A 4D feature‐tracking algorithm: A multidimensional view of cyclone systems

Abstract

An objective four-dimensional (4D) algorithm developed to track extratropical relative vorticity anomaly 3D structure over time is introduced and validated. The STACKER algorithm, structured with the TRACKER single-level tracking algorithm as source of the single-level raw tracks, objectively combines tracks from various levels to determine the 3D structure of the cyclone (or anticyclone) events throughout their life cycle. STACKER works progressively, beginning with two initial levels and then adding additional levels to the stack in a bottom-up and/or top-down approach. This allows an iterative stacking approach, adding one level at a time, resulting in an optimized 4D determination of relative vorticity anomaly events. A two-stage validation process is carried out with the ECMWF reanalysis ERA-Interim dataset for the 2015 austral winter. First the overall tracking capability during an austral winter, taking into account a set of climate indicators and their impacts on Southern Hemisphere circulation, was compared to previous climatologies, in order to verify the density and distribution of the cyclone events detected by STACKER. Results show the cyclone density distribution is in very good agreement with previous climatologies, after taking into account potential differences due to climate variability and different tracking methodologies. The second stage focuses on three different long-lived events over the Southern Hemisphere during the winter of 2015, spanning seven different pressure levels. Both GOES satellite imagery, infrared and water vapour channels, and ERA-Interim cloud cover products are used in order to validate the tracks obtained as well as the algorithm's capability and reliability. The observed 3D cyclone structures and their time evolution are consistent with current understanding of cyclone system development. Thus, the two-stage validation confirms that the algorithm is suitable to track multilevel events, and can follow and analyse their 3D life cycle and develop full 3D climatologies and climate variability studies.

A Three-Dimensional Perspective on Extratropical Cyclone Impacts

“…However, while they identified the vertically well-organized cyclones as typically larger, more intense, and longer-lived than shallower cyclones, any differences between vertically organized and shallow cyclones in surface impacts remain unclear. More recently, Lakkis et al (2019) have developed a more complex 4D cyclone tracking scheme based on the methods of Hoskins and Hodges (2005), which was applied over the Southern Hemisphere for the season June-August 2015.…”

The numerous approaches to tracking extratropical cyclones and the challenges they present

“…However, ETCs have complex 3-dimensional features that extend through multiple levels in the atmosphere. Lakkis et al (2019) have created a 4-dimensional (4D) feature-tracking algorithm that identifies and tracks ETCs across multiple levels in the atmosphere. They have adapted the method from Table 2 in Neu et al (2013), with the addition of M23, representing the results from Massey's (2016) Hodges (1995), repeating the process of identifying and tracking an ETC using relative vorticity on multiple vertical levels and then stacking these results to create a 4D representation of the track.…”

“…Using tracking algorithms allows for the analysis of long‐term trends and the lifecycle of ETCs, along with their speed and intensity (Feser et al ., 2015). Most objective feature‐tracking methods have two phases: the identification of an ETC and tracking the same system across multiple time‐steps (Raible et al ., 2008; Massey, 2016; Lakkis et al ., 2019).…”

“…Lakkis et al . (2019) have created a 4‐dimensional (4D) feature‐tracking algorithm that identifies and tracks ETCs across multiple levels in the atmosphere. They have adapted the method from Hodges (1995), repeating the process of identifying and tracking an ETC using relative vorticity on multiple vertical levels and then stacking these results to create a 4D representation of the track.…”

Early 21st Century cyclone climatology: a 3D perspective.Basic Characterization

“…A detailed description of this algorithm, named STACKER, can be found in Lakkis et al . (2019). Briefly, the algorithm determines which cyclone events independently tracked at different levels are to be combined into single (multi‐level) events, preserving all the track features detected at each level.…”