New method for detecting extratropical cyclones: the eight-section slope detecting method

Abstract

An algorithm for identifying extratropical cyclones (ECs) on the basis of gridded data is proposed in this study. The algorithm, which is named the eight-section slope detecting (ESSD) method, has five key procedures to identify an EC by using the mean sea level pressure (MSLP) or geopotential height. They are: (i) finding the location of every minimum of the MSLP/geopotential-height; (ii) establishing a targeted box for each minimum; (iii) dividing the targeted box into eight sub-regions; (iv) calculating eight relative slopes within the eight sub-regions; (v) confirming an EC only if all eight relative slopes are above an appropriate threshold. Based on the 0.75° × 0.75° ERA-Interim reanalysis field, comparisons show that the ESSD method performs better in identifying ECs than the other three previous EC detection algorithms, as it can lower the error caused by mistaking a trough for an EC. Moreover, a test of detecting ECs in the Northern Hemisphere using the ESSD method repeated 500 times (randomly distributed across 40 years) shows that the accuracy of this method varies from 79% to 91%, with an annual mean accuracy of ~85%. This means that the ESSD method can provide credible results with respect to EC identification.

Keywords

• extratropical cyclone
• cyclone detection
• era-interim