Objective tropical cyclone extratropical transition detection in high‐resolution reanalysis and climate model data

Abstract

This paper describes an objective technique for detecting the extratropical transition (ET) of tropical cyclones (TCs) in high‐resolution gridded climate data. The algorithm is based on previous observational studies using phase spaces to define the symmetry and vertical thermal structure of cyclones. Storm tracking is automated, allowing for direct analysis of climate data. Tracker performance in the North Atlantic is assessed using 23 years of data from the variable‐resolution Community Atmosphere Model (CAM) at two different resolutions ( ΔX∼55 km and 28 km), the Climate Forecast System Reanalysis (CFSR, ΔX∼38 km), and the ERA‐Interim Reanalysis (ERA‐I, ΔX∼80 km). The mean spatiotemporal climatologies and seasonal cycles of objectively detected ET in the observationally constrained CFSR and ERA‐I are well matched to previous observational studies, demonstrating the capability of the scheme to adequately find events. High‐resolution CAM reproduces TC and ET statistics that are in general agreement with reanalyses. One notable model bias, however, is significantly longer time between ET onset and ET completion in CAM, particularly for TCs that lose symmetry prior to developing a cold‐core structure and becoming extratropical cyclones, demonstrating the capability of this method to expose model biases in simulated cyclones beyond the tropical phase.

The Future of Midlatitude Cyclones

“…This is partly due to the limitations associated with objective, automated TC detection and tracking methods as well as with the definition of ET itself. In addition, a comparison of high-resolution National Center for Atmospheric Research Community Atmospheric Model hindcasts, reanalysis data, and TC best track data indicated systematic structural errors present during ET in climate models [118].…”

Towards Dynamical Seasonal Forecast of Extratropical Transition in the North Atlantic

“…This method has an objective definition of ET and requires only thermal geopotential heights at isobaric levels as inputs. It has been used in a range of individual storm analyses (e.g., Bao et al, ; Griffin & Bosart, ; Liu & Smith, ; Wang et al, ; Yang et al, ) and in ET climatology studies (Kitabatake, ; Song et al, ; Wood & Ritchie, ; Zarzycki et al, ). CPS characterizes the thermal evolution of storms with three parameters: 900‐ to 600‐hPa thermal thickness asymmetry ( B ), 900‐ to 600‐hPa thermal wind (– V L T ), and 600‐ to 300‐hPa thermal wind (– V U T ).…”

“…The onset of ET is triggered when B is larger than 10 m or – V L T is lower than 0. The completion of ET is determined when the two conditions are both satisfied (Liu et al, ; Wood & Ritchie, ; Zarzycki et al, ). For HiFLOR, we employs a simplified version of the CPS method that uses 850‐ to 500‐hPa thermal thickness asymmetry ( B ), 850‐ to 500‐hPa thermal wind (– V L T ), and 500‐ to 300‐hPa thermal wind (– V U T ) due to the availability of data.…”

A Global Climatology of Extratropical Transition. Part II: Statistical Performance of the Cyclone Phase Space

“…In contrast, the cyclone phase space (CPS) framework proposed by Hart (2003) can be used to define ET in a purely objective, automatable way. The CPS has become widely used and has been applied to operational analysis and reanalysis data (e.g., Hart 2003; Kitabatake 2011; Wood and Ritchie 2014) as well as climate model output (Zarzycki et al 2017;Liu et al 2017).…”