Satellite-observed cold-ring-shaped features atop deep convective clouds

Abstract

This paper focuses on deep convective storms which exhibit a distinct long-lived cold ring at their cloud top, as observed in enhanced infrared (IR) window satellite imagery. The feature seems to be closely linked to a similar phenomenon, cold-U/V (enhanced-V) shape, or in general to storms which exhibit an enclosed warm spot or larger warm area downwind of the overshooting tops, surrounded by colder parts of the storm anvil. While storms exhibiting some form of warm spots seem to be quite common, storms exhibiting distinct cold rings or cold-U/Vs are significantly less frequent. The cold-ring feature is described here for storms which occurred above the Czech Republic and Austria on 25 June 2006. Compared to other cold-ring-shaped storms, this case was extraordinary not only by the magnitude and duration of the cold ring and its central warm spot, but also by storm cloud-top heights, reaching 16-17 km, as determined from ground-based C-band radar observations. The paper also addresses a possible link between cold-ring-shaped storms with those exhibiting a cold-U/V (enhanced-V) feature, indicating (based on model results) that the stratification and wind shear just above the tropopause are key conditions for the cold-ring to exist. The case from 25 June 2006 also shows that the cloud top height, derived from satellite radiances, has significant error when applied to this particular type of storm. Finally, we discuss the potential of the satellite-observed cold-ring feature as an indicator of storm severity.