Gerd Baumgarten

Relevance of mountain wave cooling for the formation of polar stratospheric clouds over Scandinavia: Mesoscale dynamics and observations for January 1997

The effect of mesoscale mountain wave-induced temperature anomalies on the formation potential of polar stratospheric clouds above northern Scandinavia is analyzed with a one-month mesoscale model integration. The simulation results are contrasted with synoptic-scale analyses and compared with remote sensing and in situ observations. The mesoscale mass flux of air parcels with temperatures below the threshold for cloud formation through a control volume is compared with its synoptic-scale counterpart. A classification of the synoptic-scale flow into periods of large and small mountain wave activity in the stratosphere is proposed. The derived classification will be used for a climatology of stratospheric mountain wave activity above Scandinavia.

Quantifying Kelvin‐Helmholtz instability dynamics observed in noctilucent clouds: 1. Methods and observations

Noctilucent clouds (NLCs) have been imaged during two nights in summer 2009 from northern Germany (Kuhlungsborn, 54°N) and middle Norway (Trondheim, 64°N). For the first time a horizontal resolution of 10 to 20 m at the altitude of the clouds (about 83 km) and a temporal resolution of about 1 s was achieved. Additional imaging using a coarser resolution provided monitoring of the larger-scale (~100 km) structures observed in the clouds. Two series of NLC images are described that reveal apparent Kelvin-Helmholtz (KH) billow structures having very different morphologies and apparent transitions to turbulence and mixing. One series exhibits deep KH billows and apparent secondary instabilities in the billow exteriors having streamwise alignment (and spanwise wave number), suggesting a small initial Richardson number (Ri). A second series of images suggests a larger and less unstable Ri, a slower KH billow evolution, shallower billows, and turbulence and mixing confined to the billow cores. We suggest that systematic exploration of these dynamics employing NLC imaging may enable characterization and quantification of KH instability occurrence statistics and of their contributions to turbulence and mixing in the summer mesopause environment with unique sensitivity to their small-scale dynamics.

Mesosphere and lower thermosphere zonal wind variations over low latitudes: Relation to local stratospheric zonal winds and global circulation anomalies

Long-term observations from medium-frequency and meteor radars (1993–2012) and rocket soundings (1979–1990 and 2002–2007) are used to study mesosphere and lower thermosphere (MLT) zonal wind variations in relation to the stratospheric winds over northern low latitudes. The combined data set provides a complete height profile of amplitude of semiannual oscillation (SAO) up to 100 km, with an exception around 75–80 km. The SAO signal has maxima around 50 km and 82 km and a minimum around 65 km. The MLT zonal winds show remarkable interannual variability during northern hemispheric spring equinox and much less during fall equinox. Zonal wind mesospheric spring equinox enhancements (MSEE) appear with a periodicity of 2–3 years, suggesting a modulation by the quasi-biennial oscillation, which we identified with the strength of stratospheric westward winds. Out of 20 years of observations, the stratospheric westward winds are strong during 11 years (non-MSEE) and weak during 9 years. Six of these 9 years show large MLT winds (MSEE), and 3 years (1999, 2004, and 2006) show small MLT winds (missing MSEE). These unexpected small winds occur in years with global circulation anomalies associated with strong sudden stratospheric warmings and an early spring transition of zonal winds. With the proposed three MSEE classes, we take into account local and global forcing factors.

Bright polar mesospheric clouds formed by main engine exhaust from the space shuttle's final launch

The space shuttle launched for the last time on 8 July 2011. As with most shuttle launches, the three main engines injected about 350 t of water vapor between 100 and 115 km off the east coast of the United States during its ascent to orbit. We follow the motion of this exhaust with a variety of satellite and ground-based data sets and find that (1) the shuttle water vapor plume spread out horizontally in all directions over a distance of 3000 to 4000 km in 18 h, (2) a portion of the plume reached northern Europe in 21 h to form polar mesospheric clouds (PMCs) that are brighter than over 99% of all PMCs observed in that region, and (3) the observed altitude dependence of the particle size is reversed with larger particles above smaller particles. We use a one- dimensional cloud formation model initialized with predictions of a plume diffusion model to simulate the unusually bright PMCs. We find that eddy mixing can move the plume water vapor down to the mesopause near 90 km where ice particles can form. If the eddy diffusion coefficient is 400 to 1000 m(2)/s, the predicted integrated cloud brightness is in agreement with both satellite and ground-based observations of the shuttle PMCs. The propellant mass of the shuttle is about 20% of that from all vehicles launched during the northern 2011 PMC season. We suggest that the brightest PMC population near 70 degrees N is formed by space traffic exhaust.

