Impact of the Diurnal Radiation Contrast on the Formation, Intensification, and Structure of Hurricane Edouard (2014)

This work examines the impacts of the diurnal radiation contrast on the formation, intensification, and structure of Hurricane Edouard (2014) at different stages of its life cycle through convection-permitting simulations.

During the formation stage, nighttime destabilization through radiative cooling may promote deep moist convection that eventually leads to the genesis of the storm, while a tropical cyclone fails to develop in the absence of the night phase despite a strong incipient vortex under moderately strong vertical wind shear. The nighttime radiative cooling further enhances the primary vortex before the storm undergoes rapid intensification.

Rapid contraction of the radius of maximum wind (RMW) occurs both in the low- and mid-levels for the control run and the sensitivity run without solar insolation, while the tropical cyclone contracts more slowly in the low-levels and later in the mid-levels and thereafter fails to intensify continuously in the absence of the night phase, under weak vertical wind shear (~4 m s^-1). The clouds at the top of the boundary layer absorbs solar shortwave heating during the daytime, which enhanced the temperature inversion there and increased the convective inhibition, while nighttime destabilization and moistening in low-levels through radiative cooling decrease convective inhibition and favor more convection inside the RMW than in the daytime phase. The budget analysis of the tangential wind tendency reveals that the greater positive radial vorticity flux inside of the RMW is the key RMW contraction mechanism in the boundary level at night, due to the enhanced convection. However, the greater positive vertical advection of tangential wind inside of the RMW dominates the RMW contraction in the mid-levels. Thereafter, the nighttime radiative cooling mainly increases convective activities outside of the primary eyewall that lead to stronger/broader rainbands and larger storm size during the mature stage of the hurricane; there is, however, less impact on the hurricane’s peak intensity in terms of maximum 10-m surface wind speed.

The control forecast undergoes distinct secondary eyewall formation during the mature stage of Edouard (as observed), while there is no apparent eyewall replacement cycle as simulated in sensitivity experiments without solar insolation and the moat is narrower in those with switch-on solar insolation at night, suggesting the potential role of the diurnally varying radiative impact. In the control run, there is an area of relatively weak convection between the outer rainbands and the primary eyewall, that is, a moat region. This area is highly sensitive to solar shortwave radiative heating, mostly in the mid- to upper levels in the daytime, which leads to a net stabilization effect and suppresses convective development. Moreover, the heated surface air weakens the wind-induced surface heat exchange (WISHE) feedback between the surface fluxes (that promote convection) and convective heating (that feeds into the secondary circulation and then the tangential wind). Consequently, a typical SEF with a clear moat follows. In the sensitivity experiment without solar insolation, in contrast, net radiative cooling leads to persistent active inner rainbands between the primary eyewall and outer rainbands, and these, along with the absence of the rapid filamentation zone, are detrimental to moat formation and thus to SEF. Sawyer–Eliassen diagnoses further suggest that the radiation-induced difference in diabatic heating is more important than the vortex wind structure for moat formation and SEF. These results suggest that the SEF is also highly sensitive to solar insolation.