Wind Erosion and Wind-Erosion Research.- Wind-Erosion Climatology.- Atmospheric Boundary Layer and Atmospheric Modelling.- Land-Surface Modelling.- Basic Aspects of Wind Erosion.- The Dynamics and Modelling of Saltation.- Dust Emission.- Dust Transport and Deposition.- Integrated Wind-Erosion Modelling.- Sand Dunes, Dynamics and Modelling.- Techniques for Wind-Erosion Measurements.- Concluding Remarks.

Physics and Modelling of Wind Erosion

Complex orography of elevated terrain and numerous peaks can act as an important physical barrier to the atmospheric flow and can generate a range of meteorological phenomena. The objective of this study is to investigate the effect of the Brazilian Plateau on the spatial distribution of cold fronts and their climatology for the period between 1979 and 2010 using Climate Forecast System Reanalysis data. The cold fronts are detected and divided into three groups, according to their frequency and trajectory over South America: the continental fronts are referred to as the AN group; the fronts that pass over the coastal region of Sao Paulo are referred to as the SP group; and the fronts that pass over the two areas are referred to as the ANSP group. Composites of means and anomalies of atmospheric variables are constructed for each group. The investigation focuses on the orographic effects of the Brazilian Plateau on the climatology of the cold fronts. The Brazilian Plateau affects the spatial distribution of maximum and minimum frontal frequency. The number of cold fronts over the elevated regions of the Brazilian Plateau is significantly lower than in the surrounding regions. The maximum frontal frequency is found to the east of the Andes and a secondary maximum is found along the Southeast coast of Brazil. The main effect of the Brazilian Plateau on the mean and anomaly fields of meteorological variables is to block the dry and cool post-frontal air at lower atmospheric levels. The most significant anomalies are found in the ANSP group of cold fronts, which are more intense and move more rapidly eastward. On the other hand, the AN cold front group is the most frequent, but the weakest and easily affected by the Brazilian Plateau orography.

Interaction of cold fronts with the Brazilian Plateau: a climatological analysis

The effects of land surfaces, sea surfaces and coastlines on the dynamics of cold fronts are examined in an idealised two-dimensional confluent deformation model of frontogenesis. When the deformation field is stationary, increasing the surface roughness weakens the fronts, while increasing the cross-front ageostrophic wind and frontal updraughts. The frontogenetic effect of deformation balances the frontolytic effect of turbulent mixing, resulting in near-steady-state fronts. In steady state, the surface fronts are located near the point at which the cross-front flow vanishes. This equilibrium point moves towards the warm air as the surface roughness is increased because the cross-front ageostophic wind also increases with increasing surface roughness. Surface sensible heating weakens and slows the fronts during the day, whereas surface sensible cooling strengthens and accelerates them at night. Adding a coastline to the model results in very strong gravity-current-like fronts in the afternoon with a relative flow of cool air towards their leading edge. Above the boundary layer, the synoptic fronts remain unaffected by the coastline. As the turbulent mixing weakens in the late afternoon, the coastal fronts surge inland, advancing faster than the wind speed in the boundary layer. When the deformation field is allowed to translate, the fronts advance across the ocean towards land. If the fronts reach the coastline between mid-morning and late afternoon, the daytime heating over the land opposes the onshore advection of cold air, retarding the fronts at the coastline. If the fronts reach the coastline in the evening or early morning, they advance onshore relatively unimpeded. Hence, the fronts arrive onshore preferentially in the late afternoon or evening. Copyright © 2010 Royal Meteorological Society

Idealized modelling of landfalling cold fronts

During a wildfire, a sharp wind change can lead to an abrupt increase in fire activity and change the rate of spread, endangering firefighters working on what had been the flank of the fire. In southeastern Australia, routine forecast of cold-frontal wind change arrival times is a critical component of the fire weather forecasting service, and mesoscale NWP model predictions are integral to this forecast process. An event-based verification method has been developed in order to verify these mesoscale NWP model forecasts of wind changes. The approach is based on fuzzy-rule techniques and objectively determines the timing of significant (fire weather) wind changes from time series of observations at a single surface station. In this paper these rules are applied to observational and NWP model forecast time series at observation locations over five fire seasons to determine objective “observed wind change times” and “forecast wind change times” for significant frontal wind changes in southeastern Au...

Verification of Mesoscale NWP Forecasts of Abrupt Cold Frontal Wind Changes

We consider the dynamics of actively entraining turbulent density currents on a conical sloping surface in a rotating fluid. A theoretical plume model is developed to describe both axisymmetric flow and single-stream currents of finite angular extent. An analytical solution is derived for flow dominated by the initial buoyancy flux and with a constant entrainment ratio, which serves as an attractor for solutions with alternative initial conditions where the initial fluxes of mass and momentum are non-negligible. The solutions indicate that the downslope propagation of the current halts at a critical level where there is purely azimuthal flow, and the boundary layer approximation breaks down. Observations from a set of laboratory experiments are consistent with the dynamics predicted by the model, with the flow approaching a critical level. Interpretation in terms of the theory yields an entrainment coefficient E/ 1= where the rotation rate is . We also derive a corresponding theory for density currents from a line source of buoyancy on a planar slope. Our theoretical models provide a framework for designing and interpreting laboratory studies of turbulent entrainment in rotating dense flows on slopes and understanding their implications in geophysical flows.

/pdf/steady-turbulent-density-currents-on-a-slope-in-a-rotating-3usnzi0tvy.pdf

Steady turbulent density currents on a slope in a rotating fluid

Observations of intense, dry summertime cold fronts in southeast Australia provide evidence for frontal deformation (cold air surges) in the coastal region well to the west of the main mountain range. This compares with the severe deformation that occurs when such cold fronts, known as Southerly Busters, interact with the mountains to the east. Three-dimensional numerical simulations of unsteady gravity currents, in the absence of orography, provide some insight into the factors affecting the evolution of cold air surges, including differential friction between land and sea; boundary-layer heating over land and the magnitude of the large-scale offshore wind. The latter affects, in turn, the presence of an internal boundary layer of cool air in the prefrontal region over the sea and a sea breeze over land. Depending upon the magnitude of the offshore wind a preferred region of enhanced vertical motion occurs just ahead of the gravity-current front, and within about 200 km of the coastline. This ca...

E. Kowalczyk

Papers

The Behavior of Dry Cold Fronts Traveling Along a Coastline