Brightness Of Noctilucent Clouds Research Articles

Responses of CIPS/AIM noctilucent clouds to the interplanetary magnetic field

Abstract. This study investigates the link between the interplanetary magnetic field (IMF) By component and the noctilucent clouds (NLCs) measured by the Cloud Imaging and Particle Size (CIPS) experiment onboard the Aeronomy of Ice in the Mesosphere (AIM) satellite. The mean ice particle radius in NLCs is found to be positively correlated with IMF By in the Southern Hemisphere (SH) and negatively correlated with IMF By in the Northern Hemisphere (NH), respectively, on a day-to-day timescale in most of the 20 summer seasons during the 2007–2017 period with a near 0 d lag time, and the response in the SH is stronger than that in the NH. Moreover, the albedo, ice water content and frequency of occurrence of NLCs present positive correlation with IMF By in the SH but no significant correlation in the NH. The superposed epoch analysis (SEA) further indicates the rm on average changes by about 0.73 nm after IMF By reversals, which is significant at the 90 % confidence level in Monte Carlo sensitivity tests. Our results suggest an IMF By-driven pathway: the influence of the solar wind on the polar ionospheric electric potential affects the nucleation processes in NLCs and consequently the ice particle radius and NLC brightness.

Wave-induced variations in noctilucent cloud brightness: model and experimental studies

The paper describes a dynamical (geometrical) model of gravity-wave-induced noctilucent cloud (NLC) brightness. NLC brightness dependences on gravity wave parameters are presented. Results of the inverse-problem solution are addressed as well. It is shown that relative variations of NLC brightness are mainly dependent on the amplitude of a gravity wave, its horizontal wavelength and unperturbed thickness of an NLC layer. It is also shown that the model can be used for determining a geometric amplitude of a perturbing wave from a relative brightness distribution (inverse problem) and that the uncertainty of the geometric wave amplitude is mainly defined by an uncertainty of the cloud layer thickness.

Mesospheric Temperature During the Extreme Midlatitude Noctilucent Cloud Event on 18/19 July 2016

AbstractA rare noctilucent cloud (NLC) event was observed at 48.8∘ N, 13.7∘ E above GERman Experimental Seismic System station in southern Germany on 18/19 July 2016 using the Compact Rayleigh Autonomous Lidar. Strong southward winds due to a quasi 2‐day planetary wave allowed for the influx of mesospheric polar air to midlatitudes on this day. The NLC observed by lidar was preceded by strong NLC displays in Cloud Imaging and Particle Size (CIPS) Experiment satellite images above the North Sea and by strong mesospheric summer radar echoes 800 km north of the lidar site and was also observed visually in central Europe. The NLC occurred at low altitude and was bright and thin with strong oscillations in altitude and brightness. Darkness allowed for high‐resolution temperature measurements at NLC altitudes. The ice particles were embedded in the upper part of a cold region with temperatures below 150 K. Significantly higher temperatures were found directly above the cloud with large vertical temperature gradients of 25 K/km at the top boundary. Spectral analysis reveals that NLC particles existed within cold phases of gravity waves within a region of high static stability. In order to study the evolution of NLC brightness in this environment, we drive the microphysical model Community Aerosol and Radiation Model for Atmospheres with lidar temperature soundings. We find that NLC particles can survive and grow in the conditions defining this midlatitude event. We conclude that the ice particles did not nucleate at the site of observation but were meridionally transported and vertically confined to a thin layer due to a large vertical temperature gradient, wind reversal, and low levels of mesospheric turbulence.

On the Anthropogenic Impact on Long‐Term Evolution of Noctilucent Clouds

AbstractLittle 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.

Response of noctilucent cloud brightness to daily solar variations

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.

Long-term variations of noctilucent clouds at ALOMAR

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 2860h 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−1 sr−1/dec) from 1998 to 2015. In the same period the altitude of faint and long-term limit clouds decreases (−66m/dec and −108m/dec). Over the entire time period of 22 years strong clouds show an increasing altitude by +76m/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 83km impacts mainly the NLC altitude. Time series of RMR lidar and 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/0–9LT). This suggests longitudinal dependent trends, which is confirmed by trend investigations of longitudinal subsets of the SBUV data set.

