Vertical fluxes in katabatic flows

Katabatic flow is a dynamical process occurring on relatively calm, clear nights above sloping terrain. Its existence is dependent on long-wave radiative transfer, particularly radiative flux divergence within the air itself, for both its generation and (it is concluded here), along with advective warming, for much of its retardation.Utilising sounding data closely spaced in time, a discussion is presented of the importance of surface shear, interfacial shear, advective warming and radiative divergence in a strong katabatic flow. It is concluded that radiative divergence is important in generating static and dynamic instabilities in the flow. The role of radiative cooling in mixing of momentum has largely been ignored so far, and might explain why higher-order models tend to overestimate katabatic speeds on smooth slopes.     

On the integral modelling of katabatic flows

The integral model for katabatic flows proposed by Manins and Sawford (1979) is solved numerically. It is shown that numerical solutions can be approximated by Ball's (1956) model in the upper part of the slope, while they tend toward Manins and Sawford's simplified solution farther downslope. The importance of entrainment and ambiant stable stratification is shown. Some limitations of Manins and Sawford's model are discussed.     

Retrieval of eddy thermal conductivity in the weakly nonlinear Prandtl model for katabatic flows

Because the nonlinearity of actual physical processes can be expressed more precisely by the introduction of a nonlinear term, the weakly nonlinear Prandtl model is one of the most effective ways to describe the pure katabatic flow (no background flow). Features of the weak nonlinearity are reflected by two factors: the small parameter ε and the gradually varying eddy thermal conductivity. This paper first shows how to apply the Wentzel–Kramers–Brillouin (WKB) method for the approximate solution of the weakly nonlinear Prandtl model, and then describes the retrieval of gradually varying eddy thermal conductivity from observed wind speed and perturbed potential temperature. Gradually varying eddy thermal conductivity is generally derived from an empirical parameterization scheme. We utilize wind speed and potential temperature measurements, along with the variational assimilation technique, to derive this parameter. The objective function is constructed by the square of the differences between the observation and model value. The new method is validated by numerical experiments with simulated measurements, revealing that the order of the root mean squre error is 10^–2 and thus confirming the method’s robustness. In addition, this method is capable of anti-interference, as it effectively reduces the influence of observation error.     

Experimental and model transport and diffusion studies in complex terrain with emphasis on tracer studies

The U.S. Department of Energy (DOE) Atmospheric Studies in Complex Terrain (ASCOT) program began in the fall of 1978 as a multiple DOE and other Federal Laboratory program devoted to developing a better physical understanding of atmospheric boundary layer flows in areas of complex terrain. The first technical challenge undertaken by the program was an investigation of atmospheric boundary layer phenomena associated with the development, continuation and breakup of nocturnal drainage wind flows. This paper discusses the general objectives the program has addressed during the past several years and focuses on results from a major field experiment conducted in 1980 in The Geysers area of northern California. Specifically, results from measurements of simultaneous tracer releases are compared to calculations from a mass-consistent wind field model coupled to a particle-in-cell transport and diffusion model. Results of these comparisons show that model calculations agree with measurements within a factor of 5 approximately 50 percent of the time. Part of the difficulty faced by the models in these comparison studies is associated with large variabilities between measurements made by samplers located one or two x apart when compared to the resolution of the models. Space and time averaging improves the comparisons considerably, although the design of the field experiment did not allow the determination of optimum spacial and temporal averages.     

Forecasting the transport and transformation of atmospheric pollutants with the COSMO-ART model

The COSMO-ART model is used at the Hydrometcenter of Russia for the routine forecast of air pollutant concentrations. The COSMO-Ru7-ART system is able to simulate adequately the values of concentration of impurities in the atmosphere. The system includes the module for estimating the emission of pollutants to the atmosphere from forest fires that was successfully tested on the case of forest fires occurred in the summer of 2010 for the European part of Russia. The accurate forecast of pollutant concentration has also a positive effect on the air temperature forecast due to taking into account the aerosol feedback on radiation.     

Study of the influence of katabatic flows on the Antarctic circulation using GCM simulations

Summary A one dimensional analytical model of katabatic wind over the Antarctica has been developed. This parametric model is derived from the bulk two-layer model of Ball including the surface friction and taking into account the Earth's rotation and the geostrophic wind in the upper layer.This model is validated using the data set (70 soundings) collected during IAGO experiment at D47 (67°24S, 138°43E, altitude 1 564m), 110 km inland from the coast of Adélie Land.The parameteric model is then introduced into a GCM which is a spectral global version of the operational numerical weather prediction model used by the French weather service. The most significant effect of the parameterization is a 50 m increase of the geopotential height over the South Pole. The surface temperature at the South Pole increases (2°C) reducing the pole-midlatitude thermal gradient. The westerly circulation at 50° S is slowed down (4m/s at 850 hPa), and the surface pressure at the South Pole increases (4hPa). These results, consistent with an increase of katabatic winds, would however be improved by a better coupling between the parameterization and the GCM boundary layer.With 8 Figures     

