A LINEAR THEORY FOR LEEWAVE ROTORS

A linear theory for the prediction of rotor formation under trapped lee waves has been developed. The theory is based on the assumption that the flow can be divided into two layers: an outer region in which the flow is inviscid, and an inner region close to the ground where turbulent stresses are important. The flow in the inner region is driven by the pressure gradient due to lee waves aloft. A simple mixing-length turbulence closure is assumed and for a given lee-wave pressure field, analytic solutions to the inner-region flow are obtained. Results from the linear theory are compared with a series of two-dimensional numerical simulations in which a sharp temperature inversion is present upwind of a ridge. Lee waves may form on the inversion, which when of sufficient amplitude, give rise to flow separation and rotors underneath the wave crests. The point at which linear theory predicts flow separation underneath the wave crests is shown to correspond, approximately, to the occurrence of lee-wave rotors in the numerical results and a flow regime diagram based on linear theory is constructed and compared to that obtained from the simulations

Long-term wind resource assessment for small and medium-scale turbines using operational forecast data and measure-correlate-predict

Output from a state-of-the-art, 4 km resolution, operational forecast model (UK4) was investigated as a source of long-term historical reference data for wind resource assessment. The data were used to implement measure-correlate-predict (MCP) approaches at 37 sites throughout the United Kingdom (UK). The monthly and hourly linear correlation between the UK4-predicted and observed wind speeds indicates that UK4 is capable of representing the wind climate better than the nearby meteorological stations considered. Linear MCP algorithms were implemented at the same sites using reference data from UK4 and nearby meteorological stations to predict the long-term (10-year) wind resource. To obtain robust error statistics, MCP algorithms were applied using onsite measurement periods of 1-12 months initiated at 120 different starting months throughout an 11 year data record. Using linear regression MCP over 12 months, the average percentage errors in the long-term predicted mean wind speed and power density were 3.0% and 7.6% respectively, using UK4, and 2.8% and 7.9% respectively, using nearby meteorological stations. The results indicate that UK4 is highly competitive with nearby meteorological observations as an MCP reference data source. UK4 was also shown to systematically improve MCP predictions at coastal sites due to better representation of local diurnal effects

Wind-borne redistribution of snow across an Antarctic ice rise

Redistribution of snow by the wind can drive spatial and temporal variations in snow accumulation that may affect the reconstruction of paleoclimate records from ice cores. In this paper we investigate how spatial variations in snow accumulation along a 13 km transect across Lyddan Ice Rise, Antarctica, are related to wind-borne snow redistribution. Lyddan Ice Rise is an approximately two-dimensional ridge which rises about 130 m above the surrounding ice shelves. Local slopes on its flanks never exceed 0.04. Despite this very smooth profile, there is a pronounced gradient in snow accumulation across the feature. Accumulation is highest on the ice shelf to the east ( climatologically upwind) of the ice rise and decreases moving westward, with the lowest accumulation seen to the west ( climatologically downwind) of the ice rise crest. Superimposed on this broad-scale gradient are large ( 20-30%), localized variations in accumulation on a scale of around 1 km that appear to be associated with local variations in surface slope of less than 0.01. The broad-scale accumulation gradient is consistent with estimates of wind-borne redistribution of snow made using wind speed observations from three automatic weather stations. The small-scale variability in accumulation is reproduced quite well using a snow transport model driven by surface winds obtained from an airflow model, providing that both the wind shear and static stability of the upwind flow are taken into account. We conclude that great care needs to be exercised in selecting ice core sites in order to avoid the possibility of blowing snow transport confounding climate reconstructions

The Scope of Radiographic Practice 2008 : A research report compiled by the University of Hertfordshire in collaboration with the Institute for Employment Studies

This report was commissioned by the Society and College of Radiographers (SCoR) at the end of 2007 to identify the current scope of practice for the radiographic workforce across the spectrum of clinical imaging and radiotherapy in the United Kingdom (UK). The work undertaken for the report originates from the School of Health and Emergency Professions at the University of Hertfordshire in collaboration with the Institute for Employment Studies (IES)