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Diumal forcing is applied to the lower boundary o f CTIM through the (1,1) Hough mode which is the m ost important diumal mode propagating into the thermosphere from below. A run was carried out (1 IE), using a forcing amplitude o f 100 m geopotential height and a maximum at 12 h local time. The resulting lower boundary profiles are shown in Figure IV.l. Effects o f this forcing on the thermosphere below 200 km are shown in Figure IV.12 for temperature and horizontal winds. The plots show diumal amplitude changes and were produced as described in section IV.2, by subtracting the diumal amplitudes under in-situ forcing from those under (1,1) mode forcing.

i) TEMPERATURE

The change o f diumal temperature amplitude shows a distinct pattem which is symmetric to the equator and strongest between around ±30° latitude (Figure IV.12, top). O f the three main maxima in the pattem, the strongest lies above the equator at around 130-140 km height. In this region the diumal amplitudes have increased by about 60 °K, which is significant when compared to the in-situ amplitudes in that region which lie below 10°K (Figure IV.6, top). Two weaker maxima are located at around ±18° latitude and extend to almost 150 km altitude. The diumal amphtude increases by around 40° K which again is significant compared with the in-situ forcing found there. Outside the region o f this pattem, poleward o f around ±40° latitude, the diumal amplitudes do not change under the influence of the lower boundary diumal forcing. The “V” shape

Propagating Hough M odes_______________________________________________Chapter IV

o f the response pattem in F igure IV.12 (top) suggests that oscillations propagate upwards and simultaneously from lower to higher latitudes. This is examined further in IV .6.1.

The marked curve in Figure IV.13 (top) shows the total energy flux per 60 sec time step at 20 °N and level 7 (around 122 km altitude), where the response to diumal forcing is strongest (Figure IV.12). A breakdown of this curve into individual terms is shown in Figure IV.13 (bottom ). For comparison, the same parameters, as produced by run NT, are shown for the same location in Figure IV .l4. One sees that the total energy flux is smaller by an average factor o f 5 and mainly semidiumal in the case o f in-situ forcing (Figure IV.14, top). Comparison o f Figures IV.13 and 14 (bottom) shows that these differences are caused by stronger horizontal and vertical energy advection terms as well as increased vertical heat conduction. As shown in equation III.22 , the vertical energy advection term is given by w d {e+g-Zp)ldp and thus depends on the vertical wind in the pressure frame, w. As discussed further in Chapter VI, w is linked to the horizontal wind gradient. The (1,1) forcing at the lower boundary produces additional horizontal wind gradients which generate up- and downwelling relative to pressure levels and, in terms o f energy, adiabatic heating and cooling on pressure levels. This is confirmed in Figure IV.13 (bottom) through the vertical energy advection term which forms the dominant heat flux at that location. Therefore, the observed temperature changes are caused mainly by adiabatic heating and cooling.

ii) W INDS

Figure IV.12 (middle and bottom) shows that meridional winds respond to diumal forcing stronger than the zonal winds. The peak zonal wind amplitude change lies around 20 m/s, which is about h alf the increase observed for meridional winds. The pattem in F igure IV.12 (middle) consists mainly of two symmetric maxima located at ±26° latitude and around 115 km height. Higher up, there are two thin regions o f amplitude enhancement at mid latitude which disappear above 220 km height, but these hardly influence the total winds since in situ zonal wind amplitudes at mid latitudes are much stronger. Regions of enhanced meridional wind amplitudes form a pattem which is more complex than that o f zonal winds and temperatures, with essentially two “V” shaped patterns which again suggest simultaneous upward and poleward propagation o f the oscillations. The meridional winds respond between 80 and 220 km height, while the temperature response is limited to altitudes between around 100 and 160 km.

Propagating H ou^h M odes_______________________________________________Chapter I V

Meridional momentum terms at 20 °N latitude and pressure level 7 are plotted in Figure IV.13 (middle) and show that the observed wind changes at this location are associated primarily with stronger horizontal pressure gradients, followed by larger horizontal advection and vertical viscous drag. The peak of the horizontal pressure gradient lies at 18 h local time but is generally dependent on the choice of phase for the lower boundary forcing. As a result o f stronger horizontal winds the ion drag has also increased, but overall is not an important factor at that latitude. The relative importance of terms is very similar for the zonal momentum components, but the zonal pressure gradient is smaller than the meridional one, thus giving the different responses o f zonal- and meridional winds. .

The diurnal phases are plotted in Figure IV.15 for temperatures (solid), zonal-(dashed) and meridional (dotted) winds. The decreasing phase values with height indicate westward propagation o f the diumal tide, in agreement with expectations. The plotted values are the local times of maxima and demonstrate for all three parameters that phase changes occur only below around 160 km in the thermosphere. Above that altitude the phases are almost constant with height, implying that there is no vertical flux o f energy through oscillations. The implication o f this is that the diumal wave at higher altitudes is non-propagating only. This is confirmed by the Hough mode decomposition carried out in section IV.6.1. The plot also shows that the diumal phase changes considerably with height at a rate of around 14 h over a 20 km range which is roughly the same for all three parameters. At higher altitudes this rate falls gradually, and above 160 km suddenly approaches zero.

Finally, one can estimate the vertical wavelength o f the (1,1) mode from F ig u re IV.15 and obtains a value o f around 10 km near the lower boundary which increases to around 35 km near the height of dissipation.