Four Years of Simultaneous Observations of Noctilucent Clouds and Mesospheric Summer Echoes at a Mid-Latitude Site (Kühlungsborn/Germany, 54°N)

Abstract:

Occurrence of ice particles in the polar summer mesopause region is an intriguing phenomenon that can be observed either optically as Noctilucent Clouds (NLC) / Polar Mesospheric Clouds (PMC) or by radar as (Polar) Mesosphere Summer Echoes ((P)MSE). The relation of both phenomena is well understood and allows insights into atmospheric properties like temperature, humidity, winds, turbulence and electron density. Simultaneous observations of NLC and PMSE require sufficient electron density (for the radar observation) and therefore daylight conditions that may hinder optical observations by lidar. Up to now, simultaneous observations of NLC and PMSE are mainly limited to polar latitudes, while data from mid-latitudes are lacking.

Since 2010 we operate a new RMR lidar at our site at Kühlungsborn/Germany (54°N, 12°E). From the best of our knowledge this lidar allows for the first time observations of mid-latitude NLC independent of solar elevation, i.e. during night and day. With our new RMR lidar and the co-located OSWIN radar we are for the first time able to compare the occurrence and altitude structure of NLC and MSE at mid-latitudes. It turns out that the lower edges of simultaneously observed NLC/MSE typically agree, as expected from higher latitudes. Though, the top edge of MSE is observed about 500 m above the NLC edge, indicating the presence of particles being too small to be observed by lidar. Nevertheless, height difference is small compared to the typical layer widths and smaller than observed at higher latitudes. This hints at different size distributions and, by this, different growing conditions at mid-latitudes. We will present a statistical overview on the comparison of simultaneously observed NLC and MSE layers and their main characteristics. Simultaneous NLC and MSE are of additional importance if observed during twilight conditions. The onset or disappearance of MSE during morning and evening twilight is directly related with changing electron densities. While twilight conditions at polar latitudes are limited to the end of NLC season, at our mid-latitude station this happens twice a day. We will present case studies of the onset of MSE during ice conditions observed as NLC, e.g. showing an altitude dependence of the minimum electron density required for MSE.