals The Role of Precipitation in Controlling the Transition from Stratocumulus to Cumulus Clouds in a Northern Hemisphere Cold-Air Outbreak

2017 ◽  
Vol 74 (7) ◽  
pp. 2293-2314 ◽  
Author(s):  
Steven J. Abel ◽  
Ian A. Boutle ◽  
Kirk Waite ◽  
Stuart Fox ◽  
Philip R. A. Brown ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Liquid Water ◽  
Rapid Change ◽  
Supercooled Liquid ◽  
Cloud Layer ◽  
Liquid Water Path ◽  
Cumulus Clouds ◽  
Cold Air ◽  
Stratiform Region ◽  
Cold Air Outbreak

Abstract Aircraft observations in a cold-air outbreak to the north of the United Kingdom are used to examine the boundary layer and cloud properties in an overcast mixed-phase stratocumulus cloud layer and across the transition to more broken open-cellular convection. The stratocumulus cloud is primarily composed of liquid drops with small concentrations of ice particles and there is a switch to more glaciated conditions in the shallow cumulus clouds downwind. The rapid change in cloud morphology is accompanied by enhanced precipitation with secondary ice processes becoming active and greater thermodynamic gradients in the subcloud layer. The measurements also show a removal of boundary layer accumulation mode aerosols via precipitation processes across the transition that are similar to those observed in the subtropics in pockets of open cells. Simulations using a convection-permitting (1.5-km grid spacing) regional version of the Met Office Unified Model were able to reproduce many of the salient features of the cloud field although the liquid water path in the stratiform region was too low. Sensitivity studies showed that ice was too active at removing supercooled liquid water from the cloud layer and that improvements could be made by limiting the overlap between the liquid water and ice phases. Precipitation appears to be the key mechanism responsible for initiating the transition from closed- to open-cellular convection by decoupling the boundary layer and depleting liquid water from the stratiform cloud.

Convective Structures in a Cold Air Outbreak over Lake Michigan during Lake-ICE

2005 ◽  
Vol 62 (7) ◽  
pp. 2414-2432 ◽  
Author(s):  
Suzanne M. Zurn-Birkhimer ◽  
Ernest M. Agee ◽  
Zbigniew Sorbjan
Keyword(s):  
Boundary Layer ◽  
Wind Shear ◽  
Lake Michigan ◽  
Vertical Mixing ◽  
Internal Boundary Layer ◽  
Internal Boundary ◽  
Cold Air ◽  
Convective Structures ◽  
Cold Air Outbreak ◽  
Lake Effect

Abstract The Lake-Induced Convection Experiment provided special field data during a westerly flow cold air outbreak (CAO) on 13 January 1998, which has afforded the opportunity to examine in detail an evolving convective boundary layer. Vertical cross sections prepared from these data, extending from upstream over Wisconsin out across Lake Michigan, show the modifying effects of land–water contrast on boundary layer mixing, entrainment, heating, and moisture flux. Through this analysis, an interesting case of lake-effect airmass modification was discovered. The data show atypical differing heights in vertical mixing of heat and moisture, as well as offshore downwelling and subsidence effects in the atmosphere. Analysis shows evidence of a new observational feature, the moisture internal boundary layer (MIBL) that accords well with the often recognized thermal internal boundary layer (TIBL). The “interfacial” layer over the lake is also found to be unusually thick and moist, due in part to the upstream conditions over Wisconsin as well as the effectiveness of vertical mixing of moist plumes over the lake (also seen in the aircraft datasets presented). Results show that the atmosphere can be much more effective in the vertical mixing of moisture than heat or momentum (which mixed the same), and thus represents a significant departure from the classical bottom-up and top-down mixing formulation. Four scales of coherent structures (CSs) with differing spatial and temporal dimensions have been identified. The CSs grow in a building block fashion with buoyancy as the dominating physical mechanism for organizing the convection (even in the presence of substantial wind shear). Characteristic turbulence statistics from aircraft measurements show evidence of these multiple scales of CSs, ranging from the smallest (microscale) in the cloud-free path region near the Wisconsin shore, to the largest (mesoscale) in the snow-filled boundary layer near the Michigan shore. A large eddy simulation (LES) model has also been employed to study the effects of buoyancy and shear on the convective structures in lake-effect boundary layers. The model simulation results have been divided into two parts: 1) the general relationship of surface heat flux versus wind shear, which shows the interplay and dominance of these two competing forcing mechanisms for establishing convection patterns and geometry (i.e., rolls versus cells), and 2) a case study simulation of convection analogous to the CSs seen in the CFP region for the 13 January 1998 CAO event. Model simulations also show, under proper conditions of surface heating and wind shear, the simultaneous occurrence of differing scales of CSs and at different heights, including both cells and rolls and their coexisting patterns (based on the interplay between the effects of buoyancy and shear).