Diurnal variations of midlatitude NLC parameters observed by daylight‐capable lidar and their relation to ambient parameters

Noctilucent Clouds (NLCs) are an important phenomenon of the summer mesopause region. While relatively common in high latitudes, NLCs are sparse (≤10% occurrence rate) below 60°latitude. We present the first study of diurnal variations of midlatitude NLCs based on lidar observations with full diurnal coverage at Kühlungsborn since 2010 independent from solar elevation. Overall, ∼100h of NLCs with a backscatter coefficient of βmax,532nm>0.5·10−10m−1sr−1are observed within ∼1800h. Occurrence rates decrease regularly from 12% at 5local solar time (LST) to ∼2% at 19LST. The mean NLC brightness varies between ∼1 and ∼3·10−10m−1sr−1with maxima at 4 and 18LST. The simultaneously observed temperatures show a systematic (tidal) variation, but we do not find a direct relation to NLC rates. Comparing NLCs and ambient winds, we find strong indications for the meridional wind (advection) being the main driver for NLC occurrence above our site.

Temporal and spatial characteristics of the formation of strong noctilucent clouds

The 3-D Lagrangian model LIMA/ICE is used to track ice particles forming noctilucent clouds (NLC). Fifty strong NLC events at three different latitudes are analyzed. Visible particles are traced back to their nucleation sites as well as traced forward until sublimation. Particle nucleation occurs in bursts within areas of high supersaturation. We characterize NLC particle growth and vertical transport: Slow growth occurs below the mesopause up to ≈6h before observation. It is followed by rapid growth within the high water vapor zone around 83km during phases of upward winds. At the same time temperature perturbations in these cold phases of waves lead to a high supersaturation. Sublimation occurs quickly after maximum brightness, since sedimentation into subsaturated altitudes is accelerated by downward winds. The duration of particle visibility (β>10% of observed backscatter) is only ≈5h. The mean particle age of all NLC events at 69°N is around 36h, but particle age varies by more than 24h for the different events studied. Although the age of particles in strong NLC depends on latitude, the visibility period does not. The brightness of strong NLC depends mainly on background conditions during the last 3h before observation. This implies that local measurements, e.g. by lidar, are representative for the morphology of strong NLC on scales of several hundred kilometers.

Latitudinal and interhemispheric variation of stratospheric effects on mesospheric ice layer trends

[1] Latitudinal and interhemispheric differences of model results on trends in mesospheric ice layers and background conditions are analyzed. The model nudges to European Centre for Medium-Range Weather Forecasts data below ∼45 km. Greenhouse gas concentrations in the mesosphere are kept constant. Temperature trends in the mesosphere mainly come from shrinking of the stratosphere and from dynamical effects. Water vapor increases at noctilucent cloud (NLC) heights and decreases above due to increased freeze drying caused by temperature trends. There is no tendency for ice clouds in the Northern Hemisphere for extending farther southward with time. Trends of NLC albedo are similar to satellite measurements, but only if a time period longer than observations is considered. Ice cloud trends get smaller if albedo thresholds relevant to satellite instruments are applied, in particular at high polar latitudes. This implies that weak and moderate NLC is favored when background conditions improve for NLC formation, whereas strong NLC benefits less. Trends of ice cloud parameters are generally smaller in the Southern Hemisphere (SH) compared to the Northern Hemisphere (NH), consistent with observations. Trends in background conditions have counteracting effects on NLC: temperature trends would suggest stronger ice increase in the SH, and water vapor trends would suggest a weaker increase. Larger trends in NLC brightness or occurrence rates are not necessarily associated with larger (more negative) temperature trends. They can also be caused by larger trends of water vapor caused by larger freeze drying, which in turn can be caused by generally lower temperatures and/or more background water. Trends of NLC brightness and occurrence rates decrease with decreasing latitude in both hemispheres. The latitudinal variation of these trends is primarily determined by induced water vapor trends. Trends in NLC altitudes are generally small. Stratospheric temperature trends vary differently with altitude in the NH and SH but add up to similar trends at mesospheric cloud heights.

A comparison between ground-based observations of noctilucent clouds and Aura satellite data

A comparison is made between ground-based observations of noctilucent clouds (NLCs), obtained with a network of automated digital cameras, and Aura satellite data (the MLS instrument). The Aura data (water vapor and temperature) demonstrate reasonable values around the summer mesopause fostering NLC formation in June through August, when supersaturated air conditions occur. The temperature decrease leads, in general, to amplification of the NLC brightness. The 2- and 5-day planetary waves, extracted from the Aura temperature field, have definite influence on the brightness variations of NLCs. The temperature behavior around the summer mesopause at 60°N demonstrated a remarkable feature, namely, in 2007 the minimum of a Gaussian fitted seasonal temperature variation was observed, on average, 14 days earlier and was broader than the corresponding minimum in 2008. The different temperature climatology resulted in different seasonal variation of NLCs in 2007 and 2008; in particular, the maximum of a Gaussian fitted seasonal variation of the NLC brightness cycle in 2007 was advanced by 12–29 days relative to that in 2008.

