M. Gerding

Atmospheric Ca and Ca+ layers: Midlatitude observations and modeling

We report on a comprehensive set of observations of the upper atmospheric Ca and Ca + layers. The observations were obtained by ground-based lidars at Kuhlungsborn, Germany (54°N, 12°E), between December 1996 and December 1998. During this period, 112 nights of Ca soundings and 58 nights of Ca + soundings were realized. The Ca layer has an average column abundance of 2.1.10 7 cm -2 , centered around 90.3 km with a mean peak density of 22 cm -3 at 89.9 km altitude. The Ca + dominates the total Ca amount above 90 km and has an average column abundance of 4.9.10 7 cm -2 . Because the vaporization of cosmic dust is the most probable source of atmospheric metals, the column densities of the metals within the atmosphere are often compared with the abundance in chondritic CI meteorites. We show that the atmospheric Ca is severely depleted with respect to other metals such as Na and Fe, compared with their relative abundances in CI chondrites. We present a one-dimensional steady state chemistry model of the nighttime Ca and Ca + layers, based on new laboratory studies of CaO reaction kinetics. This model is able to reproduce satisfactorily the characteristic features of the annual mean layers and to provide a possible explanation for the unusual seasonal variation of the Ca layer which exhibits a pronounced summertime enhancement around 87 km.

NLC particle properties from a five‐color lidar observation at 54°N

On June 13/14, 1998, a first simultaneous five-color lidar measurement of a noctilucent cloud was realized. The experiment was made possible by the fact that at the site of our Institute (54°N), dark summer nights allow lidar observations of overhead noctilucent clouds (NLCs) without expensive spectral filters. We determine the size distribution of the NLC particles assuming spherical ice particles with a monomodal, lognormal size distribution. To this end we introduce a new method for calculation of the NLC particle properties. For the latter the following ranges are found: Particle number density N = 260–610 cm−3, median radius rm = 20.2–27.5 nm, and distribution width σ= 1.5–1.6. Cross checks using different wavelength combinations confirm the robustness of the NLC particle property results. The results are similar to those from recent lidar work on noctilucent clouds, observed at arctic latitudes.

Simultaneous K and Ca lidar observations during a meteor shower on March 6–7, 1997, at Kühlungsborn, Germany

We report about observations of meteor trails by ground-based lidars which were obtained with two metal resonance lidars monitoring simultaneously the same air column at meteor trail heights. The lidars are located at the site of the Leibniz-Institute of Atmospheric Physics (54°N, 12°E). More specifically, we have used K and Ca lidars to study meteor trails with respect to (1) their absolute K or Ca abundances, (2) their duration in the laser beams, (3) the altitude distributions of the K and Ca trails, and (4) the reaction of the regular K and Ca layers to the occurence of a (yet unknown) meteor shower. Lidar observations during the night of March 6–7, 1997, began around 1820 UT. They indicated the start of an unknown meteor shower at ∼2200 UT, which we could observe until 0430 UT of March 7. Within 4 h after 2200 UT, the column densities of the regular K and Ca layers increased by factors of 2 and 4, respectively. During the period of the shower, our lidars registered 26 Ca trail events, but only 2 K trail events. Hence we observe for the two metals a noticeable difference between their column density enhancements and rates of trails. The rate of Ca trails was quite similar to that observed by our lidars near the peak of the Lyrids on April 22–23, 1997. The Ca trails were observed in the altitude range from 81 to 98 km with a mean altitude of 89.6 km. In all of the lidar-observed meteor trails, it was either K or Ca which could be detected. Metal densities in the trails ranged from ∼90 to 360 cm−3 in the case of K and from 50 to 2700 cm−3 for Ca.

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

rence rate) below 60 i latitude. We present the first study of diurnal variations of midlatitude NLCs based on lidar obser- vations with full diurnal coverage at Kuhlungsborn since 2010 independent from solar elevation. Overall, � 100 h of NLCs with a backscatter coefficient ofmax,532nm >0 .5� 10 -10 m -1 sr -1 are observed within � 1800 h. Occurrence rates decrease regularly from 12% at 5 local solar time (LST) to � 2% at 19 LST. The mean NLC brightness varies between � 1a nd� 3 � 10 -10 m -1 sr -1 with maxima at 4 and 18 LST. The simultaneously observed temperatures show a system- atic (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. Citation: Gerding, M., M. Kopp, P. Hoffmann, J. Hoffner, and F.-J. Lubken (2013), Diurnal variations of midlatitude NLC parameters observed by daylight-capable lidar, and their rela- tion to ambient parameters, Geophys. Res. Lett., 40, 6390-6394,

Characteristics of stratospheric turbulent layers measured by LITOS and their relation to the Richardson number

