Harald Flentje

Evidence for inertia gravity waves forming polar stratospheric clouds over Scandinavia

[1]xa0At three successive days at the end of January 2000 the Deutsches Zentrum fur Luft- und Raumfahrt (DLR) airborne lidar Ozone Lidar Experiment explored mountain-wave-induced polar stratospheric clouds above the Scandinavian mountain ridge. Global analyses and mesoscale modeling are applied to explain their complex internal structure and their day-to-day variability. Depending on the synoptical-scale meteorological conditions, stratospheric temperature anomalies of different amplitude and horizontal extent are generated by the upward propagating mountain waves. Short-term excitation of about 6 hours resulted in localized stratospheric temperature anomalies directly above the mountain ridge as for 25 January 2000. In this case, the elevation of the observed clouds differed not much from the synoptic-scale clouds upstream above the Norwegian Sea. On the other hand, long-lasting flow past the Scandinavian mountain ridge formed huge 400-km horizontally extending stratospheric ice clouds in altitudes as much as 5 km above the elevation of the upstream clouds just 1 day later. Inertia gravity waves with horizontal wavelengths of about 350 km are responsible for their formation. For the first time a predicted temperature minimum far downstream of the mountains could be proofed by the observation of an isolated stratospheric ice cloud above Finland. The observed particles are classified in terms of their measured optical properties such as backscatter ratio and depolarization. In all cases, mountain waves generated ice clouds. In contrast to the nitric acid trihydrate tail of the ice cloud on 25 January the same classification results in a tail of liquid supercooled ternary solutions droplets 1 day later. The particle structure downstream of the mountains is very complex and needs detailed microphyical modeling and interpretation.

Vertical variability of aerosol properties observed at a continental site during the Lindenberg Aerosol Characterization Experiment (LACE 98)

[1]xa0In the summer of 1998, an aerosol column closure experiment was conducted over eastern Germany as part of the Lindenberg Aerosol Characterization Experiment (LACE 98). The experimental platforms involved were two aircraft equipped with aerosol in situ instrumentation, an airborne aerosol backscatter lidar and solar irradiance sensors (upwelling and downwelling) to measure optical and microphysical aerosol properties relevant for radiative forcing in the entire atmospheric column from the boundary layer to the tropopause region. Using Mie theory and reasonable assumptions on the particle complex refractive index, the hemispheric backscatter fraction and spectral light scattering coefficients were calculated from the measured aerosol size distribution data. A comparison between optical parameters derived from particle size distribution measurements and optical properties measured by independent techniques yields an uncertainty range of ±30% for the calculated aerosol optical parameters. Detailed information on the vertical variability of the particle size distribution, the aerosol extinction coefficient and the hemispheric backscatter fraction is given, which is applicable for a realistic representation of aerosol properties in climate models. The estimated daily averaged aerosol shortwave radiative forcing varies from −33 ± 12 to −12 ± 5 W m−2, and respective aerosol optical depths at λ = 0.710 μm range from 0.18 to 0.06.

Mountain Wave–Induced Polar Stratospheric Cloud Forecasts for Aircraft Science Flights during SOLVE/THESEO 2000

Abstract The results of a multimodel forecasting effort to predict mountain wave–induced polar stratospheric clouds (PSCs) for airborne science during the third Stratospheric Aerosol and Gas Experiment (SAGE III) Ozone Loss and Validation Experiment (SOLVE)/Third European Stratospheric Experiment on Ozone (THESEO 2000) Arctic ozone campaign are assessed. The focus is on forecasts for five flights of NASAs instrumented DC-8 research aircraft in which PSCs observed by onboard aerosol lidars were identified as wave related. Aircraft PSC measurements over northern Scandinavia on 25–27 January 2000 were accurately forecast by the mountain wave models several days in advance, permitting coordinated quasi-Lagrangian flights that measured their composition and structure in unprecedented detail. On 23 January 2000 mountain wave ice PSCs were forecast over eastern Greenland. Thick layers of wave-induced ice PSC were measured by DC-8 aerosol lidars in regions along the flight track where the forecasts predicted enh...

