Small scale structures of NLC observed by lidar at 69°N/69°S and their possible relation to gravity waves

Abstract

Lidar measurements of noctilucent clouds (NLC) were conducted by the Davis Rayleigh-/Raman-lidar in Antarctica (68.58°S, 77.97°E) and by the Rayleigh-/Mie-/Raman-lidar at the ALOMAR observatory in northern Norway (69.28°N, 16.01°E). We compare southern and northern hemisphere NLC at time scales of 10. min to several hours using multi-year datasets (four seasons at ALOMAR, 2008-2011, and nine seasons at Davis, 2001/2002 to 2009/2010). NLC characteristics studied include the vertical structure of NLC layers, the duration of NLC layers as well as the apparent downward motion of NLC layers with time. We find multiple layers during 9% of all NLC observations with vertical separations of double layers between 1.5 and 3. km. The mean downward progression of NLC with measurement time is -0.3. km/h and comparable at Davis and ALOMAR. We find no general spatial tilt of the layer at ALOMAR but individual layers show up to 2. km altitude difference at 40. km horizontal separation. Typical NLC observations at both stations last about 5 hours, hinting at horizontal extents of about 700. km, and reoccur after approximately 10 hours. This is in the range of mid-frequency gravity waves (GW). On short-time scales NLC characteristics are presumably impacted by small scale processes in the vicinity of the clouds, generated by e.g. breaking GW. In addition, we discuss a possible relation to GW by looking at the influence of stratospheric wind conditions on NLC layer characteristics at 69°S. © 2013 Elsevier Ltd.