Noctilucent Clouds

Posted by: chaamjamal on: July 17, 2018

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1. Noctilucent Clouds (NLC) occur over North America as frequently as they do in Europe and the U.S.S.R. NLC displays are persistent and last for periods up to and greater than 5 hours, but individual parts including the billow structure often form and decay within a few minutes or tens of minutes. The rapid structural changes in the clouds indicate that the layer in which they are formed is well stirred and often in wave motion.
2. In the Northern Hemisphere NLC are observed predominantly between June 1 and August 15, with the peak of activity occurring around 20 to 30 days after the summer solstice and the brightest and most widespread displays taking place between July 1 and August 15. The optimum latitude for NLC observations is around 60° N. NLC occur far more frequently than previously supposed during the month of July, NLC are seen nearly every night in some part of the Northern Hemisphere. An observer at 60° N might expect to see NLC on about 75% of the clear nights during the month of July. Occasionally NLC displays extend over an area in excess of millions of square kilometers.
3. Recent studies of NLC in the Southern Hemisphere have resulted in the proof of the existence of NLC there and in the determination of some of their characteristics. Southern Hemisphere NLC were found to have a general drift motion toward the west-north-west. NLC were observed at 53° S during the period December 25–January 20, with the brightest and most widespread displays occurring during the first four days of January.
4. A comparison of these results for 53° S with those obtained from stations at 53° N suggests that NLC in the Southern Hemisphere have the same apparent frequency of occurrence with respect to the solstice as NLC in the Northern Hemisphere and that the clouds are likely to be seen at 60° S from December 1 to February 15.
5. Geometrical considerations of NLC observations and observational results show that the clouds are likely to be seen only during the time periods when the solar depression angle (SDA) is between 6° and 16° and that they are most easily detected at SDA from 9° to 14°. At SDA greater than 16°, the 82 km level where the NLC are formed is no longer illuminated by the sun even at the observer’s horizon. An atmospheric screening height of around 30 km appears to be operative in the case of NLC.
6. The collection and statistical analysis of all available data on NLC provides the following picture of their characteristics in the Northern Hemisphere: Color: bluish white Height: (average) 82.7 km Latitude of Observations: 45° to 80°, best at about 60° Season for Observations: March through October, best in June through August Times for Observations: nautical and part of astronomical twilight, SDA = 6° to 16° Spatial Extent: 10 000 to more than 4000 000 km2. Duration: several minutes to more than 5 hours Average Velocity: 40 m/sec towards SW; individual bands often move in different directions and at different speeds than the display as a whole Thickness: 0.5 to 2 km Vertical Wave Amplitude: 1.5 to 3 km Average Particle Diameter: about 0.3 microns Number Density of Particles: 10−2 to 1 per cm3
7. Temperature in Presence of NLC: about 135° K. The available evidence suggests that the dust particles in NLC are of extraterrestrial origin and that they have a volatile coating, the nature of which is uncertain at this time although it is largely assumed to be water substance. The fact that no uncoated particles with diameters greater than 0.20 micron were found in the NLC samples obtained over Sweden in 1962 indicates that particles of this size are absent in the regions above and below the cloud layer. This result suggests that the larger particles may be formed in the NLC layer by coagulation of the smaller ones and that these particles are retained in the NLC layer by some mechanism such as large-scale vertical motions.
8. Calculations of the fall speed of NLC particles indicate that the particles are likely to be of low density (below 1 g/cm3) and/or non-spherical in shape. In view of the large uncertainties remaining as to the nature of NLC particles and the characteristics of the region in which they form, a definitive theory explaining their formation must await further experimental data.
9. A knowledge of the wind and temperature distribution would permit a decision as to whether the observed wave forms are internal gravity waves or interface waves. A knowledge of the temperature and water vapor content during the presence and absence of NLC would also be helpful in the investigation of condensation processes on NLC particles and the changes of NLC appearance. Polarization measurements at scattering angles greater than 90° would assist in determining whether NLC become visible because of an increased concentration of particles at the mesopause or because of an increase in particle size due to coating.
