Case study of an ice void structure in polar mesospheric clouds

Abstract

The Cloud Imaging and Particle Size (CIPS) instrument aboard the Aeronomy of Ice in the Mesosphere (AIM) mission produces detailed images of Polar Mesospheric Clouds (PMCs) from space and has revealed several distinct types of cloud structures. One such commonly observed structure is an ‘ice void’—a nearly circular ice-free region with diameter varying from tens to thousands of kilometers. We document the morphology of large ice voids (diameter of ∼500–1000km) as seen by CIPS in the northern hemisphere 2007 PMC season. The meteorology of the mesosphere when these voids occur is described by the Navy Operational Global Atmospheric Prediction System–Advanced Level Physics High Altitude (NOGAPS–ALPHA) forecast/assimilation system. A comprehensive study of dynamics and transport in the mesosphere using NOGAPS–ALPHA during the occurrence of a large ice void observed by CIPS on 19 July 2007 indicates that warmer temperatures (warmer than the frost point temperature of ∼144K) at the location of the void may be related to increased tidal activity and transport of warm air from low latitudes. However, a detailed analysis of all identified voids is necessary to generalize the factors that could be responsible for the formation of an ice void.