Katabatic Jet Research Articles

Katabatic Winds over Steep Slopes: Overview of a Field Experiment Designed to Investigate Slope-Normal Velocity and Near-Surface Turbulence

We describe a new field campaign over a steep, snowy \(30^{\circ }\) alpine slope, designed to investigate three recurrent issues in experimental studies of steep-slope katabatic winds. (1) Entrainment is known to be present in katabatic jets and has been estimated at the interface between the jet and the boundary layer above it. However, to our knowledge, the slope-normal velocity component has never been measured in the katabatic jet. (2) It is hard to accurately measure turbulence in the first tens of centimetres above the surface using standard sonic anemometry due to the filtering effect of the long instrument path. The present field experiment used a three-dimensional multi-hole pitot-type probe with a high sampling frequency (1250 Hz) that was positioned as close to the surface as 3 cm. It provides three-dimensional mean velocity and Reynolds stress tensor from which dissipation can be estimated, as well as spectra for the turbulent quantities. Energy spectra reveal a well-developed inertial range and capture the inertial scales and some of the dissipative scales. (3) Measuring turbulence on a mast usually provides information about mean and turbulent quantities at certain discrete heights because the sensors are sparsely located inside the jet. We present the first measurements of well-developed katabatic flows where the full wind-speed and temperature profiles acquired, from tethered balloon are available at the location of the measurement mast, which comprises three-dimensional anemometry and thermometry.

Buoyancy Effects in the Turbulence Kinetic Energy Budget and Reynolds Stress Budget for a Katabatic Jet over a Steep Alpine Slope

Katabatic winds are very frequent but poorly understood or simulated over steep slopes. This study focuses on a katabatic jet above a steep alpine slope. We assess the buoyancy terms in both the turbulence kinetic energy (TKE) and the Reynolds shear-stress budget equations. We specifically focus on the contribution of the slope-normal and along-slope turbulent sensible heat fluxes to these terms. Four levels of measurements below and above the maximum wind-speed height enable analysis of the buoyancy effect along the vertical profile as follow: (i) buoyancy tends to destroy TKE, as expected in stable conditions, and the turbulent momentum flux in the inner-layer region of the jet below the maximum wind-speed height $$z_j$$ ; (ii) results also suggest buoyancy contributes to the production of TKE in the outer-layer shear region of the jet (well above $$z_j$$ ) while consumption of the turbulent momentum flux is observed in the same region; (iii) In the region around the maximum wind speed where mechanical shear production is marginal, buoyancy tends to destroy TKE and our results suggest it tends to increase the momentum flux. The present study also provides an analytical condition for the limit between production and consumption of the turbulent momentum flux due to buoyancy as a function of the slope angle, similar to the condition already proposed for TKE. We reintroduce the stress Richardson number, which is the equivalent of the flux Richardson number for the Reynolds shear-stress budget. We point out that the flux Richardson number and the stress Richardson number are complementary stability parameters for characterizing the katabatic flow apart from the region around the maximum wind-speed height.

On the turbulence structure of deep katabatic flows on a gentle mesoscale slope.

A comprehensive analysis of the turbulence structure of relatively deep midlatitude katabatic flows (with jet maxima between 20 and 50 m) developing over a gentle (1°) mesoscale slope with a long fetch upstream of the Meteor Crater in Arizona is presented. The turbulence structure of flow below the katabatic jet maximum shows many similarities with the turbulence structure of shallower katabatic flows, with decreasing turbulence fluxes with height and almost constant turbulent Prandtl number. Still stark differences occur above the jet maximum where turbulence is suppressed by strong stability, is anisotropic and there is a large sub‐mesoscale contribution to the flux. Detecting the stable boundary‐layer top depends on the method used (flux‐ vs. anisotropy‐profiles) but both methods are highly correlated. The top of the stable boundary layer, however, mostly deviates from the jet maximum height or the top of the near‐surface inversion. The flat‐terrain formulations for the boundary‐layer height correlate well with the detected top of the stable boundary layer if the near‐surface and not the background stratification is used in their formulations; however, they mostly largely overestimate this boundary‐layer height. The difference from flat‐terrain boundary layers is also shown through the dependence of size of the dominant eddy with height. In katabatic flows the eddy size is semi‐constant with height throughout the stable boundary‐layer depth, whereas in flat terrain, eddy size varies significantly with height. Flux‐gradient and flux‐variance relationships show that turbulence data from different stable boundary‐layer scaling regimes collapse on top of each other showing that the dominant dependence is not on the scaling regime but on the local stability.

