Principal rainbands of Hurricanes Katrina and Rita as observed in RAINEX

The Hurricane Rainband and Intensity Chance Experiment (RAINEX) successfully observed the principal rainbands of Hurricanes Katrina and Rita using the National Center for Atmospheric Research (NCAR) Electra Doppler radar (ELDORA). This radar captured convective-scale features of the rainband in exceptionally high resolution. RAINEX also employed a high-resolution numerical model that was able to resolve small-scale features similar to those observed by the ELDORA radar and forecast the evolution of the hurricanes' internal structures. The data and model output have been analyzed in ways to determine how the convective-scale motions within the rainband might play an important role in affecting the intensity of the overall storm, as has been suggested by earlier theoretical and modeling studies.

The radar data were first classified into regions of convective and stratiform precipitation by an objective scheme applied to the reflectivity fields. The Doppler radar air motion fields were then analyzed for the convective regions. Statistics from this classification and analysis suggest that the principal rainbands' updrafts, when viewed in high resolution and three-dimensions, strongly resemble those in an eyewall in terms of their dynamic and precipitation structure. The dominant flow patterns within the convective regions consisted of an overturning updraft, a low-level downdraft entering the rainband from the radially outward side, and an upper-level downdraft on its radially inward side. These structures were highly recurrent within the upwind portion of the rainband convection, while downwind structures were weaker and more variable, even though the rainband reflectivity remained strong. An analysis of the high-resolution structures of the cells embedded in the principal rainbands further suggests that they help develop a wind maximum in the vicinity of the rainband. This wind maximum likely interacts with the larger-scale circulation of the hurricane vortex.

The high-resolution simulation of Katrina did not exhibit a dominant, well-connected band of intense convection suggestive of a classic principal rainband. Instead, it exhibited discrete pockets of strong convection, identified by their intense rain rates. The kinematic structure of these individual cells showed strong vertical motions similar to that which the ELDORA radar observed. However, these model vertical motions occurred in a much shallower layer than shown by radar. The eyewall outflow extended over a larger layer, and also exhibited flow reversals within this layer. Further numerical experimentation is expected to lead to better agreement with the ELDORA data. At that point, a fuller synopsis of the internal structure of the hurricane and its role in modifying the intensity of the storm should be achievable by integrated analysis of the high-resolution model output and radar data.