Abstract
This paper presents a diagnosis of the slow decay of two extratropical cyclones that occurred over the central United States in March and November 1999. Results reveal that vertical superposition of the cyclone and upper-level troughs is not required for cyclone decay to occur. In both cases, decay commenced while the upper-level trough was still upstream from, and thus upper-level cyclonic vorticity was present above, the cyclone. Failure of the upstream tilt in both trough systems to yield cyclone intensification was attributed largely to the advection of cold air, primarily at lower to middle levels, into each cyclone's interior. Also of interest is the fact that latent heat release also contributed to the decay of both cyclones. This latter result was a consequence of the location of the cyclones in the outer portion of their respective heating regions. Finally, more rapid decay of the two cyclones was prevented by middle- and lower-tropospheric adiabatic warming that occurred in the statically stable cyclone interiors.
* Current affiliation: ACES Power Marketing LLC, Indianapolis, Indiana.
Corresponding author address: Dr. Phillip J. Smith, Department of Earth and Atmospheric Sciences, Purdue University, 1397 Civil Engineering Building, West Lafayette, IN 47907-1397. Email: pjsmith@purdue.edu