On the fine vertical structure of the low troposphere over the coastal margins of East Antarctica

Abstract. In this study, 8 years of high-resolution radiosonde data at nine Antarctic stations are analysed to provide the first large-scale characterization of the fine vertical structure of the low troposphere up to 3 km altitude over the coastal margins of East Antarctica. Radiosonde data show a large spatial variability of wind, temperature and humidity profiles, with different features between stations in katabatic regions (e.g., Dumont d'Urville and Mawson stations), stations over two ice shelves (Neumayer and Halley stations) and regions with complex orography (e.g., McMurdo). At the Dumont d'Urville, Mawson and Davis stations, the yearly median wind speed profiles exhibit a clear low-level katabatic jet. During precipitation events, the low-level flow generally remains of continental origin and its speed is even reinforced due to the increase in the continent–ocean pressure gradient. Meanwhile, the relative humidity profiles show a dry low troposphere, suggesting the occurrence of low-level sublimation of precipitation in katabatic regions but such a phenomenon does not appreciably occur over the ice shelves near Halley and Neumayer. Although ERA-Interim and ERA5 reanalyses assimilate radiosoundings at most stations considered here, substantial – and sometimes large – low-level wind and humidity biases are revealed but ERA5 shows overall better performance. A free simulation with the regional polar version of the Weather Research and Forecasting model (Polar WRF) (at a 35 km resolution) over the entire continent shows too-strong and too-shallow near-surface jets in katabatic regions especially in winter. This may be a consequence of an underestimated coastal cold air bump and associated sea–continent pressure gradient force due to the coarse 35 km resolution of the Polar WRF simulation. Beyond documenting the vertical structure of the low troposphere over coastal East Antarctica, this study gives insights into the reliability and accuracy of two major reanalysis products in this region on the Earth. The paper further underlines the difficulty of modeling the low-level flow over the margins of the ice sheet with a state-of-the-art atmospheric model.

Large-Eddy Simulation of a Katabatic Jet along a Convexly Curved Slope. Part I: Statistical Results

Abstract Large-eddy simulation is performed to study a katabatic jet along a convexly curved slope with a maximum angle of about 35.5°. The design of this numerical simulation of turbulent shear flow is discussed, and a qualitative assessment of the method is proposed. The katabatic flow is artificially generated by ground surface cooling, and a stable atmospheric boundary layer with constant stratification is considered as a reference state. A quantitative statistical analysis is used to describe the present turbulent flow, with a focus on the outer-layer shear of the katabatic jet, which extends about 50 m above the jet maximum. The Prandtl model for a katabatic jet is applied to the present results, and revisited versions of the model found in the literature are discussed, with an emphasis on specific momentum and turbulent heat diffusion. The vertical and downslope variability of the turbulent kinetic energy budget is also discussed, and it is shown that advection and production contributions in the downslope direction are far from negligible in katabatic flows along curved slopes. A specific effect that the convex curvature has on the katabatic jet is one of centrifugal deceleration and an increase of the flow’s turbulent production and turbulent intensity in the outer-layer shear. A strong thickening of the outer layer is also observed.

Large‐Eddy Simulation of a katabatic jet along a convexly curved slope: 2. Evidence of Görtler vortices

AbstractThis paper is the second part of a study of katabatic jet along a convexly curved slope with a maximum angle of about 35.5°. Large‐Eddy Simulation (LES) is performed with a special focus on the outer‐layer shear of the katabatic jet. In the first part, a basic statistical quantitative analysis of the flow was performed. Here a qualitative and quantitative description of vortical structures is used to gain insight in the present 3‐D turbulent flow. It is shown that Görtler vortices oriented in the streamwise downslope direction develop in the shear layer. They spread with a specific mushroom shape in the vertical direction up to about 100 m height. They play a main role with respect to local turbulent mixing in the ground surface boundary layer. The present curved slope configuration constitutes a realistic model for alpine orography. This paper provides a procedure based on local turbulence anisotropy to track Görtler vortices for in situ measurements, which has never been proposed in the literature.

