Dynamics and Predictability of the Intensification of Hurricane Edouard (2014)

Abstract

The dynamics and predictability of the intensification of Hurricane Edouard (2014) are explored through a 60-member convection-permitting ensemble initialized with an ensemble Kalman filter that assimilates dropsondes collected during NASA’s Hurricane and Severe Storm Sentinel (HS3) investigation. The 126-h forecasts are initialized when Edouard was designated as a tropical depression and include Edouard’s near–rapid intensification (RI) from a tropical storm to a strong category-2 hurricane. Although the deterministic forecast was very successful and many members correctly forecasted Edouard’s intensification, there was significant spread in the timing of intensification among the members of the ensemble. Utilizing composite groups created according to the near-RI-onset times of the members, it is shown that, for increasing magnitudes of deep-layer shear, RI onset is increasingly delayed; intensification will not occur once a critical shear threshold is exceeded. Although the timing of intensification varies by as much as 48 h, a decrease in shear is observed across the intensifying composite groups ~6–12 h prior to RI. This decrease in shear is accompanied by a reduction in vortex tilt, as the precession and subsequent alignment process begins ~24–48 h prior to RI. Sensitivity experiments reveal that some of the variation in RI timing can be attributed to differences in initial intensity, as the earliest-developing members have the strongest initial vortices regardless of their environment. Significant sensitivity and limited predictability exists for members with weaker initial vortices and/or that are embedded in less conducive environments, under which the randomness of moist convective processes and minute initial differences distant from the surface center can produce divergent forecasts.

Dynamics and Predictability of the Rapid Intensification of Super Typhoon Usagi (2013)

“…As shown in Figure a, some members, with both early and late intensification, had quite large initial tilt magnitudes due to the moderate‐weak environmental shear. The entire ensemble exhibited rapidly decreasing tilt magnitudes after the model initialization, being smaller than 40 km at the RI onset (Figure a), which is consistent with the findings in Munsell et al (). It should be noted that RI did not always occur immediately after the vortex tilt magnitude decreased significantly (Figure c).…”

“…Previous studies show that a stronger initial vortex is more likely to intensify quickly, regardless certain unfavorable environmental conditions (Emanuel & Zhang, ; Munsell et al, , ; Torn & Cook, ; Zhang & Emanuel, ). This was also the case in the ensemble simulations of Usagi (Figure b).…”

“…During the precession stage, the TC tilt vector rotates cyclonically and gradually decreases. RI typically begins right after alignment, that is, when the magnitude of the tilt vector is small (Miyamoto & Nolan, ; Munsell et al, ; Rogers et al, ). The ensemble forecasts started under moderate‐weak shear conditions, with an ensemble mean deep layer shear of 4 m s −1 (Figure c).…”

“…Idealized simulations in a no‐shear environment suggest that storms with stronger vortices undergo RI earlier (Miyamoto & Takemi, ). With respect to real‐world cases, Munsell et al () found that a strong initial vortex can undergo RI regardless of unfavorable environmental conditions, such as when there is a strong vertical wind shear. Recent studies demonstrate that forecasting errors of TC intensity (including RI) are dominated by errors of the initial vortex intensity and inner‐core moisture rather than environmental factors for the first 3–5 forecasting days (Emanuel & Zhang, , ).…”

Sensitivity of Hail Precipitation to Ensembles of Uncertainties of Representative Initial Environmental Conditions From ECMWF

“…In recently published papers, the predictability and various causes of the loss of predictability of other hazards related to severe storms, including tornadoes, heavy rainfall, severe convective winds, and tropical cyclones, have been quantified using ensemble simulations (Durran & Weyn, ; Emanuel & Zhang, ; Fernández‐González et al, ; Flora et al, ; Miglietta et al, , ; Munsell et al, ; Nielsen & Schumacher, ; Weyn & Durran, , ; Zhang et al, , ; Zhang & Tao, ). For example, the practical and intrinsic predictability of a tornadic supercell have been explored from the perspective of convection initiation time, which was strongly modulated by the evolution of the planetary boundary layer, and local topography (Zhang et al, , ).…”