Experiencia de los Apalaches (EE.UU.)

Figure TV-48 shows global mean concentration profiles of N2, O2, O, as given by MSIS-E90 and calculated by CTIP and CMAT at equinox for low solar activity. Figure IV-49 is a similar plot for the NCAR 1-D TIME model. The agreement between CMAT, MSIS-E90, and the NCAR 1-D TIME model is relatively good. Both the NCAR 1-D TIME and CMAT overestimate number densities of Ng and O2 between about 90-100km, in comparison with MSIS-E90. CMAT calculates the peak in O density to be 4.4x10" cm^ at 99km compared to 4.2x10" cm ’ at 97km given by MSIS-E90. It should be noted that the peak in O is sensitive to the amplitude of the diumal tide {^ble and Shepherd [1997]), CMAT runs with increased lower boundary tidal forcing have smaller O peak density. The vertical distribution of O in the lower mesosphere differs between the NCAR 1-D TIME and CMAT. This is because the CMAT profiles correspond to global means of all diumal conditions, whereas the NCAR 1-D TIME incorporates a global zenith angle corresponding to the diumal mean. This difference is also responsible for the difference in 0('D ) pro filles shown in Figure TV-50 and Figure TV-51. Locations on the CMAT grid that correspond to the solar zenith angle used in the NCAR 1-D TIME model, give comparable morphologies for constituents that are in photochemical equilibrium. Another reason that these two models differ with respect to O is that CMAT does not include middle atmosphere water ion chemistry. Intermediate stages in the ion chemical scheme following ionisation can produce O. Figure TV-52 shows zonal mean O concentrations as from CMAT, MSIS-E90, and CTIP. The distribution of the peak O layer differs from that of MSIS-E90, with a more localized maximum at the equator. In comparison mns, this feature was found to be influenced by net vertical transport associated with tidal dissipation. Runs with stronger tidal amplitudes produced a less localised maximum at the equator, as shall be discussed in chapter VI. Studies with the TIME-GCM also show a local maximum at the equator, but differ from CMAT in that they show maxima at the poles {see Figure TV-53).

Yee et al. [1997] stated that this was due to downweUing of O rich air from the

thermosphere. Figure TV-31 {top) shows the vertical velocity morphology in CMAT for kp=2+ at equinox. The solar heating circulation pattern of upweUing at the equator and downweUing at midlatitudes is present, but is disrupted at the poles due to heating within the auroral oval, resulting in Httle downweUing above about 115km. Therefore there is no mechanism in CMAT for increasing O at the poles during equinox. Yee et al. [1997] pointed

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out the difficulty in confirming the high latitude maxima at equinox due to the limited number of high latitude nightglow observations. They stated that winter observations seemed to show enhancements in agreement with the proposed circulation. CMAT solstice simulations shows winter enhancement, but also show a disruption of this circulation at very high latitudes due to the auroral oval. From the simulations presented here it would seem that more study is required into the magnitude of high latitude auroral effects upon the circulation of the lower thermosphere. The local time variation in O at the equator for equinox, as calculated by CMAT is given in Figure IV-54 {top). Clearly visible is the transition at about 90km of O being long-lived to a state of photochemical equilibrium. Below this altitude O is only present during the day due to solar dissociation, at night it rapidly recombines.

IV.2.4.2 OZONE

Figure IV-50 and Figure IV-51 show global mean ozone densities as calculated by CMAT and the NCAR 1-D TIME model. CMAT gives an upper stratospheric peak ozone volume mixing ratio of about 18 ppmv at the equator, compared to about 1 1 ppmv as observed by HALOE. As discussed previously this is mostly due to the omission of chlorine catalytic cycle destruction of ozone within the CMAT photochemical scheme, as is the case with the NCAR TIME-GCM. The representation of upper stratospheric chemistry is crude within both models due to the difficulty in modelling heterogeneous chemistry. This region is a lower boundary, and therefore limits the accurate simulation of upper stratospheric processes. In the upper mesosphere both CMAT and the NCAR 1-D TIME GCM give a 1.5x10* cm ^ secondary peak in O3, though the CMAT peak is at 89km compared to 8 6km in the NCAR 1-D TIME model. Figure IV-58 shows O, mixing ratio as a function of height averaged from 11:00 to 14:00 LT as calculated by CMAT. Figure IV-57 shows a similar monthly mean profile of ozone mixing ratio from measurements between 11:30 and 14:30 LT as measured by HRDI (Marsh, private communication [2000]). The most striking difference between these plots is the absence in the HRDI data of a localised maximum at the equator between 90-100km. As discussed previously in relation to O, this is due to tidally induced vertical transport of O^. Figure IV-59 shows the variation in ozone volume mixing ratio at 90km as a function of longitude and latitude as measured by HRDI (Marsh et al. [1999]). If the HRDI zonal mean ozone plot had been made as averages of observations at sHghtiy later times, e.g. between 14-17 hrs local time, the zonal mean plot would have

