92nd American Meteorological Society Annual Meeting (January 22-26, 2012)

Tuesday, 24 January 2012: 4:30 PM
The Vertical Structure of Precipitation in Tropical Cyclones As Seen by the TRMM PR
Room 256 (New Orleans Convention Center )
Deanna A. Hence, Univ. of Washington, Seattle, WA; and R. A. Houze Jr.

Ten years of three-dimensional Tropical Rainfall Measurement Mission (TRMM) Precipitation Radar (PR) echoes reveal the typical vertical structure of precipitation features seen in tropical cyclones. Statistical analyses based on annular regions spaced evenly from the center of the cyclone indicate different vertical precipitation structures for the eyewall, the organized rainbands closest to the storm center, and the disorganized rainbands further afield. When a concentric eyewall forms outside of an existing eyewall, the vertical structure of the outer eyewall is a hybrid of typical eyewall and inner rainband structure.

The eyewall contains high reflectivities and high echo-tops, with deeper and more intense but highly intermittent echo perturbations superimposed on the basic structure. The inner rainband echoes are intense but less deep and highly uniform at all levels, mostly containing stratiform precipitation with a limited amount of vertically-constrained convection, In tropical cyclones with concentric eyewalls, the upper-troposphere portions of the outer eyewalls are weak and uniform like the inner rainbands, but the lower-tropospheric portions are more intense and uniform than rainbands of single eyewall storms. The distant rainbands are weaker, sparse, highly convective and less vertically constrained.

The PR data are normalized by the amount of radar echo in each sample and examined quadrant-by-quadrant relative to the direction of the environmental shear. The changes in these normalized statistics indicate that the eyewall convection generates in the downshear-right quadrant, matures in the downshear-left, and dissipates in the upshear-left. In the rainbands, the convection initiates in the upshear-right quadrant of the distant rainbands, matures in the downshear right quadrant as it travels inwards, and dissipates in the downshear-left quadrant of the inner rainbands. The non-normalized statistics indicate that the associated rainfall asymmetry places most of the rainfall in the downshear-left quadrant of the eyewall and rainbands, both from precipitation particles travelling downwind along the eyewall and rainbands as well as particles travelling outward from the inner regions of the storm. These results indicate that environmental wind shear is mostly responsible for the changes in convective structure around tropical cyclone eyewalls and rainbands as well as the placement of rainfall around the storm. In storms with concentric eyewalls, the inner eyewall is more strongly affected by shear than are the eyewalls of single eyewall storms, while the outer eyewalls are relatively unaffected by shear, suggesting that the outer eyewall is amplifying the shear-induced asymmetry of the inner eyewall.

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