Observations Of Nocturnal Drainage Flow In A Shallow Gully

The formation of cold air drainage flows in a shallow gully is studied during CASES-99 (Cooperative Atmosphere-Surface Exchange Study). Fast and slow response wind and temperature measurements were obtained on an instrumented 10-m tower located in the gully and from a network of thermistors and two-dimensional sonic anemometers, situated across the gully. Gully flow formed on clear nights even with significant synoptic flow. Large variations in surface temperature developed within an hour after sunset and in situ cooling was the dominant factor in wind sheltered locations. The depth of the drainage flow and the height of the down-gully wind speed maximum were found to be largest when the external wind speed above the gully flow is less than 2 m s^-1. The shallow drainage current is restricted to a depth of a few metres, and is deepest when the stratification is stronger and the external flow is weaker. During the night the drainage flow breaks down, sometimes on several occasions, due to intermittent turbulence and downward fluxes of heat and momentum. The near surface temperature may increase by 6 ° C in less than 30 min due to the vertical convergence of downward heat flux. The mixing events are related to acceleration of the flow above the gully flow and decreased Richardson number. These warming events also lead to warming of the near surface soil and reduction of the upward soil heat flux. To examine the relative importance of different physical mechanisms that could contribute to the rapid warming, and to characterize the turbulence generated during the intermittent turbulent periods, the sensible heat budget is analyzed and the behaviour of different turbulent parameters is discussed.     

Evening and Morning Transition of Katabatic Flows

An experimental investigation of the evening and morning transition phases of katabatic slope flows has been conducted to identify the mechanisms for their development and destruction over an isolated slope. The momentum and energy equations of the flow have been used to describe these mechanisms for the particular topographic features of the studied slope, and to outline the differences from the dynamics of well-developed simple slope flows. In the lowest portion of the slope, frontal characteristics have been identified in early evening periods when the local pre-existing near-surface thermal structure does not impose a katabatic acceleration. The frontal shape is determined by the near-surface thermal stability and ambient wind. The flow initiation is distinctly different when it is linked to the local surface cooling, in which case it develops gradually and produces a slight local warming.The erosion of the katabatic layer at mid-slope precedes that at the foot and is closely linked to dilution of the local surface inversion. The flow erosion at the foot is often delayed, as the warming of air proceeds uniformly at all heights near the ground, so maintaining the inversion due to warming produced by mixing and advective processes linked to the upslope flow development. The latter initiates first at mid-slope and then at the foot, where for a non-negligible time period it flows over the persistent katabatic flow. The prerequisite for the development of this structure is the maintenance of a shallow inversion in the first 2–3 m above the ground surface.The morning dilution of the katabatic flow is apparently different from common experience over simple slopes and may be attributed to the steep upper portion of the slope in association with its easterly orientation, which results in strong non-uniformity of the solar heating along the slope.     

Observations of the Relation Between Upslope Flows and the Convective Boundary Layer in Steep Terrain

Slope flow mechanisms are crucial for the transport of air pollutants in complex terrain. Previous observations in sloping terrain showed upslope flows filling the entire convective boundary layer (CBL) and reducing air pollution concentrations by venting air pollutants out of the CBL into the free atmosphere. During the Pacific 2001 Air Quality Field Study in the Lower Fraser Valley, British Columbia, Canada, we observed slope flows during weak synoptic winds, clear skies, and strong daytime solar heating. With a Doppler sodar we measured the three wind components at the foot of a slope having an average angle of 19° and a ridge height of 780 m. We operated a scanning lidar system and a tethersonde at a nearby site on the adjacent plain to measure backscatter of particulate matter, temperature, wind speed, wind direction, and specific humidity. Strong daytime upslope flows of up to 6 m s⁻¹ through a depth of up to 500 m occurred in the lower CBL, but with often equally strong and deep return flows in the upper part of the CBL. The mass transport of upslope flow and return flow approximately balanced over a 4-h morning period, suggesting a closed slope-flow circulation within the CBL. These observations showed that air pollutants can remain trapped within a CBL rather than being vented from the CBL into the free atmosphere.     