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included a strong tidal maximum at the equator, in agreement with CMAT. Therefore the disagreement between CMAT and the HRDI data with respect to the equator local maximum is most likely due to a disagreement in tidal phase. The altitude of the minimum calculated by CMAT at about 75km is about 10km higher than that observed. This difference in altitude has been found in other models {/illen et aL[1984], Zhu et al. [1999]). This region is influenced heavily by HO^ chemistry, the magnitude of the minimum being related to the magnitude and location of the peak OH mixing ratio, and the nature of its’ diumal variation, as discussed in section IV.2.4.3. The local time variation of O3 at the equator as calculated by CMAT is given in Figure IV-54 {bottom)and Figure TV-56. Clearly visible in both plots is the lack of diumal variation below about 50km. At night in the absence of solar dissociation, O3 has chemical and transport lifetimes in excess of a day, and wül therefore show little variation. During the day the chemical lifetime of O3 is of the order of 100-1000 seconds. There is however little variation because any O created from O3 dissociation rapidly recombines to reform O3. This is because the chemical lifetime of O relative to the chemical timescale of O recombination to form O3, is much smaller than the timescale of O3 dissociation {Alien et ai [1984]). Therefore there is little variation over the diumal cycle. The nighttime lifetime of O3 remains in excess of a day up to about 85km, therefore nighttime concentrations remain constant up to this altitude. With increasing altitude above 50km, solar dissociation of O3 results in a decrease of O3 densities following sunrise. This is shown in Figure TV-56 and Figure TV-55. In the 80-90km region O3 densities are also affected by interaction with HO^ chemistry. At about 85km there is a sharp rise in O3 as atomic oxygen recombines, followed by a night time decrease as it is destroyed mainly through reaction with hydrogen. Comparison with Figure TV-55 and Figure TV-56 show many similarities in diumal O3 features modelled by CMAT and the John Hopkins University Applied Physics Laboratory two-dimensional model (JHU/APL

2-D) i^hu et al [1999]), though the absolute magnitudes differ at certain altitudes. rv.2.4.3 H O ,

The global mean profiles of H, OH, and HOg as calculated by CMAT and the NCAR 1-D TIME model are given in Figure TV-60 Figure TV-61, along with the profile of H given by MSIS-E90. It should be noted that the treatment of H in CMAT and the NCAR 1-D TIME differ. CMAT is not fully self-consistent in that it relaxes to MSIS-E90 starting at about 150km {see section III.4.8), and therefore wiU show a near identical agreement with

C M A T Steady State Results________________________________________________Chapter I V

MSIS-E90 above this region. Below this altitude CMAT is self-consistent with respect to H. At about 85km the NCAR 1-D TIME model gives a peak in H of 1.9x10® cm^, compared to the CMAT peak of 9.1x10^ cm ^ at 89 km and 1.6x10® cm ^ at 84km from MSIS-E90. The smaller magnitude of the H peak calculated in CMAT is due to the HgO profile used above the top altitude of the UARS HgO climatology applied in these runs, taken firom the John Hopkins University Applied Physics Laboratory two-dimensional model (JHU/APL 2-D)

{Zhu et al. [1999]). The sharp negative gradient in HgO mixing ratio at these altitudes is critical in determining the H concentration following solar dissociation of HgO. The lower mixing ratios of water at the altitudes of the CMAT H peak are most likely due to differences between the two models with regard to photodissociation variation with altitude, and eddy diffusion. Below about 80km H is in a state of photochemical equilibrium, recombining at night {seeFigure IV-64 (/<?/>)).

The global mean concentrations of OH and HOg are in general agreement with the NCAR 1-D TIME model, though CMAT gives a stronger localized layer at about 80km. The daytime values as calculated by CMAT are of the same magnitude as those measured by the Middle Atmosphere High Resolution Spectrograph Investigation (MAHRSl) {Conway et al.

