Haraldur Ólafsson

Hálfdán Ágústsson

Sigvaldi Árnason

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TEMPORAL OSCILLATIONS IN DOWNSLOPE WINDSTORMSSigvaldi Árnason1, Hálfdán Ágústsson2 and Haraldur Ólafsson21Veðurstofa Íslands, Bústaðavegi 9, 150 Reykjavík, Iceland2Háskóli Íslands, Bústaðavegi 9, 150 Reykjavík, IcelandE-mail: halfdana@hi.isKeywords - Downslope windstorms, wind speed oscillations1. INTRODUCTIONOne of the characteristics of downslope windstorms is gustiness. Windgusts in downslope windstorms are oftenmore than twice as strong as the mean wind speed. In fact, mean gustfactors of more than 1.7 are common up toseveral km downstream of mountains (Ágústsson and Ólafsson 2004).In between gusts, there may be periods with relatively weak winds. The physics behind the gustiness ofdownslope windstorms is not well understood and until recently the only way to predict the gusts has been bystatistical methods.During a few recent downslope windstorms in Iceland, observations of wind have been made every one second,permitting an analysis of the characteristics of the temporal oscillations of the wind speed. In this short paper, twosuch windstorms are analyzed and one of these windstorms is simulated with the numerical model MM5 (Grell etal. 1995). The numerical simulation employs boundaries from the ECMWF and is run with a horizontal resolutionof 1 km and in 40 vertical levels. The turbulence is parameterized with the ETA scheme.2. THE 13–14 JANUARY 2005 WINDSTORMThe temporal evolution of the wind speed at Skrauthólar, SW-Iceland during a downslope windstorm on 13–14January 2005 is given in Fig. 1. 0 5 10 15 20 25 30 35 40 45 5001/1315:0001/1318:0001/1321:0001/1400:0001/1403:0001/1406:0001/1409:00f [m/s]TimeObs. fFigure 1. Temporal evolution of the wind speed at Skrauthólar, SW-Iceland on 13–14 January 2005.A Fourier analysis of the temporal oscillations during the most gusty part of the windstorm and a more calmpart of the windstorm is given in Figs. 2 and 3.550Figure 2. The energy spectrum of the wind during a calm period in the windstorm in Fig. 1.Figure 3. The energy spectrum of the wind during a gusty period in the windstorm in Fig. 1.During the windstorm, the wind speed oscillates between 0 and 47 m/s. The energy spectrum in Figs. 2 and 3shows a quite steady increase from the most rapid oscillations of a few seconds to oscillations of 1–2 minutes. Forperiods of longer duration the energy remains quite constant. At periods of approximately 10 seconds, the calmpart of of the windstorm (Fig. 2) has more variability (energy) than the gustier part of the windstorm. For periodsof 1–2 minutes and longer, the gusty part of the windstorm features on the other hand more energy than the calmpart.3. THE 2 FEBRUARY 2005 WINDSTORMFigure 4 shows the temporal evolution of the wind speed during a downslope windstorm at Grundarfjörður, W-Iceland. An analysis of the temporal oscillations of a calm part of the windstorm and of the gustiest part of thewindstorm is given in Figs. 5 and 6.As in the 13–14 January case, there is relatively more energy associated with the most rapid oscillations duringthe calm part of the windstorm than during the gusty part. For oscillations of 1–2 minutes and up to 12–15 minutesthere is on the other hand more energy in the oscillations in the gusty part of the windstorm.Figure 7 shows a similar analysis of the output from a numerical simulation with the MM5 model. The model551Figure 4. Temporal evolution of the wind speed at Grundarfjörður, W-Iceland on 2 February 2005.Figure 5. The energy spectrum of the wind during a calm period in the windstorm in Fig. 4.does not reproduce the observed variability, neither at high-frequency (10 sec.), nor at lower frequencies (1 minuteand more).4. DISCUSSIONThe data presented here indicates that the character of the temporal oscillations of the windspeed is differentduring the climax of the windstorm than during more calm periods. When the windstorms are at their maximumthere is relatively large variability at periods from 1–2 minutes and upward. A possible reason for this may berelated to oscillations in the gravity waves that generate the downslope windstorm at the surface, but this is hardto verify. In order to establish more knowledge about how the temporal variability of the wind relates to localtopographic conditions and to elements of the atmospheric flow, data from more windstorms is needed and willbe collected.The poor performance of the numerical model is not unexpected for the high-frequency oscillations (less thana minute). As the physical reason for the oscillations with longer periods is unclear, it is difficult to indicatewhether the numerical model should be able to reproduce this feature. There is nothing inhibiting oscillationsrelated to buildup and breakdown of mountain waves to be reproduced in a numerical model, but the associated552Figure 6. The energy spectrum of the wind during a gusty period in the windstorm in Fig. 4.Figure 7. The energy spectrum of the wind as simulated with the MM5 during the windstorm in Fig. 4.wind speed oscillations and generation of turbulence is inevitably dependent upon the turbulence parameterizationand the diffusion in the model.5. CONCLUSIONSObservational data indicates that during the gustiest part of downslope windstorms, there is relatively large vari-ability on the time scale of 1–2 minutes and upwards. The observed variability is not reproduced in a numericalsimulation, not even at periods of several minutes. A method to reproduce this variability needs to be developed.REFERENCESÁgústsson, H. and Ólafsson, H.: 2004, Mean gust factors in complex terrain, Meteorol. Z. 13(2), 149–155.Grell, G. A., Dudhia, J. and Stauffer, D. R.: 1995, A Description of the Fifth-GenerationPennState/NCAR Mesoscale Model (MM5), NCAR Technical Note NCAR/TN-398+STR. Available athttp://http://www.mmm.ucar.edu/mm5/doc1.html (May 2004).553

TEMPORAL OSCILLATIONS IN DOWNSLOPE WINDSTORMS

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