The IOP 5 simulation is initialized at 8901190000 UTC and the IOP 8 simulation is initialized at 8902240000 UTC. Both simulations are made for 36 hours.
Using this grid of trajectories, the 12-h contraction rate is determined. Basically, the contraction rate will be large when two nearby trajectories have vastly different histories. Since airstreams are characterized by similar trajectories, high contraction rates represent airstream boundaries.
The loops show the airstream boundaries (shaded in red) at 1 km (sigma) every 15 minutes from 12-36 hours into the simulation. Superimposed upon these airstream boundaries are selected trajectories from a coarse grid (1.0 degree lat/lon) 24 hours into the simulation. Thus, from 12-24 hours, the arrows represent backward trajectories. From 24-36 hours, the arrows represent forward trajectories. At 24 hours, there is a slight break in the loop as it switches from backward to forward trajectories. At this time (24 hours into the simulation), all of the trajectories are at a height of 1 km (sigma).
The length of each trajectory corresponds to one hour of motion. The arrows represent the position of the trajectory every 15 minutes. The larger the arrow, the higher (altitude) the trajectory.
Each trajectory is color-coded according to its position relative to the airstream boundaries at 24 hours. All trajectories that lie in regions of high contraction rate are colored yellow. All others are colored according to the airstream structure identified in Robert Cohen's PhD thesis and summarized in The Analysis of Airstream Boundaries (presented at the American Meteorological Society's 15th Weather Analysis and Forecasting Conference in Norfolk, Virginia, August 19-23, 1996):
Each storm has black parcels that follow the storm center. It appears that this air is more closely associated with the green in-situ airstream than the brown airstream. This is probably due to the ascent that occurs just ahead of the center, which removes the (old brown) air between the black air and the (newer) brown air. In this sense, it is not "secluded" but rather "left behind".
Each storm also has a three-airstream structure. The blue/brown boundary is associated with the surface cold front, the green/blue boundary is associated with the secondary trough (and upper cold front) and the green/brown boundary is associated with the surface warm front.
In IOP 5, the brown/green boundary is much less defined than the other two boundaries. Both airstreams tend to wrap cyclonically around the low center (although most of the brown airstream ascends and turns anticyclonically).
In IOP 8, the brown/green boundary is very defined. It is the blue/brown boundary that is less defined. None of the brown air wraps around the low center.
The airstream boundaries in IOP 5 have a north-south orientation compared to the east-west orientation of the boundaries in IOP 8. This orientation difference is typical of large-scale diffluent (IOP 5) vs. confluent (IOP 8) flow. This difference is seen more clearly in loops of idealized 2-dimensional diffluence (0.8 MB) and confluence (0.7 MB) flow superimposed upon a vortex.