428 The Role of the WISHE Mechanism in Secondary Eyewall Formation

Tuesday, 24 January 2017
4E (Washington State Convention Center )
Chieh-Jen Cheng, National Taiwan University, Taipei, Taiwan; and C. C. Wu

     Concentric eyewalls (CEs) and the eyewall replacement cycle (ERC) in tropical cyclones (TCs) have been widely documented by both observational and numerical studies. The accompanied change of TC characteristics associated with CE and ERC is of scientific importance, and is an interesting issue for TC intensity prediction.   In this study, high-resolution numerical simulations (based on WRF model) are conducted to examine the role of the WISHE (wind-induced surface heat exchange) mechanism in SEF.  Analyses of the control experiment (CTL) show that about 12 hours prior to SEF, a region of increasing diabatic heating due to active convections is established around and outside the SEF region, leading to an increase of inertial stability and an expansion of tangential winds, while the former enhances the heating efficiency, and the later induces a pathway to SEF in the boundary layer.
     To examine the sensitivity of SEF to surface flux (i.e., the WISHE mechanism) in both storm core and outer rainband regions, in the sensitivity experiments the surface wind is capped at several designated values in the calculation of total surface heat flux.  The values of the capped surface winds are set at 15, 10, 5, and 1 m s-1. In addition, the selected regions of suppressed heat flux are OSC (R>50km), OBC (R>80km, InC (R<50km), and RiR (55km<R<90km), respectively.  Analyses show that the WISHE mechanism can both quantitatively and qualitatively affect the evolution and characteristics of the vortex, especially the outer eyewall.  When the heat fluxes are suppressed around or outside the SEF region, the onset timing of the outer eyewall is delayed and the intensity of the eyewall is weaker (OSC-10, OSC-15, OBC-10, OBC-15), and in some experiments (OSC-01, OSC-05, OBC-01, and OBC-05) no SEF occurs. Features of progressively strengthening supergradient forces and the accompanied strengthening convection in the SEF region are found in the cases with SEF.  While for InC cases, the WISHE mechanism around the inner region is not crucial for the SEF, but the experiment design can result in a short-lived inner eyewall and affect the ERC.  Moreover, the relative intensity between the inner and outer eyewalls can also affect the onset of the SEF and the intensifying rate of the outer eyewall. These results show that the WISHE mechanism plays a crucial role in both SEF and ERC.
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