The intensity and motion of hybrid cyclones in the Australian region in a composite potential vorticity framework

Abstract

Hybrid cyclones (HCs) in the Australian region typically reach their peak intensity in an amplified flow comprising upper-tropospheric ridges upstream and downstream of the cyclone and a north-south elongated trough. Nonetheless, there is considerable case-to-case variability. Taking a composite viewpoint, the present study investigates how such variations in the upper-tropospheric potential vorticity (PV) anomalies affect the subsequent intensity and motion of HCs in the Australian region. First, cyclones are grouped into four clusters with structurally similar environments through a k-means clustering of the 315 K PV anomaly. The clusters reveal that HCs can be associated with a north-south elongated trough (Cluster 1), a PV cut-off (Cluster 2), and cyclonically breaking troughs (Clusters 3 and 4). Second, the effect of these features on the intensity and tracks is quantified using piecewise PV inversion. The maximum intensity of cyclones in Cluster 1 is largely determined by their upper-tropospheric cyclonic PV anomaly. Conversely, diabatically generated lower-tropospheric PV anomalies dominate the intensity of cyclones in Clusters 3 and 4. In these two clusters, the cyclonically breaking trough and a downstream ridge induce an anomalous northeasterly low-level flow across the cyclone centre. The downstream ridge is most pronounced in Cluster 4, leading to the greatest poleward cyclone displacement compared to the other clusters. In Clusters 1 and 2, the upper-level PV anomaly primarily slows the eastward motion of the cyclones. In agreement with recent idealized studies, the analysis suggests that the effect of upper-tropospheric PV anomalies on the poleward motion of HCs is analogous to the beta-gyres that influence the motion of tropical cyclones. KEYWORDS cut-off, hybrid cyclone, piecewise potential vorticity inversion, wave breaking 1 Q J R Meteorol Soc. 2019;145:273-287.wileyonlinelibrary.com/journal/qj

The Future of Midlatitude Cyclones

“…The relative contribution of diabatic processes to ETC intensification can be quantified via the pressure tendency equation [27], the omega equation [28,29], or the Zwack-Okossi equation [30]. Another perspective on the influence of LH on cyclones can be obtained from the potential vorticity (PV) framework [31]: diabatic PV generation associated with LH in clouds leads to the formation of a cyclonic PV anomaly in the lower troposphere that contributes to the cyclonic circulation [25,26,[32][33][34]. More recent climatological studies have used this lower-tropospheric PVanomaly as a measure of the relevance of diabatic processes for ETCs in different regions [35] and of different intensities [36,37], indicating that such lower-tropospheric PVanomalies are dominant factors in the development of many intense ETCs [37].…”

Synoptic climatology of hybrid cyclones in the Australian region

“…Its structure is less clear, although it points to a weaker downstream ridge in events with a strongly positively tilted trough. The information of this EOF analysis is used in a companion paper (Quinting et al ) to separate all HCs into four clusters, and to address the question how the upper‐level PV structure affects the intensity and motion of HCs.…”

Characterizing Australia's east coast cyclones (1950–2019)

“…However, this method focused on point data and did not assess or group by track characteristics. Previous studies in Australia (Quinting, Reeder, & Catto, 2019b; Ramsay et al, 2012) have demonstrated how advantageous cluster analysis using tracks has been in investigating spatial and temporal trends of tropical cyclone (TC) events, a field of research that has not been applied in‐depth for ECC events.…”