Alignment of mountain lee waves viewed using NOAA AVHRR imagery, MST radar, and SAR

Recent radar observations of mountain waves in the troposphere and lower stratosphere above Aberystwyth (52.4 degreesN, 4.0 degreesW) indicate that, on average, the wave alignment is related more closely to the wind direction within the boundary layer than to the alignment of mountain ridges. This is investigated using independent data-NOAA AVHRR imagery of both mountain-wave clouds and convective cloud streets, combined with surface synoptic wind measurements. The mountain-wave cloud bands are found to be aligned not at exactly 90 degrees to the surface wind but rotated a further 18 degrees clockwise. Similarly, in an important back-up test, the cloud streets are found not to be parallel to the surface wind but rotated 12 degrees clockwise, which agrees with over 30 years of observations, most recently of wind rows on the ocean by synthetic aperture radar (SAR). Because the wind rotates, on average, clockwise with increasing height in the northern-hemisphere boundary layer, the mountain-wave clouds will be at 90 degrees to the wind direction in the middle of the boundary layer. Therefore, the satellite images independently confirm earlier mesosphere-stratosphere-troposphere (MST) radar observations. Mountain lee waves may corrupt SAR measurements of surface wind above the ocean, so knowledge of their alignment is useful; two examples of lee waves modulating the sea roughness west of Aberystwyth are discussed.