Observations of 250-km-wavelength clear-air eddies and 750-km-wavelength mesocyclones associated with a synoptic-scale midlatitude cyclone

Abstract

Satellite images of a decaying synoptic-scale cyclone over the North Pacific reveal two distinct types of multiple mesoscale cyclonic disturbances. The approximate positions of these disturbances within the synoptic-scale cyclone are determined using analyses from an operational global-scale numerical model. One type, a set of four 250-km-wavelength eddies, occurred in clear air and represent perturbations within the cyclonic shear side of a 40-45 m s-1 upper-level jet-front system. Their propagation at 28.5 ± 3.5 m s-1 roughly matched the wind speed and direction within the jet at their position and provided evidence of stretching deformation along their axis. Their growth is documented over 18 h, and is measured in terms of horizontal displacements of a preexisting moisture boundary in water vapor imagery. Their e-folding time increased from 6 to 9 h as horizontal displacements exceeded 100 km and horizontal billows indicative of wave breaking formed. The billow-like structures most likely represent areas of enhanced mixing of stratospheric and tropospheric air by means of a quasi-horizontal process acting in the vicinity of a tropopause fold. Because they developed in a region of significant horizontal shear and because the absence of clouds suggests vertical motions were small or absent, the behavior of these eddies is consistent with barotropic instability on a 50-km-wide shear zone or potential vorticity strip. The other type, a set of five 750-km-wavelength cyclonic disturbances (mesocyclones), is also evident in the satellite images, but because they modulated the cloud field they appear in both the WV and infrared images. They wrapped fully around the synoptic-scale cyclone, forming a wavenumber 5 perturbation that later became a wavenumber 4 perturbation propagating cyclonically about its center at 19 ± 1 m s-1 and likely formed on an occluded front. Two of these waves are distinguished by the development of their own comma clouds, indicating they had deep vertical circulations and suggesting that moist baroclinic instability or CISK was active. These 750-km-wavelength mesocyclones most likely affected the deformation and shear along the upper front, which could have modified the barotropic stability of the region, and thus influenced where the 250-km-wavelength eddies formed.