Large-Eddy Simulation of Katabatic Flows

A large-eddy simulation model with rotated coordinates and an open boundary is used to simulate the characteristics of katabatic flows over simple terrain. Experiments examine the effects of cross winds on the development of the slope-flow boundary layer for a steep (20°) slope and the role of drainage winds in preventing turbulence collapse on a gentle slope (1°). For the steep flow cases, comparisons between model average boundary-layer velocity, temperature deficit, and turbulence kinetic energy budget terms and tower observations show reasonable agreement. Results for different cross slope winds show that as the cross slope winds increase, the slope flow deepens faster and behaves more like a weakly stratified, sheared boundary layer. Analysis of the momentum budget shows that near the surface the flow is maintained by a balance between downslope buoyancy forcing and vertical turbulence flux from surface drag. Above the downslope jet, the turbulence vertical momentum flux reverses sign and acceleration of the flow by buoyancy is controlled by horizontal advection of slower moving ambient air. The turbulence budget is dominated by a balance between shear production and eddy dissipation, however, buoyancy and pressure transport both are significant in reducing the strength of turbulence above the jet. Results from the gentle slope case show that even a slight terrain variation can lead to significant drainage winds. Comparison of the gentle slope case with a flat terrain simulation indicates that drainage winds can effectively prevent the formation of very stable boundary layers, at least near the top of sloping terrain.     

The KABEG’97 field experiment: An aircraft-based study of katabatic wind dynamics over the Greenland ice sheet

The aircraft-based experiment KABEG97 (Katabatic wind and boundary-layer front experiment around Greenland) was performed in April/May 1997. During the experiment, surface stations were installed at five positions on the ice sheet and in the tundra near Kangerlussuaq, West Greenland. A total of nine katabatic wind flights were performed during quite different synoptic situations and surface conditions, and low-level jets with wind speeds up to 25m s^-1 were measured under strong synoptic forcing of the katabatic wind system. The KABEG data represent a unique data set for the investigation of katabatic winds. For the first time, high-resolution and accurate aircraft measurements can be used to investigate the three-dimensional structure of the katabatic wind system for a variety of synoptic situations.Surface station data show that a pronounced daily cycle of the near-surface wind is present for almost all days due to the nighttime development of the katabatic wind. In a detailed case study the stably-stratified boundary layer over the ice and the complex boundary-layer structure in the transition zone ice/tundra are investigated. The katabatic wind system is found to extend about 10 km over the tundra area and is associated with strong wind convergence and gravity waves. The investigation of the boundary-layer dynamics using the concept of a two-layer katabatic wind model yields the results that the katabatic flow is always a shooting flow and that the pure katabatic force is the main driving mechanism for the flow regime, although a considerable influence of the large-scale synoptic forcing is found as well.     

Second-Order Modelling of Turbulence in Katabatic Flows

A complete one-dimensional second-order closure model is used to simulate katabatic flows observed on glaciers and ice caps. The model is tested with two different closure assumptions for the viscous dissipation, one based on a prognostic equation for and the other on a diagnostic buoyant length scale. Both formulations give quite similar results. Model simulations are compared to observations made over sloping ice surfaces during periods dominated by katabatic flow. In general, good agreement is found for both mean wind and temperature profiles as well as eddy correlation measurements. It is also found that the turbulent transport terms play an important role in katabatic flows as opposed to the classical stable boundary layer where these terms are usually ignored. Even the turbulent transport of temperature variance, which leads to the well-known countergradient term in unstable boundary layers, is relatively important for modelling the observed temperature profiles. The effect of these terms on the flux-profile relationships, using observed and simulated profiles, is also discussed.     

