Near-monochromatic ducted gravity waves associated with a convective system close to the Pyrenees

Abstract

Near-monochromatic gravity waves (GWs) associated with a mesoscale convective system (MCS) were detected during the Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST) field campaign in Lannemezan (France) on 21 June 2011. These GWs are analyzed using available instrumental data (e.g. an array of microbarometers, a microwave system Humidity And Temperature PROfiler (HATPRO) and an ultra-high-frequency (UHF) wind profiler). Pressure oscillations of up to 0.5 hPa were recorded after a pronounced pressure drop of 1.4 hPa, identified as the MCS weak low. Wavelet analysis and evaluated wave parameters confirm the occurrence of such GWs (period approximate to 9 min, horizontal wavelength approximate to 7 km), which propagated from southwest to northeast, i.e. in the same direction of propagation as the MCS. Observational evidence suggests the downdraughts associated with the rear-inflow jet at the weak low zone of the MCS as the most likely generator mechanism of the GWs. However, the complex orography and proximity of the Pyrenees to the field campaign could also play an important role. Wave propagation was possible through the ducting mechanism, favoured by the existence of a critical level in a wind-sheared environment around 2000 m above ground level. Wave-like motions related to the passage of the GWs were also observed in other atmospheric parameters close to the surface and within the lower troposphere. The effects of GWs on the surface fluxes have also been analyzed through Multi-Resolution Flux Decomposition (MRFD) methods.