A Diurnal Cycle in the Energetics and Efficiencies of Tropical Cyclones

Abstract The oceanic tropical cyclone (TC) diurnal cycle includes oscillations in the total rainfall and boundary layer inflow. A diurnal cycle may also occur in the TC energetics, including the wind field kinetic energy. The TC mechanical efficiency, a property of the TC bounded by the Carnot efficiency, depends on these energetics and so could experience a diurnal cycle. We use output from a hurricane nature run to investigate whether there is a diurnal cycle in the mechanical and Carnot efficiencies and energetics of TCs. Results from the nature run show some statistically significant indications of diurnal variability in the efficiencies and TC wind field kinetic energy but a lack of statistical significance for other energetics. We compare our results from the highly realistic simulation to two longer-lived, idealized simulated TCs using the Cloud Model 1, where the diurnal cycle can be isolated from environmental shear and forward motion. The first simulation is forced by the solar diurnal cycle, whereas the second is a control under perpetual-afternoon conditions. Results from the first simulation show statistically significant diurnal variability in several TC energetics and in the mechanical and Carnot efficiencies. Comparison to the control confirms that for most variables, this diurnal oscillation is not an artifact of the experimental setup. Finally, a comparison of the daily mean TC energetics and efficiencies between the two idealized simulations yields no statistically significant differences. This may indicate a lack of rectification of these variables by the diurnal cycle, although further ensemble studies should confirm this. Significance Statement The main purpose of this work is to examine the diurnal variability of the “mechanical efficiency” of tropical cyclones, which measures how well tropical cyclones convert the heating they experience into the strength of their winds. This matters because tropical cyclones are known to experience significant variability over the course of a day, which has important implications for tropical cyclone forecasting. However, the diurnal variability in the mechanical efficiency and related energetics has not received much attention. Our study could improve the forecasting of tropical cyclones by documenting another aspect of their 24-h variability. We find diurnal cycles in both the mechanical efficiency of tropical cyclones and in several related tropical cyclone energetics, such as the energy of the winds.