Leibniz Institute of Atmospheric Physics at the Rostock University

The spread of mineral particles over southwestern, western, and central Europe resulting from a strong Saharan dust outbreak in October 2001 was observed at 10 stations of the European Aerosol Research Lidar Network (EARLINET). For the first time, an optically dense desert dust plume over Europe was characterized coherently with high vertical resolution on a continental scale. The main layer was located above the boundary layer (above 1‐km height above sea level (asl)) up to 3–5‐km height, and traces of dust particles reached heights of 7–8 km. The particle optical depth typically ranged from 0.1 to 0.5 above 1‐km height asl at the wavelength of 532 nm, and maximum values close to 0.8 were found over northern Germany. The lidar observations are in qualitative agreement with values of optical depth derived from Total Ozone Mapping Spectrometer (TOMS) data. Ten‐day backward trajectories clearly indicated the Sahara as the source region of the particles and revealed that the dust layer observed, e.g., over Belsk, Poland, crossed the EARLINET site Aberystwyth, UK, and southern Scandinavia 24–48 hours before. Lidar‐derived particle depolarization ratios, backscatter‐ and extinction‐related Ångström exponents, and extinction‐to‐backscatter ratios mainly ranged from 15 to 25%, −0.5 to 0.5, and 40–80 sr, respectively, within the lofted dust plumes. A few atmospheric model calculations are presented showing the dust concentration over Europe. The simulations were found to be consistent with the network observations.

More than 130 observation days of the horizontal and vertical extent of Saharan dust intrusions over Europe during the period May 2000 to December 2002 were studied by means of a coordinated lidar network in the frame of the European Aerosol Research Lidar Network (EARLINET). The number of dust events was greatest in late spring, summer, and early autumn periods, mainly in southern (S) and southeastern (SE) Europe. Multiple aerosol dust layers of variable thickness (300–7500 m) were observed. The center of mass of these layers was located in altitudes between 850 and 8000 m. However, the mean thickness of the dust layer typically stayed around 1500–3400 m and the corresponding mean center of mass ranged from 2500 to 6000 m. In exceptional cases, dust aerosols reached northwestern (NW), northern (N), or northeastern (NE) Europe, penetrating the geographical area located between 4°W–28°E (longitude) and 38°N–58°N (latitude). Mean aerosol optical depths (AOD), extinction‐to‐backscatter ratios (lidar ratios, LR), and linear depolarization ratios of desert aerosols ranged from 0.1 to 0.25 at the wavelength of 355 or 351 nm, 30 to 80 sr at 355 or 351 nm, and 10 to 25% at 532 nm, respectively, within the lofted dust plumes. In these plumes typical Saharan dust backscatter coefficients ranged from 0.5 to 2 Mm −1 sr −1 . Southern European stations presented higher variability of the LR values and the backscatter‐related Ångström exponent values (BRAE) (LR: 20–100 sr; BRAE: −0.5 to 3) than northern ones (LR: 30–80 sr; BRAE: −0.5 to 1).

Abstract. Polar mesosphere summer echoes (PMSE) are very strong radar echoes primarily studied in the VHF wavelength range from altitudes close to the polar summer mesopause. Radar waves are scattered at irregularities in the radar refractive index which at mesopause altitudes is effectively determined by the electron number density. For efficient scatter, the electron number density must reveal structures at the radar half wavelength (Bragg condition for monostatic radars; ~3 m for typical VHF radars). The question how such small scale electron number density structures are created in the mesopause region has been a longstanding open scientific question for almost 30 years. This paper reviews experimental and theoretical milestones on the way to an advanced understanding of PMSE. Based on new experimental results from in situ observations with sounding rockets, ground based observations with radars and lidars, numerical simulations with microphysical models of the life cycle of mesospheric aerosol particles, and theoretical considerations regarding the diffusivity of electrons in the ice loaded complex plasma of the mesopause region, a consistent explanation for the generation of these radar echoes has been developed. The main idea is that mesospheric neutral air turbulence in combination with a significantly reduced electron diffusivity due to the presence of heavy charged ice aerosol particles (radii ~5–50 nm) leads to the creation of structures at spatial scales significantly smaller than the inner scale of the neutral gas turbulent velocity field itself. Importantly, owing to their very low diffusivity, the plasma structures acquire a very long lifetime, i.e., 10 min to hours in the presence of particles with radii between 10 and 50 nm. This leads to a temporal decoupling of active neutral air turbulence and the existence of small scale plasma structures and PMSE and thus readily explains observations proving the absence of neutral air turbulence at PMSE altitudes. With this explanation at hand, it becomes clear that PMSE are a suitable tool to permanently monitor the thermal and dynamical structure of the mesopause region allowing insights into important atmospheric key parameters like neutral temperatures, winds, gravity wave parameters, turbulence, solar cycle effects, and long term changes.

