Fluid Trapping

Fluid Trapping Parameter: In regions of strong vorticity (i.e., cyclones), air parcels tend to become trapped within the vortex. This can produce a boundary within which air parcels are trapped and so follow the vortex over extended periods. In regions with strong deformation, some air parcels are brought closer together but others become increasingly separated. The trapping parameter (TRAP) is defined according to the simple formula: TRAP = 1/4 [(DEFres)^2 - (VOR)^2]. Negative values are identified regions where the vorticity is larger than the deformation, and it is within such regions that air parcels are likely to be trapped. This parameter is useful for identifying the development of strong vortices and for tracking them.

Reference:
Cohen, R. A., and D. M. Schultz, 2005: Contraction rate and its relationship to frontogenesis, the Lyapunov exponent, fluid trapping, and airstream boundaries, Mon. Wea. Rev., 133, 1353-1369.