SWIRL'99
A contribution to the Mesoscale Alpine Programme

Project Goals
Instrumentation and Measurement Sites
Methods
References
Funding

Quick-Look Data

Results

Project Description

Project Goals

The scintillometers of the Paul Scherrer Institute (PSI) are designed for wind measurements. The peak wavelength of the emitted light is in the infrared at 890 nm. 15-cm spherical mirrors are used both in the transmitter and the receiver. This configuration permits path lengths of up to 10 km. The large aperture (15-cm mirrors) of the scintillometer makes it less prone to optical saturation, a phenomenon that destroys the relation between refractive index fluctuations and wind velocity. With three mirrors arranged in an L-shape (see picture below) it is possible to measure the horizontal and vertical component simultaneously. Crosswind data is recorded every 7 s, but some averaging is necessary to gain reliable measurements. The instruments have been calibrated empirically by comparison with an array of conventional anemometers.
 
 

A receiver of PSI's scintillometer. The three-mirror design allows for the simultaneous measurement of horizontal and vertical crosswind components.

PSI scintillometer characteristics Technical details   Mirror diameter * 0.15 m Mirror focal length * 0.30 m Light source pulsed infrared diode Peak wavelength 890 nm Number of mirrors transmitter: 1 receiver: 3 (for vertical and horizontal crosswind components) Measurement details   Recorded parameters horizontal and vertical crosswind components standard deviation of light intensity Measurement range 0..40 m s-1 Measurement range for which calibration has been verified + 0..10 m s-1 Accuracy 12 % (0.25 m s-1 for weak wind speeds) Precision 0.25 m s-1 Sampling rate 0.15 Hz (1 measurement every 7 s) * for both, transmitter and receiver + the calibration depends on the sampling rate and holds for path lengths of up to 10 km

Two large-aperture scintillometers were set up across the Rhine Valley near Sevelen (click for a site map) . Site characteristics are given in this Table. The transmitters were located in Triesenberg, Liechtenstein, on the right-hand side of the Rhine River. The receivers were located on the opposite side of the valley, i.e. in Oberschan and in Sevelen. This yielded a V-shape for the arrangement of the light paths. This setup allowed for the derivation of the area-averaged horizontal crosswind and the vertical crosswind.

The instruments were in operation from 1 September to 17 November 1999, covering the whole MAP Special Observing Period. Weekly maintenance trips were made to keep possible instrument failures and data losses to a minimum. Except for a 4-day gap in the data in the second week, no data was lost till the end of the MAP SOP.

Methods

We applied simple geometric relationships to infer the area-averaged horizontal wind vector (Furger, 2000). For foehn situations with wave activity we expected reasonable signal-to-noise ratios permitting the determination of the 3d wind vector. Temporal variations of this vector revealed interesting phenomena of the foehn flow, but also the non-uniformity of the wind field within the valley.

Our wind measurements revealed aspects of foehn onset at about 500 m above ground in the Rhine Valley, like timing and intermittency. We also gained information on mountain-induced waves within the valley.

Qualitative information on turbulence at higher elevations in the valley could be gained from our scintillometer measurements for complete diurnal cycles.

Funding

This project was entirely funded by PSI. The interest in the MAP activities and support by the farmer families Sulser (Oberschan-Ergellen) and Giger (Obersevelen-Flusa), as well as the support of the Community of Triesenberg, Liechtenstein, were much appreciated.

Contacts

For further information please contact Markus Furger

Links

MAP - MAP-FORM - Gasphase and Aerosol Chemistry Group - Laboratory of Atmospheric Chemistry - PSI

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