Modeling the concentration of pollutants using the WRF-ARW atmospheric model and CHIMERE chemistry transport model

Described is a system for analyzing and forecasting the air quality in the central regions of Russia, During the operation of the system, the detailed meteorological information provided by the WRF-ARW model is used by the CHIMERE chemistry transport model for simulating the processes of transport, chemical transformation, and deposition of atmospheric minor constituents. Considered is the quality of retrieved and forecasted (with the lead time up to three days) concentrations of O₃, NO₂, NO, CO, and PM₁₀. The presented verification scores of pollutant concentrations demonstrate a relative success of the system. Demonstrated is a need in improving the data on the emissions of the air pollutants used for simulations. A procedure for the statistical correction of computed concentrations is described and verification scores of its results are given.     

Evidence of katabatic flows deduced from a 84 m meteorological tower in Athens,Greece

The identification of katabatic flows and their characteristics observed on a simple slope (the western side of Hymettos mountain) for a six-month period (January to June, 1990) are presented. This is the first application of data obtained from an 84 m high meteorological research tower recently erected at the National Center for Scientific Research Demokritos in Athens, Greece. The tower is described with respect to construction and instrumentation. The topography of the region and the observational site are also described. Criteria for the identification of katabatic flows are presented, while the frequency of occurrence of such flows is estimated. Finally, two case studies are analysed in order to reveal the special characteristics of the katabatic flows.     

The Vertical Transport of Air Pollutants by Convective Clouds. Part I: A Non-Reactive Cloud Transport Model

A convective cloud transport model, without chemical processes, is developed by joining a set of concentration conservative equations into a two-dimensional, slab-symmetric and fully elastic numerical cloud model, and a numerical experiment is completed to simulate the vertical transport of ground-borne, inert gaseous pollutant by deepthunderstorm. The simulation shows that deep convective storm can very effectively transport high concentrated pollutant gas from PBL upward to the upper troposphere in 30 to 40 minutes, where the pollutant spreads laterally outward with strong anvil outflow, forming an extensive high concentration area. Meanwhile, relatively low concentration areas are formed in PBL both below and beside the cloud, mainly caused by dynamic pumping effect and sub-cloud downdraft flow. About 80% of the pollutant gas transported to the upper troposphere is from the layer below 1.5 km AGL (above ground level).     

Sulfur distribution and transport studies in East Asia using eulerian model

A three-dimensional regional Eulerian model of sulfur deposition and transport is developed. Processes treated in the model include emission, transport, diffusion, gas-phase and aqueous-phase chemical process, dry deposition, rainout and washout or sulfur. A “looking up table” method is provided to deal with the gas-phase chemical process including sulfur transfer. Dry-depositon velocity considers the influence of underlying surface, wind, degree of stabil-ity by parameterization. Model calculated values reasonably agrees with observation. Distribution of sulfur deposi-tion and transport in East Asia are also analyzed in this paper. Some amount of sulfur emission of different countries transport across boundaries, but the main origin of sulfur deposition in each country in East Asia is from itself. Furthermore, some transport paths on different layers and outlet or inlet zones are found. According to sulfur bal?ance and budget we concluded that sulfur outlets are bigger than inlets across boundary and emissions are more than deposition in most places of East Asia.     

Numerical simulations of Cross-Appalachian transport and diffusion

Numerical studies of short-term transport and diffusion over regional distances have been made with a particle-in-cell model and compared with observations from the Cross-Appalachian Tracer Experiment. The plume configurations of all seven simulations show general agreement with the observed patterns. Frequency distributions of observed and predicted values are broadly similar, although correlation coefficients of points paired in space and time are poor; this results from small dislocations in predicted versus observed plume orientations.     

The extension of a density current model of katabatic winds to include the effects of blowing snow and sublimation

A density current model was extended for use in katabatic flow over the steep slopes of Antarctica through the inclusion of the Coriolis effect and weight flux terms corresponding to blowing snow and cooling caused by sublimation. The model was calibrated and tested against data obtained during two flights in Adelie Land, Antarctica, along a trajectory starting about 170 km inland and extending to Dumont d'Urville. The predicted trend in water vapor flux agrees with measurements of this flux, lending support to empirical formulae for both snow flux and sublimation rate. Model predictions of velocity were in good agreement with measured quantities when reasonable estimates of radiation divergence and surface heat exchange were provided as input to the model. The potential temperature gradient above the katabatic layer was found to play a major role in flow stability for high velocity and deep katabatic flows. Velocity predictions were in better agreement with the data when a locally determined value was used for the coefficient in the empirical snow flux expression.     

Dynamical aspects of katabatic wind evolution in the Antarctic coastal zone

The spatial evolution of katabatic winds along idealized slopes representative of Antarctic terrain is examined using a hydrostatic, two-dimensional primitive equation model with high resolution. A downslope momentum-forces analysis is made of simulations in which katabatic flow reaches steady state, with emphasis on physical mechanisms in the coastal zone. The importance of the reversal of the pressure gradient force in the coastal zone, causing the sudden decay of katabatic winds, is discussed.     

