How do outer spiral rainbands
affect tropical cyclone structure and intensity?

Yuqing Wang, IPRC


A long-standing issue on how outer spiral rainbands affect the structure and intensity of tropical cyclones is studied through a series of numerical experiments using the cloud-revolving tropical cyclone model TCM4. Since diabatic heating due to phase changes is the main driving force of outer spiral rainbands, their effect on the tropical cyclone structure and intensity is evaluated by artificially modifying the heating/cooling rate from cloud microphysical processes in the model. The effect of diabatic heating due to phase changes in outer spiral rainbands on the storm structure and intensity is understood based mainly on the hydrostatic adjustment. Namely, heating (cooling) in an atmospheric column would result in a decreasing (increasing) tendency in surface pressure underneath. This, in turn, would affect the horizontal pressure gradient and thus winds across the radius of maximum wind in the lower troposphere. The results show that cooling in outer spiral rainbands is important to both the intense tropical cyclone and the maintenance of a relatively compact inner core structure, while heating in outer spiral rainbands is negative to the storm intensity but favors the increase in storm size. Overall, the presence of active outer spiral rainbands limits the intensity of tropical cyclones. Since heating and cooling outside the inner core in outer spiral rainbands depends strongly on the relative humidity in the near core environment, implications of the results to the formation of the annular hurricane structure, the concentric eyewall cycle, and the size change of tropical cyclones are discussed.