Based on high-resolution turbulence measurements performed with the newly established balloon-borne instrument Leibniz Institute Turbulence Observations in the Stratosphere (LITOS) during the Balloon Experiments for University Students (BEXUS) 6 and BEXUS 8 campaigns from Kiruna, we derived characteristics of stratospheric turbulence layers, like their thickness and distance in between. Typically, the layers are ∼15–130 m thick and have a distance of ∼60–270 m, and their number increases with altitude. Due to the very high measurement resolution of LITOS in the range of millimeters, we obtain energy dissipation rate profiles with unprecedented precision. Within the turbulent layers we get a mean dissipation rate of 3.4×10−2W/kg (BEXUS 6) and 1.1 × 10−2 W/kg (BEXUS 8) corresponding to a heating rate of 1 to ∼3 K/d. The profiles show an increase of the energy dissipation with altitude. Comparisons with the Richardson number Ri preclude a clear correlation between the occurrence of turbulence and Ri 1/4 and far beyond, independent of the scale over which Ri has been determined.

Lidars With Narrow FOV for Daylight Measurements

Daytime lidar operation in the middle atmosphere requires a narrow field of view (FOV) of the receiving telescope for effective background reduction and a high-transmission narrow-band detection. The laser beam position in the atmosphere relative to the optical axis of the receiving telescope is subject to high-frequency disturbances such as turbulence, vibration, and wind as well as comparable slow drift (thermal effects of the laser, stability of the building, etc.). We developed a beam stabilization system (BSS) that ensured a pulse-to-pulse stabilization of the laser beam with ~ 3 μrad remaining jitter, allowing ~ 60 μrad FOV. With BSS and single-pulse data acquisition system, the optimal alignment of the laser and telescope can be controlled, and information on the FOV and laser divergence in the far field can be derived. The capability of the BSS is to stabilize the laser against all internal and external disturbances below the repetition rate of the laser.

A study of the dissociative recombination of CaO + with electrons: Implications for Ca chemistry in the upper atmosphere

Abstract The dissociative recombination of CaO+ ions with electrons has been studied in a flowing afterglow reactor. CaO+ was generated by the pulsed laser ablation of a Ca target, followed by entrainment in an Ar+ ion/electron plasma. A kinetic model describing the gas‐phase chemistry and diffusion to the reactor walls was fitted to the experimental data, yielding a rate coefficient of (3.0 ± 1.0) × 10−7 cm3 molecule−1 s−1 at 295 K. This result has two atmospheric implications. First, the surprising observation that the Ca+/Fe+ ratio is ~8 times larger than Ca/Fe between 90 and 100 km in the atmosphere can now be explained quantitatively by the known ion‐molecule chemistry of these two metals. Second, the rate of neutralization of Ca+ ions in a descending sporadic E layer is fast enough to explain the often explosive growth of sporadic neutral Ca layers.

Characterization of a Double Mesospheric Bore Over Europe

Observations of a pair of mesospheric bore disturbances that propagated through the nighttime mesosphere over Europe are presented. The observations were made at the Padua Observatory, Asiago (45.9°N, 11.5°E) by the Boston University all-sky imager on 11 March 2013. The bores appeared over the north-west horizon, approximately 30 minutes apart, and propagated towards the south-east. Using additional satellite and radar data, we present evidence indicating the bores originated in the mesosphere from a single, larger-scale mesospheric disturbance propagating through the mesopause region. Furthermore, the large-scale mesospheric disturbance appeared to be associated with an intense weather disturbance that moved southeastwards over the United Kingdom and Western Europe during 10 and 11 March.

A new model of meteoric calcium in the mesosphere and lower thermosphere

Meteoric ablation produces layers of metal atoms in the mesosphere and lower thermosphere (MLT). It has been known for more than 30 years that the Ca atom layer is depleted by over 2 orders of magnitude compared with Na, despite these elements having nearly the same elemental abundance in chondritic meteorites. In contrast, the Ca ion abundance is depleted by less than a factor of 10. To explain these observations, a large database of neutral and ion– molecule reaction kinetics of Ca species, measured over the past decade, was incorporated into the Whole Atmosphere Community Climate Model (WACCM). A new meteoric input function for Ca and Na, derived using a chemical ablation model that has been tested experimentally with a Meteoric Ablation Simulator, shows that Ca ablates almost 1 order of magnitude less efficiently than Na. WACCM-Ca simulates the seasonal Ca layer satisfactorily when compared with lidar observations, but tends to overestimate Ca measurements made by rocket mass spectrometry and lidar. A key finding is that CaOH and CaCO3 are very stable reservoir species because they are involved in essentially closed reaction cycles with O2 and O. This has been demonstrated experimentally for CaOH, and in this study for CaCO3 using electronic structure and statistical rate theory. Most of the neutral Ca is therefore locked in these reservoirs, enabling rapid loss through polymerization into meteoric smoke particles, and this explains the extreme depletion of Ca.