In recent times it has become increasingly clear that releases of trace gases from human activity have a potential for causing change in the upper atmosphere. However, our knowledge of systematic changes and trends in the temperature of the mesosphere and lower thermosphere is relatively limited compared to the Earth's lower atmosphere, and not much effort has been made to synthesize these results so far. In this article, a comprehensive review of long‐term trends in the temperature of the region from 50 to 100 km is made on the basis of the available up‐to‐date understanding of measurements and model calculations. An objective evaluation of the available data sets is attempted, and important uncertainly factors are discussed. Some natural variability factors, which are likely to play a role in modulating temperature trends, are also briefly touched upon. There are a growing number of experimental results centered on, or consistent with, zero temperature trend in the mesopause region (80–100 km). The most reliable data sets show no significant trend but an uncertainty of at least 2 K/decade. On the other hand, a majority of studies indicate negative trends in the lower and middle mesosphere with an amplitude of a few degrees (2–3 K) per decade. In tropical latitudes the cooling trend increases in the upper mesosphere. The most recent general circulation models indicate increased cooling closer to both poles in the middle mesosphere and a decrease in cooling toward the summer pole in the upper mesosphere. Quantitatively, the simulated cooling trend in the middle mesosphere produced only by CO 2 increase is usually below the observed level. However, including other greenhouse gases and taking into account a “thermal shrinking” of the upper atmosphere result in a cooling of a few degrees per decade. This is close to the lower limit of the observed nonzero trends. In the mesopause region, recent model simulations produce trends, usually below 1 K/decade, that appear to be consistent with most observations in this region.

Abstract This study analyzes a new high‐resolution general circulation model with regard to secondary gravity waves in the mesosphere during austral winter. The model resolves gravity waves down to horizontal and vertical wavelengths of 165 and 1.5 km, respectively. The resolved mean wave drag agrees well with that from a conventional model with parameterized gravity waves up to the midmesosphere in winter and up to the upper mesosphere in summer. About half of the zonal‐mean vertical flux of westward momentum in the southern winter stratosphere is due to orographic gravity waves. The high intermittency of the primary orographic gravity waves gives rise to secondary waves that result in a substantial eastward drag in the winter mesopause region. This induces an additional eastward maximum of the mean zonal wind at z ∼ 100 km. Radar and lidar measurements at polar latitudes and results from other high‐resolution global models are consistent with this finding. Hence, secondary gravity waves may play a significant role in the general circulation of the winter mesopause region.

