Vacillation Cycles in WRF Simulations of Hurricane Katrina

Abstract

The intensification of hurricane Katrina (2005) in non-hydrostatic simulations with the WRF model is compared to a hydrostatic simulation conducted in a previous study with the Australian Bureau of Meteorology's Tropical Cyclone Limited Area Prediction System (TCLAPS). During the intensification both simulations exhibit inner-core vacillation cycles between highly asymmetric and more symmetric phases in the low-level potential vorticity (PV) and vertical velocity, although their evolution and structure are more complex in WRF. Although in WRF the lowlevel PV vacillates between symmetric and more asymmetric stages, similar to the hydrostatic TCLAPS simulation, no fully monopolar structure develops in asymmetric phases; the PV maintains its ring-like structure during the intensification. The asymmetric phases in WRF are characterised by isolated PV anomalies breaking from the eyewall and propagating towards the centre. While stirring high PV into the centre episodically, these isolated PV anomalies cause a stepwise drop in central pressure.

Intensity fluctuations in Hurricane Irma (2017) during a period of rapid intensification

“…In terms of trying to understand what these fluctuations are, there are similarities to vacillation cycles particularly with the simulation conducted in Reif et al (2014) which exhibits transitions from ring-like to monopolar PV distributions but with a more ring-like state than Nguyen et al (2011). Although one significant difference compared to the vacillation cycles in Hardy et al (2021) is that the more monopolar state during the weakening phases were transient with PV 0 /PV max peaking at the end of the weakening phase before dropping rapidly.…”

“…In addition to the radial PV structure the PV also varies azimuthally with the intensity fluctuations. One way of describing the azimuthal PV symmetry is the method of Nguyen et al (2011) and Reif et al (2014), where the azimuthal standard deviation of PV is calculated at each radius and the maximum value is taken. A high standard deviation of PV implies a less azimuthally symmetrical storm.…”

“…Hankinson et al (2014) showed that the asymmetric states were associated with radially inward moving isolated PV anomalies. The cause of the asymmetric states was further examined by Reif et al (2014) who related asymmetric periods to both convective and barotropic instability. Hardy et al (2021) showed similar processes occurring during the rapid intensification of Typhoon Nepartak (2016) with monopolar states associated with near stagnant tangential wind tendency and weaker eyewall updrafts than in the ring-like phase.…”

Wind Distribution in the Eye of Tropical Cyclone Revealed by a Novel Atmospheric Motion Vector Derivation

“…Note that the transition cannot be diagnosed based solely on AMVs because they do not provide the low‐level wind distribution in the eyewall region. Another factor to consider is the process of alternating transitions between regime 1 and regime 2, known as the “inner‐core vacillation cycle,” which has been investigated in numerical simulations (Hankinson et al., 2014; Hardy et al., 2021; Nguyen et al., 2011; Reif et al., 2014). However, in the eye of Lan, there were no apparent alterations in the shape of the radial profile of the angular velocity within the eye following the rapid increase in angular velocity (Figure 19a).…”

Moist Static Energy and Secondary Circulation Evolution Characteristics during the Rapid Intensification of Super Typhoon Yutu (2007)

“…High-resolution numerical weather prediction models have been used to study the relationship between TC inner core structure and RI [21][22][23]. For example, Nguyen et al [21] examined the inner core structure evolution in the simulation of a high-impact hurricane Katrina (2005).…”