Plot of the month for January 2018 is from Alexander Baker, who asks: how many wintertime heavy precipitation events can be attributed to extratropical cyclones?

Using ERA-Interim reanalysis data for the period 1979-2016, the number of heavy precipitation events coinciding with cyclones was quantified for the Northern Hemisphere, following a published method by Pfahl and Wernli (2012).

For each December-February season, we defined heavy precipitation events in 6-hourly precipitation accumulations as exceedances of the 95th percentile, and then identified where these exceedances occurred simultaneously with the passage of an extratropical cyclone, whose positions are identified and tracked using vorticity data. We therefore consider that such heavy precipitation events are cyclone-associated, and are able to compute the climatological frequency of such events.

The figure below shows the climatological frequency (as a percentage) of extratropical cyclone-associated winter (DJF) heavy precipitation events for the Northern Hemisphere. The storm track regions are clearly represented and many populous regions are clearly affected by heavy precipitation due to cyclones. We are currently extending this analysis to global climate model simulations.

Figure | Northern Hemisphere extratropical cyclone-associated winter (DJF) heavy precipitation events, expressed as a percentage of all heavy precipitation events. (Click the figure for a larger version.)