Report on momentum budget Andrew + Elisa
Andrew Bushel presents result for parametrized drag versus explicit drags in the midlatitudes from the met-office model, monthly series that could be easily compared between models. Elisa have already comparable datas from CMCC. To analyse the intras-sea&sonal variabilities of the drag between models, around 20years of AMIP type runs would be fine. Also, separation between orographic and non orographic drag would be nice to have, since Orographic drags seem to be precursor of split SSW.
For the relation between parameterized drags and warmings or warming type, some daily datas over selected monthes could be useful. Exemple of relations between parameterized drag and vortex strength index illustrates how this can be done (??).
For the midlatitudes and in relation with the warmings, it might be better to compare the vertical momentum fluxes through the corresponding level rather than the drag at that level (downward control view).
Comparison should also be made of the tendencies in the tropics, and not only for the QBO, since the models show variability in the SAO as well.
Elisa points that the rôle of diffusive terms could also be adressed, for instance by showing plots of tendencies, and by comparaison with the drag themselves.
Manuel Pulido
shows diagnostics of momentum budget in the MERRA re-analysis, which has the advantage of being incremental: the model is not restarted each 6hrs, but rather smoothly corrected by a constant tendency term, which can be interpreted as a missing wave drag at high altitude (although it
certainly overestimates it since this terms is also related to model errors, natural drift from intial values????). In any case, the missing drag largely compensate at high altitude a may be too large orographic drag near the modles top. Do mountain waves break to high ? In the SH hemisphere, the resolvbed drag is less than in the NH and the GWD is huge near the top, and is essentially non orographic this time. Again the missing drag partly compensates again this too large drag.
Comparaison between two analysis techniques, one using PV explicitely shows large differences, still not much explain. Maybe nevertheless, a PV view of the drag on the mean flow and their effect on the zonal mean PV could be developped with the monthly mean files we have (??).
The series provided could be used directly to analyse drag between the models, and also issues stay about trends.
Julio Bacmeister
present simulations at 1° and 0.25° horizontal resolution (and L60?) where is shown that GWs parameterization are still needed. At leat to produce a QBO. He also propose a WGNE initiative on surface drag intercomparison between models. So far, the models are quite coherent in the
representation of the surface parameterized torques, but present a lot of spread in the subgrid scale
orography contribution to the torques. For instance, the SSO mountain torques in IFS may well be
well below the others, because the « boundary layer » form drag introduced recently make up a
large part of this torque.
Thiese terms show a lot of variability between years, and the major changes are related to changes in the NAO for instance.
Naftali cohen
present a systematic analysis of the compensations that occur in models between parameterized drags and explicit ones. The compensation in the NH mainly concern the mountain wave drag schemes, and the relation is complicated by the fact that all the drags are coupled. For instance a change in parameterized drag, affects the planetary waves index, and therefore affect the explicit drag. Three mechanisms are associated with these interactions a
stability constraint, a potential vorticity mixing constraint, and a nonlocal interaction associated with changes in the refractive index of planetary-wave propagation. The first mechanism is dominant for
strong-amplitude and meridionally-narrow parameterized torques, the second is most likely for parameterized torques applied inside the
winter-hemisphere surf-zone region, a key breaking region for planetary waves. The third mechanism, on the other hand, is most relevant for
parameterized torques just outside the surf zone. Accordingly, the interactions between resolved and parameterized torques depend on where is deposited the parameterized drag (e.g. far or near the surf zone), and its time-scale.
This compensation is also present when one consider climate change scenarios, it should be nice to analyse this compensations between models (e.g. Does the total drags are comparable, but the contribution of each varies). Everyone agrees that it is a good innovative entrance to analyse the mom budgets. A modified approach is suggested, which explicitly considers
the impact of wave driving on the potential vorticity of the stratosphere (see comparable ideas in Manuel's talk)
Kaoru Sato
Using gravity-wave resolving GCM the n mechanism of
barotropically/baroclinically unstable fields in the winter northern mesosphere are examined. The unstable fields
appear in the large planetary wavenumbers 1-3 and coincide with positive EPFD and negative PV gradient. It is argued that
poleward shift of the GWD change the refractive index properties allowing more vertical planetary wave propagation and result in a poleward shift of the jet. When the GWD meridionally shift is substantial it cause local instability that is manifested in a negative PV gradient. This region get compensated by a local generation of large scale planetary waves. This process of interaction between PW and GW is not zonally uniform.
