Comercialización de frutas

Global mean temperatures from CMAT and MSIS-E90 for F10.7=76 and F10.7=180 at equinox are given in Figure IV-19. CMAT agrees reasonably well with MSIS-E90 over most of the mesosphere-thermosphere height range. There is however a difference in exospheric temperature at low FI 0.7. CMAT has an exospheric temperature of 697K compared to 790K given by MSIS-E90 at F10.7=76. At very high F10.7 [e.g. 243), CMAT gives higher exospheric temperatures than MSIS-E90. There are many factors that could contribute to this offset, such as uncertainties in EUV flux, absorption cross-sections, neutral heating efficiency, molecular diffusion coefficients, and perhaps most importantly high latitude electric field strength. Similar comparisons have been made with the NCAR TIE-GCM

i^ b le et al. [1987]). Close agreement with MSIS exospheric temperatures were achieved

through the tuning of electric field strength at high solar activity. The reduction by up to a factor of two applied to the electric field magnitude subsequently reduced Joule heating and therefore exospheric temperature. Such tuning would improve agreement between MSIS- E90 and CMAT at very high FI 0.7, but has not currently been applied in CMAT. Whilst improving the thermospheric energetics and temperature morphology in comparison with MSIS, the implementation of electric field variability that has been applied is a crude first approximation. Constant field variability has been assumed for all locations at which the field strength exceeds a critical value {see III.2.8). No attempt has been made to include changes in variability associated with changes in solar cycle, location and geomagnetic activity. Since the variability wiU be a function of electron density, wind and compositional structure, it is possible that using a fixed degree of variability over the solar cycle may introduce systematic errors in overall Joule heating.

The CTIP runs used for most of the comparisons in this chapter include lower boundary tidal forcing at 80km. Figure IV-20 shows global mean temperature profiles as calculated by CMAT, MSIS-E90, CTIP with-, and CTIP without- tidal forcing. The introduction of tides in CTIP tends to cool the thermosphere, by up to 190K in the exosphere. CMAT runs with

C M AT Steady State Results________________________________________________Chapter I V

doubled lower boundary tidal forcing, and associated increased tidal signatures in the 80km region, do not produce any significant change in global mean thermospheric temperature. As shall be discussed in the following sections the most likely cause of the CTIP tidal temperature trend would appear numerical rather than physical, associated with the inability of the one scale height vertical resolution to adequately resolve the diumal tide.

In the absence of tidal forcing CTIP overestimates global mean temperature in the lower thermosphere by up to lOOK, and fails to simulate a mesopause at about 100km. The reason for these discrepancies is mostly likely due to the omission of COg and NO radiative cooling. Sensitivity studies by Robk et al [1987] have shown that switching off these cooling mechanisms can result in a decrease in exospheric temperature of 172K at solar minimum. CMAT was run for equinox at solar minimum with no COg cooling {see Figure IV-23). As expected the omission of CO^ cooling acts to increase the temperature by 150K at 110km. The shape of the profile also differs in that there is a turning point rather than a local minima in the 100-110km height region. This is similar to the CTIP profile shown in Figure IV-20. The exospheric temperature is seen to increase by nearly lOOK, which implies that such an offset must also exist in the CTIP upper thermosphere temperature profiles.

Global mean CMAT and MSIS-E90 middle atmosphere temperatures for F10.7=76 are given in Figure IV-22. There is reasonably close agreement in the upper stratosphere and lower mesosphere up to about 80km. MSIS-E90 gives a stratopause temperature of 267K compared to 272K in CMAT, which is at a slightly higher altitude. Although this value is in reasonably close agreement with MSIS-E90, it is worth noting that the upper stratosphere / lower mesosphere ozone densities calculated in CMAT are higher than those observed {see

rV.2.4.2). In the upper stratosphere this is mainly due to the omission of the chlorine catalytic cycle destruction of ozone, as is the case with the NCAR TIME-GCM {see Yee et al

[1997]). Therefore solar heating due to absorption by ozone may be overestimated, and the stratopause temperature higher than MSIS-E90. Above 80km there is disagreement between CMAT and MSIS-E90. CMAT calculates a mesopause temperature of 185K at about 100 km for F10.7=76, and 189K for FI 0.7=180. The variation of lower mesosphere temperature with FI 0.7 is discussed in IV.2.1.2. MSIS-E90 gives no solar cycle variation in temperature in the mesosphere, with a mesopause temperature of 177K at about 97km. The eddy diffusion climatology applied has a peak value of 75 m^s'^ at about 105km. Although

C M A T Steady State Results________________________________________________Chapter JK

this profile is based on calculations due to Garcia and Solomon[1985], it is low in comparison to eddy diffusion profiles applied in other models, which can have peak values up to 300 cm V \ CMAT runs incorporating higher values have a colder mesopause temperature, in closer agreement with MSIS-E90, but have higher turbulent viscous damping of the tides. O f interest is the turning point at pressure level 27 (~83km) calculated by CMAT. This is a global feature caused by exothermic neutral chemistry, and is in agreement with the NCAR

1-D TIME GCM {seeFigure IV-21).