Abstract
In this study, we compare two different cyclone-tracking algorithms to detect North Atlantic polar lows, which are very intense mesoscale cyclones. Both approaches include spatial filtering, detection, tracking and constraints specific to polar lows. The first method uses digital bandpass-filtered mean sea level pressure (MSLP) fields in the spatial range of 200–600 km and is especially designed for polar lows. The second method also uses a bandpass filter but is based on the discrete cosine transforms (DCT) and can be applied to MSLP and vorticity fields. The latter was originally designed for cyclones in general and has been adapted to polar lows for this study. Both algorithms are applied to the same regional climate model output fields from October 1993 to September 1995 produced from dynamical downscaling of the NCEP/NCAR reanalysis data. Comparisons between these two methods show that different filters lead to different numbers and locations of tracks. The DCT is more precise in scale separation than the digital filter and the results of this study suggest that it is more suited for the bandpass filtering of MSLP fields. The detection and tracking parts also influence the numbers of tracks although less critically. After a selection process that applies criteria to identify tracks of potential polar lows, differences between both methods are still visible though the major systems are identified in both.
Highlights
The discrete cosine transforms (DCT) is more precise in scale separation than the digital filter and the results of this study suggest that it is more suited for the bandpass filtering of mean sea level pressure (MSLP) fields
The points used for the tracking are typically chosen as local extremes of some field, for example, minima of the mean sea level pressure (MSLP) field (Serreze, 1995; Gulev et al, 2001; Muskulus and Jacob, 2005; Wernli and Schwierz, 2006; Zahn and von Storch, 2008a), the 1000 hPa geopotential height surface (Z1000) (Blender et al, 1997), maxima of the relative vorticity field (Hodges, 1995; Scharenbroich et al, 2010) in the northern hemisphere (NH) and minima in the southern hemisphere (SH) and geostrophic vorticity computed as the Laplacian of pressure or geopotential (Murray and Simmonds, 1991)
The polar low identification criteria of MZ were applied to both techniques to assign tracks to potential polar lows
Summary
Automatic tracking methods provide a convenient way to perform the analysis of weather systems in long-term datasets to explore their spatial and temporal variability (Murray and Simmonds, 1991; Hodges, 1994, 1995; Serreze, 1995; Blender et al, 1997; Gulev et al, 2001; Muskulus and Jacob, 2005; Wernli and Schwierz, 2006; Zahn and von Storch, 2008a). The methods explored are those of Hodges (1994, 1995, 1999) and Zahn and von Storch (2008a) These were applied to detect polar lows in the North Atlantic which are small intense maritime mesoscale cyclones forming poleward of the Polar Front in both hemispheres (Rasmussen and Turner, 2003).
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
