Karacostas, Theodore S. (Aristotelian University of Thessaloniki, Department of Meteorology and Climatology, Thessaloniki, Greece)
A meteorological "bomb" is defined as an explosive deepening extratropical surface cyclone, whose central pressure falls very rapidly, at a rate of at least 1 hPa.hr-1, continuously for 24 hours. This drastically developed cyclogenesis, which is a rare event over the Mediterranean area, became responsible for setting up the record of minimum atmospheric surface pressure in Athens and Thessaloniki, Greece.
The presented case study of 19 March 1981 is analyzed by means of cross-sections and time-sections, in order to investigate the dynamic structure of the cyclone and the nature of the atmospheric processes that are responsible for the development of it. The system, which was set up as an open long wave, became a close-of cyclone all the way to 300 hPa. The warm air advenction behind the upper air trough, coupled with the abundance of moist air from the Mediterranean Sea, and the continuously increasing cyclonic vorticity, first due to shear and then due to shear and curvature, are the main reasons for the development of a negatively tilted baroclinic zone.
Vertical cross-section analyses of radiosonde data indicate the dynamic structure of the baroclinity, as the baroclinic zone is inclined backwards, towards the cold air, with increasing height. Strong cyclonic and vertical shear are diminishing down-stream within the upper troposphere. The distinguishing characteristics of the frontal layer are the great horizontal temperature gradient and the pronounced vertical stability. The system reached the mature stage and became vertical as the kinetic energy converted into potential, intensifying thus the baroclinity and rising the baroclinic zone.
It is concluded that both aspects, baroclinic and convective nature, are present in the development of the bomb.
However, the former event is the dominant one.