J. Fiedler

Size distributions of NLC particles as determined from 3‐color observations of NLC by ground‐based lidar

From June to August 1998 the ALOMAR Rayleigh/Mie/Raman lidar, located at 69°N and 16°E in Northern Norway, repeatedly observed noctilucent clouds (NLCs) overhead the lidar. Due to a recent upgrade in detector technology, the lidar was able to obtain 151 hours of NLC observations, simultaneously at 355, 532, and 1064 nm. For the 11 strongest NLC events, we have calculated size distributions for the NLC particles from the backscatter ratios measured at the 3 wavelengths and using the assumptions of spherical ice particles with a monomodal lognormal size distribution. For all events evaluated at the layer maxima, we obtain well-defined median radii rmed, width parameters σ, and particle number densities NNLC for the NLC particle distributions. Mean values for 10 out of the 11 events are rmed = 51 nm, σ = 1.42, and NNLC = 82 cm−3.

Noctilucent clouds above ALOMAR between 1997 and 2001: Occurrence and properties

[1]xa0We report on observations of noctilucent clouds (NLCs) by a ground-based lidar located in northern Norway at 69°N, 16°E. The ALOMAR Rayleigh/Mie/Raman (RMR) lidar conducted measurements of the Arctic middle atmosphere from 1 June to 15 August during each year from 1997 to 2001. This data set contains 1122 hours of lidar observations whereof 408 hours include NLC signatures. The interannual variation of the NLC occurrence frequency shows a decrease of strong NLCs, while weak NLCs occur more frequent. The seasonal variation of the NLC occurrence shows a well pronounced core period where NLCs appeared during 43% of the time. The basic properties of NLCs are characterized by three parameters: maximum value of the volume backscatter coefficient βmax (≡brightness), centroid altitude zc, and half width δz (≡thickness). A typical NLC above ALOMAR during the 5-year period reported here owns a brightness of βmax = 9.6 × 10−10 m−1 sr−1, an altitude of zc = 83.3 km, and a thickness of δz = 1.2 km. The interannual variation of the parameters shows a decrease of the brightness, an increase of the altitude, and a nearly constant thickness, while seasonal variability is higher than these interannual changes. During the core period, the NLCs are noticeably brighter than at the beginning as well as the end of the season. Altitude and thickness of NLCs decrease during the season.

Tidal variations of noctilucent clouds measured at 69°N latitude by groundbased lidar

During June/July 1997 we observed with a groundbased lidar noctilucent clouds (NLCs) overhead the ALOMAR observatory in Northern Norway (69°N, 16°E). These daylight observations of NLC were made possible by use of a very small field-of-view for the lidar receiving telescope in combination with a narrowband double-etalon Fabry-Perot interferometer in front of the photon counting detectors. During 148 hours of lidar measurements, NLCs were observed for 69 hours. Thus, the mean occurrence frequency of NLC was 46%. The observed NLCs exhibited marked variations of the layer altitude and backscatter ratio with local time (LT). The temporal behaviour of both the NLC altitude and NLC backscatter ratio are dominated by a semidiurnal variation. The NLC altitude attains maxima near 13 LT and 01 LT and minima near 07 LT and 19 LT. The backscatter ratio clearly maximizes at times of minimum NLC layer altitude. These observations point to a strong control of variations in NLC altitude and backscatter ratio by a stable semidiurnal tide. The mean centroid NLC altitude is 82.7 km.

Particle properties and water content of noctilucent clouds and their interannual variation

[1]xa0Noctilucent clouds (NLC) have been observed by a multicolor lidar in northern Norway (69°N, 16°E). From three backscatter coefficients we calculate the parameters of a monomodal particle size distribution. We deduce the mean of the size distribution, the width, and the average number density of the ensemble. Using the backscatter coefficients at the peak of the layer the particle size above ALOMAR is investigated by comparing the observations with model results for spherical and aspherical particles assuming either lognormal or Gaussian size distribution. From the analysis of 645 particle size soundings (142 h of measurements) we find that the average size of all NLC particles above ALOMAR from 1998 to 2005 is 47.7 ± 1 nm for cylinders with Gaussian distribution while it is 39.7 ± 1 nm for the traditional model having spherical particles with lognormal distribution. The distribution width is 16.6 ± 0.5 nm for Gaussian distributed cylinders while the particle number density is 85 ± 6 cm−3. We compare our results in detail to previously published measurements and find a satisfying agreement between the observations taking into account the limitations of previous studies and the different locations of the measurements. From the particle properties we calculate a mean surface density of (4.4 ± 0.2) × 10−8 cm2/cm3 and a mean volume density of (6.0 ± 0.2) × 10−14 cm3/cm3. The mean volume density of faint and strong clouds is 1.6 × 10−14 cm3/cm3 and 7.9 × 10−14 cm3/cm3, respectively. From the volume density we calculate the year-to-year variation of the seasonal mean cloud water content to be about 40% and the average observable NLC ice mass flux through 70°N to be about 11 kilotons.

