| Following earlier comparisons using the Canadian Middle Atmosphere Model
(CMAM, without interactive chemistry), the dynamical characteristics of the
model are assessed with interactive chemistry activated. Time-sequences of
temperatures and winds at Tromsø (70° N) show that the model has more
frequent and earlier stratospheric winter warmings than typically observed.
Wavelets at mesospheric heights (76, 85 km) and from equator to polar
regions show that CMAM tides are generally larger, but planetary waves (PW)
smaller, than medium frequency (MF) radar-derived values.
Tides modelled for eight geographic locations during the four seasons are
not strikingly different from the earlier CMAM experiment; although monthly
data now allow these detailed seasonal variations (local combinations of
migrating and non-migrating components) within the mesosphere (circa
50–80 km) to be demonstrated for the first time. The dominant semi-diurnal
tide of middle latitudes is, as in the earlier papers, quite well realized
in CMAM. Regarding the diurnal tide, it is shown here and in an earlier
study by one of the authors, that the main characteristics of the diurnal
tide at low latitudes (where the S (1,1) mode dominates) are well captured
by the model. However, in this experiment there are some other unobserved
features for the diurnal tide, which are quite similar to those noted in the
earlier CMAM experiment: low latitude amplitudes are larger than observed at
82 km, and middle latitudes feature an unobserved low altitude (73 km)
summer maximum. Phases, especially at low and middle (circa 42° N)
latitudes, do not match observations well.
Mesospheric seasonal tidal variations available from the CUJO (Canada U.S.\
Japan Opportunity) radar (MFR) network (sites 40–45° N) reveal
interesting longitudinal differences between the CMAM and the MFR
observations. In addition, model and observations differ in the character of
the vertical phase variations at each network-location.
Finally, the seasonal variations of planetary wave (PW) activity available
from CMAM and the MFR show quite good agreement, apart from the amplitude
differences (smaller in CMAM above 70 km). A major difference for the 16-d
PW is that CMAM shows large amplitudes before the winter solstice; and for
the 2-d PW, while both CMAM and MFR show summer and winter activity, the
observed summer mesopause and winter mesospheric wave activities are
stronger and more extended in height.
Models such as CMAM, operated without data-assimilation, are now able to
provide increasingly realistic climatologies of middle atmosphere tides and
PW activity. Differences do exist however, and so discussion of the various
factors affecting tidal and PW characteristics in atmospheres, modelled and
observed, is provided. Other diagnostics of model-characteristics and of
future desirable model experiments are suggested. |