Finite element computation of pollutant transport in thermally stratified flows

A finite element model for the computation of thermally stratified flows is described. The model includes a special treatment of the pressure gradient term by splitting it into hydrostatic and hydrodynamic contributions. Four-node elements are used for space-discretization and a semi-implicit scheme for time integration. The computed flow is used to simulate pollutant transport by a finite element model. Numerical results from such simulations, in particular for urban heat islands are described and compared with experimental data.     

A new technique for the solution of diffusion equation systems for reacting pollutants

In this paper it is shown how equation systems describing the vertical diffusion of reacting pollutants can be efficiently solved without employing operator splitting. The proposed technique is based on a selfadaptive predictor-corrector method the idea being to replace each inversion of a (K·J)²-matrix by 2K-1 inversions of J²-matrices. As an application, simulation results for the transport and the chemical transformation of motorcar emissions are presented.     

Numerical simulations of katabatic jumps in coats land,Antartica

A non-hydrostatic numerical model, the Regional Atmospheric Modeling System (RAMS), has been used to investigate the development of katabatic jumps in Coats Land, Antarctica. In the control run with a 5 m s^-1downslope directed initial wind, a katabatic jump develops near the foot of the idealized slope. The jump is manifested as a rapid deceleration of the downslope flow and a change from supercritical to subcritical flow, in a hydraulic sense, i.e., the Froude number (Fr) of the flow changes from Fr > 1 to Fr> 1. Results from sensitivity experiments show that an increase in the upstream flow rate strengthens the jump, while an increase in the downstream inversion-layer depth results in a retreat of the jump. Hydraulic theory and Bernoulli's theorem have been used to explain the surface pressure change across the jump. It is found that hydraulic theory always underestimates the surface pressure change, while Bernoulli's theorem provides a satisfactory estimation. An analysis of the downs balance for the katabatic jump indicates that the important forces are those related to the pressure gradient, advection and, to a lesser extent, the turbulent momentum divergence. The development of katabatic jumps can be divided into two phases. In phase I, the t gradient force is nearly balanced by advection, while in phase II, the pressure gradient force is counterbalanced by turbulent momentum divergence. The upslope pressure gradient force associated with a pool of cold air over the ice shelf facilitates the formation of the katabatic jump.     

Wind flows and forces in a model spruce forest

Wind tunnel tests have been conducted on a 1:75 scale model of a Sitka spruce forest in a correctly scaled turbulent boundary-layer flow. 12000 tree models were manufactured with mass, flexibility and aerodynamic drag characteristics chosen to give dynamical similarity with typical 15 m trees in a 30ms^–1 gale. To measure the dynamic response of a sample tree, set within this model forest, a miniature, fast response strain-gauge balance was designed and built. Linked to a computer for on-line data sampling, this balance provided measurements of the fluctuating along-wind and acrosswind components of the overturning moment at ground level, leading to values of mean and extreme moments and the frequency spectrum of the sway motion. Associated measurements of local wind flow characteristics were made with hot-wire anemometers and a laser anemometer. The response of the tree has the characteristics of classical lightly damped vibration and there is evidence that resonant sway motion increases the extreme overturning moments significantly above the values produced by wind gust forces alone.     

A mechanistic model of isopycnal diffusion in the ocean

A two-dimensional (xz) box ocean model, is formulated to examine the mechanistic role of isopycnal diffusion (i.e. diffusion along a constant density surface) as compared to that of horizontal/vertical diffusion. A large-scale surface temperature anomaly forces a steady solution. The presence of isopycnal diffusion substantially increases the vertical penetration in the steady state: the vertical heat flux is increased by an order of magnitude in some locations. The time scale of the transient response is also modified, and the nature of this response is scale dependent. It is thus not possible in general to reproduce the transient response with isopycnal diffusion by adjusting diffusitives in a lateral diffusion formulation. Implications of the results to climate modelling are considered.     

Effects of stability on the profiles of vertical velocity and its variance in katabatic flow

The atmospheric katabatic flow in the foothills of the Front Range of the Rocky Mountains has been monitored by a network of towers and sodars for several years as part of the Atmospheric Studies in COmplex Terrain (ASCOT) program. We used three years of data from the network to explore the dependence on surface cooling and channeling by winds above the canyon of (1) profiles of the mean and variance of the vertical (perpendicular to the geopotential) component of motion and (2) the mean component of the wind perpendicular to the local terrain of Coal Creek Canyon. Previously we found that the magnitude of the near-surface temperature difference decreases with increasing surface cooling in light winds, apparently because of increasing turbulence caused when increasing drainage winds interact with surface topography. The variance of vertical velocity exhibits three types of vertical profiles, corresponding to different cooling rates and external wind speeds. The mean variance was found to depend strongly on a locally derived Richardson number.