Major sudden stratospheric warmings ( SSWs ) are striking phenomena of wintertime stratospheric circulation usually defined as a reversal of zonal mean circulation from westerlies to easterlies. SSWs often have significant impact on tropospheric circulation and cause anomalies in surface climate lasting for up to 2 months. For this reason, dynamics and predictability of SSW receive considerable attention. It is however well‐known that not all SSWs cause significant, long‐lasting impact on the troposphere. In order to explain differences in tropospheric impacts following SSWs , several reasons have been previously proposed, including differences in type of SSW (split or displacement), persistence of stratospheric anomalies, preconditioning of the tropospheric circulation, and whether or not SSW was accompanied by a planetary wave reflection in the stratosphere. Here we address the predictability of tropospheric impacts by SSWs by seeking early precursors of the impacts. We separate midwinter SSWs into two groups: those which are followed by significant, long‐lasting impacts on the tropospheric circulation (defined in terms of anomalous Northern Annular Mode) and those not followed by significant anomalies in the annular mode. We show that SSWs characterised by a more negative Northern Annular Mode index in the lower stratosphere around 150 hPa and enhanced wave activity propagation to the stratosphere during the first few days following the central date have a larger probability of being followed by tropospheric impact, both in reanalyses and in climate model runs. These anomalies play a more important role in the subsequent downward propagation of the signal to the troposphere than the type of SSW : whether it is a split or a displacement, or absorptive or reflective SSW . We propose that using these anomalies as precursors of tropospheric impacts of SSW can enhance climate predictability.

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 r med , width parameters σ, and particle number densities N NLC for the NLC particle distributions. Mean values for 10 out of the 11 events are r med = 51 nm, σ = 1.42, and N NLC = 82 cm −3 .

Physicochemical processes that generate and transform aerosols in jet aircraft plumes are discussed on the basis of theoretical models and recent observations of young contrails in the upper troposphere. The initial evolution of optical depth and ice water content under threshold contrail formation conditions is studied. Constrained by the measurements, a lower bound is deduced for the number density of ice crystals initially present in contrails. This bound serves as a visibility criterion for young contrails. An analysis of the primary contrail particles (aqueous solution droplets nucleated in situ, emitted insoluble combustion aerosols, and entrained background aerosols) reveals that only soot must he involved as ice forming nuclei if the visibility criterion is to be fulfilled. Possible activation pathways of the soot aerosols are investigated, including an analysis of their wetting behavior and droplet scavenging and heterogeneous nucleation properties. To support these investigations, results of laboratory experiments concerning contact angles of acidic solution droplets on carbonaceous surfaces and the freezing probability of sulfuric acid tetrahydrate are presented. Assuming that the soot particles acquire a liquid coating, heterogeneous freezing rates and their sensitivity on important parameters are studied.

Abstract We examine the characteristics of secondary gravity waves (GWs) excited by a localized (in space) and intermittent (in time) body force in the atmosphere. This force is a horizontal acceleration of the background flow created when primary GWs dissipate and deposit their momentum on spatial and temporal scales of the wave packet. A broad spectrum of secondary GWs is excited with horizontal scales much larger than that of the primary GW. The polarization relations cause the temperature spectrum of the secondary GWs generally to peak at larger intrinsic periods τ I r and horizontal wavelengths λ H than the vertical velocity spectrum. We find that the one‐dimensional spectra (with regard to frequency or wave number) follow lognormal distributions. We show that secondary GWs can be identified by a horizontally displaced observer as “fishbone” or “>” structures in z − t plots whereby the positive and negative GW phase lines meet at the “knee,” z knee , which is the altitude of the force center. We present two wintertime cases of lidar temperature measurements at McMurdo, Antarctica (166.69°E, 77.84°S) whereby fishbone structures are seen with z knee =43 and 52 km. We determine the GW parameters and density‐weighted amplitudes for each. We show that these parameters are similar below and above z knee . We verify that the GWs with upward (downward) phase progression are downward (upward) propagating via use of model background winds. We conclude that these GWs are likely secondary GWs having ground‐based periods τ r =6–10 hr and vertical wavelengths λ z =6–14 km, and that they likely propagate primarily southward.

The Arctic polar vortex exhibited widespread regions of low temperatures during the winter of 2005, resulting in significant ozone depletion by chlorine and bromine species. We show that chemical loss of column ozone (ΔO 3 ) and the volume of Arctic vortex air cold enough to support the existence of polar stratospheric clouds (V PSC ) both exceed levels found for any other Arctic winter during the past 40 years. Cold conditions and ozone loss in the lowermost Arctic stratosphere (e.g., between potential temperatures of 360 to 400 K) were particularly unusual compared to previous years. Measurements indicate ΔO 3 = 121 ± 20 DU and that ΔO 3 versus V PSC lies along an extension of the compact, near linear relation observed for previous Arctic winters. The maximum value of V PSC during five to ten year intervals exhibits a steady, monotonic increase over the past four decades, indicating that the coldest Arctic winters have become significantly colder, and hence are more conducive to ozone depletion by anthropogenic halogens.

