Trends in noctilucent clouds

Franz-Josef Lübken,Gerd Baumgarten

doi:10.5194/egusphere-egu21-4607

Abstract

&lt;p&gt;Noctilucent clouds are often cited as potential indicators of climate change in the middle&lt;br&gt;atmosphere. They owe their existence to the very cold summer mesopause region (~130K) at mid&lt;br&gt;and high latitudes. We analyze trends derived from the Leibniz-Institute Middle Atmosphere&lt;br&gt;Model (LIMA) and the MIMAS ice particle model (Mesospheric Ice Microphysics And tranSport model)&lt;br&gt;for the years 1871-2008 and for middle, high and arctic latitudes, respectively.&lt;br&gt;Model runs with and without an increase of carbon dioxide and water vapor (from methane oxidation)&lt;br&gt;concentration are performed. Trends are most prominent after ~1960 when the increase of both&lt;br&gt;carbon dioxide and water vapor accelerates. Negative trends of (geometric) NLC altitudes are primarily&lt;br&gt;due to cooling below NLC altitudes caused by carbon dioxide increase. Increases of ice particle&lt;br&gt;radii and NLC brightness with time are mainly caused by an enhancement of water vapor.&lt;br&gt;Several ice layer and background parameter trends are similar at high and arctic latitudes but are&lt;br&gt;substantially different at middle latitudes. This concerns, for example, occurrence rates, ice water&lt;br&gt;content (IWC), and number of ice particles in a column. Considering the time period after 1960,&lt;br&gt;geometric altitudes of NLC decrease by approximately 260m per decade, and brightness increases by&lt;br&gt;roughly 50% (1960-2008), independent of latitude. NLC altitudes decrease by approximately 15-20m&lt;br&gt;per increase of carbon dioxide by 1ppmv. The number of ice particles in a column and also at the&lt;br&gt;altitude of maximum backscatter is nearly constant with time. At all latitudes, yearly mean NLC&lt;br&gt;appear at altitudes where temperatures are close to 145+/-1K. Ice particles are present nearly&lt;br&gt;all the time at high and arctic latitudes, but are much less common at middle latitudes. Ice water&lt;br&gt;content and maximum backscatter are highly correlated, where the slope depends on latitude. This&lt;br&gt;allows to combine data sets from satellites and lidars. Furthermore, IWC and the concentration of&lt;br&gt;water vapor at the altitude of maximum backscatter are also strongly correlated. Results from&lt;br&gt;LIMA/MIMAS agree nicely with observations.&lt;/p&gt;

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