Symmetric Instability in the Outflow Layer of a Major Hurricane

Abstract

A set of 327 dropsondes from the NOAA G-IV aircraft was used to create a composite analysis of the azimuthally averaged absolute angular momentum in the outflow layer of major Hurricane Ivan (2004). Inertial instability existed over a narrow layer in the upper troposphere between the 350-and 450-km radii. Isolines of potential and equivalent potential temperature showed that the conditions for both dry and moist symmetric instability were satisfied in the same region, but over a deeper layer from 9 to 12 km. The radial flow maximized at the outer edge of the unstable region. The symmetrically unstable state existed above a region of outward decrease of temperature between the cirrus overcast of the storm and clear air outside. It is hypothesized that the temperature gradient was created as a result of longwave heating within the cirrus overcast and longwave cooling outside the cloudy region. This produced isentropes that sloped upward with radius in the same region that absolute momentum surfaces were flat or sloping downward, thus creating symmetric instability. Although this instability typically follows rather than precedes intensification, limited numerical evidence suggests that the reestablishment of a symmetrically neutral state might influence the length of the intensification period.

Monthly variations of tropical cyclone rapid intensification ratio in the western North Pacific

“…Moreover, TC RI is one of the most difficulties in tropical weather forecasting. This is primarily ascribable to the inadequate understanding of internal dynamics and the interaction with the large‐scale environment (Bosart, Velden, Bracken, Molinari, & Black, ; Ge, ; Ge, Guan, & Zhou, ; Molinari & Vollaro, ; Reasor, Rogers, & Lorsolo, ; Shay, Goni, & Black, ; Wang & Wu, ; Wang & Zhou, ). During the RI period, TCs are affected by multi‐scale processes that govern the inner‐core structure and interactions between the storm and environmental conditions.…”

“…During the RI period, TCs are affected by multi‐scale processes that govern the inner‐core structure and interactions between the storm and environmental conditions. For instance, the processes include TC interaction with subsurface oceanic thermal conditions (Mei, Xie, Primeau, McWilliams, & Pasquero, ; Shay et al, ; Wang, Wang, Weisberg, & Black, ; Wang, Wang, Zhang, & Wang, ), responses to vertical wind shears (VWSs; Ge, Li, & Peng, ; Reasor et al, ), concentric eyewall replacement cycles (Willoughby, Clos, & Shoreibah, ; Kossin & Sitkowski, ; Ge et al, ), modifications in upper‐level outflow (Molinari & Vollaro, ), global warming (Sun et al, ), and many others.…”

Evolution of the upper tropospheric outflow in Hurricanes Iselle and Julio (2014) in the Navy Global Environmental Model (NAVGEM) analyses and in satellite and dropsonde observations

“…Azimuthal u θ wind components were used to calculate inertial stability. This study only focused on outflow that was radially inward of 6° from the storm center, most directly following the choice of 600 km by MV96 and Molinari and Vollaro []. That limit was set to isolate outflow that was connected to processes in the TC inner core.…”

“…Here we seek to build on observational outflow studies such as Molinari et al . [] and Molinari and Vollaro [], theoretical outflow studies such as Emanuel and Rotunno [] and Emanuel [], and modeling outflow studies such as Wang et al . [] by using observations coupled with 6 h high‐resolution gridded analyses from the Navy Global Environmental Model (NAVGEM) [ Hogan et al ., ] to describe the evolution of outflow during the life cycles of two hurricanes.…”

Repeated Eyewall Replacement Cycles in Hurricane Frances (2004)

“…An alternative set of mechanisms for ERC initiation have been proposed that differ dramatically from the above papers: those excited by external forcing. Molinari and Vollaro (1989;p. 1104) argued that interaction with an upper-level midlatitude trough produced midlevel spinup outside the core driven by lateral eddy flux convergence of angular momentum.…”