Gravity waves (GWs) are a ubiquitious dynamical feature of the polar summer mesopause region. During three summer campaigns, in 1991, 1993 and 1994, we launched seven sounding rockets from the north Norwegian island Andøya. Each of these payloads carried an ionization gauge capable of measuring the total atmospheric density at a high spatial resolution. From these measurements, temperature profiles were determined for altitudes between 70 and 110 km, with an altitude resolution of 200 m. The temperature profiles reveal significant rms variations that are as large as 6 K at 80 km, 10 K at 85 km, and even 20 K at 95 km. During three out of the seven launches a bright noctilucent cloud (NLC) was simultaneously detected by our ground‐based lidar and by rocket‐borne in situ experiments. During these flights, the NLC is located close to a local temperature minimum below the mesopause. We then estimated gravity wave parameters from accompanying falling sphere and chaff wind observations and found signatures that the wave periods during the NLC cases were on the order of 7–9 hours, with corresponding horizontal wavelengths of 600–1000 km. Motivated by these observations, we used a microphysical model of NLC generation and growth to study the interaction between GWs and NLC. Based on recently measured and modeled temperatures and water vapor mixing ratios, and our gravity wave parameter estimates, we find that the NLC layer indeed follows the motion of the cold phase of the wave by means of a complex interplay between ice crystal sedimentation, transport by the vertical wind, and simultaneous growth. It turns out that the history of individual particles significantly influences the observed properties of NLC. Furthermore, we find that GWs with periods longer than 6.5 hours amplify NLC while waves with shorter periods tend to destroy NLC. In addition, we can only find a correlation between local temperature minima and the location of the NLC provided that the wave periods are longer than ∼6 hours, which is consistent with our wave parameter estimates.

Since 2000, regular lidar observations of the vertical aerosol distribution over Europe have been performed within the framework of EARLINET, the European Aerosol Research Lidar Network. A statistical analysis concerning the vertical distribution of the volume light extinction coefficients of particles derived from Raman lidar measurements at 10 EARLINET stations is presented here. The profiles were measured on a fixed schedule with up to two measurements per week; they typically covered the height range from 500 m to 6000 m above ground level (agl). The analysis is made for the planetary boundary layer (PBL) as well as for several fixed layers above ground. The results show typical values of the aerosol extinction coefficient and the aerosol optical depth (AOD) in different parts of Europe, with highest values in southeastern Europe and lowest values in the northwestern part. Annual cycles and cumulative frequency distributions are also presented. We found that higher aerosol optical depths in southern Europe compared to the northern part are mainly attributed to larger amounts of aerosol in higher altitudes. At 9 of the 10 sites the frequency distribution of the aerosol optical depth in the planetary boundary layer follows a lognormal distribution at the 95% significance level.

Abstract Several studies show that the anomalous long-lasting Russian heat wave during the summer of 2010, linked to a long-persistent blocking high, appears mainly as a result of natural atmospheric variability. This study analyzes the large-scale flow structure based on the ECMWF Re-Analysis Interim (ERA-Interim) data (1989–2010). The anomalous long-lasting blocking high over western Russia including the heat wave occurs as an overlay of a set of anticyclonic contributions on different time scales. (i) A regime change in ENSO toward La Niña modulates the quasi-stationary wave structure in the boreal summer hemisphere supporting the eastern European blocking. The polar Arctic dipole mode is enhanced and shows a projection on the mean blocking high. (ii) Together with the quasi-stationary wave anomaly, the transient eddies maintain the long-lasting blocking. (iii) Three different pathways of wave action are identified on the intermediate time scale (~10–60 days). One pathway commences over the eastern North Pacific and includes the polar Arctic region; another one runs more southward and crossing the North Atlantic, continues to eastern Europe; a third pathway southeast of the blocking high describes the downstream development over South Asia.

