Discussion
The correct answer is "b". The atmosphere (both real modeled)
is generally trying to achieve geostrophic balance. However, for comparison
consider the familiar case of air flowing through a jet streak. Momentum
from the jet core is advected forward into the exit region, so the exiting
parcels are supergeostrophic and therefore get turned toward higher heights.
The crossing of height contours represents conversion of kinetic energy
to geopotential energy, allowing the parcels to slow down. This generates
the classic exit-region secondary circulation, with jet exit parcels turning
to the right. Conservation of mass (continuity equation) implies sinking
motion at a lower level to the right of the exit region and rising motion
to the left. Thus, a mass field whose geostrophic wind contains a speed
maximum results in a smooth flow that contains as ageostrophic secondary
circulation. Similarly, even more complicated patterns and dynamics involving
things such as curved flow, asymmetric height and wind fields, friction,
latent heating, and smooth evolution of the flow are all part of the "balanced
state" in the real atmosphere and NWP models.
Gravity waves are not part of this balance flow. The balanced state is
not achieved until adjustment-related gravity waves have passed through
the region (of size 
).
Other gravity waves caused by factors such as flow over topography or convection,
are common but are transient, and are not part of the long-lived balanced
state achieved through the adjustment process.