Quantifying Kelvin‐Helmholtz instability dynamics observed in noctilucent clouds: 2. Modeling and interpretation of observations

A companion paper describes high-resolution, ground-based imaging of apparent Kelvin-Helmholtz instabilities (KHI) observed in noctilucent clouds (NLCs) near the polar summer mesopause. Here we employ direct numerical simulations of KHI at Richardson numbers from Ri = 0.05 to 0.20 and relatively high Reynolds numbers to illustrate the dependence of KHI and secondary instabilities on these quantities and interpret and quantify the KHI events described by Baumgarten and Fritts (2014). We conclude that one event triggered by small-scale gravity waves provides clear evidence of strong KHI initiated at Ri ~0.05–0.10. Events arising in a more uniform shear environment exhibit KHI and small-scale dynamics that compare reasonably well with modeled KHI initiated at Ri ~0.20. Our application of numerical modeling in quantifying KHI dynamics observed in NLCs suggests that characteristics of KHI, and perhaps other small-scale dynamics, that are defined well in NLC displays can be used to quantify the dynamics and spatial scales of such events with high confidence. Specifically, our comparisons of KHI observations and modeling appear to indicate a “turbulent” viscosity ~5–40 times the true kinematic viscosity at the NLC altitude. This offers an alternative, or an augmentation, to more traditional radar, lidar, and/or airglow measurements employed for such studies of small-scale dynamics at coarser spatial scales during polar summer.

Solar Variability and Trend Effects in Mesospheric Ice Layers

In this paper we summarize results from the SOLEIL project (SOLar variability and trend Effects in Ice Layers) which was part of the CAWSES priority program in Germany. We present results from LIMA/ICE which is a global circulation model concentrating on ice clouds (NLC, noctilucent clouds) in the summer mesopause region. LIMA/ICE adapts to ECMWF data in the lower atmosphere which produces significant short term and year-to-year variability. The mean ice cloud parameters derived from LIMA/ICE generally agree with observations. The formation, transport, and sublimation of ice particles causes a significant redistribution of water vapor (‘freeze drying’). Model results are now available for all years since 1961 for various scenarios, e.g., with and without greenhouse gas increase etc. Temperatures and water vapor are affected by solar activity. In general it is warmer during solar maximum, but there is a small height region around the mesopause where it is colder. This complicates the prediction of solar cycle effects on ice layers. The magnitude of the solar cycle effect is ∼1–3 K which is similar to the year-to-year variability. Therefore, only a moderate solar cycle signal is observed in temperatures and in ice layers. Temperature trends at NLC altitudes are partly caused by stratospheric trends (‘shrinking effect’). Trends are generally negative, but are positive in the mesopause region. Again, this complicates a simple prediction of temperature trends on ice layers and requires a complex model like LIMA/ICE. Trends in CO2 and stratospheric O3 enhance mesospheric temperature trends but have comparatively small effects in the ice regime. Comparison of contemporary and historic observations of NLC altitudes leads to negligible temperature trends at NLC altitudes (∼83 km). For the time period of satellite measurements (1979–2009) LIMA/ICE predicts trends in ice cloud brightness and occurrence rates, consistent with observations. Temperature trends are not uniform in time but are stronger until the mid 1990s, and weaker thereafter. This change is presumably related to stratospheric ozone recovery. The accidental coincidence of lowest temperatures and solar cycle minimum in the mid 1990s led to large NLC activity. It is important to consider the time period and the height range when studying temperature and ice cloud trends. In the mesosphere temperature trends can be as large as −(3–5) K/decade (in agreement with observations) or rather small, depending on the time period and height range.

