Geophysical Research Letters

On the horizontal and temporal structure of noctilucent clouds as observed by satellite and lidar at ALOMAR (69N)



[1] Simultaneous and common volume observations of Noctilucent Clouds (NLC) and Polar Mesospheric Clouds (PMC) have been performed above the ALOMAR research station in Northern Norway (69°N, 16°E) from ground and space, respectively. A detailed case study on August 5, 2008 shows that the measured particle sizes and T-matrix simulations of the optical properties allow to combine the two observation techniques. From the ground, the observations were performed by lidar sounding of the temporal evolution of the cloud at two locations separated by about 40 km, before, during and after the coincidence. From space, the CIPS instrument onboard the AIM satellite observed the horizontal structure of the cloud. Using mesospheric radar wind measurements at ALOMAR the advection of the cloud particles is calculated and the temporal evolution of the cloud as seen from ground is compared with the horizontal structure observed from satellite. This comparison allows estimation of the timescales during which the clouds behave as passive tracers. We find that during this case study cloud structures larger than about 5 km × 5 km and oscillations slower than about one minute behaved like a passive tracer for up to one hour corresponding to horizontal scales of about 300 km. However, if the cloud shows wave structures with brightness modulations of 20% microphysical changes might take place on scales of minutes and kilometers.

1. Introduction

[2] Noctilucent clouds (NLC), also called Polar Mesospheric Clouds (PMC) when observed from space, have attracted attention for more than a century [Gadsden and Schröder, 1989]. These clouds have been well studied because of their appearance at unusually high altitudes in the atmosphere, enhancing the sky brightness after sunset. PMC/NLC provide an excellent tracer to study processes in the middle atmosphere, e.g., the wavy structure in the clouds directly reveals dynamical processes in the vicinity of the clouds that are otherwise difficult to study [Jesse, 1896; Witt, 1962; Dalin et al., 2010]. Although the main question about their nature (icy particles) seems to be solved, they still attract a wide community of researchers [Hervig et al., 2001; Rapp and Thomas, 2006]. The clouds seem to be an excellent indicator for multi decadal to short scale variations in the mesopause region where our knowledge about the processes are still limited [Thomas et al., 1991; Lübken et al., 2009; Chandran et al., 2010]. Different remote sensing methods are used to investigate the clouds, spanning a wide spectrum of resolution from a few hundred meters for ground based observations to several hundred kilometers for space based methods [Witt, 1962; Reimuller et al., 2011]. Previous studies using combined lidar and satellite based data usually had to introduce an extended coincidence criterion (∼2.5 hours, ∼500 km) [e.g., von Savigny et al., 2009]. Besides the aspect of comparing different observation methods it is unclear to what extent the temporal evolution of NLC at a single location is caused by advection or in-situ changes. While it was often assumed that advection plays a major role, it was observed that due to huge horizontal gradients in the background conditions fast in-situ changes can occur [Fritts et al., 1993; Baumgarten et al., 2009]. We report on the first coincident observation of NLC/PMC by lidar and satellite allowing to combine the information about the temporal evolution as observed by lidar with the horizontal information observed from space.

2. Method and Database

[3] The ALOMAR RMR lidar is a twin system with two 1.8 m diameter steerable telescopes. It utilizes a combination of strong spatial and spectral filtering to obtain daylight capability [von Zahn et al., 2000]. As a result NLC can be detected during all local times of the Arctic summer [e.g., Fiedler et al., 2011]. The lidar was operated with a range resolution of 50 m and a temporal resolution of 30 seconds. For the data shown here both telescopes were tilted 20 degrees off zenith. One of the telescopes was pointed towards north, the other towards east. This setup was chosen for strato- and mesospheric wind measurements and results in a horizontal separation of the two measurement volumes at the height of the NLC layer of 39 km. We use the 532 nm backscatter coefficient to monitor the cloud evolution after smoothing the data with a FWHM = 475 m binomial filter in range and a one minute temporal averaging. The particle size is determined from spectral observations at 355 nm, 532 nm and 1064 nm [Baumgarten et al., 2010]. We have analyzed 431 hours of lidar observations acquired between June 1 and August 15, 2008.

