Vertical profiles of semidiurnal southward wind amplitudes and local times o f maxima are shown in Figures V.3 and V.4 for Run 1 and in Figures V.5 and V.6 for Run 2. In the following, the comparisons are described for the low-, mid- and high latitudes. The results are discussed in section V.3.5.
i) LOW LATITUDE
The upper image in Figure V.3 shows a comparison o f semidiurnal amplitudes at 18.0°N, as sim ulated by CTIM (solid) and the TIGCM (dashed). The CTIM wind amplitude has stronger variation with height, reaching from roughly 5 m/s to 35 m/s, than the TIGCM amplitude which varies only by less than 4 m/s around an average o f 20 m/s. Amplitude maxima are found near 100 and 120 km altitude in the CTIM values, while TIGCM amplitudes peak near 110 and 160 km. Above around 240 km the amplitude reaches a constant value in both models which is roughly 18 m /s in the TIGCM- and 6 m/s in the CTIM output. In the zonal direction (not shown here) the variability o f amplitude with height is similar in both models, but shifted to lower values in the TIGCM run, where values go from 2 m/s at 100 km to 13 m/s near 125 km and back to around 2 m/s at 200 km. Zonal wind amplitudes in CTIM range from 6 m/s near 110 km to 26 m/s at 120 km and around 16 m/s at 140 km. The CTIM zonal winds reach a constant amplitude o f 17 m/s above 240 km, while the TIGCM wind amplitudes increase up to an altitude o f around 340 km until they reach their saturation value of around 8 m/s which is, in contrast to the case o f meridional wind, smaller than the value in CTIM. A number o f local maxima can be found between 80 an 100 km in the CTIM zonal wind amplitudes and lie below the height range o f the TIGCM.
The local times o f semidiurnal southward wind maxima are shown in Figure V.4 for CTIM and the TIGCM. Both models have almost identical phase values below 150 km and above 320 km. Between those heights the TIGCM wind maxima occur later by up to 2 hours between 180 and 220 km altitude. The models reach their constant southward wind phase value at a different altitude. The CTIM phase is shifted backwards in time more quickly above 150 km than is the case for the TIGCM and therefore reaches constant local time of maximum o f around 0:30 h (or 12:30 h) already at around 200 km altitude, while the same happens in the TIGCM only 100 km above that.
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Southward wind amplitudes produced by CTIM in Run 2 are shown in F igure V.5 along with values from the GSWM and HWM (see V.3.1 for description o f these models).The plot shows that amplitudes are similar at 80 km in all three models but then increase much stronger with height in the GSWM and HWM than in CTIM. Between around 100 and 125 km the CTIM amplitudes reach values of 10 m/s and those o f the GSWM and HWM approach 70 and 25 m/s, respectively. The CTIM amphtudes peak near 105 km, 120 km and 155 km with values o f around 5 m/s, 10 m/s and 15 m/s, respectively. The GSWM amplitudes continuously rise with altitude and the HWM values have one maximum near 115 km of around 25 m/s which is more distinct than the relatively weak maxima in the CTIM output. Between 120 and 145 km altitude the CTIM and HWM values are roughly the same. In the zonal direction (not shown here) the CTIM wind amplitudes peak at around 5 m/s at roughly 105 km altitude and between 120 and 200 km rise continuously from alm ost 0 to 15 m/s. The GSWM zonal winds also reach a peak at 105 km altitude with around twice the value o f the CTIM peak, then fall off and above 110 km rise again to a peak o f 10 m/s at 120 km. The agreement between CTIM and the GSWM is thus much better below 115 km than for meridional winds. Zonal wind amplitudes produced by the HWM are similar to the GSWM values up to 105 km, but then increase much stronger than the GSWM and CTIM winds to a value of around 50 m/s near 120 km. They remain large also above 120 km and do not agree as well with the CTIM values as meridional winds at that height.
The phase plot in Figure V.6 shows that CTIM local times o f maxima are up to 2 hours ahead of the GSWM values, but they change with altitude at nearly the same rate. In contrast, the HWM phase at 80 km lies around 2 hours behind that of the other models and then changes with height at a slower rate. Above 130 km the vertical phase change is roughly similar in the CTIM and HWM, although the HWM southward winds peak around 4 hours before the CTIM values. This height regime o f similar vertical phase change coincides roughly with the region o f similar amplitudes described earlier.
ii) M ID -LA TITU D E
At 42.0°N the meridional winds produced by the CTIM and TIGCM (Figure V.3, middle plot) agree better than at low latitudes. Although the values at a fixed height differ by up to 20 m/s, the overall vertical patterns are very similar, though shifted upwards for the TIGCM values. One difference between the vertical patterns is that the CTIM winds make two oscillations, thus giving
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them two local maxima (at 110 and 160 km), while the TIGCM winds make only one oscillation with one main wind amplitude maximum at 120 km. The saturation wind amplitudes are more similar than at low latitude with values o f around 12 m/s and 16 m/s for the CTIM and TIGCM winds, respectively, above 300 km. Zonal wind amplitudes (not shown) behave similar to the meridional winds, with the only difference being that the saturated amplitudes are identical in both models at around 4 m/s above 320 km altitude.
