Diurnal Variability of an Axisymmetric Hurricane in Statistical Equilibrium

New evidence from recent observational and modeling studies suggests that the diurnal cycle of radiation may be fundamentally linked to structural changes in the lifetime of a tropical cyclone. Although a diurnal response in high cloudiness has been well documented in the past, its effect on tropical cyclone intensity and variability remains unknown and the dynamical response within the storm is still largely unexplained. Previous modeling studies attribute part of these inconclusive findings to experimental setup (e.g., initial and boundary conditions) as well as to radiative parameterizations in the model. A new investigation in an idealized numerical model equipped with interactive radiation is performed to reevaluate the role of the daily cycle of radiation on axisymmetric hurricane structure.

A tropical cyclone lasting 500 days is generated in George Bryan's Cloud Model 1 (CM1, see Bryan and Rotunno 2009), which, after a transient spin-up period of 30 days, reaches radiative-convective equilibrium with its surroundings. This simple framework excludes the effects of environmental influences, such as wind shear and varying sea surface temperatures, and thus allows for the isolation of fundamental characteristics of the storm itself. Composites of the radial structure at 6 hour increments shows an increase in convective activity in the eyewall region just after sunset, which then increases in width and intensity overnight. In the early morning this activity begins propagating outward and eventually reaches larger radii by mid-afternoon. This pattern is consistent with recent observations of the diurnal cycle in real storms.

Comparison to a similar solution excluding interactive solar radiation shows that the diurnal cycle reduces overall storm intensity by 25%. Additionally, variability about the radius of maximum wind is observed to occur in conjunction with strong winds originating outside of the eyewall in the exterior environment (i.e., greater than 100km from storm center) on a larger, non-periodic timescale. These anomalous winds are linked with strong environmental convection and in most cases, eyewall replacement cycles. These cycles are more frequent in, though are not unique to, the diurnal solution. Vertical profiles of heating at various radii from the storm center are considered for both the diurnal and longwave-only solutions, and both solutions are compared to analyses from an additional case using idealized, sinusoidal longwave cooling.