Structure and evolution of Subtropical Cyclone Anita as evaluated by heat and vorticity budgets

This paper explores the evolution of subtropical cyclone Anita, which occurred near the east coast of Brazil (~19.S-37.W) in March 2010. Thermodynamic and dynamic processes during Anita's lifecycle are investigated using the heat and vorticity budget equations. The cyclone developed with hybrid characteristics and moved anomalously to the southwest where it coupled with an upper level cut-off low during the mature phase. This coupling was the main dynamical mechanism for further cyclone deepening. Anita then remained quasi-stationary about 30.S -47.W for two days due to an upper level dipole pattern which prevented earlier displacement of the upper level low counterpart. When the dipole pattern dissipated, the cyclone moved southeast and underwent extratropical transition whilst merging with a mid-latitude frontal cyclone. Diabatic heating and horizontal temperature advection are found to be essential for the subtropical development. During extratropical transition, it is instead diabatic cooling together with adiabatic cooling and warm air advection that act as the main mechanisms to influence the local temperature tendencies at low levels. Low level cyclonic tendencies were mostly due to convergent flow, and the residual vorticity partially destroyed the vorticity tendencies produced by the divergence term. Moreover, in regions and levels where convection could explain some of the vorticity tendencies, it is found that apparent sinks of cyclonic vorticity were related to negative vorticity due to divergence (i.e., convergent flow), whilst apparent sources were related to positive vorticity due to divergence (i.e., divergent flow)

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

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.Key PointsAn objective detection technique for tracking tropical cyclone extratropical transition in gridded climate data is describedObjectively calculated extratropical transition climatology in high‐resolution reanalyses closely match observational studiesTropical cyclones in CAM take too long to undergo extratropical transition highlighting model biases requiring further investigationPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/136754/1/jame20355_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/136754/2/jame20355.pd