Impacts of Diurnal Radiation Cycle on Secondary Eyewall Formation

This work examines the sensitivity of the secondary eyewall formation (SEF) of Hurricane Edouard (2014) to diurnal solar insolation cycle through convection-permitting simulations. A control run with real diurnal radiation cycle and a sensitivity experiment without solar insolation are conducted. For the former, there exists an area with relative weak convection between the outer rainbands and the primary eyewall. This area is highly sensitive to the solar shortwave radiative heating. The radiative heating mostly in the mid- to upper-level at daytime leads to a net stabilization effect, which suppresses convective development, causing a negative feedback between convective heating (that feeds to the secondary circulation and then the wind) and the surface fluxes (that promotes convection). Warming induced by subsidence in the area also helps to sharpen the radial gradient of low-level equivalent potential temperature into a front-like zone, resulting in active convective bursts on its inner edge. A positive feedback between front-like zone and active convection there results in organization of the outer rainbands and eventually a typical SEF with a clear moat region. On the contrary, the active inner rainbands due to net radiative cooling in the sensitivity experiment occurs persistently between the eyewall and outer rainbands, so there is no space for moat formation and no SEF consequently. A Sawyer-Eliassen diagnosis further suggests that the diabatic heating forcing is more important for moat formation and SEF than the wind structure of background vortex. These results suggest that the SEF might be highly sensitive to solar insolation.