Seasonal and latitudinal variation of noctilucent cloud altitudes

[1]xa0We present a summary of ∼1500 hours of lidar measurements of noctilucent cloud (NLC) altitudes at Kuhlungsborn (54°N), ALOMAR (69°N), and Spitsbergen (78°N). Mean centroid altitudes (zc) are 82.75 km, 83.33 km, and 83.68 km, respectively. Standard deviations σ representing geophysical variability are ∼0.7–1.6 km. Errors of mean heights Δzc are much smaller (60–340 m). Several processes which are not yet fully understood influence zc. Therefore, σ is a better measure of uncertainties in zc compared to Δzc. The increase of NLC heights with latitude is very small (43 ± 65 m/deg) and statistically not significant. At ALOMAR zc varies with season by up to ∼1–1.5 km. NLC heights accumulate around climatological temperatures of 145 K at 69°N and 78°N (higher at 54°N). The LIMA model nicely reproduces observations and indicates that non-variation of NLC heights at polar latitudes is caused by temperature uniformity: the 145 K isotherm varies in height by less than 250 m from the pole to 60°N.

Noctilucent clouds: One‐ and two‐color lidar observations

From early July until mid-August 1995 we observed six noctilucent clouds (NLC) above the ALOMAR observatory in Northem Norway (69°N, 16°E) with ground based lidar. The NLC layers had maximum backscatter ratios between 17 and 190 at 532 nm, their mean altitude was 83.0 km. The latest of these events (August 12/13, 1995) was observed by two lidars, both, the Rayleigh- and the ozone lidar, located in the ALOMAR observatory, on wavelengths of 532 nm and 308 nm, respectively. During the period of common-volume observations from 23:55 UT until 01:30 UT, maximum averaged backscatter ratios of 56 and 28 were measured at 532 and 308 nm, respectively. If we assume spherical particles and a lognormal particle size distribution of width s, standard Mie theory yields median radii r m of the NLC particles between 16 nm and 44 nm with number densities between 2780 cm -3 and 160 cm -3 for assumed values of s between 1.6 and 1.2, respectively.

Gravity wave influence on NLC: experimental results from ALOMAR, 69 N

The influence of gravity waves on noctilucent clouds (NLC) at ALOMAR (69 N) is analysed by relating gravity wave activity to NLC occurrence from commonvolume measurements. Gravity wave kinetic energies are derived from MF-radar wind data and filtered into different period ranges by wavelet transformation. From the dataset covering the years 1999–2011, a direct correlation between gravity wave kinetic energy and NLC occurrence is not found, i.e., NLC appear independently of the simultaneously measured gravity wave kinetic energy. In addition, gravity wave activity is divided into weak and strong activity as compared to a 13 yr mean. The NLC occurrence rates during strong and weak activity are calculated separately for a given wave period and compared to each other. Again, for the full dataset no dependence of NLC occurrence on relative gravity wave activity is found. However, concentrating on 12 h of NLC detections during 2008, we do find an NLC-amplification with strong long-period gravity wave occurrence. Our analysis hence confirms previous findings that in general NLC at ALOMAR are not predominantly driven by gravity waves while exceptions to this rule are at least possible.

Laser-Wavelength and Laser-Beam-Direction Stabilization of the ALOMAR Rayleigh/Mie/Raman Lidar

The ALOMAR Rayleigh/Mie/Raman twin lidar is developed, built and operated jointly by research groups from Germany, the United Kingdom and France. The instrument is located near Andenes in Northern Norway (69°N, 16°E) and is designed for simultaneous multi-parameter and multi-color observations both night and day of the Arctic middle atmosphere in the altitude range from 10–90 km. For Doppler wind and temperature measurements in two directions simultaneously both power lasers are driven by a single narrow-bandwidth seeder laser. The seeder laser is actively stabilized against an iodine absorption line. For daylight measurements during the Arctic summer, receiving telescopes with a very narrow field-of-view are used. To match this condition, an active stabilization of the directions of both laser beams is used. For stable long-term observation on a climatological basis the entire transmitter side is designed for automatic operation. To this end all functions are controlled by computers which allows an automatic operation of the laser systems.