China is a key region for understanding fire activity and the drivers of its variability under strict fire suppression policies. Here, we present a detailed fire occurrence dataset for China, the Wildfire Atlas of China (WFAC; 2005-2018), based on continuous monitoring from multiple satellites and calibrated against field observations. We find that wildfires across China mostly occur in the winter season from January to April and those fire occurrences generally show a decreasing trend after reaching a peak in 2007. Most wildfires (84%) occur in subtropical China, with two distinct clusters in its southwestern and southeastern parts. In southeastern China, wildfires are mainly promoted by low precipitation and high diurnal temperature ranges, the combination of which dries out plant tissue and fuel. In southwestern China, wildfires are mainly promoted by warm conditions that enhance evaporation from litter and dormant plant tissues. We further find a fire occurrence dipole between southwestern and southeastern China that is modulated by the El Niño-Southern Oscillation (ENSO).

Water vapour data inside cirrus clouds from in‐situ measurements with an aircraft‐borne frost‐point hygrometer are analysed. These data have been obtained during two field campaigns, performed in the Southern and Northern hemisphere mid latitudes. There were many occurrences of ice supersaturation inside the investigated cirrus, with a higher frequency of occurrences in the Southern Hemisphere. The source of the differences in the humidity data from the two hemispheres is not clear, and it is speculated that these differences may be related to different levels of pollution. A distribution law for the relative humidity inside cirrus clouds is inferred.

Abstract. A direct detection Doppler lidar for measuring wind speed in the middle atmosphere up to 80 km with 2 h resolution was implemented in the ALOMAR Rayleigh/Mie/Raman lidar (69° N, 16° E). The random error of the line of sight wind is about 0.6 m/s and 10 m/s at 49 km and 80 km, respectively. We use a Doppler Rayleigh Iodine Spectrometer (DoRIS) at the iodine line 1109 (~532.260 nm). DoRIS uses two branches of intensity cascaded channels to cover the dynamic range from 10 to 100 km altitude. The wind detection system was designed to extend the existing multi-wavelength observations of aerosol and temperature performed at wavelengths of 355 nm, 532 nm and 1064 nm. The lidar uses two lasers with a mean power of 14 W at 532 nm each and two 1.8 m diameter tiltable telescopes. Below about 49 km altitude the accuracy and time resolution is limited by the maximum count rate of the detectors used and not by the number of photons available. We report about the first simultaneous Rayleigh temperature and wind measurements by lidar in the strato- and mesosphere on 17 and 23 January 2009.

We report on initial measurements of potassium density and temperature profiles of the mesopause region by a containerized and hence transportable lidar instrument. The centerpiece of this lidar is a new, all‐solid‐state laser system. We employed a cw seeded and pulsed alexandrite ring laser working at the 770 nm wavelength of the K(D 1 ) finestructure line. We show that, different from the Na case, in K soundings the effects of a minor isotope have to be taken into account in the data analysis. We point out that we observe on average considerably smaller K column densities than those published in the late 1970s by a French group. We study quantitatively the influence of the Hanle and potential saturation effects on the derived temperatures. We give an example of a mesopause temperature profile taken with our transportable K lidar.

The first global model of meteoric iron in the atmosphere (WACCM‐Fe) has been developed by combining three components: the Whole Atmosphere Community Climate Model (WACCM), a description of the neutral and ion‐molecule chemistry of iron in the mesosphere and lower thermosphere (MLT), and a treatment of the injection of meteoric constituents into the atmosphere. The iron chemistry treats seven neutral and four ionized iron containing species with 30 neutral and ion‐molecule reactions. The meteoric input function (MIF), which describes the injection of Fe as a function of height, latitude, and day, is precalculated from an astronomical model coupled to a chemical meteoric ablation model (CABMOD). This newly developed WACCM‐Fe model has been evaluated against a number of available ground‐based lidar observations and performs well in simulating the mesospheric atomic Fe layer. The model reproduces the strong positive correlation of temperature and Fe density around the Fe layer peak and the large anticorrelation around 100 km. The diurnal tide has a significant effect in the middle of the layer, and the model also captures well the observed seasonal variations. However, the model overestimates the peak Fe + concentration compared with the limited rocket‐borne mass spectrometer data available, although good agreement on the ion layer underside can be obtained by adjusting the rate coefficients for dissociative recombination of Fe‐molecular ions with electrons. Sensitivity experiments with the same chemistry in a 1‐D model are used to highlight significant remaining uncertainties in reaction rate coefficients, and to explore the dependence of the total Fe abundance on the MIF and rate of vertical transport.