Statistical Characterization of the Flow Structure in the Rhine Valley

The flow structure at the intersection between the Rhine and the Seez valleys nearthe Swiss city of Bad Ragaz has been documented by means of wind and pressuremeasurements collected from 9 September to 10 November 1999 during the MesoscaleAlpine Programme (MAP) experiment. To understand better the dynamics of theageostrophic winds that develop in this part of the Rhine valley, some key questionsare answered in this paper including the following: (i) How does air blow at theintersection of the Rhine and Seez valleys? and (ii) what are the dynamical processes(mechanical or thermal) driving the flow circulations in the valleys? Statistical analysis of the wind and pressure patterns at synoptic scale and at the scaleof the valley shows that five main flow patterns, SE/S, NW/W, NW/N, NW/S, SE/N(wind direction in the Seez valley/wind direction in the Rhine valley) prevail. The SE/S regime is the flow splitting situation. It is mainly driven by a strong pressure gradient across the Alps leading to foehn, even though some nocturnal cases are generated bylocal thermal gradients. The NW/W and NW/N regimes are mechanically forced bythe synoptic pressure gradient (as the flow splitting case). The difference between thetwo regimes is due to the synoptic flow direction [westerly (northerly) synoptic flowfor the NW/W (NW/N) regime], showing that the Rhine valley (particularly from BadRagaz to Lake Constance) is less efficient in channelling the flow than the Seez valley.The NW/S (occurring mainly during nighttime) and SE/N (occurring mainly duringdaytime) regimes are mainly katabatic flows. However, the SE/N regime is also partlyforced at the synoptic scale during the foehn case that occurred between 18 October and 20 October 1999, with a complex layered vertical structure. This analysis also shows that, contrary to what was observed in a broad section of theupper Rhine valley near Mannheim, very few countercurrents were observed near BadRagaz where the valley width is much smaller.     

Large-Eddy Simulation Of The Stably Stratified Planetary Boundary Layer

In this work, we study the characteristics of a stably stratifiedatmospheric boundary layer using large-eddy simulation (LES).In order to simulate the stable planetary boundary layer, wedeveloped a modified version of the two-part subgrid-scalemodel of Sullivan et al. This improved version of themodel is used to simulate a highly cooled yet fairly windy stableboundary layer with a surface heat flux of(W)_o = -0.05 m K s^-1and a geostrophic wind speed of U_g = 15 m s^-1.Flow visualization and evaluation of the turbulencestatistics from this case reveal the development ofa continuously turbulent boundary layer with small-scalestructures. The stability of the boundary layercoupled with the presence of a strong capping inversionresults in the development of a dominant gravity wave atthe top of the stable boundary layer that appears to be relatedto the most unstable wave predicted by the Taylor–Goldsteinequation. As a result of the decay of turbulence aloft,a strong-low level jet forms above the boundary layer.The time dependent behaviour of the jet is compared with Blackadar'sinertial oscillation analysis.     

Contrasting vertical structures of nocturnal boundary layers

This study analyzes eight levels of sonic anemometerdata collected on a 60-m towerduring CASES-99, toward the goal of understanding thevertical structure of thenocturnal boundary layer. Several different regimesare found. Thin boundarylayers are often observed where fluxes decrease with height and approximately vanish between 20 and 30 m aboveground. The flow above the thin boundary layeraccelerates and increasing shear oftengenerates significant turbulence in the middle ofthe night. Thisshear-generated turbulence is often stronger thanthat near the surface corresponding to an upside-downboundary layer. During these conditions,the turbulent transport of turbulence is downwardtoward the surface. The turbulence in this regimeshows features of z-less turbulence to the extentthat neither the height above groundnor the boundary-layer depth are primary scalingvariables. This layer isdifferent from a `residual layer' in thatturbulence is actively generated byshear associated with nocturnal accelerationsand often is stronger than that inthe surface-based boundary layer.In many cases, the turbulence does not varysignificantly across the towerlayer, implying that the boundary layer ismuch deeper than the 60-m towerlayer. Several case studies are presentedto illustrate the largevariation of vertical structure betweennights.     