10. Better information about the nature of the particles would help in making more definite theoretical deductions from ground-based optical measurements and more reliable theoretical estimates about the sinking velocities important for the theory of NLC formation. A measurement of the height of the turbopause (altitude below which turbulence is seen) and the turbulent state of the atmosphere in the region in question, by means of artificial vapor trails, could make an important contribution to the Chapman-Kendall theory which postulates a descent of the turbopause to the NLC region. Because of the sometimes observed disappearance of NLC when auroral displays occur, a particularly interesting experiment would be a sequence of temperature measurements in the NLC region when aurora and NLC occur together in order to see whether a warming of the region due to auroral heating can, in fact, be discovered and whether such a warming leads to significant changes or even disappearance of the NLC by removing the coating from the nuclei or by greater turbulence which would reduce the particle concentration.
11. NLC are, even in the latitudes and seasons when they occur, relatively rare phenomena, but their study is related to many other unresolved research questions connected with the mesopause, the lowest layer of the ionosphere, the lowest fringe of the auroral layer, and with the influx of cosmic dust. Thus their continued exploration can contribute greatly to our knowledge, not only of this particular level, but of our whole atmospheric and space environment.  (source: Fogle, & Haurwitz 1966)  [LINK] . [This was Fogle’s PhD dissertation written at a time before all PhD dissertations on atmospheric phenomena had to relate to AGW climate change one way or another.]

 

 

NLC BIBLIOGRAPHY 

1. 1962: Witt, Georg. “Height, structure and displacements of noctilucent clouds.” Tellus 14.1 (1962): 1-18.Observations of noctilucent clouds have been carried out during the summer of 1958 at Torsta (63.3° N; 14.6° E) in Central Sweden. Simultaneous pairs of cloud photographs have been taken with accurate phototheodolite cameras from the end-points of a geodetically determined base-line of length 51.5 km. The picture pairs were subsequently analyzed in stereo instruments (autographs) by which Cartesian space coordinates were obtained for various points in the cloud system. These coordinates, duly corrected for atmospheric refraction, were used for determination of the height of the individual features. Through the stereoscopic effect, measurements could be made on diffuse parts of the cloud system as well as on marked details. Additional information about movements of the cloud system was obtained from a time-lapse film in Kodachrome. The results were plotted and analyzed by conventional methods and maps of the cloud topography at consecutive time intervals could be prepared. In addition to these maps, vertical cross-sections through the cloud system were made as well as detailed studies of particularly interesting cloud features. This paper gives a presentation and interpretation of the results obtained so far and a brief description of the photogrammetric technique applied. The results presented below were obtained during a very bright cloud display with good visibility conditions on August 10–11th, 1958. Thirty pairs of pictures were taken of various parts of the cloud system, which covered the entire northern horizon. Eight of these have been analyzed so far. The results can be summarized as follows. The cloud system moved in a direction north-east to south-west with velocities of the order of 50 to 100 m/s. It consisted of a continuous diffuse layer interchanging with regions of sharply defined features such as systems of parallel billows and bands, blobs and other smaller-scale irregularities of various shapes. The measured heights varied between 81.5 and 85.5 km. The long parallel bands were identified as a system of waves with wavelengths of the order of 50 km and amplitudes up to 4 km which propagated in a direction nearly opposite to that of the cloud system with absolute velocities of the order of 10 to 20 m/s. The wave crests were oriented nearly perpendicular to the main air flow and were continuous over distances of hundreds of kilometers and exhibited local refraction effects. The smaller billows had wavelengths of the order of 5–10 km and amplitudes about 0.5–1.0 km; they moved with the cloud system. The billows showed no preferred orientation and were observed to pass through the crests of the longer waves. It is indicated by the analysis the regular changes in the brightness of these clouds are due to changes of the optical thickness of the cloud layer.