The noctilucent cloud (NLC) display during the ECOMA/MASS sounding rocket flights on 3 August 2007: morphology on global to local scales

Abstract. During the ECOMA/MASS rocket campaign large scale NLC/PMC was observed by satellite, lidar and camera from polar to mid latitudes. We examine the observations from different instruments to investigate the morphology of the cloud. Satellite observations show a planetary wave 2 structure. Lidar observations from Kühlungsborn (54° N), Esrange (68° N) and ALOMAR (69° N) show a highly dynamic NLC layer. Under favorable solar illumination the cloud is also observable by ground-based cameras. The cloud was detected by cameras from Trondheim (63° N), Juliusruh (55° N) and Kühlungsborn. We investigate planetary scale morphology and local scale gravity wave structures, important for the interpretation of the small scale rocket soundings. We compare in detail the lidar observations with the NLC structure observed by the camera in Trondheim. The ALOMAR RMR-lidar observed only a faint NLC during the ECOMA launch window, while the camera in Trondheim showed a strong NLC display in the direction of ALOMAR. Using the high resolution camera observations (t~30 s, Δx<5 km) and the wind information from the meteor radar at ALOMAR we investigate the formation and destruction of NLC structures. We observe that the NLC brightness is reduced by a factor of 20–40 within 100 s which can be caused by a temperature about 15 K above the frostpoint temperature. A horizontal temperature gradient of more than 3 K/km is estimated.

NLC observations during one solar cycle above ALOMAR

Between 1997 and 2007 noctilucent clouds (NLC) have been observed by the Arctic Lidar Observatory for Middle Atmosphere Research (ALOMAR) Rayleigh/Mie/Raman (RMR) lidar in Northern Norway at 69 ∘ N , 16 ∘ E . During a total of 3322 measurement hours, being well distributed over the 11 years, season and local time, NLC were detected for 1272 h. The seasonal extreme values of NLC occurrence, altitude, brightness and vertical extent are found 15–30 days after summer solstice. The year-to-year occurrence frequency of strong NLC is anti-correlated to solar activity until 2004 and decreases monotonously thereafter. This drop in cloud occurrence is not seen by satellite-borne SBUV instruments and raises the question about the interpretation of cloud occurrences observed by different methods. The cloud water content is unchanged or even decreasing after 2004. Considering the entire 11 year period there is no significant anti-correlation between the occurrence of strong clouds and solar activity. The mean NLC altitudes are 83.2 km ± 20 m and 82.5 km ± 30 m for all clouds and strong clouds, respectively. Today's cloud altitudes differ by only a few hundred meters from observations ∼ 115 years ago (82.1 km). Taking into account current geophysical cloud altitude variability of ∼ 1 km , the mean NLC altitude appears to be unchanged. This behavior falls short of an expected altitude decrease of more than 2 km following from temperature trends calculated by current global atmosphere models.

Reduced meteoric smoke particle density at the summer pole – Implications for mesospheric ice particle nucleation

Noctilucent clouds (NLC) and polar mesospheric summer echoes (PMSE) are phenomena that occur in the summertime polar regions due to the presence of ice particles around the mesopause. That ice particles are able to form in a region with such low water vapour concentration as the mesopause is noteworthy. Even though the summer mesopause is the coldest region on Earth, temperatures are generally not low enough for homogeneous nucleation to occur, which necessitates the presence of pre-existing condensation nuclei. The nature of these nuclei has long puzzled the scientific community and many candidates have been suggested, such as particles of meteoric origin, ion clusters, sodium bi-carbonate, sulfate aerosols and soot particles. Out of these the so-called “smoke particles”, i.e. particles re-condensed from ablated meteoritic material, have long been considered the most likely. Generally, it has been believed that these particles exist in numbers of the order of thousands per cubic centimetre at the mesopause. This belief is based on 1-dimensional studies of meteoric material. A recent 2-dimensional model study, which includes the atmospheric circulation from summer to winter pole however, suggests much lower number densities at the summer mesopause. We here investigate the implications of low number densities for the formation of ice particles. We find that even though resulting ice particle distribution may produce typical NLC brightness, the number density of ice particles is not consistent with what is expected for NLC and PMSE. In particular, it is much lower than the ice particle concentration (>1000 cm −3) typically expected to explain the “electron bite-outs” that are frequently observed in the vicinity of PMSE’s. We therefore re-examine the assumptions and parameters that determine the smoke distribution. We show that even though the number of condensation nuclei at the polar summer mesopause can be increased within the uncertainties, the results in most scenarios remain insufficient. We show that charged particles, perhaps in combination with significant deviations from the mean mesospheric state, may be necessary for condensation of ice particles in the polar summer mesosphere. Hence, we raise the question whether the conventional ideas of nucleation on meteoric smoke, which are used in current mesospheric ice models, are correct.

