Observational analysis of gravity waves associated with a convective system near the Pyrenees during the BLLAST campaign

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). A deep analysis of this event has been performed using data from several instruments, e.g. an array of high sampling rate microbarometers, a microwave system Humidity And Temperature PROfiler (HATPRO), an ultra-high frequency (UHF) wind profiler, additional barometers and other standard meteorological instruments). Surface pressure records from the microbarometers showed several large amplitude oscillations (up to 0.5 hPa). Wavelet analysis and evaluated wave parameters confirm the occurrence of such GWs (period ~ 9 minutes, wavelength ~ 7 km) propagating from southwest to northeast, i.e. the same direction of propagation of the MCS. The origin of these GWs has been proposed to be related to the downdrafts associated with the rear-inflow jet at the posterior part of the MCS, although the complex orography of a mountainous area very close to the Pyrenees could also play an important role. The propagation of the GWs was possible through the ducting mechanism, favoured by the existence of a critical level in a wind shear zone around 2000 m above ground level. Wave-like motions related to the GWs passage were also observed in other atmospheric parameters close to the surface and within the lower troposphere. The GWs effects on the surface fluxes have also been studied from Multi-Resolution Flux Decomposition (MRFD) analysis.