SUMMARYNumerical simulations of neutral, turbulent flow over periodic forested hills are carried out using a firstorder turbulence closure scheme. The forest is represented by the inclusion in the model of a canopy near the surface where additional drag is exerted on the flow as a result of the trees. Simulations with a more traditional roughness-length parametrization of the surface, which does not explicitly model the flow within the forest, are also carried out for comparison. Comparison of simulations with existing wind-tunnel data shows that the simple mixing-length canopy closure scheme provides a good estimate of the mean flow over a forested hill and is a significant improvement on a traditional roughness-length parametrization in this case.The numerical results are seen to be in close agreement with existing linear analytical solutions for flow over a low and wide forested hill. The model is used to investigate the breakdown of the analytical solution when the hill becomes steep and the perturbations to the flow are no longer linear. Even in the case of a gentle slope, the analytical solution can break down when the hill is narrow or the canopy is deep and the vertical velocities near the top of the canopy become large. This leads to a downwind shift in the surface pressure perturbation and an enhanced drag compared to a roughness-length model. Model results also show enhanced flow separation in the presence of a canopy. These results are important for a range of meteorological problems including the interpretation of flux measurements, parametrization of orographic drag, predicting wind damage to trees and calculating the yield from wind turbines.
The eastern side of the Antarctic Peninsula (AP) mountain range and the adjacent ice shelves are frequently affected by föhn winds originating from upwind of the mountains. Six automatic weather stations (AWSs) and archived model output from 5 km resolution Antarctic Mesoscale Prediction System (AMPS) forecasts have been combined to identify the occurrence of föhn conditions, and their spatial distribution over the Larsen C Ice Shelf (LCIS) from 2009 to 2012. Algorithms for semi‐automatic detection of föhn conditions have been developed for both AWS and AMPS data. The frequency of föhn varies by location, being most frequent at the foot of the AP and in the north of the ice shelf. They are most common in spring, when they can prevail for 50% of the time. The results of this study have important implications for further research, investigating the impact of föhn on surface melting, and the surface energy budget of the ice shelf. This is of particular interest due to the collapse of Larsen A and B ice shelves in 1995 and 2002 respectively, and the potential instability issues following a large calving event on Larsen C in 2017.
Abstract.Observations of the Saharan boundary layer, made during the GERBILS field campaign, show that mesoscale land surface temperature variations (which were related to albedo variations) induced mesoscale circulations. With weak winds along the aircraft track, land surface temperature anomalies with scales of greater than 10 km are shown to significantly affect boundary-layer temperatures and winds. Such anomalies are expected to affect the vertical mixing of the dusty and weakly stratified Saharan Residual Layer (SRL). Mesoscale variations in winds are also shown to affect dust loadings in the boundary layer.Using the aircraft observations and data from the COSMO model, a region of local dust uplift, with strong along-track winds, was identified in one low-level flight. Large eddy model (LEM) simulations based on this location showed linearly organised boundary-layer convection. Calculating dust uplift rates from the LEM wind field showed that the boundary-layer convection increased uplift by approximately 30%, compared with the uplift rate calculated neglecting the convection. The modelled effects of boundary-layer convection on uplift are shown to be larger when the boundary-layer wind is decreased, and most significant when the mean wind is below the threshold for dust uplift and the boundary-layer convection leads to uplift which would not otherwise occur.Both the coupling of albedo features to the atmosphere on the mesoscale, and the enhancement of dust uplift by boundary-layer convection are unrepresented in many cliCorrespondence to: J. H. Marsham (jmarsham@env.leeds.ac.uk) mate models, but may have significant impacts on the vertical transport and uplift of desert dust. Mesoscale effects in particular tend to be difficult to parametrise.
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