Spatial and Temporal Occurrences of Intermittent Turbulence During CASES-99

Occurrences of intermittent turbulence in very stable conditions during theCASES-99 field study near Leon, Kansas were detected at several sites separatedby horizontal distances from 1 km to 25 km using sonic anemometers, minisodarsand a laser scintillometer. Periods with significant turbulent heat fluxes wereseparated by extended quiescent periods with little or no flux, and most of theflux during a night was realized in relatively small fractions (<20%) of thetotal time. There appeared to be no relationship between this intermittencyfraction and the median z/L (z being height and L the Obukhov length)value for the night, although overall sensible heat flux values on very stablenights were significantly less than those on less stable nights. The intermittencyfraction at 7 m was found to increase with mean wind speed at 20 m and, to alesser extent, with wind shear between 20 m and 30 m. While correspondenceof turbulent episodes at two sites separated by 1 km was common, it was less common at separations on the order of 20 km. There were time periods, however, during which enhanced turbulence levels were seen nearly simultaneously at large separation distances. Turbulence episodes were found to propagate upward or downward at different times with no readily defined large-scale controlling mechanism.     

Extremely Weak Mixing in Stable Conditions

Transport by extremely weak turbulence occurring on nights with clear skies and weak winds is examined from seven tower levels of eddy-correlation data taken from each of two field programs. The very small flux is systematic, provided that the perturbations are computed from a record-dependent averaging length, which must be as small as 10 s in very stable conditions. With traditional methods for computing the flux, these fluxes were considered too small to estimate, in that the computed values behaved erratically. For extremely weak turbulence, the fluxes decrease systematically with height and often indicate very shallow boundary-layer depths on the order of 10 m. However, in one field program, the turbulence slowly increases with height above the surface flux-based boundary layer apparently due to horizontal advection of stronger turbulence driven by modest surface heterogeneity. For very weak turbulence, the eddy diffusivity for momentum is systematically greater than that for heat in both field programs. The dependence of the turbulence strength and its variability with stability is examined in some detail.     

Intermittency and the Exchange of Scalars in the Nocturnal Surface Layer

The determination of nocturnal surface fluxes in low wind conditions is a major problem for micrometeorological studies. The eddy correlation technique, extensively used in field measurements, becomes inappropriate if not enough turbulent activity exists. At the same time, the phenomenon of turbulence intermittency is responsible for the existence of localized events of short duration within which a large fraction of the total nighttime scalar exchange occurs. The scalar flux within a certain intermittent event varies considerably depending on the window used for the flux calculation. In many cases, events with very different time durations occur in the same night, and therefore, the proper determination of the surface flux would require averaging within data windows of different sizes for each event. In this work, the surface exchanges of temperature, moisture and carbon dioxide are analysed at a micrometeorological tower at southern Brazil. Intermittent turbulence is a common occurrence at the location. The analysis shows that the fluxes vary with turbulence intensity and the estimation technique. A variable-window size method for flux estimation is suggested and shown to cause an increase in the magnitude of the nocturnal surface fluxes     

Characteristics of Submeso Winds in the Stable Boundary Layer

The characteristics of submeso motions in the stable boundary layer are examined using observations from networks of sonic anemometers with network sizes ranging from a few hundred metres to 100 km. This study examines variations on time scales between 1 min and 1 h. The analysis focuses on the behaviour of the spectra of the horizontal kinetic energy, the ratios of the three velocity variances, their kurtosis, the dependence of horizontal variability on time scale, and the inter-relationship between vertical vorticity, horizontal divergence and deformation. Motions on larger time and space scales in the stable boundary layer are found to be nearly two-dimensional horizontal modes although the ratio of the vorticity to the divergence is generally on the order of one and independent of scale. One exception is a small network where stronger horizontal divergence is forced by a decrease in surface roughness. The horizontal variability, averaged over 1 h, appears to be strongly influenced by surface heterogeneity and increases with wind speed. In contrast, the time dependence of the horizontal structure on time scales less than one hour tends to be independent of wind speed for the present datasets. The spectra of the horizontal kinetic energy and the ratio of the crosswind velocity variance to the along-wind variance vary substantially between networks. This study was unable to isolate the cause of such differences. As a result, the basic behaviour of the submeso motions in the stable boundary layer cannot be generalized into a universal theory, at least not from existing data.     