The phases at mid latitude (middle plot in Figure V.4) differ more between the two models than at low latitude. Below around 120 km they have similar values, but then a reversal o f the phase propagation is seen in the CTIM winds, producing a phase difference o f around 7 hours between the models. This difference decreases with altitude and for the satui ated phases above around 240 km remains at a 2 hour phase lead o f the CTIM southward winds.
The meridional wind amphtudes produced in CTIM’s Run 2 at 42.0 °N (middle plot in F igure V.5) are, as expected from the already different amplitudes at the lower boundary (see T able V.3), smaller than in Run 1 and have less variability with height. The amplitudes are also lower than those o f the GSWM and HWM. The reason for 12 h amplitudes being lower than in the GSWM is that CTIM does not use semidiurnal forcing modes higher than the (2,5) mode, while the GSWM gives also the (2,6) and higher modes at 80 km. A similar feature in the vertical profiles from all three models is the local amplitude peak near 110 km. Largest amplitude values are predicted by the GSW M (at 65 m/s), followed by the HWM (at 35 m/s) and CTIM (10 m/s). In the zonal direction the CTIM predictions are again smaller than those o f the other two models. A local peak is again found near 100 km, but here the HWM peak is higher by around 10 km. The zonal wind amphtudes are again generally largest in the GSWM, but the vertical amplitude growth occurs at a similar rate in the HWM and GSWM.
The vertical phase profiles (middle plot in Figure V.6) show the largest vertical phase velocity in the GSWM values and the smallest in the HWM data. The CTIM phase velocity lies between the two. The phase reversal seen in CTIM values near 140 km altitude is not reproduced by the HWM and GSWM. A similar phase reversal was however seen in the TIGCM values (middle plot in Figure V.4). One interesting feature in Figure V.6 is the almost constant phase between 90 and 100 km altitude. In weaker form this feature is also seen at the low latitude location. It is neither present in any o f the other models or in CTIM Run 1. Possible causes for this are discussed in
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section V.3.5.
iii) H IG H LA TITU D E
A t high latitudes the meridional wind amplitudes and phases are very similar below 160 km in CTIM and the TIGCM (lower plots in Figures V.3 and V.4). The shape o f the vertical amplitude pattern is almost identical in both models, with a wind amplitude o f around 40 m/s near 130 km altitude. Above 160 km the CTIM amplitudes continues to fall towards the saturation value o f 25 m/s, while the TIGCM amphtudes reach a minimum at 180 km and then rise again towards 32 m/s above 300 km. The southward wind maxima above 200 km occur around I hour earlier in CTIM. The zonal winds (not shown) also follow a very similar vertical pattern, but the amplitude peak near 110 km altitude is smaller in CTIM at 33 m/s, as opposed to 42 m/s in TIGCM. The TIGCM zonal winds after a minimum at 150 km rise with altitude towards a saturation amplitude o f 42 m/s, while at the same time the CTIM amplitudes continue to fall after the peak at 110 km and reach their saturation amplitude o f 10 m/s above 200 km.
Although CTIM meridional wind amplitudes become larger when moving poleward they are smaller by up to 50 m/s below 120 km altitude when compared to the GSWM values and by up to 30 m/s when compared to the HWM. A wind amplitude maximum is found in the HWM output at 120 km altitude, but neither CTIM nor the GSWM show this feature. Below 110 km the CTIM amphtudes are almost zero and then rise continuously with altitude, reaching values o f 65 m/s at 200 km. In spite o f these differences in absolute value between CTIM and the other two models the rate o f amplitude growth with altitude is more similar in the three models than at lower latitudes. The same applies to zonal winds which are also smaller in CTIM and largest in the GSWM.
The meridional wind phase change with altitude at high latitudes (lower plot in Figure V.6) is generally strongest in the GSWM. The rate above 100 km is roughly similar in CTIM and the HWM, and the phase values of both models are almost identical between 110 and 140 km altitude. Below that, the CTIM meridional wind phase is characterized by a phase reversal between 90 and 100 km, resulting in a shift of the southward wind maxima from 11 hours near 90 km towards later times, by around 2.5 hours. Above this phase reversal the waves are again westward travelling. A weaker phase reversal can be found in the GSWM values near 85 km, but not in the HWM.
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