Observed influence of moist convection and cloudiness on boundary layer wind and momentum flux profiles.

2020 ◽  
Author(s):  
Mariska Koning ◽  
Louise Nuijens ◽  
Fred Bosveld ◽  
Pier Siebesma ◽  
Remco Verzijlbergh ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Surface Layer ◽  
Wind Shear ◽  
Momentum Flux ◽  
Cloud Cover ◽  
Cloud Layer ◽  
Momentum Transport ◽  
Clear Sky ◽  
Momentum Fluxes ◽  
Wind Profiles

<p>Convective momentum transport (CMT) measurements are scarce, but important to constrain the impacts of CMT on wind profiles, variability of the wind and possibly the large-scale circulation.</p><p>We investigate how wind profiles and momentum fluxes change with cloudiness and convection. With stronger convection, we expect that the wind shear in the lowest 200m, wherein wind turbines are located, reduces. Cumulus days are generally strongly convective and hence well mixed. They are expected to differ from clear-sky days: the boundary layer is deeper, and cumulus may induce a different (thermal) circulation in the sub-cloud layer. Comparing cumulus and other days fairly, we must be mindful of the changes in convection strength with cloud cover, time of the day, seasons, and the wind strength that impacts the wind shear magnitude.</p><p>This study uses nine years of data from the Cabauw observatory, The Netherlands, containing 10-minute averages of wind speed, wind direction, and momentum fluxes from a 200 m tall tower along with cloud-base heights from a ceilometer. Realistic fine-scale Large Eddy Simulation (LES) hindcasts over the same time period and a 5km<sup>3</sup> domain over Cabauw provide insight into the processes at higher altitude. In both observations and LES, days with rooted clouds, which have strong connection to the sub-cloud layer, are separated from clear-sky days and days in which clouds only impact the convection through radiation effects. Days with rooted clouds are subsequently divided into three groups of increasing cloud cover: 5-30% (shallow clouds), 30-70% (somewhat deeper clouds) and >70% (overcast).</p><p>Both observations and LES show that shear in the near-surface wind speed (NSWS) reduces with stronger insolation, which is expected: more insolation causes a more unstable atmosphere, stronger convection, thus more mixing. In a weakly unstable atmosphere, rooted clouds (5-70% cloud cover) generally have better mixed winds (less normalised shear). The NSWS accelerates more from morning to afternoon on these days, indicating that not only the mixing is stronger, but also that downward mixing of higher momentum by the clouds affects the wind in the lowest 200m. If this is true, the assumption of Monin-Obukhov Similarity Theory (MOST) that large convective eddies are not important in the surface layer, does not hold. This possibly has a great impact on surface-flux parametrizations based on MOST, which are used by many numerical models, from local and mesoscale to global models. Analysing surface-layer scaling for momentum, we test whether this assumption is indeed violated in such cases.</p><p>Momentum transport profiles in LES show that when deeper clouds with larger cloud cover are present, transport in the cloud layer is larger. In the cross-wind component of the profile, the four categories show different deceleration in the mixed layer, and different acceleration near the top of the mixed layer. Likely, the stronger inversion-jump in the cross-wind causes this momentum flux character.</p><p>With this study, we provide an overview of the effects that have been observed in different cloudiness and convective conditions and gained understanding of the important processes and implications of the cloud effects on momentum transport.</p>

scholarly journals An Observational Study of Hurricane Boundary Layer Small-Scale Coherent Structures

