Hg. Mayr et al., EQUATORIAL OSCILLATIONS MAINTAINED BY GRAVITY-WAVES AS DESCRIBED WITHTHE DOPPLER SPREAD PARAMETERIZATION - I - NUMERICAL EXPERIMENTS, Journal of atmospheric and solar-terrestrial physics, 60(2), 1998, pp. 181-199
Gravity Waves (GW) as described with the Doppler Spread Parameterizati
on (DSP) of Hines have been shown to produce significant Quasi-biennia
l and Semi-annual oscillations (QBO and SAO) in the zonal circulation
at equatorial latitudes. To elucidate the process involved, we discuss
computer experiments performed without the external, seasonal variati
ons in solar differential heating, and the conclusions are summarized
here. (1) As reported earlier, two kinds of oscillations are then gene
rated spontaneously: one at lower altitudes in the stratosphere which
has a long period approaching that of the QBO, the other one al higher
altitudes in the mesosphere which has a shorter period close to that
of the SAO. The amplitudes of these oscillations are weaker, however,
than those generated with seasonal heating. (2) When the time independ
ent solar healing is turned off, the oscillations continue but with si
gnificantly reduced period in the mesosphere where the vertical winds
driven by solar differential heating produce Doppler shifting. The ass
ociated temperature variations are then small, consistent with the one
dimensional prototype calculations discussed by Lindzen and Holton. (
3) When the GW momentum source is turned off above 40 km to suppress t
he short-period oscillation, the long-period oscillation below continu
es with enhanced amplitude and period. When the source is turned off b
elow 35 km, the amplitude of the short-period oscillation increases at
higher altitudes, indicative of increased acceleration. (4) The wind
oscillations change systematically and dramatically with changing eddy
diffusivity/viscosity (K). The amplitudes and periods of both short-
and long-period oscillations decrease (increase) with increasing (decr
easing) K, and above a certain value of viscosity, both oscillations c
ompletely cease. Lowering K thus helps in generating a larger and long
er period QBO at lower altitudes, but can be detrimental to the SAO wh
en the natural oscillation period increase significantly above six mon
ths. (5) Under oscillation conditions, the height variations in the GW
vertical wave numbers reveal a staircase pattern reflecting the inter
mittence characteristic of the wave interaction. It reveals the nonlin
earity of the GW interaction to be of third (odd) order, which has the
distinctive property that it can feed and maintain the fundamental ha
rmonic of the oscillation-considered a prerequisite for sustaining the
oscillations without external time dependent source. In the companion
paper, an analytical model of the equatorial oscillation is presented
which describes some of the trends reported here. (C) 1998 Elsevier S
cience Ltd. All rights reserved.