Ground-Based Observations of Polar Mesospheric Clouds: Structure, Radiance and Gravity Wave Effects

Abstract

Ground-based observations of Polar Mesospheric Clouds, PMC (often termed noctilucent clouds) have provided a wealth of information on their occurrence frequency, latitudinal extent, radiance, structure, and optical properties (particle sizes). First detected over 100 years ago, mesospheric clouds are a direct manifestation of major dynamical processes taking place in the summer polar mesosphere. Noctilucent clouds (NLC) are easily visible in the summer twilight sky at latitudes ranging from typically 55-65° and have been studied intensely using photometric, polarimetric and imaging instrumentation. In particular, NLC are characterized by their radiance structure and they provide a natural "laboratory" for the investigation of gravity waves, wave breaking effects and instability processes occurring near the high-latitude summer mesopause. In this talk I will summarize current knowledge of mesospheric cloud structure and dynamics, focusing on the effects of gravity waves on their visibility, formation and dissipation. Recent ground-based and satellite measurements will be described which extend significantly these measurements well into the sunlit polar cap region. New ground-based measurement in coordination with the NASA Aeronomy of Ice in the Mesosphere (AIM) satellite will provide a unique opportunity for quantifying their global properties.