2. 1964: Hemenway, C. L., R. K. Soberman, and G. Witt. “Sampling of noctilucent cloud particles.” Tellus 16.1 (1964): 84-88. Sampling of noctilucent cloud particles by means of sounding rockets has been successfully carried out from northern Sweden in the Summer of 1962. Two successful flights were achieved, one in the presence of noctilucent clouds and one when no such clouds could be visually observed from the ground or from aircraft. The collecting surfaces were exposed between the altitudes of approximately 75 and 98 kilometers during ascent only. The particle concentration in a vertical column through the noctilucent cloud display is found to be greater than 8 × 10¹⁰particles per square meter which is at least one thousand times greater than in the case when no clouds were observed. The integral size distribution of the cloud particles is of the form N = Ad^−p where 3 < p < 4. A significant fraction of the collected cloud particles had a volatile coating prior to collection. The particles were analyzed by electron diffraction, neutron activation, and electron beam microprobe techniques. Electron-beam microprobe analysis has given evidence for iron particles with high nickel content. Calcium films were used as indicators of moisture associated with the collected particles. Study of the exposed and unexposed films flown in the sampling experiments has revealed evidence for moisture. Laboratory simulation of a ring- or halo-patterns found in the electron microscopic examination of the noctilucent cloud particles has been attempted. This was done by impacting ice-coated nickel particles on collecting surfaces similar to those used in the sampling experiment. Ring patterns similar to those observed on the rocket sampling surfaces have been produced. The primary conclusions are that the cloud particles are probably of extraterrestrial origin and that a significant fraction appears to have been coated with terrestrial ice. Plans for future experiments are briefly outlined
3. 1972: Donahue, Thomas Michael, B. Guenther, and J. E. Blamont. “Noctilucent clouds in daytime: Circumpolar particulate layers near the summer mesopause.” Journal of the Atmospheric Sciences 29.6 (1972): 1205-1209. Observations with a horizon scanning airglow photometer on OGO-6 have revealed the presence of a dense scattering layer near 80 km over the geographic pole during the local summer. The layer is detected on all satellite passes above 80° latitude beginning 15 days before the solstice. The optical depth of the layer increases by more than a factor of 50 between 70° and 85°. It is suggested that noctilucent clouds are weak sporadic manifestations of these persistent polar layers.
4. 1982: Turco, R. P., et al. “Noctilucent clouds: Simulation studies of their genesis, properties and global influences.” Planetary and Space Science 30.11 (1982): 1147-1181. Extremely cold mesopause temperatures (<140K) are necessary to form noctilucent clouds; such temperatures only exist at high latitudes in summer. A water vapor concentration of 4–5 ppmv is sufficient to form a visible cloud. However, a subvisible cloud can exist in the presence of only 1 ppmv of H₂O. Ample cloud condensation nuclei are always present in the mesosphere; at very low temperatures, either meteoric dust or hydrated ions can act as cloud nuclei. To be effective, meteoric dust particles must be larger than 10–15 Å in radius. When dust is present, water vapor supersaturations may be held to such low values that ion nucleation is not possible. Ion nucleation can occur, however, in the absence of dust or at extremely low temperatures (<130K). While dust nucleation leads to a small number (<10cm⁻³) of large ice particles (>0.05 μm radius) and cloud optical depths (at 550 nm) ∼10⁻⁴, ion nucleation generally leads to a large number (∼10³cm⁻³) of smaller particles and optical depths ∼10⁻⁵). However, because calculated nucleation rates in noctilucent clouds are highly uncertain, the predominant nucleus for the clouds (i.e., dust or ions) cannot be unambiguously established. Noctilucent clouds require several hours-up to a day-to materialize. Once formed, they may persist for several days, depending on local meteorological conditions. However, the clouds can disappear suddenly if the air warms by 10–20 K. The environmental conditions which exist at the high-latitude summer mesopause, together with the microphysics of small ice crystals, dictate that particle sizes will be ≲ 0.1 μm radius. The ice crystals are probably cubic in structure. It is demonstrated that particles of this size and shape can explain the manifestations of noctilucent clouds. Denser clouds are favored by higher water vapor concentrations, more rapid vertical diffusion and persistent upward convection (which can occur at the summer pole). Noctilucent clouds may also condense in the cold “troughs” of gravity wave trains. Such clouds are bright when the particles remain in the troughs for several hours or more; otherwise they are weak or subvisible. Ambient noctilucent clouds are found to have a negligible influence on the climate of Earth. Anthropogenic perturbations of the clouds that are forecast for the next few decades are also shown to have insignificant climatology implications
5. 1989: Gadsden, Michael, and Wilfried Schröder. “Noctilucent clouds.” Springer, Berlin, Heidelberg, 1989. 1-12. Noctilucent clouds are immediately recognizable, even when being seen for the first time. The name suggests it all: they are night-shining clouds. From mid-latitudes(ø > 50°), they can be seen during the summer in the twilight arch which moves around the north (or south, in the southern hemisphere) horizon as the night progresses. In form much like cirrostratus clouds, they are usually silvery-white or pale blue in colour and they stand out clearly behind the darker twilight sky. Ordinary (i.e. tropospheric) clouds are dark silhouettes under these conditions; noctilucent clouds shine. The reason for this is that noctilucent clouds are very high in the atmosphere and remain in sunlight long after the Sun has set at ground level.
