Observations of rotors and downslope winds in the Falkland Islands

Strong downslope winds and turbulence represent a hazard to aviation in the lee of mountains. In this study results from a field campaign aimed at observing the near-surface flow across and downwind of a mountain ridge will be presented.

Mount Pleasant Airfield (MPA) is located to the south of the Wickham range (height 643 m), a long east-west running ridge on East Falkland, south Atlantic. When the wind is northerly, aviation traffic at MPA is frequently hampered by low-level turbulence associated with lee waves. This is often termed "rotor streaming" by forecasters. An array of automated weather stations (AWS) recording surface pressure, wind speed and direction (at 2 m), temperature and relative humidity were used to record data on East Falkland from Nov 2000 until Oct 2001. The AWSs were arranged in a rough transect oriented normal to the Wickham ridge, with a dense cluster located across MPA. Twice daily radiosonde measurements and web-cam time-lapse photography were also employed.

The surface wind measurements show three flow behaviours during episodes of northerly flow:

1. Uniform steady flow to the lee of the mountains with no net lee-side acceleration relative to the upwind flow.

2. Strongly accelerated flow to the lee of the mountains.

3. Highly divergent and unsteady flow downwind of the mountains, during which the flow exhibits a high degree of spatial variability and regions of flow reversal occur.

Flow behaviour 3 is thought to correspond to the presence of rotors aloft. Radiosonde temperature profiles suggest that the presence of a strong temperature inversion at a height similar to the mountain height is a necessary condition for behaviours 2 or 3.

In this study key quantities expressing the measured lee-side flow acceleration and variability will be related to the stability structure of the observed vertical profiles. Specifically it is found that the fractional speed-up downwind of the mountains is proportional to the non-dimensional mountain height (based on upstream near-surface winds and a depth averaged Brunt-Vaisala frequency diagnosed from radiosonde data). A relationship is also established between a quantity which describes the spatial variability of the downwind flow and the upstream wind and Brunt-Vaisala frequency.

The measurements will be compared with the results from an idealised numerical modelling study by the same authors. Specifically, by categorising the flow into the above three behaviours, the measurements will be used to construct flow regime diagrams which will be compared to those obtained from the numerical simulations.