Buoyant Turbulent Kinetic Energy Production in Steep-Slope Katabatic Flow

We develop several critical concepts that should be considered when interpreting, modelling and designing future experiments for flows over sloping terrain. Vertical buoyancy fluxes in katabatic flows can be positive and a source of turbulent kinetic energy (TKE) despite the statically stable, thermal stratification that drives these flows. This phenomenon occurs when the ratio of along-slope to slope-normal kinematic heat fluxes is greater than the cotangent of the slope angle, suggesting a critical value of slope-angle steepness found in earlier studies. We provide field-data-based evidence that the along-slope heat flux may dominate the variables in this inequality, and therefore in generating buoyant TKE production or suppression over a steep slope. These data show the along-slope heat flux can be more variable and significantly larger in magnitude than the slope-normal component. The gradient Richardson number does not include the effects of the along-slope buoyancy; furthermore, none of the canonical stability parameters can properly reflect the TKE redistribution from turbulent transport divergence and the sink of TKE in cases of counter-gradient momentum fluxes, which we frequently observe near the peak of the katabatic jet. In such cases, canonical stability parameters inadequately represent the physical mechanisms associated with stability. These results have broad implications related to accurately modelling turbulence and surface exchanges over sloping terrain and illustrate the need to more thoroughly investigate the along-slope heat flux and its drivers, the meaning and definitions of stability, and the effects of non-local turbulent transport.

Weakly nonlinear Prandtl model for simple slope flows

The Prandtl model couples, probably in the most succinct way, basic boundary‐layer dynamics and thermodynamics for pure anabatic and katabatic flows over inclined surfaces by assuming a one‐dimensional steady‐state balance between buoyancy and turbulent friction. Although the classic Prandtl model is linear, having an a priori assigned vertically constant eddy diffusivity and heat conductivity, K, in this analytic work we partly relax both of these restrictions. The first restriction is loosened by using a weakly nonlinear approach where a small parameter, ε, controls feeding of the flow‐induced potential temperature gradient back to the environmental potential temperature gradient, because the former, below the katabatic jet, can be 20–50 times stronger than the latter, background or free‐flow gradient. An appropriate range of values for ε, controlling the weak nonlinearity for pure katabatic flow, is provided. In this way, the near‐surface potential temperature gradient becomes stronger and the corresponding katabatic jet somewhat weaker (at a slightly lower height) than that in the classic Prandtl solution. The second restriction is partly relaxed by using a prescribed, gradually varying K with distance from the underlying surface, all within the usual validity of the zero‐order Wentzel–Kramers–Brillouin approximation to solve the coupled differential equations. The new model is compared with the glacier wind data from the Pasterze experiment (PASTEX‐94), Austria. Further discussion includes gradient Richardson number consideration and an application to simple anabatic flows. The model may be applied for estimation and interpretation of the wind affecting glacier mass balance and air pollution.

Analytical and Numerical Investigation of Two-Dimensional Katabatic Flow Resulting from Local Surface Cooling

The analysis of katabatic flows is often complicated by heterogeneity in surface characteristics. This study focuses on an idealized type of katabatic flow driven by a simple form of inhomogeneous surface forcing: a buoyancy or buoyancy flux that varies down the slope as a top-hat profile (cold strip). We consider the two-dimensional Boussinesq system of governing flow equations with the slope angle, Brunt–Vaisala frequency, and coefficients of eddy viscosity and diffusivity treated as constants. The steady-state problem is solved analytically in a linearized boundary-layer framework. Key flow structures are a primary katabatic jet (essentially the classical one-dimensional Prandtl jet), a rotor-like feature straddling the upslope end of the strip, and two nearly horizontal jets: an inward jet of environmental air feeding into the primary jet on the upslope end of the strip and an outward jet resulting from the intrusion of the primary katabatic jet into the environment on the downslope end of the strip. Next, the corresponding nonlinear initial value problem is solved numerically until a steady state is reached at low levels. The main features of the linear solution are seen in the numerical results, but with some notable differences: (i) the primary jet in the numerical simulation requires a longer distance to attain a one-dimensional boundary-layer structure and extends further downslope off the strip before intruding into the environment; (ii) the numerically simulated outward environmental jet is narrower and more intense than the inward jet, and has a pronounced wave-like structure.