[4] The Cloud Imaging and Particle Size experiment (CIPS) on board the Aeronomy of Ice in the Mesosphere (AIM) satellite is a downward looking panoramic imager with a field of view of 120° (along-track) by 80° (across-track) or about 2000 × 1000 km [Russell et al., 2009; McClintock et al., 2009]. The CIPS observations of Orbit 06972 between 66° N and 72° N are shown in Figure 1. CIPS has an unprecedented spatial resolution. In order to derive the PMC morphology and cloud particle size, CIPS measures scattered sunlight at 265 nm. The observed signals include Rayleigh scattering by molecules as well as Mie-scattering by the PMC particles. The Rayleigh scattering signal is removed from the observed signal to infer PMC albedo. We show Level 2 data from the most recent V4.20R4 data available with a horizontal resolution of 25 km2 accessible through We have compared the lidar observations at ALOMAR with CIPS observations around ALOMAR (±0.5° Latitude, ±1° Longitude) for the summer 2008. We found 122 orbits with observations above ALOMAR between June 1 and August 15. 51 of those orbits show albedo enhancements above 2.8 × 10−6/sr. The threshold was chosen by the non-PMC albedo fluctuations outside the season. We selected the case of August 5, 2008 12:13 UT because (1) the CIPS image in the nadir showed the best overlap with the two lidar measurement volumes and (2) the lidar observed a strong NLC [Fiedler et al., 2003]. CIPS data V4.20R4 includes a particle size determined by albedo measurements at different scattering angles. The particle size retrievals make use of a phase function based on mean and width observations of the particle size ensemble [Baumgarten et al., 2010]. To estimate the amount of lidar operation that can be compared to CIPS observations we assume that one CIPS overpass is comparable to 1 h of lidar data (cf. discussion and conclusions). We find that about 4.2 % of lidar data in 2008 have simultaneous CIPS observations at ALOMAR. On the other hand about 15% of the CIPS overpasses are covered with lidar operation. So although the lidar operation is limited by the weather conditions, the combination of the two instruments leads to a number of coincident observations at ALOMAR. To compare the temporal and horizontal structures observed by the different methods we calculate two simple back and forward trajectories under the assumption that an air parcel is advected by the local mean wind. At the time of the coincident observation the trajectories go through the two lidar beams. We use the hourly mean wind as measured with the ALOMAR MF-radar at the altitude of the NLC [Singer et al., 1997]. During the coincident observation we measured horizontal wind vectors of (u = −54.4 m/s, v = −4.0 m/s) and (u = −48.1 m/s, v = −4.5 m/s) at 82 km and 84 km respectively.

Figure 1.

CIPS observation of PMC albedo observed during Orbit 06972 on August 5, 2008 between about 12:10 and 12:20 UT above northern Scandinavia. The color scale is linear from 0 to 40 × 10−6/sr. The data is plotted from CIPS Version 4.20R4.

3. Observation and Results

[5] The lidar observations around the time of the satellite pass over ALOMAR are shown in Figure 2. Both lidar systems observed a strong NLC with a centroid altitude of 83 km. We observed modulation of the centroid altitude by about 1 km. These modulations occurred on scales smaller than the separation of the lidar measurement volumes of about 40 km so that the NLC was observed, e.g., 1 km lower in the northward pointing system than in the eastward pointing system, indicating a tilt of the cloud layer. However the maximum tilt we observed was less than 1.3°. There were periodic instances of a double layer structure as well as periodic enhancements in brightness in the cloud, however these small scale modulations were embedded in a wider layer most of the time.

Figure 2.

Temporal evolution of NLC as observed by the two systems of the RMR-Lidar. The line of sight is tilted 20° off zenith towards the East and the North. The time of the AIM overpass is indicated by the vertical line at 12:13 UT.

[6] Simultaneous observations of particle properties were performed with the northward pointing lidar system. The standard analysis based on 14 minute mean profiles at the peak of the layer gave mean volume equivalent particle radii of 29 nm to 74 nm. Distribution widths range from 7 nm to 18 nm and the number densities fall between 100 and 300/cm3 during the lidar operation on August 5, 2008. These are in the range of typical values for strong NLC [Baumgarten et al., 2008].