2008 ◽  
Vol 136 (8) ◽  
pp. 2871-2893 ◽  
Author(s):  
Sylvie Lorsolo ◽  
John L. Schroeder ◽  
Peter Dodge ◽  
Frank Marks
Keyword(s):  
Boundary Layer ◽  
Observational Study ◽  
Wind Field ◽  
Relative Frequency ◽  
Surface Wind ◽  
High Energy ◽  
Small Scale ◽  
Near Surface ◽  
Hurricane Boundary Layer ◽  
Surface Wind Field

Abstract Data with high temporal and spatial resolution from Hurricanes Isabel (2003) and Frances (2004) were analyzed to provide a detailed study of near-surface linear structures with subkilometer wavelengths of the hurricane boundary layer (HBL). The analysis showed that the features were omnipresent throughout the data collection, displayed a horizontal and vertical coherency, and maintained an average orientation of 7° left of the low-level wind. A unique objective wavelength analysis was conducted, where wavelength was defined as the distance between two wind maxima or minima perpendicular to the features’ long axis, and revealed that although wavelengths as large as 1400 m were observed, the majority of the features had wavelengths between 200 and 650 m. The assessed wavelengths differ from those documented in a recent observational study. To evaluate the correlation between the features and the underlying near-surface wind field, time and spectral analyses were completed and ground-relative frequency distributions of the features were retrieved. High-energy regions of the power spectral density (PSD) determined from near-surface data were collocated with the features’ ground-relative frequency, illustrating that the features have an influence on the near-surface wind field. The additional energy found in the low-frequency range of the PSDs was previously identified as characteristic of the hurricane surface flow, suggesting that the features are an integral component of the HBL flow.

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 Sensitivity of Hurricane Intensity Forecast to Convective Momentum Transport Parameterization

2006 ◽  
Vol 134 (2) ◽  
pp. 664-674 ◽  
Author(s):  
Jongil Han ◽  
Hua-Lu Pan
Keyword(s):  
Wind Shear ◽  
Vertical Wind Shear ◽  
Momentum Transport ◽  
Gradient Force ◽  
Pressure Gradient Force ◽  
Momentum Exchange ◽  
Forecast System ◽  
Hurricane Intensity ◽  
Convective Momentum Transport ◽  
The Many

Abstract A parameterization of the convection-induced pressure gradient force (PGF) in convective momentum transport (CMT) is tested for hurricane intensity forecasting using NCEP's operational Global Forecast System (GFS) and its nested Regional Spectral Model (RSM). In the parameterization the PGF is assumed to be proportional to the product of the cloud mass flux and vertical wind shear. Compared to control forecasts using the present operational GFS and RSM where the PGF effect in CMT is taken into account empirically, the new PGF parameterization helps increase hurricane intensity by reducing the vertical momentum exchange, giving rise to a closer comparison to the observations. In addition, the new PGF parameterization forecasts not only show more realistically organized precipitation patterns with enhanced hurricane intensity but also reduce the forecast track error. Nevertheless, the model forecasts with the new PGF parameterization still largely underpredict the observed intensity. One of the many possible reasons for the large underprediction may be the absence of hurricane initialization in the models.

Experiments on wind-driven heat exchange processes over melting snow

2021 ◽  
Author(s):  
Michael Haugeneder ◽  
Tobias Jonas ◽  
Dylan Reynolds ◽  
Michael Lehning ◽  
Rebecca Mott
Keyword(s):  
Snow Cover ◽  
Surface Wind ◽  
Infrared Camera ◽  
Snow Accumulation ◽  
Internal Boundary ◽  
Small Scale ◽  
Snow Melt ◽  
Two Dimensional ◽  
Atmospheric Conditions ◽  
Near Surface

