Turbulent flux calculation in the polar stable boundary layer: Multiresolution flux decomposition and wavelet analysis

Airborne measurements in the Arctic stable boundary layer were performed using the helicopter-borne turbulence measurement system Helipod. The observations were carried out during the ARK-XII summer expedition of the research icebreaker Polarstern in the Arctic ocean. The boundary layer over closed sea ice exhibited mainly weakly stable stratification. Only flights in continuous turbulence were chosen for the analysis. The measurements of sensible heat flux H and latent heat flux lambda E were made 200 km away from land to reduce the influence of any captured mesoscale flux. Nevertheless, the analysis of multiresolution (MR) cospectra yield some small mesoscale flux contribution, visible at the larger timescales. A MR gap timescale tau(gap) was defined to separate the turbulent flux from the mesoscale flux and used for the recalculation of the turbulent fluxes after high-pass filtering. This procedure also reduced the statistical error of the fluxes significantly. For comparison, wavelet transformations were performed that led to wavelet gap scales of the fluxes in good agreement with the MR gap scale. Additionally, the wavelet covariance gave a good representation of the homogeneity of the kinematic heat flux w'theta' and the kinematic moisture flux w'm' in time.