Climatological aspects of cyclogenesis near Adélie Land Antarctica

The Ad´elie Land coastal region of Antarctica is one of the most prominent cyclogenesis regions in the Southern Hemisphere, and is adjacent to the continent’s most intense katabatic wind regime. However, the physical mechanisms responsible for cyclogenesis are not known. A manual analysis of cyclogenesis for the 2003–2005 period using output from the Antarctic Mesoscale Prediction System (AMPS) identifies two primary patterns of cyclogenesis near the Ad´elie Land coast. For “secondary development” cyclones, enhanced low-level cyclonic vorticity and baroclinicity result from the combination of an existing synoptic-scale cyclone to the west, coastal barrier winds and katabatic winds. “Lee cyclogenesis” occurs near 152◦E on the cyclonic-shear side of the Ad´elie Land katabatic jet, where a low-level warm potential temperature anomaly sets up a lee trough that becomes mobile with the arrival of upper-level synopticscale forcing. The representation of both “secondary development” and “lee cyclogenesis” cyclones in an automated cyclone-tracking scheme is explored, where it is found that the automated scheme overestimates cyclogenesis for this region. The location of the Antarctic coastal cyclogenesis maximum near Ad´elie Land is due to the unique juxtaposition of the extraordinary katabatic wind regime and dissipating synoptic-scale cyclones to the west.

Katabatic glacier wind

Katabatic wind is the wind that appears over a cool sloping surface. Owing to the cooling of this surface, the atmospheric air over the surface is also cooled and flows down over the background of the surrounding warmer air mass due to gravity. A jet current appears the thickness of which can range from a few meters (on the mountain glaciers) to a few hundred meters, for example, in the Antarctic. Such types of local winds exist over mountainous valley glaciers and over the domes of the ice covers; therefore, these winds are also called glacier winds. The maximum wind velocity of the katabatic jet flow corresponds to its longitudinal axis. The height of this maximum over the underlying sloping surface, as well as the dependence of this height on the meteorological and other conditions, first of all, on the wind velocity, are practically impor� tant. In other words, the temperature and dynamic regime of the lower air layer (below the height of the maximum velocity) in the katabatic jet current are the main issues. These problems are discussed in this study.

Katabatic flow induced by a cross-slope band of surface cooling

This paper investigates the behavior of katabatic flow induced by an idealized, thermally inhomogeneous surface; a strip of surface cooling that has a finite width in the along-slope direction and is infinitely long in the cross-slope direction. Numerical simulations using the Boussinesq equations of motion and the thermodynamic energy equation are performed for various slope angles and strip lengths. The underlying dynamical processes in the katabatic jet and the near environment are explored by considering the along-slope momentum balance after a steady state has been achieved.

Profiles of katabatic flow in summer and winter over Coats Land, Antarctica

AbstractObservations from a novel autonomous Doppler sodar wind profiling system are described and analysed. These include the first continuous wintertime soundings of katabatic winds over Antarctica—a continent with which they are synonymous. During 2002 and 2003 over 2600 wind profiles were taken during ‘case‐studies’ of high‐resolution sounding lasting hours to days. These case‐studies have been subjectively classified as: synoptically driven, katabatically influenced (28 days); primarily katabatically driven flows (a subset of 16 days); or other flow types. The Doppler sodar observations were augmented by automatic weather station observations at the field site and further up the slope, as well as synoptic and upper‐air observations at Halley Research Station, some 50 km distant on the Brunt Ice Shelf.In primarily katabatic flows there is a systematic change in the shape and depth of the low‐level katabatic jet with wind speed. Relatively strong katabatic flows (maximum winds of typically 8–10 m s−1) have a jet maximum between 20 and 60 m above the surface and are relatively deep (up to 200 m); while moderate katabatic flows (4–8 m s−1) typically have a jet maximum between 3 and 30 m and are shallower (∼100 m), although they can also be more diffuse in structure with a wind speed maximum at higher altitude. In all katabatic flows there is backing of wind direction with height, consistent with decreasing friction away from the surface. During summertime katabatic flows there is a clear diurnal signature at all heights, although this is less pronounced in the surface layer where there seems to be a persistent 2–4 m s−1 katabatic flow during all case‐studies. Where the diurnal forcing results in an abrupt katabatic flow deceleration, i.e. what may be a katabatic ‘jump’, there is a concurrent vertical acceleration. Wind profiles from a recent numerical weather prediction study of idealized katabatic flows at this site compare favourably with selected mean profiles; the only significant difference is that the model's wind speed is too low over the lowest ∼10 m. Copyright © 2006 Royal Meteorological Society.