The influence of nonstationarity on the turbulent flux–gradient relationship for stable stratification

Extensive eddy-correlation datasets are analyzed to examine the influence of nonstationarity of the mean flow on the flux–gradient relationship near the surface. This nonstationarity is due to wavelike motions, meandering of the wind vector, and numerous unidentified small-scale mesoscale motions. While the data do not reveal an obvious critical gradient Richardson number, the maximum downward heat flux increases approximately linearly with increasing friction velocity for significant stability. The largest of our datasets is chosen to more closely examine the influence of stability, nonstationarity, distortion of the mean wind profile and self-correlation on the flux-gradient relationship. Stability is expressed in terms of z/L, the gradient Richardson number or the bulk Richardson number over the tower layer. The efficiency of the momentum transport systematically increases with increasing nonstationarity and attendant distortion of the mean wind profile. Enhancement of the turbulent momentum flux associated with nonstationarity is examined in terms of the nondimensional shear, Prandtl number and the eddy diffusivity.     

Turbulence in a Katabatic Flow

Turbulence measurements performed in a stable boundary layer over the sloping ice surface of the Vatnajökull in Iceland are described. The boundary layer, in which katabatic forces are stronger than the large-scale forces, has a structure that closely resembles that of a stable boundary layer overlying a flat land surface, although there are some important differences. In order to compare the two situations the set-up of the instruments on an ice cap in Iceland was reproduced on a flat grass surface at Cabauw, the Netherlands. Wind speed and temperature gradients were calculated and combined with flux measurements made with a sonic anemometer in order to obtain the local stability functions _m and _h as a function of the local stability parameter z/L. Unlike the situation at Cabauw, where _m was linear as a function of z/L, in the katabatically forced boundary layer, the dependence of _m on stability was found to be non-linear and related to the height of the wind maximum. Thermal stratification and the depth of the stable boundary layer however seem to be rather similar under these two different forcing conditions.Furthermore, measurements on the ice were used to construct the energy balance. These showed good agreement between observed melt and components contributing to the energy balance: net radiation (supplying 55% of the energy), sensible heat flux (30%) and latent heat flux (15%).Local sources and sinks in the turbulent kinetic energy budget are summed and indicate a reasonable balance in near-neutral conditions but not in more stable situations. The standard deviation of the velocity fluctuations _u, _v, and _w, can be scaled satisfactorily with the local friction velocity u_* and the standard deviation of the temperature fluctuation with the local temperature scale _*.     

Nocturnal stable boundary layer height model and its application

A model of the evolution of the nocturnal stable boundary layer height, based on the heat conservation equation for a turbulent flow, is presented. This model is valid for nights with weak winds and little cloudiness in rural areas. The model includes an expression of vertical profile of potential temperature within the boundary layer, which is obtained using micrometeorological information from Prairie Grass, Wangara and O'Neill Projects. The expression turned out to be a second-grade polynomial of the dimensionless height of the nocturnal stable boundary layer. The resulting model is a function of the Monin–Obukhov length, the surface potential temperature of air and the roughness length. This model was satisfactorily compared with micrometeorological data. It was applied at three stations of Argentina, using surface hourly meteorological information. From the results that were obtained, the monthly average values of the stable boundary layer thickness were analysed. The maximum monthly average values occur during the cold season and the minimum ones take place during the hot season. It was observed that the monthly average thickness increases with latitude.     

