Synoptic-Dynamic Climatology of the “Bomb”

TLDR

This predominantly maritime, cold‑season event is usually found about 400 n mi downstream from a mobile 500 mb trough, within or poleward of the maximum westerlies, and within or ahead of planetary‑scale troughs. The authors defined a bomb as an extratropical surface cyclone with a central pressure fall of at least 1 mb h⁻¹ over 24 h and examined such events in the Northern Hemisphere from September 1976 to May 1979, including a detailed 12‑hour development analysis during the 1978–79 season. Explosive development occurs over a wide range of sea‑surface temperatures but is favored near strong gradients, while both quasi‑geostrophic diagnostics and National Meteorological Center models underestimate the observed rapid pressure falls.

Abstract

By defining a “bomb” as an extratropical surface cyclone whose central pressure fall averages at least 1 mb h−1 for 24 h, we have studied this explosive cyclogenesis in the Northern Hemisphere during the period September 1976–May 1979. This predominantly maritime, cold-season event is usually found ∼400 n mi downstream from a mobile 500 mb trough, within or poleward of the maximum westerlies, and within or ahead of the planetary-scale troughs. A more detailed examination of bombs (using a 12 h development criterion) was performed during the 1978–79 season. A survey of sea surface temperatures (SST's) in and around the cyclone center indicates explosive development occurs over a wide range of SST's, but, preferentially, near the strongest gradients. A quasi-geostrophic diagnosis of a composite incipient bomb indicates instantaneous pressure falls far short of observed rates. A test of current National Meteorological Center models shows these products also fall far short in attempting to capture observed rapid deepening.