科研之友
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摘要
Mountain waves can cause surface wind variations shown by synthetic aperture radar measurements of sea surface roughness. Since mountain waves are quasi-stationary, it can be difficult to identify the mountain-wave component of a surface anemometer time series. However, orographic gravity waves are non-stationary on rare occasions. This study examines two anomalous case studies found in four years of data, when surface wind downwind of mountains oscillates with a period of 20-60 min. Meso-strato-troposphere radar shows vertical wind oscillations of the same period, the amplitude a maximum in the lower troposphere, apparently trapped waves. Satellite images show ordinary mountain-wave clouds with horizontal wavelengths 9.0 and 5.7 km in the two case studies. Phase speeds are 5.3 and 4.8 m s(-1), assuming a fixed wavelength. Surface wind direction reverses a few times through 180A degrees in one instance, consistent with propagating rotors. A multi-beam method is used to find if phase propagation is upwind or downwind for one case study, based on cross-correlation of the high-frequency component of wind time series in off-vertical radar beams. Surprisingly, phase propagation is found to be mostly upwind. Both changing background wind and/or stability, and non-linear wave interactions, can cause mountain waves to drift upwind or downwind, and the exact cause could be resolved by numerical modelling.
- 出版日期2015-6
