There are five different vertical motion options in HYSPLIT. The
suggested default (0:data) is to use the vertical velocity field that is
included with most meteorological data. Other options may be required
for special situations such as following the transport of a balloon
on a constant density surface, comparing isobaric flow fields between
data sets, or situations when the meteorological data’s vertical
velocity field may be too noisy compared with the time step at
which the data are available (high spatial resolution simulations).
In the sigma option the trajectory remains on its
original terrain following sigma surface. In the isobaric, isentropic, and constant
density (isopycnic) options, the vertical velocities are computed
from the equation,
W = ( ∂q/∂t – u ∂q/∂x – v ∂q/∂y) / (∂q/∂z)
where W is the velocity required for
the trajectory to remain on the q surface (pressure, potential
temperature, density). Note that the equation results in only
an approximation of the motion and a trajectory may drift from
the desired surface.
Shown below left is
the same trajectory of the previous example using the NAM 12 km vertical
velocity fields. To the right is the same trajectory computed using the
isentropic flow assumption and choosing the Theta Vertical Coordinate option from the
Trajectory Display menu. This graphic shows that the potential temperature
varied by only about 1 degree, however by assuming adiabatic flow conditions the second trajectory
ended in northeastern Lousiana after 84 hours instead of northcentral Texas. The validity of the adiabatic flow
assumption would need to be assessed for this case.