Nocturnal Boundary-Layer Regimes

This study analyzes turbulence data collected over a grassland site in the nocturnal boundary layer. Examination of the dependence of the nocturnal boundary layer on stability suggests three regimes: a) the weakly stable case, b) a transition stability regime where many of the variables change rapidly with increasing stability and c) the very stable case. The value of z/L where the downward heat flux is a maximum defines the stability boundary between the weakly stable and transition regimes, where L is the Obukhov length. In the present analysis, the downward heat flux reaches a maximum at z/L approximately equal to 0.05 for 10 m, although comparison with other data indicates that this is not a universal value. For weaker stability, the heat flux decreases with decreasing z/L due to weaker temperature fluctuations. In the transition stability regime, the heat flux decreases rapidly with increasing stability due to restriction of vertical velocity fluctuations by the increasing stratification.For weakly stable conditions, the variances scale according to Monin-Obukhov similarity theory. For very stable conditions, the variances are contaminated by non-turbulent horizontal motions and do not follow the scaling laws. An alternative length scale based on variances is developed which explains more of the variance of the transfer coefficients compared to the Obukhov length.     

Turbulence Characteristics Of The Stable Boundary Layer Over A Mid-Latitude Glacier. Part Ii: Pure Katabatic Forcing Conditions

Observations obtained over a glacier surface in a predominantlykatabatic flow and with a distinctwind maximum below 13-m height are presented. The data werecollected using a 13-m high profilemast and two sonic anemometers (at about 2.5-m and 10-m heights).The spectra at frequencies belowthat of the turbulence range appear to deviate considerably fromthe curves obtained by Kaimal andco-workers during the 1968 Kansas experiment. The characteristicsof these deviations are compared tothe observations of others in surface-layers disturbed by anykind of large-scale outer-layer (orinactive) turbulence. In our case the disturbances arelikely to be induced by the highmountain ridges that surround the glacier. Moreover, the deviationsobserved in the cospectra seemto result from an, as yet, unspecified interaction between theinactive outer-layer turbulenceand the local surface-layer turbulence. Near the distinctwind maximum turbulence production ceasedwhile turbulence itself did not, probably the result ofturbulence transport from other levels. Consequently, we studied thelocal similarity relations using _w instead of u_* as an alternative velocity scale. Wellbelow the wind maximum, and for relatively low stability(0< Ri_g <0.2), the flow behaves accordingto well established local-scaling similarity relationshipsin the stable boundary layer. For higherstability (Ri_g > 0.2), and near or above the wind maximum, the boundary-layer structure conforms tothat of z-less stratification suggesting that the eddy sizeis restricted by the local stability ofthe flow. In line with this we observed that the sensibleheat fluxes relate remarkably well to thelocal flow parameters.     

Intermittent Turbulence Associated with a Density Current Passage in the Stable Boundary Layer

Using the unprecedented observational capabilities deployed duringthe Cooperative Atmosphere-Surface Exchange Study-99 (CASES-99),we found three distinct turbulence events on the night of 18October 1999, each of which was associated with differentphenomena: a density current, solitary waves, and downwardpropagating waves from a low-level jet. In this study, we focus onthe first event, the density current and its associatedintermittent turbulence. As the cold density current propagatedthrough the CASES-99 site, eddy motions in the upper part of thedensity current led to periodic overturning of the stratifiedflow, local thermal instability and a downward diffusion ofturbulent mixing. Propagation of the density current induced asecondary circulation. The descending motion following the head ofthe density current resulted in strong stratification, a sharpreduction in the turbulence, and a sudden increase in the windspeed. As the wind surge propagated toward the surface, shearinstability generated upward diffusion of turbulent mixing. Wedemonstrate in detail that the height and sequence of the localthermal and shear instabilities associated with the dynamics ofthe density current are responsible for the apparent intermittentturbulence.     

Large-Eddy Simulation of air Pollution Dispersion in the Nocturnal Cloud-Topped Atmospheric Boundary Layer

Effects of stratocumulus clouds on the dispersion of contaminants are studied in the nocturnal atmospheric boundary layer. The study is based on a large-eddy simulation (LES) model with a bulk parametrization of clouds. Computations include Lagrangian calculations of atmospheric dispersion of a passive tracer released from point sources at various heights above the ground. The results obtained show that the vertical diffusion is non-Gaussian and depends on the location of a source in the boundary layer.