Observational Evidence for Asymmetric Inertial Instability

Abstract

We present a detailed analysis of a mesoscale circulation phenomenon that appeared on satellite images as a group of propagating cloud wavelets. These wavelets developed on the anticyclonic side of a strong, upper-level subtropical jet in a region of negative potential vorticity (P). During their lifetime, the wavelets were located over the North American synoptic sounding network; the meteorological conditions surrounding their occurrence are examined. We investigate the various physical mechanisms that may have been responsible for generating the observed cloud wavelets. In consideration of the time scale of the wavelets and their proximity to the region where P < 0, inertial instability appears to be the most likely dynamical mechanism for explaining their existence. Using hourly satellite images, we determined that the wavelets propagated downstream at a phase speed 25 m s?1 slower than the maximum wind speed of the jet with life cycles of 4?5 h, horizontal wavelengths of ?400 km, and periods of about 1.4 h. To substantiate further our contention, the observed characteristics of the wavelets are compared to, and found to be consistent with, a theoretical model of asymmetric inertial instability.