6. 1989: Garcia, Rolando R. “Dynamics, radiation, and photochemistry in the mesosphere: Implications for the formation of noctilucent clouds.” Journal of Geophysical Research: Atmospheres 94.D12 (1989): 14605-14615. The nature of noctilucent clouds, which occur at very great heights and high latitudes during summer, has remained something of a mystery for over 100 years. The realization that the summer mesopause is the coldest region of the Earth’s atmosphere, together with the possibility that transport by atmospheric motions could maintain a substantial mixing ratio of water vapor against very rapid chemical destruction, has led to the present consensus that noctilucent clouds are formed of water ice. A number of recently developed microphysical models have been successful in simulating cloud particle distributions whose characteristics are consistent with satellite radiance observations. However, because of the scarcity of data on temperature, dynamics, and water vapor abundances, these models have had to rely on a number of assumptions about the behavior of these quantities. This paper attempts to illustrate by means of model calculations how various dynamical and photochemical processes interact to produce the unique environment that makes possible the existence of noctilucent clouds. In particular, it focuses on how thermal relaxation influences the altitude and strength of gravity wave breaking and on the effects of such wave breaking on the circulation, temperature distribution, and transport of water vapor near the summer mesopause. It is also shown that, if present understanding of hydrogen chemistry in the mesosphere is even approximately correct, variations in Lyman α radiation should have a significant effect on water vapor abundances near the summer mesopause and, therefore, on the occurrence of noctilucent clouds.
7. 1990: Gadsden, M. “A secular change in noctilucent cloud occurrence.” Journal of Atmospheric and Terrestrial Physics52.4 (1990): 247-251. Evidence is given for a secular change now taking place in the frequency of occurrence of noctilucent clouds. Separate lines of argument lead to the strong supposition that this change occurs as the result of a small, systematic cooling of the upper mésosphère in summertime. The change is likely to have amounted to 7 K over the last 20–30 years. While changes in water vapour concentration will affect the frequency of occurrence, it is just as likely that the changes may be taking place in the mean mesopause temperature. These changes in mean temperature increase the probability of occurrence of a low (threshold) temperature which allows cloud formation.
8. 1993: Fritts, David C., et al. “Wave breaking signatures in noctilucent clouds.” Geophysical Research Letters 20.19 (1993): 2039-2042. Results of a recent modeling study of gravity wave breaking in three dimensions byAndreassen et al. and Fritts et al. showed wave saturation to occur via a three‐dimensional instability oriented normal to the direction of wave propagation. The instability was found to occur at horizontal scales comparable to the depth of unstable regions within the wave field and to lead to substantial vertical displacements and tilting of isentropic surfaces. Because of strong similarities between the wave and instability structures in the simulation and the structure observed in noctilucent cloud layers near the summer mesopause, we have used these model results to compute the advective effects on cloud visibility and structure for a range of viewing angles and cloud layer widths. Our results show the gravity wave breaking signature to provide a plausible explanation of the observed structures and suggest that noctilucent cloud structures may be used in turn to infer qualitative properties of gravity wave scales, energy and momentum transports, and turbulence scales near the summer mesopause.
