Northern Hemisphere Storm Tracks Diagnosed by Synoptic-Scale Ascent

We present a climatology of Northern Hemisphere storm tracks wherein synoptic-scale ascent is used to measure storm location and intensity. Values for ascent are derived from the ERA-interim reanalysis (1979-2011) using the alternative balance omega equation, which helps to remove sub-synoptic scale features from the data. The resulting climatology is consistent with the well-documented Atlantic and Pacific storm tracks, but provides a more complete representation of storm tracks over the continental landmasses, including mountainous regions. By including ascent from both open and closed waves, this climatology is more inclusive than those addressing only closed cyclones.

To better understand the comparative contributions of processes occurring aloft and near the surface, the ascent-based storm track is partitioned into an upper (700-300 hPa) and lower (1000-700 hPa) layer. Results show that lower tropospheric baroclinicity contributes strongly to the storm track proximal to the Kuroshio and Gulf Stream, while lee cyclogenesis is an important factor downstream of both the Himalaya and Rocky Mountains. Aloft, the storm track is more continuous and shifted north from that at the surface. While small-scale orographic features are filtered from the data, the impact of large-scale orography on the distribution of storms is apparent. Preliminary results exploring the relation of synoptic-scale ascent and the distribution of precipitation on both global and regional scales are also presented.