Diurnal Modulation of Tropical Cyclone Intensity

Diurnal variability of tropical cyclone intensification rate and intensity has been a subject of interest for over 3 decades. Observational evidence for such variability has been inconsistent, but suggests a nocturnal maximum often exists. In this study, a series of simple axi-symmetric and idealized 3D numerical simulations are conducted to understand how diurnal forcing affects tropical cyclones. In each case, the immediate impact of the diurnal cycle is the modulation of the net radiative loss of energy to space at cloud-top. This occurs in the upper troposphere and lower stratosphere, primarily within the anticyclonic cirrus outflow. The first order impact is to destabilize the upper troposphere during the night compared to the daytime. The second order impacts on the storm can vary, depending on the environmental thermal structure, the manner in which the storm is interacting with the environment and the stage of the storm's lifecycle. Some second order impacts found were::

(1) Enhancement of vertical convection outside the storm center due to lower stability (2) Enhancement of slantwise convection or overturning near storm center (3) Vertical trapping of gravity waves below the anvil top, leading to the spreading of convective activity away from the convective core possibly leading to triggering of eye-wall replacement cycle (4) Increase (decrease) in the efficiency of the Carnot engine during the night (day) and so increase (decrease) in storm intensity (5) Decrease (increase) in potential vorticity of the outflow during the night (day) leading to a lower (higher) inertial stability tax on storm circulation

Simulation results suggest that the impact of the diurnal cycle is most important during the intensification stages of the storm when vertical convective activity is most important to the storm dynamics.