Equatorial mountain torques, Equatorial Angular Momentum and cold surges in a GCM, F. Lott and S. Mailler, LMD Paris, France. [email protected],
AAM Budget and
Equatorial Mountain Torques
Mass AAM
Wind AAM Parametrized torque
Mountain torque The mountain torques are the
dominant forcing of the equatorial components of the Atmospheric
Angular Momentum budget
(compared to the frictional torques):
T
M1<0
F: reactive force applied by the mountain on the atmosphere
(almost perpendicular to the local wind)
T
M: Torques applied by the mountain on the atmosphere
90°E Greenwich
axis
90°E
T
M2<0
Greenwich axis
Abstract :
The dynamical relations between the equatorial atmospheric angular momentum, the equatorial mountain torque and the cold surges are analysed in a General Circultaion Model (GCM). First we show that the global equatorial atmospheric momentum budget is very well closed in the model which is a clear benefit when we compare with results from the NCEP reanalysis (not shown in this poster).
We then confirm that the equatorial torques due to the Tibetan plateau, the Rockies and the Andes are well related to the cold surges developping over South Eastern China, North America, and the Southern South America respectively. For all these mountains, a peack
in the Equatorial mountain torque component that points locally toward the pole preceeds by few days the development of the cold surges, yielding a predictive interest to our results.
We also analyse the contributions to the torques of the parameterized mountain stresses and find that they contribute substantially. In experiments without the parameterized stresses, we also find that the explicit terms partly compensate the parameterized contributions
to the torque, and the cold surges are not much affected. This shows that the cold surges can be well captured by models, providing that the synoptic conditions prior to their onset are well represented. The compensation between torques is nevertheless not complete
and some weekening of the cold surges is found when the mountain forcings are reduced. This illustrates how the exact torques are needed at a given time to produce the correct synoptic scale dynamics at a later stage.
Surface Pressure anomaly composite in the LMDz - GCM
(almost identical Results from NCEP)
la g = 0d ay
The composites in LMDz are keyed to the first component of the Eq. Mountain torque due to the himalayas
la g = 2d ay s
Dynamical origin
H P
Ug
F
U
g: Geostrophic wind
F: Reactive force on the flow Mountain is in green
P
s: Surface pressure
H P
F
LP
P
s=cte
H P
Ug
F
H P
Ua
F
U
aageostrophic wind:
Equilibrate F via Coriolis
Transport mass from left to the right here
t
P
s>0
LP t
P
s<0
PH
F
PH
LP
U
a
F
t
P
s
>0
t
P
s
