3.5 The Influence of Cloud-Radiation Feedback on Tropical Cyclone Intensity and Structure

Tuesday, 6 August 2013: 2:45 PM
Multnomah (DoubleTree by Hilton Portland)
Yizhe Peggy Bu, Univ. of California, Los Angeles, Los Angeles, CA; and R. G. Fovell

Our past work has demonstrated that cloud microphysical processes can materially influence tropical cyclone (TC) structure, motion and intensity. A large part of the sensitivity emerges owing to how condensed water particles interact with longwave and shortwave radiation. Microphysics schemes tend to vary with respect to the number and size of particles they generate, and different hydrometeor species interact with radiation in different ways.

We have been using two versions of the Weather Research and Forecasting (WRF) model, WRF-ARW and the Hurricane WRF (HWRF), to assess the influence of cloud-radiation feedback (CRF) on the structure, motion and intensity of idealized and real-world TCs. Despite relatively small magnitudes of its diabatic forcing (< 1 K/hr), CRF can profoundly alter the TC structure and intensity by encouraging radial outflow in the upper troposphere, leading to more extensive anvils. This is a positive feedback as the radial outflow transports the hydrometeors that effectively carry the radiative forcing.

The influence of CRF-enhanced anvils on storm structure is clear: hydrometeor advection causes progressive moistening of the TC's outer core, encouraging greater convective activity. The impact of this enhanced activity on storm intensity and inner core structure is more complex. On the one hand, outer core activity appears to complete with the inner core convection, leading to TC weakening. On the other hand, TCs without CRF tend to have stronger downdrafts immediately outward from the eyewall, which can ventilate low entropy air into the updraft. The net result appears to depend on which effect comes to dominate.

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