Describing surface fluxes in katabatic flow on Breidamerkurjökull, Iceland

AbstractFor very stable boundary layers there is no well‐accepted theory today. In this study, an improved Prandtl model with varying diffusivity is applied to less than ideal conditions for pure katabatic flow pertaining to very stable boundary layers. We find that the improved Prandtl model adequately describes the usual and persistent katabatic glacier wind on Breidamerkurjökull. This is true even for flows with very different heights and strengths of the jet. A theoretical estimate of the katabatic jet height, based on temperature deficit and lapse rate, is verified. The calculated surface fluxes compare well with the measured turbulence parameters. A possible reason for the robustness of the katabatic jet (and other low‐level jets) is given in terms of the Scorer parameter. Copyright © 2004 Royal Meteorological Society.

Momentum Budget of the East Antarctic Atmospheric Boundary Layer: Results of a Regional Climate Model

Output of a regional atmospheric climate model is used to quantify the average January and July momentum budget of the atmospheric boundary layer (ABL) over the East Antarctic ice sheet and the surrounding oceans. Results are binned in nine elevation intervals over the ice sheet and six distance intervals over the ocean. In January, when surface cooling is weak, the large-scale pressure gradient force dominates the ABL momentum budget. In July, under conditions of strong surface cooling, a shallow katabatic jet develops over the gentle slopes of the interior ice sheet and a strong, deep jet over the steep coastal slopes. In the coastal regions the ABL thickens considerably, caused by the piling up of cold air over the adjacent sea ice and ice shelves. This represents the main opposing force for the katabatic winds. Horizontal and vertical advection are generally small. In the cross-slope direction the momentum budget represents a simple balance between surface drag and Coriolis turning. Intraseasonal variability of the large-scale wind field in the ABL can be explained in terms of the strength of the polar vortex, the background baroclinicity, and the topography of the ice sheet. Subsidence is found over the interior ice sheet and rising motion in the coastal zone, reflecting the acceleration and deceleration of the katabatic circulation. However, vertical velocities are generally small, because the downslope mass flux in the ABL is confined to a shallow layer below the wind speed maximum.

A theory for the estimation of surface fluxes in simple katabatic flows

The classical Prandtl model describes the basic dynamics of katabatic flows, but it fails in determining the gradients near the surface because the eddy diffusivity, K, is independent of height, z. An improvement is proposed to the Prandtl model by using the WKB method to solve the dynamic equation for any prescribed, reasonably smooth, K(z) profile. The surface fluxes are obtained by extrapolation from the katabatic jet down to the surface, thus the fluxes depend primarily on the jet which is well captured by the WKB method. The profiles of wind, temperature, momentum and heat fluxes, as well as the surface fluxes calculated using the model, are tested against observations from a meteorological field campaign on the Pasterze glacier, Austria, in 1994, and their numerical simulations. Generally there is a good agreement between observations and theory.

A theory for the estimation of surface fluxes in simple katabatic flows

AbstractThe classical Prandtl model describes the basic dynamics of katabatic flows, but it fails in determining the gradients near the surface because the eddy diffusivity, K, is independent of height, z. An improvement is proposed to the Prandtl model by using the WKB method to solve the dynamic equation for any prescribed, reasonably smooth, K(z) profile.The surface fluxes are obtained by extrapolation from the katabatic jet down to the surface, thus the fluxes depend primarily on the jet which is well captured by the WKB method. The profiles of wind, temperature, momentum and heat fluxes, as well as the surface fluxes calculated using the model, are tested against observations from a meteorological field campaign on the Pasterze glacier, Austria, in 1994, and their numerical simulations. Generally there is a good agreement between observations and theory.

Analysis of Surface Winds at Mawson, Antarctica

An analysis is made of the wind regime at Mawson, Antarctica, to distinguish the meteorological characteristics leading to the gale force winds associated with blizzards as distinct from calm conditions or the relatively moderate and fairly steady surface katabatic winds. Investigation of relationships between the surface and midtropospheric winds over Mawson reveal that strong upper-level winds do not necessarily strengthen the surface wind, reinforcement of which varies with upper wind direction and season. Blizzard, katabatic, and calm wind categories and combinations of these are defined to allow investigation of different characteristics of winds. The importance of considering the katabatic near-surface jet component, which generally strengthens the surface wind, is demonstrated for categories of both weak and strong winds. Features of development in the midtropospheric flow during and 48 h prior to occurrences of blizzard, katabatic, and calm conditions are highlighted. Upper-level forcings associated with blizzard or calm conditions are related to slow-moving high pressure cells downstream. Similarities in forcing mechanisms of calm and katabatic periods suggest a relationship with the efficient drainage of low-level air, although there is evidence that upper-level opposition to the surface flow produces calm conditions.