[7] In Figure 3 we show the CIPS observations localized around ALOMAR. We observe a small patch of an enhanced PMC with a peak albedo of 37 × 10−6/sr just between the two lidar volumes, and about 50 km to the south-east of ALOMAR. The albedo shows wave structures with north-east oriented wave fronts. The horizontal wavelength of the PMC structures is about 50 km with an albedo modulation of about 15 × 10−6 /sr peak to valley. To compare the temporal and horizontal scales of the different instruments we extract the CIPS albedo along a linear trajectory of air parcels that were moving with the mean wind between 82 km and 84 km of 51.4 m/s (u = −51.3 m/s, v = −4.3 m/s). These are indicated by dashed lines in Figure 3. For an actual comparison of the cloud brightness as observed by CIPS and lidar we calculate the vertical integrated backscatter coefficient (βint) and compare those with the cloud albedo along the trajectories going through the two measurement volumes of the lidar. In Figure 4 we show βint for the two time series observed by the lidars to the north and to the east of ALOMAR. Here t0 = 0 corresponds to the time of the coincidence. Negative distances correspond to locations west of ALOMAR while positive values are found to the east. In each panel we also show the CIPS albedo at a scattering angle of θ = 90° (Aλ=265nmθ=90°) and the equivalent βint(CIPS) as calculated from Aλ=265nmθ=90° albedo and particle size retrieval. To calculate

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we take the particle size (r: mean of a Gaussian size distribution) as measured by CIPS and calculate color ratios (CR) as:

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using differential scattering cross sections ( inline image) from T-matrix calculations. The distribution widths comes from parametrized dependency of s on r as observed by Baumgarten et al. [2010]. The ratio of the left and right scale in Figure 4 was chosen to match CR265532(r, s) for particles of volume equivalent radius r = 45 nm and a corresponding distribution width of s = 15.5 nm [Baumgarten et al., 2010]. The range of the blue lines spans the range of particle sizes between r = 40 nm and r = 50 nm with corresponding distribution widths. We observe that Aλ=265nmθ=90° and βint(lidar) agree at the time of the coincident observations (t0 = 0) better than 10% for the north volume, while they agree in the east volume only by about 25%. The agreement is improved when taking into account the actual particle size as observed by CIPS. In the north and east measurement volumes CIPS observed a particle size of 41 nm and 49 nm respectively. The lidar observed a mean particle size of 37.5 ± 7 nm in the north volume at the time of the overpass ±15 minutes. The value of the lidar is calculated from the vertically integrated signals under the assumption of a Gaussian particle size distribution, similar to the method used by CIPS. In both systems we find an agreement of βint(lidar) and βint(CIPS) of better than 5% where βint(CIPS) shows the lower value in both systems.

Figure 3.

CIPS observation of PMC albedo observed around ALOMAR. Black dotted lines indicate the trajectories of air parcels at the altitude of NLC as derived from MF-radar wind observations at ALOMAR. The air parcels move from the east to the west. The measurement volumes are indicated by black/white circles. The color scale is linear from 0 to 40 × 10−6/sr.

Figure 4.

Vertically integrated lidar backscatter coefficient (green line, left scale) compared to the CIPS 90° albedo (blue line, right scale) extracted along the trajectories for the (top) northward and (bottom) eastward pointing lidar system. The red line (left scale) shows the backscatter coefficient calculated from the CIPS 90° albedo and CIPS particle size retrieved using the width over size ratio as measured by the lidar [Baumgarten et al., 2010].

[8] For the north volume we observe an agreement of the NLC brightness from about t0 − 0.75 h to t0. Even after the coincidence to about t0+ 20 min lidar and CIPS agree reasonably well. The time-frame for a good agreement is less for the east volume and is only from aboutt0 − 30 min to t0.

[9] There are several reasons for a difference of βint(CIPS) and βint(lidar) at times other than t0: 1) different sampling areas of the two systems, 2) the optical model to calculate CR265532(r, s) is not appropriate, 3) the air parcels do not follow the wind as observed by the radar, 4) the NLC is not inert during the time-frame investigated. For CIPS the footprint is about 5 × 5 km while it is only about 0.02 × 3.6 km for the lidar [Baumgarten and Fiedler, 2008]. Since we find a good agreement between CIPS and lidar at t0 we consider the different size of the sounding volumes to play only a minor role. Similarly the optical model appears to be valid and allows to calculate βint(CIPS) from Aλ=265nmθ=90° and vice versa. This conversion seems to work at least for βint(CIPS) in the range of 2 × 10−6/sr to 5.5 × 10−6/sr. It is noteworthy that the NLC layer showed an unusual large vertical extent of about 4 km including a double (triple) layer structure in the east (north) volume at t0. Multiple layers, introduced by small-scale gravity waves, could be an indication of different populations of particles in the layer. Nevertheless the optical model using a mono-modal Gaussian size distribution leads to a good agreement of lidar and CIPS. For times other than t0 we do not expect a perfect agreement. But still between t0 + 0.25 h and t0 + 1 h, when the most intense patch of NLC/PMC is observed by the lidar and CIPS in the north volume, a modulation of 20% in NLC (PMC) brightness is observed with a duration of about 30 minutes (150 km) from peak to peak. For the east volume the lidar shows a brightness modulation with a period of 30 minutes between t0 − 0.5 h and t0 + 0.75 h. Around the east volume CIPS observes a brightness modulation with a separation of the brightness bands of 150 km and a relative amplitude comparable to the amplitude seen by the lidar. At t0 − 0.5 h and t0 + 1 h a strong brightness increase and decrease is found respectively. Again the relative brightness changes agree for both instruments. This qualitative agreement of the brightness modulation indicates that the temporal variation of the cloud brightness as seen by the lidar is partly an advected horizontal structure.