Interhemispheric comparison of mesospheric ice layers from the LIMA model

The new LIMA/ice model is used to study interhemispheric temperature differences at the summer upper mesosphere and their impact on the morphology of ice particle related phenomena such as noctilucent clouds (NLC), polar mesosphere clouds (PMC), and polar mesosphere summer echoes (PMSE). LIMA/ice nicely reproduces the mean characteristics of observed ice layers, for example their variation with season, altitude, and latitude. The southern hemisphere (SH) is slightly warmer compared to the NH but the difference is less than 3 K at NLC/PMC/PMSE altitudes and poleward of 70 ∘ N / S . This is consistent with in situ temperature measurements by falling spheres performed at 69 ∘ N and 68 ∘ S . Earth's eccentricity leads to a SH mesosphere being warmer compared to the NH by ∼ 2 – 3 K up to approximately 85 km and fairly independent of latitude. In general, NH/SH temperature differences in LIMA increase with decreasing latitude and reach ∼ 10 K at 50 ∘ . The latitudinal variation of NH/SH temperature differences is presumably caused by dynamical forcing and explains why PMSE are basically absent at midlatitudes in the SH whereas they are still rather common at similar colatitudes in the NH. The occurrence frequency and brightness of NLC and PMC are larger in the NH but the differences decrease with increasing latitude. Summer conditions in the SH terminate earlier compared to NH, leading to an earlier weakening and end of the ice layer season. The NLC altitude in the SH is slightly higher by 0.6–1 km, whereas the NLC altitudes itself depend on season in both hemispheres. Compared to other models LIMA/ice shows smaller interhemispheric temperature differences but still generates the observed NH/SH differences in ice layer characteristics. This emphasizes the importance of temperature controlling the existence and morphology of ice particles. Interhemispheric differences in NLC/PMC/PMSE characteristics deduced from LIMA/ice basically agree with observations from lidars, satellites, and radars.

Comparison of long-term Moscow and Danish NLC observations: statistical results

Abstract. Noctilucent clouds (NLC) are the highest clouds in the Earth's atmosphere, observed close to the mesopause at 80–90 km altitudes. Systematic NLC observations conducted in Moscow for the period of 1962–2005 and in Denmark for 1983–2005 are compared and statistical results both for seasonally summarized NLC parameters and for individual NLC appearances are described. Careful attention is paid to the weather conditions during each season of observations. This turns out to be a very important factor both for the NLC case study and for long-term data set analysis. Time series of seasonal values show moderate similarity (taking into account the weather conditions) but, at the same time, the comparison of individual cases of NLC occurrence reveals substantial differences. There are positive trends in the Moscow and Danish normalized NLC brightness as well as nearly zero trend in the Moscow normalized NLC occurrence frequency but these long-term changes are not statistically significant. The quasi-ten-year cycle in NLC parameters is about 1 year shorter than the solar cycle during the same period. The characteristic scale of NLC fields is estimated for the first time and it is found to be less than 800 km.

Mean diurnal variations of noctilucent clouds during 7 years of lidar observations at ALOMAR

Abstract. From 1997 to 2003, noctilucent clouds (NLC) were observed by lidar above the ALOMAR observatory in Northern Norway (69° N) during a total of 1880 measurement hours. This data set contains NLC signatures for 640h, covering all local times, even during the highest solar background conditions. After data limitation imposing a threshold value of 4x10-10m-1sr-1 for the volume backscatter coefficient of the NLC particles, a measure for the cloud brightness, local time dependencies of the NLC occurrence frequency, altitude, and brightness were determined. On average, over the 7 years NLC occurred during the whole day and preferably in the early morning hours, with a maximum occurrence frequency of ~40% between 4 and 7 LT. Splitting the data into weak and strong clouds yields almost identical amplitudes of diurnal and semidiurnal variations for the occurrence of weak clouds, whereas the strong clouds are dominated by the diurnal variation. NLC occurrence, altitude, as well as brightness, show a remarkable persistence concerning diurnal and semidiurnal variations from 1997 to 2003, suggesting that NLC above ALOMAR are significantly controlled by atmospheric tides. The observed mean anti-phase behavior between cloud altitude and brightness is attributed to a phase shift between the semidiurnal components by ~6h. Investigation of data for each individual year regarding the prevailing oscillation periods of the NLC parameters showed different phase relationships, leading to a complex variability in the cloud parameters.

