Vertical distribution of radar reflectivity in intense Atlantic tropical cyclone eyewalls observed by the TRMM Precipitation Radar

Previous studies have examined how environmental factors and changes in intensity affect the horizontal rainfall distribution in tropical cyclones, but there has been limited work on how these factors change the vertical distribution of cloud and precipitation processes. Yet, the vertical distribution of these processes in a tropical cyclone eyewall relates directly to the internal dynamics of the storm, and high-resolution models must eventually represent the vertical distribution of cloud and precipitation processes accurately.

This study assesses how environmental wind shear, sea surface temperature (SST), the storms motion, and the storms intensity change the vertical distribution of radar reflectivity within the eyewalls of tropical cyclones. High-resolution reflectivity data from overpasses of the Tropical Rainfall Measurement Mission satellite for the 1998-2007 hurricane seasons in the Atlantic, Gulf of Mexico and Caribbean basins are sorted into categories based on: the National Hurricane Centers best track data for storm location, motion, eye diameter, and intensity; National Oceanic and Atmospheric Administration (NOAA) Comprehensive Large Array-data Stewardship System (CLASS) for SST data; and the National Centers for Environmental Prediction (NCEP) Reanalysis for the environmental wind data used to calculate vertical wind shear. We focus on intense storms that ultimately reach Category 4 or 5 (Saffir 2003) in their lifetime, but utilize all of the overpasses collected while the storms are considered hurricane strength. The reflectivity data are further divided into quadrants calculated around the track and shear vectors and annuli calculated from the eye radius. The first annulus, beginning at the eye radius, encompasses the eyewall region.

The annulus 1 total reflectivity distribution is distinctly marked by a large concentration of high reflectivity values (> 35 dBZ) in low levels, echo reaching high altitudes (> 10 km), and much more uniform distribution than typically seen in convective frequency distribution profiles. Intense outliers of the distribution typically reach close to 50 dBZ in low levels and heights of 12-14 km, suggesting the presence of intense convective towers superimposed upon the general structure. This general uniformity mixed with intense outliers marks the annulus 1 distribution as unique in comparison to ordinary non-tropical-cyclone convection. When separated into quadrants relative to the shear vector, the overall intensity and height of the echoes maximize in the downshear-left quadrant of the storm, with the downshear-right quadrant similar in height but less in low-level intensity. The upshear-left quadrant is less in height and intensity than either of the downshear quadrants, and the upshear-right quadrant has the weakest and shortest distribution of the four. Both upshear quadrants have prominent brightbands in their distributions. These are in line with previous results that vertical wind shear creates a rainfall asymmetry with a maximum of precipitation in the downshear left quadrant. This asymmetry, as suggested by Black et al. (2002), may result from updrafts forming in the downshear region of the storm, creating a precipitation asymmetry as they and their associated precipitation particles travel around the eyewall, weakening in the rear quadrants before dissipating.

The asymmetry associated with vertical windshear, with similar quadrant-to-quadrant variations in the vertical distribution of radar reflectivity, appears to some degree in all of the categories defined for this study. However, in all quadrants, category 1 or 2 storms have eyewalls that are generally shorter and weaker in intensity at all levels than the total distribution, while category 4-5 storms have taller and more intense eyewalls. SST mostly affects the outlier distribution: eyewalls in colder water have almost completely suppressed occurrence of very tall and intense outliers, whereas the eyewalls in warmer waters tend to have an abundance of them. The asymmetry resulting from the shear only changes in low shear and high SST categories, where the track speed is near or greater in magnitude than the shear vector and nearly opposite in direction. These distributions are more intense in the downshear quadrants than the upshear quadrants, but have less of a right-left asymmetry. These results suggest that although the shear is the dominant control of the vertical distribution of precipitation processes, the track motion can work to shift the asymmetry of these processes relative to the storm center depending on its strength and direction relative to the shear. Storm intensity and SST are the dominant controls of precipitation intensity but do not affect the spatial distribution. This analysis will be continued for all annuli of the storm lying outside the eyewall. This further analysis will gauge how the factors examined here affect the vertical distribution of tropical cyclone precipitation processes as a function of radius and azimuth outside the eyewall out to the farthest radii at which cyclone structure is evident. Future work will also expand the database of this study to include intense tropical cyclones from all of the world basins.

Black, M. L., J. F. Gamache, F. D. Marks, Jr., C. E. Samsury and H. E. Willoughby, 2002: Eastern Pacific Hurricanes Jimena of 1991 and Olivia of 1994: The effect of vertical shear on structure and intensity. Mon. Wea. Rev., 130, 2291-2312.

Saffir, H.S. (2003), Communicating damage potentials and minimizing hurricane damage, Hurricane! Coping With Disaster, edited by R. Simpson, pp. 155 164, AGU, Washington, D.C.