For typical long-duration balloon flights, target payload
recovery zones are located on the launch-site latitude.
And, long-duration flight launch sites are chosen such
that few overflight problems are expected from constant-latitude
flights. Thus, minimum latitude variation is a desirable
flight trajectory characteristic.
Conventional wisdom says that stratospheric summer
polar (as opposed to stratospheric polar winter or stratospheric
tropical) circulation is the most favorable for long-duration
balloon flight trajectories with minimum latitude variation.
In the summer, the polar vortex is typically unified,
and the consistent, low-velocity, east-to-west flight
path yields little north-south motion; whereas in the
winter, the polar vortex is typically fragmented into
two or more circulations and significant disturbances
(stratwarms) can disrupt the prevailing west-to-east
flow. (Note that the twice-yearly reversal of the prevailing
polar stratospheric winds is known as "turnaround."
During the local summer, polar vortices exhibit steady
east-to-west flows. During local winter, polar vorticies
develop often-fragmented west-to-east flow.)
The preliminary trajectories shown on this page largely
confirm conventional wisdom. However, stable west-to-east
winter trajectories are observed for two cases in the
southern hemisphere.
Summer Trajectories
The June 1981 Fairbanks trajectories show little latitude
variation until approximately 80 days into the flight
(end of August) where the balloon encounters turnaround
conditions. Similarly, the February 1981 Alice Springs
and Christchurch trajectories exhibit near-constant
latitude until about the 55th day where stratospheric
turnaround conditions are observed. In the February
1981 Alice Springs trajectory, the balloon loses velocity
and meanders for a few days until the west-to-east circulation
is established. Thereafter, the balloon flies eastward.
In the February 1981 Christchurch trajectory, a short
direction reversal occurs: a likely result of a small
stratospheric warming event.
Winter Trajectories
In contrast to the near-constant-latitude June 1981
Fairbanks trajectory, the February 1981 Fairbanks trajectory
exhibits an erratic and undesirable trajectory: significant
latitude excursion and overflight of geopolitically
sensitive areas are observed.
The June 1981 Alice Springs and Christchurch trajectories
show little latitude variation, relative to the February
1981 Fairbanks trajectory. Until the 80th day (end of
August 1981), the Alice Springs trajectory is remarkably
stable. The June 1981 Christchurch trajectory is more
stable in latitude compared with the February 1981 Fairbanks
trajectory.
Note that the average zonal velocity for the February
1981 Christchurch and Alice Springs trajectories is
quite different. For Alice Springs, the average zonal
velocity for the first 70 days of flight is 35 m/s,
whereas the average zonal velocity for the first 70
days of the Christchurch flight is 70 m/s. This observed
zonal wind velocity difference is not inconsistent with
data shown in Global Atmospheric Circulation Statistics,
1000-1 mb , William J. Randel, Atmospheric Chemistry
Division, National Center for Atmospheric Research,
Boulder, CO, NCAR/TN-366+STR, 1992. Randel shows strong
latitude gradients for zonal winds between 20 deg. S
and 60 deg. S at 35 km in the southern hemisphere winter.
Average velocities near Christchurch latitudes are as
high as 70 m/s.
|
