Mesospheric temperature during the extreme mid-latitude noctilucent cloud event on 18/19 July 2016

Abstract

A rare noctilucent cloud (NLC) event was observed at 48.8° N, 13.7° E above GERES station in southern Germany on 18/19 July 2016 using the CORAL Rayleigh lidar. Strong southward winds due to a quasi 2-day planetary wave allowed for the influx of mesospheric polar air to mid-latitudes on this day. The NLC observed by lidar was preceded by strong NLC displays in CIPS satellite images above the North Sea and by strong MSE radar echoes 800 km north of the lidar site, and was also observed visually in central Europe. The NLC occurred at low altitude and was bright and thin with strong oscillations in altitude and brightness. Darkness allowed for high-resolution temperature measurements at NLC altitudes. The ice particles were embedded in the upper part of a cold region with temperatures below 150 K. Significantly higher temperatures were found directly above the cloud with large vertical temperature gradients of 25 K/km at the top boundary. Spectral analysis reveals that NLC particles existed within cold phases of gravity waves within a region of high static stability. In order to study the evolution of NLC brightness in this environment we drive the microphysical model CARMA with lidar temperature soundings. We find that NLC particles can survive and grow in the conditions defining this mid-latitude event. We conclude that the ice particles did not nucleate at the site of observation, but were meridionally transported and vertically confined to a thin layer due to a large vertical temperature gradient, wind reversal, and low levels of mesospheric turbulence.