Explosive Cyclogenesis over the Eastern United States

Cases of explosive cyclogenesis (“bombs”) were identified over the central and eastern United States and were compared with nonexplosive cyclone development in the same region. The tendency equations for vorticity and geopotential thickness, and a modified divergence equation were used to find signatures of distinction between these two types of cyclogenesis, whose recognition might improve forecasting skills. Bombs tend to show a marked decrease of vorticity with height during their incipient and explosive stages, whereas regular cyclones reveal only weak vertical vorticity gradients in the troposphere. The low-tropospheric spin-up in bombs precedes significantly that in the upper troposphere. Preexisting low-tropospheric vorticity maxima are associated with low-level jet streaks. Whereas regular cyclones possess an ill-defined level of nondivergence (i.e., a broad region between 800 and 400 mb, of divergence values close to zero), incipient bombs have a well-marked zero-divergence level near 500 mb, associated with a sharp maximum of rising motions. We observed a marked increase in large-scale latent beat release between the incipient and the explosive phases of bomb development. The convective component of latent beating shows a distinct maximum in the incipient phase of bombs and a decrease which continues through the explosive and mature phases. Regular cyclones show much less heating and much less change between the phases of development than is observed with the bombs. Static stability reveals little change as the bomb grows and mature. There also is little difference between the stability associated with bombs and regular cyclones.