Analysis of an Ensemble of High-Resolution WRF Simulations for the Rapid Intensification of Super-Typhoon Rammasun (2014) (#1062)
Diagnostics are presented from an ensemble of high resolution forecasts that differed markedly in their predictions of the rapid intensification (RI) of typhoon Rammasun. We show that the basic difference stems from subtle differences in initializations of (a) 500–850 hPa environmental winds, and (b) mid-level moisture and ventilation. We then describe how these differences impact on the evolving storm structure, convective organization and the timing of RI.
The evolution of vortex cloudiness from the member that best forecasts the RI is similar to the actual imagery, with the development of an inner cloud band wrapping inwards to form the eyewall. We illustrate that this structure is related to the tilt and associated dynamics of the developing inner-core in shear. For the most accurate ensemble member : (a) initially in easterly shear, inhibition of ascent and a reduction in convection over the up-shear sector allows moistening of the boundary layer air, which is transported to the down-shear sector to feed a developing convective asymmetry; (b) minimal ventilation provides favorable conditions for development of the convective asymmetry; (c) undiluted clouds and moisture from the down-shear left quadrant are then wrapped inwards to the up-shear left quadrant to form the eyewall cloud; (d) this process seems related to a critical down-shear tilt of the vortex structure from mid-levels, and the vertical synchronization of low- and mid-level circulations over up-shear quadrants; and (e) the eyewall cloud continues to develop episodically via these processes. For the member that forecasts a much-delayed RI, these processes are inhibited by stronger shear, lack of vertical coherence of the circulation, lesser moisture and larger ventilation.
Analysis suggests that ensemble prediction for RI is needed to account for the sensitivity to a relatively narrow range of environmental wind shear, moisture and vortex inner-structure.