9. 1996: Thomas, G. E. “Is the polar mesosphere the miner’s canary of global change?.” Advances in Space Research 18.3 (1996): 149-158. The polar mesosphere is an atmospheric region located between latitude 50° and the pole, and between 50 and 90 km. During summer it becomes the coldest region on earth (<130K). This review focuses on past and future alterations of the temperature and water vapor content of this extremely cold region. These two influences are crucial for the formation of mesospheric ice particles in noctilucent clouds (NLC). A recent two-dimensional model study has been conducted of how long-term changes in carbon dioxide (CO₂) and methane (CH₄) concentrations may modify the temperature and water vapor concentration at mesopause heights. The model is a version of the well-known Garcia-Solomon model, modified to include accurate non-LTE cooling in the CO₂ 15 μm band. The existence region of NLC is defined as a domain where water-ice is supersaturated. Reduced levels of CO₂ and CH₄ are found to confine the model NLC existence region to within the perpetually-sunlit polar cap region, where the clouds would no longer be visible to a ground observer. A doubling of CO₂ and CH₄ could extend the NLC region to mid-latitudes, where they would be observable by a large fraction of the world’s population.
10. 1997: Cho, John YN, and Jürgen Röttger. “An updated review of polar mesosphere summer echoes: Observation, theory, and their relationship to noctilucent clouds and subvisible aerosols.” Journal of Geophysical Research: Atmospheres102.D2 (1997): 2001-2020. Peculiar atmospheric radar echoes from the high‐latitude summer mesosphere have spurred much research in recent years. The radar data (taken on frequency bands ranging from 2 to 1290 MHz) have been supplemented by measurements from an increasing arsenal of in situ (rocket borne) and remote sensing (satellites and lidars) instruments. Theories to explain these polar mesosphere summer echoes (PMSEs) have also proliferated. Although each theory is distinct and fundamentally different, they all share the feature of being dependent on the existence of electrically charged aerosols. It is therefore natural to assume that PMSEs are intimately linked to the other fascinating phenomenon of the cold summer mesopause, noctilucent clouds (NLCs), which are simply ice aerosols that are large enough to be seen by the naked eye. In this paper we critically examine both the data collected and the theories proposed, with a special focus on the relationship between PMSEs and NLCs.
11. 2001: Rosenlof, K. H., et al. “Stratospheric water vapor increases over the past half‐century.” Geophysical research letters 28.7 (2001): 1195-1198. Ten data sets covering the period 1954–2000 are analyzed to show a 1%/yr increase in stratospheric water vapor. The trend has persisted for at least 45 years, hence is unlikely the result of a single event, but rather indicative of long‐term climate change. A long‐term change in the transport of water vapor into the stratosphere is the most probable cause.
12. 2002: Wickwar, Vincent B., et al. “Visual and lidar observations of noctilucent clouds above Logan, Utah, at 41.7 N.” Journal of Geophysical Research: Atmospheres 107.D7 (2002). Noctilucent clouds (NLCs) were observed from a midlatitude site (Logan, Utah) on the evenings of 22 and 23 June 1999 mountain daylight time. On both nights the clouds were seen for approximately an hour by experienced observers, and they were photographed. The NLC was also observed on the second evening for approximately an hour in the zenith with the Rayleigh‐scatter lidar at the Atmospheric Lidar Observatory, which is operated by the Center for Atmospheric and Space Sciences on the campus of Utah State University. These observations enabled several of the properties of the cloud to be determined. They were within the range of those observed at higher latitudes, but notably the NLC was very weak and thin. These combined visual and lidar observations unequivocally support the identification of the cloud as a noctilucent cloud. The midlatitude location (41.74°N, 111.81°W) is ∼10° equatorward of previous observations. This equatorward penetration is significant because of potential implications about global change or the global circulation.
