Dissipative Waves Excited by Gravity-Wave Encounters with the Stably Stratified Planetary Boundary Layer

Abstract

We suggest that the strata of strong echo returns frequently revealed by remote-sensor records of the stably stratified planetary bound layer (PBL) represent the wavefronts of dissipative waves (viscous and thermal-conduction waves) excited by gravity-wave encounters with the PBL and the earth's surface. The viscous waves appear to be more strongly forced and should therefore dominate the observations. This simple picture accounts for the following observed properties of the strata: 1) their nearly ubiquitous presence within the stably stratified PBL, 2) their nearly horizontal orientation, 3) the small spacing (some tens of meters, typically) separating the strata, 4) variability in that spacing in both height and time, and 5) the high shears and temperature gradients associated with the strata. Preliminary calculations of the energy fluxes and stresses associated with the wave motions, also presented here, suggest strongly that such waves are not mere curiosities of the PBL but reveal important dynamical processes. In addition, their great similarity to the sheets of temperature and current gradients in the thermocline suggests an analogous dynamical origin for the ocean phenomenon.