Type 1 and 2 mountain waves observed by MST radar and AVHRR

Abstract

Mountain waves can be assumed to be forced directly by mountains, neglecting the boundary layer (type 1) or indirectly by mountain boundary-layer turbulence and convection (type 2). This study investigates if the two types of mountain wave can be identified objectively. Data are from the Aberystwyth meso-strato-troposphere (MST) radar, and advanced very high resolution radiometer. Mountains around the MST radar are fairly low and isotropic, except for the Cadair Idris ridges. Waves downwind of these mountains, previously simulated by a type 1 numerical model, appear to be type 1 in reality. However, another case study shows interacting convective rolls and mountain waves (type 2) even above Cadair Idris, as also observed occasionally at high, ridge-like Cross Fell. Over seven years of MST radar data on wave alignment, compared to surface wind, show type 2 waves for most wind directions. However, there is a deviation for near-northerly winds consistent with the Cadair Idris ridges producing type 1 waves. For other wind directions, wave alignment is examined as a function of boundary-layer temperature gradient and surface wind speed, showing little dependence on either, which is similar to the Ekman rotation’s lack of dependence. It is suggested that not only Ekman rotation but also type 2 mountain waves involve the ubiquitous turbulent surface layer, which acts as an effective mountain a few hundred metres higher than the actual mountain, even in the absence of convection, and commonly causes type 2 mountain waves.