13. 2003: Zahn, Ulf. “Are noctilucent clouds a “Miner’s Canary” for global change?.” EOS, Transactions American Geophysical Union84.28 (2003): 261-264. Noctilucent clouds (NLC) occur close to 83 km altitude during summer at polar, high, and mid‐latitudes. They are frequently visible to Earth‐bound observers, provided the observers are on the night side of Earth and the clouds are still illuminated by the Sun. Under these conditions, NLC can become a quite impressive sight. NLC owe their existence to the extremely low temperatures (well below 150 K) which prevail during summer over a wide latitude band in the 82‐ to 90‐km altitude region. For a major review of NLC science, the reader is referred to Gadsden and Schröder [1989].
14. 2007: Karlsson, Bodil, Heiner Körnich, and Jörg Gumbel. “Evidence for interhemispheric stratosphere‐mesosphere coupling derived from noctilucent cloud properties.” Geophysical Research Letters 34.16 (2007). We investigate the link between the cold summer mesopause region and the dynamics in the stratosphere. In particular, we use Odin observations of noctilucent cloud (NLC) properties as a proxy for the state of the summer mesosphere and ECMWF winter stratospheric temperatures as a proxy for the residual circulation in the stratosphere. Large areas of strong anticorrelation between winter stratospheric temperature and summer mesospheric NLC indicate that there is an interhemispheric connection. Time‐lagged cross correlation shows that the wave activity flux at 100 hPa leads the NLC response by several weeks. The presented findings are consistent with recent model studies where the modulation of the mesospheric gravity wave drag by the stratospheric planetary waves yields an interhemispheric stratosphere‐mesosphere coupling.
15. 2017: Kuilman, Maartje, et al. “Exploring noctilucent cloud variability using the nudged and extended version of the Canadian Middle Atmosphere Model.” Journal of Atmospheric and Solar-Terrestrial Physics 164 (2017): 276-288. Ice particles in the summer mesosphere – such as those connected to noctilucent clouds and polar mesospheric summer echoes – have since their discovery contributed to the uncovering of atmospheric processes on various scales ranging from interactions on molecular levels to global scale circulation patterns. While there are numerous model studies on mesospheric ice microphysics and how the clouds relate to the background atmosphere, there are at this point few studies using comprehensive global climate models to investigate observed variability and climatology of noctilucent clouds. In this study it is explored to what extent the large-scale inter-annual characteristics of noctilucent clouds are captured in a 30-year run – extending from 1979 to 2009 – of the nudged and extended version of the Canadian Middle Atmosphere Model (CMAM30). To construct and investigate zonal mean inter-seasonal variability in noctilucent cloud occurrence frequency and ice mass density in both hemispheres, a simple cloud model is applied in which it is assumed that the ice content is solely controlled by the local temperature and water vapor volume mixing ratio. The model results are compared to satellite observations, each having an instrument-specific sensitivity when it comes to detecting noctilucent clouds. It is found that the model is able to capture the onset dates of the NLC seasons in both hemispheres as well as the hemispheric differences in NLCs, such as weaker NLCs in the SH than in the NH and differences in cloud height. We conclude that the observed cloud climatology and zonal mean variability are well captured by the model.
16. 2017: Fiedler, Jens, et al. “Long-term variations of noctilucent clouds at ALOMAR.” Journal of Atmospheric and Solar-Terrestrial Physics 162 (2017): 79-89. Noctilucent clouds (NLC) are measured by the Rayleigh/Mie/Raman-lidar at the ALOMAR research facility in Northern Norway (69°N, 16°E) since 1994. The data set contains about 2860 h of NLC detections and is investigated for the first time regarding trends. NLC properties depend on cloud brightness which is taken into account by the use of several cloud classes, related to brightness ranges. For NLC brighter than the long-term detection limit and strong NLC, respectively, the trend terms show increasing occurrence frequency (+9%/dec and+5%/dec) and brightness (+1.7×10⁻¹⁰ m⁻¹ sr⁻¹/dec and +1.5×⁻¹⁰ m⁻¹sr⁻¹/dec) from 1998 to 2015. In the same period the altitude of faint and long-term limit clouds decreases (−66 m/dec and −108 m/dec). Over the entire time period of 22 years strong clouds show an increasing altitude by +76 m/dec. NLC properties are affected differently by solar and atmospheric parameters. In general, Lyman-α and stratospheric ozone impact all three NLC parameters, temperature at 83 km impacts mainly the NLC altitude. Time series of RMR lidarand SBUV satellite instruments match best for NLC occurrence frequency and brightness when restricting SBUV to the morning data at longitudes around ALOMAR (64–74°N, 8–24°E/0–9 LT). This suggests longitudinal dependent trends, which is confirmed by trend investigations of longitudinal subsets of the SBUV data set.
