Rainband structures observed in RAINEX

Although hurricane track forecasting has gradually improved over the past 30 years, the poor understanding of hurricane intensity changes continues to negatively affect forecasts of hurricane intensity. This is largely because intensity forecasts are highly influenced by the dynamics of mesoscale features (eyewalls and rainbands) within the hurricane, whereas track forecasts are more heavily influenced by the large-scale flow patterns of the storm's environment. Forecasting rapid intensification cycles are especially problematic, since most of the major hurricanes exhibit rapid intensification at some stage during their life cycle.

Some studies have suggested how the spiraling rainbands may affect the internal structure, dynamics and intensity of a tropical cyclone. However, a variety of hypotheses exist regarding the evolution and structure of rainbands, as well as their purpose in regard to the hurricane's vortex dynamics. For example, Atlas et al. (1963) suggest that in their radially-inward spiraling configuration, rainbands have upwind ends consisting mostly of active convective cells, with increasingly stratiform structure toward the downwind ends. In contrast, Willoughby et al. (1984) suggest that the spiraling rainbands contain discrete cells of active convection of varying intensity all along their entire length. Some rainbands exhibit an eyewall-like tangential wind maxima (Samsury and Zipser 1995). We have further noted that occasionally the spiraling rainbands may also take on a more constant radius of curvature and develop the continuous solid convective structure of an eyewall. These configurations suggest that the rainbands may play a role in the eyewall replacement cycle, and associated hurricane intensity changes. A preliminary study of the Tropical Rainfall Measurement Mission Precipitation Radar (TRMM PR) data for the 1998-2005 hurricane seasons of the North Atlantic basin suggest the existence of these types of precipitation structures within rainbands by the reflectivity signatures seen through a large sample of storms.

The structure of rainbands is likely relevant to changes in hurricane intensity. When portions of a rainband become stratiform, cyclonic vorticity is likely concentrated in midlevels in the stratiform regions, as happens in a mesoscale convective system. Another process by which a rainband develops a cyclonic vorticity maximum occurs when a rainband develops a tangential wind maximum, which has a zone of cyclonic shear vorticity on its inner edge. These mesoscale vorticity features of rainbands are likely fed into the larger storm circulation, thus contributing to storm intensification. This study explores the development of mesoscale vorticity maxima in the context of observed hurricane rainbands.

This study is based on aircraft data obtained in the 2005 Hurricane Rainband and Intensity Experiment (RAINEX), three Doppler radar equipped P3 aircraft (including two NOAA P3s and the NRL P3) flew coordinated flight tracks in Hurricanes Katrina, Ophelia, and Rita. Several flight tracks were aimed at observing rainbands near the central core of the storm. Some tracks provided quadruple Doppler radar coverage. In addition to the Doppler radar data, the aircraft obtained dropsonde data at 5-10 min intervals along each side of the rainband. The dropsonde data indicated the thermal and moisture stratification of the inflow and outflow to the rainbands.

This paper will present analysis of the airborne Doppler radar and dropsonde data for rainbands observed during RAINEX, to further examine the precipitation distribution of a rainband and the associated circulation within the rainband. Vertical velocity and vorticity fields will be calculated from the Doppler radar data. The Doppler-synthesized winds, interpolated reflectivity, vertical velocity and vorticity fields will be ingested into the NCAR Zebra visualization software, which allows the user to view and analyze the horizontal structure of the radar parameters, overlay the radar data with observations of other instruments, and create user-defined vertical cross-sections. The dropsonde data will also be ingested into Zebra.

Using Zebra, we will examine the rainbands to determine their convective/stratiform structure, and how this structure was related to the mesoscale dynamics of the rain band. In particular, we will determine how the mesoscale vorticity field corresponded to the precipitation structure. The three-dimensional distribution of vorticity within the rainband will be examined to determine if the vorticity concentrates at midlevels in the stratiform regions of rainbands and on the inner sides of rainbands with secondary wind maxima. The kinematic structures seen in the Doppler radar analysis of the rainbands will be further compared with high-resolution wind fields from three-dimensional MM5 model simulations of RAINEX hurricanes carried out at the University of Miami. The model output will be ingested into Zebra to facilitate these comparisons. The model output will provide details of rainband evolution not captured in the aircraft data snapshots of storm structure. The overall synthesis of Doppler wind fields, dropsondes, and model representations of the rainbands accomplished by this analysis provides a comprehensive analysis of the precipitation structure, dynamics, and thermodynamics of the rainbands in Katrina, Ophelia, and Rita.

REFERENCES

Atlas, D., K. R. Hardy, R. Wexler, and R. J. Boucher, 1963: On the origin of hurricane spiral bands. Geofis. Int., 3, 123-132

Samsury, C. E., and E. J. Zipser, 1995: Secondary wind maxima in hurricanes: Airflow and relationship to rainbands. Mon. Wea. Rev., 123, 3502-3517

Willoughby, H. E., F. D. Marks, Jr., and R. J. Feinberg, 1984: Stationary and moving convective bands in hurricanes. J. Atmos. Sci., 41, 3189-3211