Diurnal propagating tide in the presence of mean winds and dissipation : a numerical investigation

The propagation characteristics of the first symmetric or (1,1) tidal mode are investigated numerically for a background atmosphere characterized by mean winds, meridional temperature gradients, and mechanical and thermal dissipation. Mechanical and thermal dissipation are parameterized through effective Rayleigh friction and Newtonian cooling coefficients. The primary effects of dissipation are to produce an amplitude peak in the vicinity of 90 km, an increase in the vertical wavelength with height from 80 to 100 km and a broadening or spreading of the tidal oscillation to higher latitudes. This latter effect occurs via coupling into the first symmetric and antisymmetric evanescent modes, commonly known as (1, -2) and (1, -1), respectively, as the (1,1) mode is mechanically damped. These features are consistent with the behaviour of the diurnal tide as observed by partial reflection, meteor and incoherent scatter radars. During solstice conditions the presence of mean winds manifests itself by introducing significant asymmetries about the equator in amplitude and phase, which can be viewed as a coupling into the first antisymmetric propagating and evanescent modes, (1,2) and (1, -1), respectively. Available data are not inconsistent with these latter results, but true verification is precluded by the inadequacy of data below 30° latitude where effects of the mean winds are most pronounced. © 1988.