Idealized Modeling of the Diurnal Cycle of Tropical Cyclones

The diurnal cycle of tropical cyclones (TCs) is a fundamental process whose impact on storm structure and intensity is not well understood. While previous work has concerned observational and modeling studies describing the TC diurnal cycle, open questions include the dynamical mechanism as well as the sensitivity of the diurnal response in individual storms. For real TCs, the diurnal cycle exhibits large variance, with certain storms showing a clear daily cycle and others showing none. Here we explore mechanisms for the sensitivity of storms to the diurnal cycle using an idealized modeling framework.

The relationship between the TC diurnal cycle and storm structure is tested first by examining the perturbation streamfunction response to time-varying heating on a diurnal timescale. Two heat sources are considered, based on previous idealized modeling: an upper-level feature located in the TC outflow, and a boundary-layer feature located near the radius of maximum wind. Sensitivity to the location, shape, and strength of the heat sources are also tested. For a fixed amplitude in the heat sources, results show that weak initial storms exhibit weaker perturbation responses, while stronger storms exhibit stronger perturbation responses. Magnitudes of perturbation tangential wind in the boundary layer for strong initial storms approach 7 m/s near the radius of maximum wind. Distance between the center of heating and the location of the radius of maximum wind is also important, with shorter distances leading to stronger perturbation tangential wind responses in the low levels. Periodic heating in smaller storms (as measured by the outer decay length of the wind profile) projects strongly onto outward propagating inertia--gravity waves, with energy radiating away from the storm.

These results are compared to a long, statistically steady simulation produced in Cloud Model 1 (CM1), which exhibits a spatially coherent TC diurnal cycle with large variance. This simulation is produced without influence from the exterior storm environment, which provides a framework to address the relationship between the diurnal cycle of radiation and the internal variability of the storm. Sensitivity of the diurnal signal to storm strength, size, and location of anomalous heating are presented, and hourly composite anomalies for the strongest, weakest, and middle 10% of diurnal responses are discussed.