Downstream development during the rapid intensification of hurricanes Opal and Katrina: the distant trough-interaction problem

Evidence is presented that the rapid intensification of Hurricanes Opal and Katrina was linked to the group propagation of Rossby waves in the upper troposphere. Rapid intensification commenced as a long wave amplified in the vicinity of the storms, with the trough to the west of the storms and the downstream ridge collocated with the storms. This large-scale structure thus resembles the “favorable distant trough” category described in Hanley et al. (2001). Analysis of the structure of the waves, and the group and phase propagation characteristics suggest that large-amplitude, long waves, and the associated jet structure, can establish a unique and favorable environment for intensification. Under these conditions it is possible to have large group propagation to change the storm's environment, and small phase propagation so that the storm's encounter with the high wind shear zones in eastward propagating troughs is delayed.

A hierarchy of numerical simulations is used to diagnose the environmental flow changes and explore the possible influence of downstream events on storm structure and rapid intensification. The main conclusions are:

1. Regions of upper tropospheric anticyclogenesis within the eastward-propagating downstream events provide a low vertical wind shear environment for intensification. 2. The downstream events are mostly defined by dry dynamics and independent of the presence of the storm. 3. During rapid intensification, the storms move into diagnosed environments characterized by (a) a developing upper level anticyclonic vorticity anomaly, (b) regions of enhanced low to midlevel cyclonic vorticity, (c) increasing conditional instability, and (d) periods of both ascent and descent. 4. The simulations indicate that boundary layer moistening within the storm occurs during the period of environmental descent, prior to rapid intensification. 5. There is little evidence from the simulations of enhanced environmental upper divergence in the vicinity of the storms during rapid intensification.

We hypothesize that a short-term partial suppression of ascent within the storm by the environment allows the storm's boundary layer to moisten via sustained surface fluxes. Once the period of external inhibition to ascent passes, deeper, more active convection develops, with rapid intensification in the low wind-shear, increasingly-cyclonic, low-level environment.

We speculate that the increasing skill of Global Forecast Models to predict these waves might provide a means of evaluating intensification potential at longer lead times.