Characteristics of rotor streaming in the Falkland Islands

During 2000 and 2001, an array of 20 automatic weather stations was used to collect high temporal resolution surface data in order to characterise the occurrence and properties of so-called "rotor streaming". This particular type of severe turbulence occurs in the lee of two ridges on East Falkland, both of which are about 600 m in height.The phenomenon can be associated with, but is not exclusively limited to, closed rotors and eddy shedding. It is known to be closely associated with strong low-level temperature inversions and presents a severe aviation hazard. The inversions are routinely observed using radiosondes launched from the island airport at Mount Pleasant.

In the accompanying paper by Sheridan et al, the relationship between inversion characteristics (mainly strength and height) an rotor streaming is discussed. As part of the development of a predictive tool for forecasting rotor streaming at the airport, it is necessary to understand why the phenomenon is so prevalent in the Falkland Islands, particularly because the hills themselves are quite unremarkable. A one dimensional boundary-layer modelling study has been carried out in order to understand the key factors which frequently give rise to inversions with temperature differences of over 5 degrees over a depth of 100 m. The role of sea surface temperature and its effect on seasonality of rotor streaming will be discussed.

Analysis of the surface data, concentrating on temporal characteristics, will be presented. The spectrum of the surface pressure signal downwind of the hills at times shows clear dominant frequencies associated with rotor streaming. These travelling disturbances can be analysed using lagged cross-correlations across the automatic weather station array, revealing propagation characteristics. The wind data has been used to calculate gust factors; during rotor streaming values in excess of 5 have been observed. There is direct evidence that rotor streaming can be associated with hydraulic jump behaviour, which appears in the data as large pressure perturbations in the lee of the hills.

The relationship of these surface phenomena to the three-dimensional modelling studies discussed in the accompanying poster by Vosper and Sheridan will also be discussed.