17. 2017: Von Savigny, Christian, Matthew T. DeLand, and Michael J. Schwartz. “First identification of lunar tides in satellite observations of noctilucent clouds.” Journal of Atmospheric and Solar-Terrestrial Physics 162 (2017): 116-121. Noctilucent clouds (NLCs) are optically thin ice clouds occurring near the polar summer mesopause. NLCs are a highly variable phenomenon subject to different sources of variability. Here we report on a poorly understood mechanism affecting NLCs, i.e., the lunar gravitational tide. We extract remarkably clear and statistically highly significant lunar semidiurnal tidal signatures in NLC occurrence frequency, NLC albedo and NLC ice water content from observations with the Solar Backscatter Ultraviolet (SBUV) satellite instruments using the superposed epoch analysis method applied to a data set covering more than 3 decades. The lunar semidiurnal tide is identified in NLC measurements in both hemispheres. In addition, lunar semidiurnal tidal signatures in polar summer mesopause temperature were extracted from space borne observations with the Microwave Limb Sounder (MLS) and the phases of the lunar tidal signatures in NLC parameters and temperature are demonstrated to be consistent. To our best knowledge these results constitute the first identification of the lunar tide in non-visual NLC observations.
18. 2017: Ugolnikov, Oleg S., et al. “Noctilucent cloud particle size determination based on multi-wavelength all-sky analysis.” Planetary and Space Science 146 (2017): 10-19. The article deals with the analysis of color distribution in noctilucent clouds (NLC) in the sky based on multi-wavelength (RGB) CCD-photometry provided with the all-sky camera in Lovozero in the north of Russia (68.0°N, 35.1°E) during the bright expanded NLC performance in the night of August 12, 2016. Small changes in the NLC color across the sky are interpreted as the atmospheric absorption and extinction effects combined with the difference in the Mie scattering functions of NLC particles for the three color channels of the camera. The method described in this paper is used to find the effective monodisperse radius of particles about 55 nm. The result of these simple and cost-effective measurements is in good agreement with previous estimations of comparable accuracy. Non-spherical particles, Gaussian and lognormal distribution of the particle size are also considered
19. 2018: Köhnke, Merlin C., Christian von Savigny, and Charles E. Robert. “Observation of a 27-day solar signature in noctilucent cloud altitude.” Advances in Space Research 61.10 (2018): 2531-2539. Previous studies have identified solar 27-day signatures in several parameters in the Mesosphere/Lower thermosphere region, including temperature and Noctilucent cloud (NLC) occurrence frequency. In this study we report on a solar 27-day signature in NLC altitude with peak-to-peak variations of about 400 m. We use SCIAMACHY limb-scatter observations from 2002 to 2012 to detect NLCs. The superposed epoch analysis method is applied to extract solar 27-day signatures. A 27-day signature in NLC altitude can be identified in both hemispheres in the SCIAMACHY dataset, but the signature is more pronounced in the northern hemisphere. The solar signature in NLC altitude is found to be in phase with solar activity and temperature for latitudes ≳70°N. We provide a qualitative explanation for the positive correlation between solar activity and NLC altitude based on published model simulations.