Noctilucent clouds and the mesospheric water vapour: the past decade

Abstract. The topic of this paper is the sensitivity of the brightness of noctilucent clouds (NLC) on the ambient water vapour mixing ratio f(H2O). Firstly, we use state-of-the-art models of NLC layer formation to predict NLC brightness changes in response to changes in the 80km mixing ratio f(H2O) for the two cases of ground-based 532nm lidar observations at 69° N and for hemispheric satellite SBUV observations at 252nm wavelength. In this study, we include a re-evaluation of the sensitivity of NLC brightness to changes in solar Lyman α flux. Secondly, we review observations of episodic changes in f(H2O) and those in NLC brightness, the former being available since 1992, the latter since 1979. To this review, we add a new series of observations of f(H2O), performed in the Arctic summer at the ALOMAR observatory. The episodic change exhibited by the Arctic summer means of f(H2O) turns out to be quite different from all those derived from annual means of f(H2O). The latter indicate that since 1996 a significant reduction of annually averaged upper mesospheric water vapour has occurred at low, mid, and high latitudes. These decreases of f(H2O) have been observed over the same time period in which a slow increase of SBUV NLC albedo has occurred. From this scenario and additional arguments we conclude that the cause for the observed long-term increase in NLC albedo remains to be identified. We close with comments on the very different character of decadal variations in NLC brightness and occurrence rate.

Forty years of noctilucent cloud observations near Moscow: Database and simple statistics

The observation procedure and relevant database of Moscow (56°N, 37°E) noctilucent cloud (NLC) systematic observations (1962–2001) are described. The long‐term series of Moscow NLC data are analyzed and compared to the observations in western Europe. Statistical analysis of seasonally averaged parameters of the NLCs is carried out. Characteristic periods in interannual variability of the integral NLC brightness fluctuation are extracted. Particular attention is paid to decadal (∼10 years) periodicity in the NLC occurrence and their brightness. A distinct difference between decadal periodicity in NLC parameters and solar activity is found and its possible origin is discussed.

Noctilucent clouds getting brighter

Noctilucent cloud (NLC) observations in northwest Europe show a significant secular increase in the occurrence frequency between 1964 and 1994. Although model computations predicted a brightness increase due to increasing stratospheric methane concentrations and a consequent increase in the number of observed NLCs, earlier investigations of the histograms of the solar depression angles at the first and last NLC sightings did not reveal any noticeable brightness change. However, a reinvestigation of the histograms based on statistical model predictions, χ2 tests, and least squares fitting clearly indicates that the NLC brightness increased by a factor of the order of 5 between the periods 1964–1974 and 1985–1994. The brightness increase goes along with a significant lengthening of the NLC season. Considering recently published trends in the upper mesospheric water vapor mixing ratio and temperature strongly suggests that the brightness increase is caused by an increase of the water vapor concentration rather than a decrease in the temperature. For 1985–1994 the analysis also predicts at least a doubling of the NLC brightness between dusk and dawn, in good agreement with lidar observations.

The North-West Europe data on noctilucent clouds: a survey

There are published data from a set of observers who have been, and are, observing under an almost-unchanged protocol without interruption from 1964. These data have already been used to demonstrate that there has been a secular increase in frequency of occurrence of noctilucent clouds. There is a 10.4-year modulation of this increase which lags on the solar activity cycle. It could be that this modulation is related to a periodic variation in the height of the 30 mb pressure level, as described by Labitzke and van Loon. There is no indication that the general increase over the years arises from a change in brightness of noctilucent clouds. The seasonal variation shows no change over the last three decades; there has been no change in the length of the season concomitant with either the secular change or with the 10.4-year periodic change seen superposed on the secular change. There are no preferred days for noctilucent clouds appearance, e.g., there is no connection with meteor showers. Noctilucent clouds are seen more frequently after midnight. There is no long term change in the latitude of the southern edge to be seen in the data; there is a tendency for noctilucent clouds to be slightly further North (by 0.9°) in the last third of the season. During the night, the noctilucent clouds appear to drift northwards by about 3° before midnight and return southwards by the same amount after midnight. Their azimuth of symmetry in the twilight sky is 351° i.e., the southern edge of noctilucent clouds appears to be aligned 9° from due East-West.