The midlatitude sporadic E layers form when metallic ions of meteoric origin in the lower thermosphere are converged vertically in a wind shear. The occurrence and strength of sporadic E follow a pronounced seasonal dependence marked by a conspicuous summer maximum. Although this is known since the early years of ionosonde studies, its cause has remained a mystery as it cannot be accounted for by the windshear theory of E s formation. We show here that the marked seasonal dependence of sporadic E correlates well with the annual variation of sporadic meteor deposition in the upper atmosphere. The later has been established recently from long‐term measurements using meteor radar interferometers in the Northern and Southern Hemispheres. Knowing that the occurrence and strength of sporadic E layers depends directly on the metal ion content, which apparently is determined primarily by the meteoric deposition, the present study offers a cause‐and‐effect explanation for the long‐going mystery of sporadic E layer seasonal dependence.

Abstract. We report on the development and current capabilities of the ALOMAR Rayleigh/Mie/Raman lidar. This instrument is one of the core instruments of the international ALOMAR facility, located near Andenes in Norway at 69°N and 16°E. The major task of the instrument is to perform advanced studies of the Arctic middle atmosphere over altitudes between about 15 to 90 km on a climatological basis. These studies address questions about the thermal structure of the Arctic middle atmosphere, the dynamical processes acting therein, and of aerosols in the form of stratospheric background aerosol, polar stratospheric clouds, noctilucent clouds, and injected aerosols of volcanic or anthropogenic origin. Furthermore, the lidar is meant to work together with other remote sensing instruments, both ground- and satellite-based, and with balloon- and rocket-borne instruments performing in situ observations. The instrument is basically a twin lidar, using two independent power lasers and two tiltable receiving telescopes. The power lasers are Nd:YAG lasers emitting at wavelengths 1064, 532, and 355 nm and producing 30 pulses per second each. The power lasers are highly stabilized in both their wavelengths and the directions of their laser beams. The laser beams are emitted into the atmosphere fully coaxial with the line-of-sight of the receiving telescopes. The latter use primary mirrors of 1.8 m diameter and are tiltable within 30° off zenith. Their fields-of-view have 180 µrad angular diameter. Spectral separation, filtering, and detection of the received photons are made on an optical bench which carries, among a multitude of other optical components, three double Fabry-Perot interferometers (two for 532 and one for 355 nm) and one single Fabry-Perot interferometer (for 1064 nm). A number of separate detector channels also allow registration of photons which are produced by rotational-vibrational and rotational Raman scatter on N2 and N2+O2 molecules, respectively. Currently, up to 36 detector channels simultaneously record the photons collected by the telescopes. The internal and external instrument operations are automated so that this very complex instrument can be operated by a single engineer. Currently the lidar is heavily used for measurements of temperature profiles, of cloud particle properties such as their altitude, particle densities and size distributions, and of stratospheric winds. Due to its very effective spectral and spatial filtering, the lidar has unique capabilities to work in full sunlight. Under these conditions it can measure temperatures up to 65 km altitude and determine particle size distributions of overhead noctilucent clouds. Due to its very high mechanical and optical stability, it can also employed efficiently under marginal weather conditions when data on the middle atmosphere can be collected only through small breaks in the tropospheric cloud layers.Key words: Atmospheric composition and structure (aerosols and particles; pressure · density · and temperature; instruments and techniques)