<p>Snowmelt runoff predictions in alpine catchments are challenging because of the high spatial variability of t<span>he snow cover driven by </span>various snow accumulation and ablation processes. In spring, the coexistence of bare and snow-covered ground engages a number of processes such as the enhanced lateral advection of heat over partial snow cover, the development of internal boundary layers, and atmospheric decoupling effects due to increasing stability at the snow cover. The interdependency of atmospheric conditions, topographic settings and snow coverage remains a challenge to accurately account for these processes in snow melt models.<br>In this experimental study, we used an Infrared Camera (VarioCam) pointing at thin synthetic projection screens with negligible heat capacity. Using the surface temperature of the screen as a proxy for the air temperature, we obtained a two-dimensional instantaneous measurement. Screens were installed across the transition between snow-free and snow-covered areas. With IR-measurements taken at 10Hz, we capture<span> the dynamics of turbulent temperature fluctuations</span><span> </span>over the patchy snow cover at high spatial and temporal resolution. From this data we were able to obtain high-frequency, two-dimensional windfield estimations adjacent to the surface.</p><p>Preliminary results show the formation of a stable internal boundary layer (SIBL), which was temporally highly variable. Our data suggest that the SIBL height is very shallow and strongly sensitive to the mean near-surface wind speed. Only strong gusts were capable of penetrating through this SIBL leading to an enhanced energy input to the snow surface.</p><p>With these type of results from our experiments and further measurements this spring we aim to better understand small scale energy transfer processes over patch snow cover and it’s dependency on the atmospheric conditions, enabling to improve parameterizations of these processes in coarser-resolution snow melt models.</p>

A New Time-dependent Theory of Tropical Cyclone Intensification

2021 ◽  
Author(s):  
Yuqing Wang ◽  
Yuanlong Li ◽  
Jing Xu
Keyword(s):  
Boundary Layer ◽  
Tropical Cyclone ◽  
Numerical Simulations ◽  
Strong Dependence ◽  
Surface Wind ◽  
Time Dependent ◽  
Quasi Equilibrium ◽  
Free Atmosphere ◽  
Near Surface ◽  
Tangential Wind

AbstractIn this study, the boundary-layer tangential wind budget equation following the radius of maximum wind, together with an assumed thermodynamical quasi-equilibrium boundary layer is used to derive a new equation for tropical cyclone (TC) intensification rate (IR). A TC is assumed to be axisymmetric in thermal wind balance with eyewall convection becoming in moist slantwise neutrality in the free atmosphere above the boundary layer as the storm intensifies as found recently based on idealized numerical simulations. An ad-hoc parameter is introduced to measure the degree of congruence of the absolute angular momentum and the entropy surfaces. The new IR equation is evaluated using results from idealized ensemble full-physics axisymmetric numerical simulations. Results show that the new IR equation can reproduce the time evolution of the simulated TC intensity. The new IR equation indicates a strong dependence of IR on both TC intensity and the corresponding maximum potential intensity (MPI). A new finding is the dependence of TC IR on the square of the MPI in terms of the near-surface wind speed for any given relative intensity. Results from some numerical integrations of the new IR equation also suggest the finite-amplitude nature of TC genesis. In addition, the new IR theory is also supported by some preliminary results based on best-track TC data over the North Atlantic and eastern and western North Pacific. Compared with the available time-dependent theories of TC intensification, the new IR equation can provide a realistic intensity-dependent IR during weak intensity stage as in observations.

scholarly journals Simulations of a cold-air pool associated with elevated wintertime ozone in the Uintah Basin, Utah

2014 ◽  
Vol 14 (11) ◽  
pp. 15953-16000 ◽  
Author(s):  
E. M. Neemann ◽  
E. T. Crosman ◽  
J. D. Horel ◽  
L. Avey
Keyword(s):  
Boundary Layer ◽  
Snow Cover ◽  
Layer Structure ◽  
Inversion Layer ◽  
Cloud Microphysics ◽  
Photochemical Reactions ◽  
Near Surface ◽  
Cold Air ◽  
Air Temperatures ◽  
Uintah Basin

Abstract. Numerical simulations are used to investigate the meteorological characteristics of the 1–6 February 2013 cold-air pool in the Uintah Basin, Utah, and the resulting high ozone concentrations. Flow features affecting cold-air pools and air quality in the Uintah Basin are studied, including: penetration of clean air into the basin from across the surrounding mountains, elevated easterlies within the inversion layer, and thermally-driven slope and valley flows. The sensitivity of the boundary layer structure to cloud microphysics and snow cover variations are also examined. Ice-dominant clouds enhance cold-air pool strength compared to liquid-dominant clouds by increasing nocturnal cooling and decreasing longwave cloud forcing. Snow cover increases boundary layer stability by enhancing the surface albedo, reducing the absorbed solar insolation at the surface, and lowering near-surface air temperatures. Snow cover also increases ozone levels by enhancing solar radiation available for photochemical reactions.

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