Die Farbe leuchtender Nachtwolken

<p>Das Phänomen leuchtender Nachtwolken (engl. Noctilucent clouds, NLCs) zeichnet sich durch die silbrig-blau oder hell-blau schimmernde Farbe aus. Unter der Verwendung des Strahlungstransfermodells SCIATRAN lassen sich unter der Annahme sphärischer NLC-Partikel die Spektren der durch NLCs gestreuten solaren Strahlung für einen bodengestützten Beobachter simulieren. Um die resultierenden Farben der NLCs möglichst objektiv bestimmen und darstellen zu können, werden die CIE (International Comission on Illumination) Farbanpassungsfunktionen und die CIE-Farbwerte verwendet. Die Darstellung erfolgt in sogenannten 2-D CIE-Farbtafeln, welches eine Standardmethode zur Visualisierung von Farbinformationen ist. Verschiedene Prozesse und Parameter, die die Farbe der NLCs beeinflussen können, wie unter anderem die Größe der NLC-Partikel, stratosphärisches Ozon und die Bedeutung von Einfach- und Mehrfachstreuung werden untersucht und diskutiert. Die Simulationen zeigen, dass die Größe der NLC-Partikel die Farbe der Wolken wesentlich beeinflusst und dass unrealistisch große Wolkenpartikel zu einer rötlichen Färbung führen. Darüber hinaus kann gezeigt werden, dass Ozon bei einer ausreichend großen optischen Tiefe der NLCs und bei bestimmten Beobachtungsgeometrien nur eine untergeordnete Rolle für die bläuliche Farbe der NLCs spielt. Des Weiteren zeigen die Berechnungen, dass der Beitrag der Mehrfachstreuung zur Gesamtstreuung nur von geringer Bedeutung ist, was eine zusätzliche Rechtfertigung für frühere Studien zu diesem Thema darstellt, die alle auf der Näherung der Einfachstreuung basieren.</p>

Capability of Airline Jets as an Observation Platform for Noctilucent Clouds at Middle Latitudes

The exact occurrence frequency of noctilucent clouds (NLCs) in middle latitudes is significant information because it is thought to be sensitive to long-term atmospheric change. We conducted NLC observation from airline jets in the Northern Hemisphere during the summer 2019 to evaluate the effectiveness of NLC observation from airborne platforms. By cooperating with the Japanese airline All Nippon Airways (ANA), imaging observations of NLCs were conducted on 13 flights from Jun 8 to Jul 12. As a result of careful analysis, 8 of these 13 flights were found to successfully detect NLCs from middle latitudes (lower than 55°N) during their cruising phase. Based on the results of these test observations, it is shown that an airline jet is a powerful tool to continuously monitor the occurrence frequency of NLCs at midlatitudes which is generally difficult with a polar orbiting satellite due to sparse sampling in both temporal and spatial domain. The advantages and merits of NLC observation from jets over satellite observation from a point of view of imaging geometry is also presented.

What caused the frequent and widespread occurrences of noctilucent clouds at middle latitudes in 2020?

The 2020 summer season has revealed frequent occurrences of noctilucent clouds (NLCs) around the Northern hemisphere at middle latitudes (45–55° N), with the lowest latitude at which NLCs were seen being 34.1° N. In order to investigate a reason for this NLC extraordinary summer season, we have analyzed long-term Aura/MLS satellite data for all available summer periods from 2005 to 2020. Both Aura/MLS summer temperature and water vapor in the upper mesosphere and the mesopause region, between 74 and 89 km altitude, have been considered. We have found that there has been a moderate decrease in the upper mesosphere temperature between 2016 and 2020 and no dramatic changes have been observed in temperature in the summer of 2020 at the middle latitude mesopause. At the same time, water vapor concentration has significantly increased (by about 12–15 %) in the zonal mean H2O value in the 2020 summer compared to 2017, meaning that the summer mesopause at middle latitudes has become more wet. At the same time, no increase in water vapor has been detected at the high latitude high altitude mesopause. A combination of lower mesopause temperature and water vapor concentration maximum at middle latitudes is the main reason for frequent and widespread occurrences of NLCs seen around the globe at middle latitudes in the summer of 2020. The 24th solar cycle minimum cannot explain the H2O maximum in 2020 since the correlation between Lyman-α flux and the amount of water vapor is low. The increase in volcanic activity from 2013 to 2015 (and its recent maximum occurred in 2015) explains the increased amount of water vapor in the upper mesosphere for the past years and its maximum in 2020 due to volcanic water vapor being injected into the atmosphere and transported into the upper mesosphere. The 5-year delay between volcanic activity and water vapor maximum is well explained by a general meridional-vertical atmospheric circulation.

Noctilucent Clouds Light Up Northern Germany

A shift in the tropopause jet may have triggered the unusual number of high-altitude clouds that briefly appeared in the early summer of 2019.

On the observation of very bright and abundant Noctilucent Clouds at Kühlungsborn/Germany (54°N, 12°E) in June 2019

<p>Noctilucent Clouds (NLC) are observed since 1997 by a RMR lidar at a mid-latitude site at Kühlungsborn/Germany (54°N, 12°E). In June 2019, we detected the brightest NLC so far, having a backscatter coefficient at 532 nm of ~50<sup>-10</sup> /m/sr, while 2.5<sup>-10</sup> /m/sr is a typical value at this location. Another three NLC in that period reached a backscatter coefficient of more than 20<sup>-10</sup> /m/sr. These strong NLC allow, e.g., for high-resolved studies with temporal resolution of 10 seconds and vertical resolution of 45 m. We will show examples of high-frequency oscillations in our data that cannot be found with typical integration times of several minutes. The period in June 2019 was not only unique in terms of NLC brightness, but also regarding NLC occurrence. While the all-year average is ~6 %, the occurrence rate in 2019 was 13 % and, and 20% if we consider June only. In the past, we found an anti-correlation between solar activity and NLC occurrence: Increasing solar UV radiation results in enhanced radiative heating and photolytic water vapor destruction. However, the high number of NLC in 2019 can only partly be explained by solar activity, even if the Lyman-alpha flux was slightly lower compared to previous years. TIMED/SABER monthly averaged temperature profiles showed an unusual low mesopause in June 2019, related to lower-than-average temperatures below 83 km. We claim that this as the main reason for the comparatively frequent and bright NLC. At the same time, meridional wind data of our nearby meteor radar show only weak southward winds and even a wind reversal at 93 km, which is not typical for the season. We will discuss potential reasons for the strange dynamical situation. We note that the weather dependent lidar observations are in good agreement with the radar observations of ice particles, so-called Mesospheric Summer Echoes (MSE). Co-located radar observations also showed unusually large occurrence rates of MSE in June 2019 as well as the occasion of many MSE below 83 km altitude.</p>

Trends in noctilucent clouds

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