Numerical Experiments of the Diurnal Cycle of Axisymmetric Tropical Cyclones

Abstract:

Recent observational and modeling studies have shown that the diurnal cycle of radiation may be fundamentally linked to structural changes in the lifetime of a tropical cyclone. While these studies suggest that an underlying mechanism within the storm may exist, the dynamics for this response are still largely unexplained. Previous modeling studies were limited due to model configuration (e.g., initial and boundary conditions) as well as to radiative parameterization schemes. In this presentation, two new investigations are discussed to reexamine the role of the daily cycle of radiation on axisymmetric hurricane structure.

In the first study, a tropical cyclone lasting 324 days is generated in Cloud Model 1 (CM1, see Bryan and Rotunno 2009) to quantify a tropical cyclone diurnal signal. A coherent response is observed in the temperature, wind, and cloud ice fields that accounts for up to a third of the overall variance. Composite analysis of each hour of the day shows a diurnal cycle in the storm intensity that, relative to the mean, intensifies in the early hours of the morning and is consistent with observational studies. Examination of the radial and vertical wind suggests two distinct circulations forced by the diurnal cycle: (1) a radiatively-driven circulation in the outflow layer due to absorption of solar radiation, and (2) a convectively-driven circulation within the storm due to latent heating. These responses are coupled and are periodic with respect to the diurnal cycle.

In the second study, following the method of Pendergrass and Willoughby (2009) and Willoughby (2009), hypothesis tests using various prescribed, periodic heating distributions are performed to examine the dynamical response of the storm to radiation. Results reveal significant changes to the secondary-circulation structure of the storm, as well as to the intensification of the primary vortex. Sensitivity to the chosen heating distribution as well as to the initial vortex are discussed, and a possible explanation for the diurnal response in axisymmetric tropical cyclones is introduced.