A Study on the Asymmetric Rapid intensification of Hurricane Earl (2010) Using the HWRF system

In this study, the result from a forecast of Hurricane Earl (2010) from the operational Hurricane Weather Research and Forecasting (HWRF) system is verified against available observations and, further, analyzed to understand the asymmetric rapid intensification of a storm in a sheared environment. The verification shows that the HWRF forecast compare favorably in many aspects to the best-track analysis, as well as P-3s radar observations. In particular, it reproduces the convection asymmetry, the wind field asymmetry and the tilt in terms of both magnitude and direction in pre rapid intensification (pre-RI) stage and RI stage. The tilt is large in pre-RI stage and small in RI stage. A further examination of the high-resolution HWRF forecast reveals that the tilt is still large at the RI onset and decreases rapidly once RI commences, suggesting that vertical alignment is the result instead of the trigger of RI. The RI onset is associated with the development of upper-level warming in the eye center, which results from upper-level storm-relative flow advecting the subsidence warming in the upshear-left region towards the low-level storm center. This scenario does not occur until some persistent convective bursts (CBs) are concentrated in the downshear-left quadrant. The budget calculation also indicates that the horizontal advection plays an important role in the development of upper-level warming at the early RI stage. The upper level warming associated with the asymmetric intensification process occurs by means of the cooperative interaction of the convective-scale subsidence, resulting from favored region for CBs, and the shear-induced meso-scale subsidence. When CBs are concentrated in the downshear-left and upshear-left quadrants, the subsidence warming could be maximized in the upshear and then advected towards the low-level storm center by the storm-relative flow at upper level. Subsequently, the surface pressure would fall and RI may occur.