[10] After t0 + 1.5 h CIPS and lidar disagree where the brightness of the lidar appears to be enhanced compared to the CIPS observations. Furthermore the CIPS albedo shows no structures similar to those observed by the lidar. While this could be caused by actual microphysical changes of the cloud, it could also be caused by the too simple trajectory calculation used. As the horizontal structure of the cloud (Figure 3) shows stronger albedo enhancements only south and slightly east of ALOMAR a hypothetical trajectory would have to be oriented roughly 90° off the actual wind measurements to find a better agreement between CIPS and lidar. Following this idea, we exemplarily applied different trajectories. However the resulting CIPS albedo timeseries compare worse to the lidar as the one shown in Figure 4. So there are strong indications that the differences are actually caused by microphysical changes in the cloud.

[11] As the maximum βint(CIPS) at +50 km in the north volume arrives about 10 minutes later at the lidar than expected from the simple advection model we speculate that the structure arises from a wave traveling against the mean wind, with a component of the intrinsic phase speed of about 20 m/s along the direction of the mean wind. To improve the agreement of the triple structure of βint(lidar) in the east volume with CIPS we would require the PMC structures to move northwards instead of southwards. For example microphysical changes of the cloud properties induced by a wave in the background temperature with uwave,intrinsic ∼ +20 m/s and vwave,intrinsic∼ +10 m/s could generate cloud structures similar to the ones observed in CIPS. Including the advection (Doppler-shift) such cloud structures would be traveling towards the north-west when observed from space. The fact that the separation of the two brightness peaks is smaller in the lidar observations as compared to CIPS could be an indication that the wave is bent by the wind field in a way that its propagation direction is stronger against the wind.

4. Discussion and Conclusions

[12] We have performed for the first time a coincident observation of NLC/PMC by lidar and satellite where the temporal and horizontal overlap is better than 1 minute and 5 km. This coincidence was possible due to the extensive lidar observations regularly performed at ALOMAR combined with the big field of view and a high resolution of the cameras of the CIPS instrument on board the AIM satellite. We observe unexpected good agreement between the two instruments, where the brightness of the cloud observed by both instruments in the two sounding volumes agrees to better than 5 % when taking the particle size properties into account. Furthermore we find a qualitatively good agreement between the temporal variations observed by the lidar and the horizontal structure as observed by CIPS. The structures can be compared using a simple advection model, indicating that the periodic structures in the PMC are embedded in the PMC layer and being advected by the horizontal wind. As the structures agree pretty well from about t0 − 0.75 h to t0 = +20 min for the north volume this means that the vertical column of the cloud can be treated as passive tracer for ±0.5 h corresponding to about ±150 km during this observation. On the other hand the agreement is less good for the east volume showing a much lower correlation length. So the clouds can remain more or less passive and are advected sometimes, but this is not necessarily always the case. During the observation in the north volume we find a modulation of the cloud brightness with two consecutive brightness maxima after about 30 minutes. These structures can be traced to horizontal variations along the trajectory with horizontal scales of about 150 km. In fact these bands seem to be traces of waves with a wavelength of about 50 km, where the normal to these bands is inclined by 60°–70° to the trajectory of the air. We find the combination of the two instruments together with wind information from radar a valuable tool to investigate the noctilucent cloud evolution on scales of minutes to hours and 5 km to 100 km. Taking the particle size properties into account when comparing even the brightness observations of different instruments is essential.


[13] We gratefully acknowledge the support of the ALOMAR staff in running the instrument and maintaining the infrastructure. The AIM mission is supported by NASA's Small Explorer's Office.

[14] The Editor thanks two anonymous reviewers for their assistance in evaluating this paper.