[1996]), as shown in Figure lV-62 (MARSI)and Figure lV-63 {CMAT). Below about 65km CMAT is close agreement with the data, with daytime values in the region of 1.0x10^ cm^ at 50km, and 6x10^ cm ’ at 60km. CMAT produces the characteristic mesospheric OH layer between 65-75km as seen in the data, though the growth rate would appear to differ. The MARHSl data shows little increase in the layer between 8.40 and 10.40 hrs, whereas CMAT shows a continual growth at 70km through and past this period, levelling out at about 12 hrs local time. It should however be noted that the CMAT plot relates to one specific location, whereas the MARSHl plot is taken from observations at different local times at different latitudes. Figure lV-64 {bottom) shows the local time variation of OH at the equator. O f interest is the development of a night time layer associated with the reaction of H and O,. This layer is seen to decrease past about 0 hrs local time, due to a decrease of H

{seeFigure lV-64 {top)).

rv.2.4.4 N O ,, N ('D ), AND N ( ‘S)

The global mean profiles of NO, NOg, N('*S), and N(^D) as calculated by CMAT and the NCAR 1-D TIME model are given in Figure lV-65 and Figure lV-6 6, along with the

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profile of N(‘^S) given by MSIS-E90. CMAT gives a peak in N('^S) of 2.3x10^ cm ^ at 168km compared to 2.1x10^ cm^ at 180km from MSIS-E90, and 2.0x10^ cm^ at 160km from the NCAR 1-D TIME model. CMAT gives a peak in N(^D) of 3.2x10^ cm^ at 203km, compared to 3.8x10^ cm^ at 200km from the NCAR 1-D TIME model. The peak in NO number density as calculated by CMAT is 1.7x10^ cm ^ at 109km compared to 9x10*^ cm'^at 105 km as calculated by the NCAR 1-D TIME model. Roble[1995] stated that the peak NO density in the NCAR 1-D TIME was in reasonable agreement with daytime low-latitude measurements made by the Solar Mesosphere Explorer satellite. However, the model used a global particle precipitation input that included the effects of high latitude precipitation. This should increase the global production of NO. At low latitudes, the region in which the NCAR 1-D TIME model is in agreement with SME observations, precipitation is a minor contributor to the production of NO. This would suggest that the NCAR 1-D TIME model was perhaps underestimating the production of NO. In the lower mesosphere and upper stratosphere, CMAT calculates a local maximum in NO of about 7ppbv compared to about 10 ppbv as measured by HALOE. The main source of NO in this region is the oxidation of N^O by 0 ( ’D). Since the N^O climatology used within CMAT is based on a UARS climatology, this deficit is most likely due to an underestimation of 0(^D) associated with the crude representation of upper stratospheric chemistry within the first few levels of CMAT. Figure IV-67 shows sunset NO mixing ratios as measured by HALOE, and calculated by the NCAR TIME-GCM (Marsh [1999]), and Figure IV- 6 8 as calculated by CMAT. Clearly visible in the upper mesosphere and lower thermosphere is the larger NO volume mixing ratios at higher latitudes, associated with auroral production. In the HALOE data the high latitude maxima decrease in altitude above about 135km, whereas in CMAT this is not the case. Also the minimum in NO as measured occurs in the 60-70km region, in comparison to 70-80km as calculated by CMAT. Figure IV-73 shows NO densities as calculated by CMAT under similar conditions to the period in which the Student Nitric Oxide Explorer (SNOE) satellite observed NO as shown in Figure TV-74 {Earth et al.

[1999]). Whilst the morphology of CMAT and the observations compare well, the NO densities as calculated by CMAT are consistently about a factor of 4-10 smaller than those observed. This offset occurs at aU latitudes suggesting that it is due to overall photochemistry rather than the magnitude of solar or auroral ionisation. Similar low NO densities have been calculated using detailed 1-D photochemical models. Swaminathan et al

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Swaminathan et aL stated that this may be due to a variety of sources of atomic nitrogen not

currendy included in current photochemical schemes, such as double ionisation at the hard X-ray edge of the solar spectrum, and reactions of Ng with high energy O f P) atoms. With regard to CMAT, another potential cause of this deficit may be the profiles of secondary electron production relative to initial ionisation. These are derived from a one dimensional electron transport model, and are invariant with FI 07 and latitude {see section III.4.1).

Figure IV-71 shows the sunset and sunrise volume mixing ratios of NO at the equator as calculated by CMAT at equinox, and Figure IV-72 shows similar profiles as measured by HALOE. Clearly visible is an asymmetry, the variation of which appears to have a vertical wavelength of about 25-30km, similar to the diumal tide. Figure IV-71 and Figure IV-72 show the ratio of the two profiles as calculated by CMAT and measured by HALOE {Marsh

and Kusell III [2000]). CMAT is in close agreement with the HALOE data above 90km,

though the peak is smaller due to the smaller diurnal tidal amplitudes simulated in CMAT. The diumal tide gives rise to areas of localized up- and down-welling. NO has a sharp gradient at these altitudes, therefore vertical advection plays an important role in the local time variation. Since the HALOE observations at dawn and dusk are separated by 12 hrs, vertical advection will be in the opposite sense for any given location. This gives rise to the observed anomaly.