Abstract
Abstract. Polar Mesosphere Summer Echoes (PMSE) and Noctilucent Clouds (NLC) have been routinely measured at the ALOMAR research facility in Northern Norway (69° N, 16° E) by lidar and radar, respectively. 2900 h of lidar measurements by the ALOMAR Rayleigh/Mie/Raman lidar were combined with almost 18 000 h of radar measurements by the ALWIN VHF radar, all taken during the years 1999 to 2008, to study simultaneous and common-volume observations of both phenomena. PMSE and NLC are known from both theory and observations to be positively linked. We quantify the occurrences of PMSE and/or NLC and relations in altitude, especially with respect to the lower layer boundaries. The PMSE occurrence rate is with 75.3% considerably higher than the NLC occurrence rate of 19.5%. For overlapping PMSE and NLC observations, we confirm the coincidence of the lower boundaries and find a standard deviation of 1.26 km, hinting at very fast sublimation rates. However, 10.1% of all NLC measurements occur without accompanying PMSE. Comparison of occurrence rates with solar zenith angle reveals that NLC without PMSE mostly occur around midnight indicating that the ice particles were not detected by the radar due to the reduced electron density.
Highlights
The low temperatures of the polar summer mesopause region allow for the formation of ice particles, giving rise to phenomena termed polar mesospheric summer echoes (PMSE) and noctilucent clouds (NLC)
The presence of ice particles is crucial to either, PMSE and NLC differ in the physical mechanisms that create them and require different
As a function of solar zenith angle influence of electron density becomes apparent. Both the PMSE occurrence rate is significantly correlated with electron density and the probability to observe NLC without PMSE is significantly anti-correlated to the PMSE occurrence rate
Summary
The low temperatures of the polar summer mesopause region allow for the formation of ice particles, giving rise to phenomena termed polar mesospheric summer echoes (PMSE) and noctilucent clouds (NLC). Li et al (2010) have recently derived size distributions of ice particle radii from radar measurements consistent with this picture In this way it can be understood that PMSE occur more frequently than NLC, in a larger altitude range and that NLC are embedded into the lower part of the PMSE layer. Open questions are related to the interplay of parameters of the background atmosphere such as temperature, water vapour and electron density and dynamical processes such as turbulence and gravity waves regarding their role in the formation of PMSE and NLC. Such understanding is crucial to the interpretation of PMSE and NLC measurements. We define several cases based on occurrence and relative layer altitudes, calculate occurrence rates of joint and sole PMSE/NLC observations, in total as well as as function of year, local time and solar zenith angle, and quantify the previously observed coincidences of lower boundaries
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