20. 2018: Langowski, M. P., et al. “First results on the retrieval of noctilucent cloud albedo and occurrence rate from SCIAMACHY/Envisat satellite nadir measurements.” Journal of Atmospheric and Solar-Terrestrial Physics 175 (2018): 31-39. We present the first results on the retrieval of noctilucent cloud (NLC) albedosand occurrence rates from SCIAMACHY (SCanning Imaging Absorption spectroMeter for Atmospheric CHartography) nadir data. The applicability of already available algorithms is discussed and necessary changes are reasoned. The occurrence rates for different latitude ranges are presented. During the summer period, when NLCs occur, the NLC occurrence rates show a maximum which is strongest at the highest latitudes. This is consistent with other observation methods. For the spring and autumn period, however, false NLC detections are observed at latitudes between 45°N and 65°N, where no NLCs are expected. The reason for this, and why it does not affect the retrieval during the NLC season is discussed. We also compared the SCIAMACHY nadir NLC occurrence rates with the ones retrieved from the SCIAMACHY limb measurements and the ones of SBUV and found qualitative agreement of these data sets.
21. 2018: Lübken, Franz‐Josef, Uwe Berger, and Gerd Baumgarten. “On the Anthropogenic Impact on Long‐Term Evolution of Noctilucent Clouds.” Geophysical Research Letters (2018). Little is known about climate change effects in the transition region between the Earth’s atmosphere and space, roughly at 80–120 km. Some of the earliest observations in this region come from noctilucent clouds (NLC) at ∼83‐km altitude. There is a long‐standing dispute whether NLC are indicators of climate change. We use model simulations for a time period of 138 years to study the impact of increasing CO₂ and H₂O on the development of NLC on centennial time scales. Since the beginning of industrialization the water vapor concentration mixing ratio at NLC heights has increased by ∼40% (1 ppmv) due to methane increase, whereas temperatures are nearly constant. The H₂O increase has led to a large enhancement of NLC brightness. NLC presumably existed centuries earlier, but the chance to observe them by the naked eye was extremely small before the twentieth century, whereas it is likely to see several NLC per season in the modern era. Non-technical explanation for the layman: In our paper we address a problem that is controversially disputed since several decades, namely, whether noctilucent clouds (NLC) in the middle atmosphere are indicators of climate change. NLC are a spectacular optical phenomenon in the summer season at midlatitudes. We show in our paper that (i) NLC are indeed indicators of anthropogenic activity, (ii) the reason for this is increasing water vapor (caused by methane increase), which significantly enhances the visibility of NLC; and (iii) contrary to common understanding, cooling of the middle atmosphere due to increased reduces(!) the visibility of NLC. NLC constitute the earliest observations in this height region. In our model we expose 40 million dust/ice particles to long‐term changes in the middle atmosphere, namely, for 138 years starting with the beginning of industrialization. The model is nudged to the real world in the lower atmosphere. Since the beginning of industrialization,the chance to observe a bright NLC has increased from just one per several centuries(!) to a few per year. We conclude that NLC are indeed an indicator for climate change.
22. 2018: Dalin, P., et al. “Response of noctilucent cloud brightness to daily solar variations.” Journal of Atmospheric and Solar-Terrestrial Physics 169 (2018): 83-90. For the first time, long-term data sets of ground-based observations of noctilucent clouds (NLC) around the globe have been analyzed in order to investigate a response of NLC to solar UV irradiance variability on a day-to-day scale. NLC brightness has been considered versus variations of solar Lyman-alpha flux. We have found that day-to-day solar variability, whose effect is generally masked in the natural NLC variability, has a statistically significant effect when considering large statistics for more than ten years. Average increase in day-to-day solar Lyman-α flux results in average decrease in day-to-day NLC brightness that can be explained by robust physical mechanisms taking place in the summer mesosphere. Average time lags between variations of Lyman-α flux and NLC brightness are short (0–3 days), suggesting a dominant role of direct solar heating and of the dynamical mechanism compared to photodissociation of water vapor by solar Lyman-α flux. All found regularities are consistent between various ground-based NLC data sets collected at different locations around the globe and for various time intervals. Signatures of a 27-day periodicity seem to be present in the NLC brightness for individual summertime intervals; however, this oscillation cannot be unambiguously retrieved due to inevitable periods of tropospheric cloudiness.