Size distribution time series of a polar stratospheric cloud observed above Arctic Lidar Observatory for Middle Atmosphere Research (ALOMAR) (69°N) and analyzed from multiwavelength lidar measurements during winter 2005

A case study of a polar stratospheric cloud (PSC) is described using multiwavelength (355, 532, and 1064 nm) lidar measurements performed at the Arctic Lidar Observatory for Middle Atmosphere Research (ALOMAR) on 6 December 2005. Rotational Raman signals at 529 and 530 nm are used to derive a temperature field within the cloud using the rotational Raman technique (RRT). The PSC size distributions are retrieved between 1500 and 2000 UTC through a combination of statistical filtering and best match approaches. Several PSC types were detected between 22 and 26 km during the measurement session. Liquid ternary aerosols are identified before about 1600 and after 1900 UTC typically; their averaged retrieved size distribution parameters and associated errors at the backscatter peak are: No 1–10 cm3 (50%), rm 0.15 mm (20%), and s 1.2 (15%). A mode of much larger particles is detected between 1600 and 1900 UTC (No 0.04 cm3 (30%), rm 1.50 mm (15%), and s 1.37 (10%). The different PSC types are also identified using standard semiempirical classifications, based on lidar backscatter, temperature, and depolarization. Overall, the characteristics of the retrieved size distributions are consistent with these classifications. They all suggest that these very large particles are certainly nitric acid trihydrate that could have been generated by the strong gravity wave activity visible in the temperature profiles. The results demonstrate that multiwavelength lidar data coupled to both RRT temperatures and our size distribution retrieval can provide useful additional information for identification of PSC types and for direct comparisons with microphysical model simulations.

Lidar observations of temperatures, waves, and noctilucent clouds at 69° N

The ALOMAR Rayleigh/Mie/Raman (RMR) lidar is an active remote sensing instrument for the investigation of the Arctic middle atmosphere during day and night. It is located in Northern Norway and operated on a routine basis to measure relative density profiles and aerosol properties in the stratosphere and mesosphere since 1995. Temperature profiles derived from the density measurements assuming hydrostatic equilibrium are used to investigate the mean temperature structure as well as gravity waves in the polar middle atmosphere. During the last two years, temperature data were acquired for approximately 2100 hours. A subset of this data basis was used to determine the potential energy density to characterize the gravity wave activity above the station. Noctilucent clouds (NLC) are the highest clouds of the Earths atmosphere and a visible sign of extreme atmospheric conditions with temperatures far below radiative equilibrium. During the last 7 years a continuous data set with 1880 measurement hours was acquired during the summer seasons, of which 640 hours contain NLC signatures. This actually most extensive lidar acquired NLC archive was analyzed regarding brightness, altitude, vertical extent, as well as occurrence frequency of noctilucent clouds above ALOMAR.

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.

A new method to infer the size, number density, and charge of mesospheric dust from its in situ collection by the DUSTY probe

We present a new method of analyzing measurements of mesospheric dust made with DUSTY rocket-borne Faraday cup probes. It can yield the variation in fundamental dust parameters through a mesospheric cloud with an altitude resolution down to 10 cm or less if plasma probes give the plasma density variations with similar height resolution. A DUSTY probe was the first probe that unambiguously detected charged dust and aerosol particles in the Earth’s mesosphere. DUSTY excluded the ambient plasma by various biased grids, which however allowed dust particles with radii above a few nanometers to enter, and it measured the flux of charged dust particles. The flux measurements directly yielded the total ambient dust charge density. We extend the analysis of DUSTY data by using the impact currents on its main grid and the bottom plate as before, together with a dust charging model and a secondary charge production model, to allow the determination of fundamental parameters, such as dust radius, charge number, and total dust density. We demonstrate the utility of the new analysis technique by considering observations made with the DUSTY probes during the MAXIDUSTY rocket campaign in June– July 2016 and comparing the results with those of other instruments (lidar and photometer) also used in the campaign. In the present version we have used monodisperse dust size distributions.