A Possible Mechanism for the Diurnal Oscillations of Tropical Cyclones

Abstract

A possible mechanism for the diurnal oscillations of tropical cyclones is presented. In the conceptual model developed to explain these features, the diurnal cycle of net radiation at the cloud tops is identified as the primary cause of the oscillations. Radiative cooling of the cloud tops at night steepens the lapse rate and increases convection. This generates a slight intensification in the storm. The reverse occurs during the daytime as the cloud tops absorb solar radiation. This process may be augmented by differential cooling of cloudy and clear areas. This conceptual model is tested through the use of a numerical model. The basic model reproduces well the development of a strong hurricane from a weak tropical depression. The model storm exhibits strong, cyclonic low-level inflow and weaker anticyclonic upper-level outflow. In addition, spiral rainbands and an eye are observed during the simulation. When the fluxes of longwave and shortwave radiation are added into the model, a definite diurnal fluctuation of intensity is evident during the early stages of the simulation. These fluctuations vary in the manner suggested by the conceptual model. This is confirmed by the oscillation of the latent heating, which peaks at night and diminishes during the day. As the storm intensifies, the fluctuations become less evident. This is to be expected, since the radiative fluxes comprise a smaller portion of the total energy budget during the later stages of the simulation.