Water vapor heterogeneity related to tropopause folds over the North Atlantic revealed by airborne water vapor differential absorption lidar

[1]xa0Airborne differential absorption lidar (DIAL) measurements of tropospheric water vapor and aerosol/clouds are presented from transfers across the North Atlantic on 13–15 May and 16–18 June 2002. The intense dynamical activity over the Atlantic is reflected in complex structures like deep tropopause folds, extended dry layers, and tilted aerosol filaments. Intrusions with H2O mixing ratios below 0.03 g kg−1 regularly develop along the storm track over intense cyclones and correspond to analyzed (European Centre for Medium-Range Weather Forecasts (ECMWF)) or simulated (MM5) potential vorticity anomalies. Sloping folds are typically 1 km thick in vertical profiles but on horizontal sections resemble long filaments with widths of a few hundred kilometers. Filaments of solid, roughly micron-sized particles occur inside and along the axes of the tropopause folds. The particles most likely originate in forest fires, from where they are injected to the lower stratosphere by deep convection and later on reenter the troposphere in developing baroclinic disturbances. Their laminar appearance indicates that these intrusions have not yet been mixed but are eroded on timescales significantly larger than a few days by stretching-thinning, stirring, and only finally, turbulent diffusion. MM5 mesoscale model simulations and, with less accuracy, also ECMWF analyses are capable of reproducing the dynamical structures associated with the observed dry intrusions. However, deep inside the intrusions where scales are small, even the MM5-simulated water vapor mixing ratios are more than twice those observed. The large dynamical range of water vapor mixing ratios (3 orders of magnitude) and large gradients (>2 g kg−1 km−1 at the intrusions boundaries) constitute a challenge for future space-borne water vapor DIAL systems.

Aerosol-radiation interaction in the cloudless atmosphere during LACE 98. 2. Aerosol-induced solar irradiance changes determined from airborne pyranometer measurements and calculations

[1]xa0One of the goals of the aerosol column closure experiment Lindenberg Aerosol Characterization Experiment (LACE 98) that was carried out over the eastern part of Germany in summer 1998 was to reduce uncertainties in the determination of the radiative effects of aerosol particles over land. For this purpose, a variety of closure experiments was carried out during LACE 98 to determine the chemical, physical, optical, and radiative properties of the aerosols during situations of clean and strongly polluted continental air. By coordinated flights of three aircraft, measurements of aerosol microphysical and optical properties, of spectral surface reflectance, and of upwelling and downwelling solar irradiances were performed at altitude levels between 200 m and 11 km. The measurements are used for a comparison with results from a radiative transfer model. The calculations are based on measured aerosol properties such as the total number concentration and particle size distribution. The chemical composition of the aerosol was obtained from an analysis of filter samples in order to obtain the optical properties of the aerosol throughout the relevant spectral range by use of Mie theory. The results for the daily averaged solar radiative forcing due to aerosol particles reveal cooling of the total Earth/atmosphere system ranging from −4 to −13 W m−2 in cloud-free conditions.

Polar stratospheric clouds during SOLVE/THESEO: Comparison of lidar observations with in situ measurements

Polar stratospheric clouds (PSCs) were observed on several flights during the 1999/2000 SOLVE/THESEO mission. Here we present an analysis of PSC size distribution, composition, and particle phase based on near coincident measurements using the Multiangle Aerosol Spectrometer Probe (MASP) and Focused Cavity Aerosol Spectrometer (FCAS III) on board the ER-2 as well as lidar instruments on board the DC-8 and Falcon aircrafts. We calculate the aerosol backscatter ratios and aerosol depolarization ratios at infrared and visible wavelengths based on the particle size distributions measured by the MASP and FCAS III instruments. We then compare our calculations to observed lidar measurements taken from on board the DC-8 and Falcon aircraft, which flew the same flight paths as the ER-2 on 20 January and 3 February 2000, respectively. Our comparison shows that calculations based on the mixed clouds containing small (submicrometer) spherical particles and large (2.5–22 μm diameter) nonspherical particles seen by the FCAS III and MASP are consistent with lidar observations. Our analysis shows that the infrared aerosol depolarization ratio must be measured to detect and identify mixed PSC clouds and that relying only on measurements of depolarization in the visible causes incorrect identification of mixed clouds as solution droplets. This study suggests that mixed clouds of supercooled ternary solutions were observed in coexistence with larger solid nitric acid hydrate particles.

Denitrification inside the stratospheric vortex in the winter of 1999–2000 by sedimentation of large nitric acid trihydrate particles

[1]xa0Synoptic-scale polar stratospheric clouds (PSCs) have been measured with an aircraft-borne lidar south of Spitsbergen in early February 2000. The PSCs of moderately depolarizing stratified layers extended from near the tropopause (10–11 km) up to 20 km altitude. Their internal structure in backscatter ratio reflects the variability of the particles concentration rather than manifold sizes and phases of different optical efficiency. Actually, their size distribution was nearly monodisperse. According to T-matrix calculations, low backscatter ratios (γ < 3), color ratios of 1–1.5, and moderate depolarization ratios (δa ≈ 0.15–0.2) indicate that the observed layers were composed of aspherical nitric acid trihydrate (NAT) particles larger than 2 μm. During three turning point missions into the cold vortex core a significant settling of the particle layers was observed between aligned outgoing and incoming legs. Low stratospheric winds and quasi-Lagrangian flight heading minimized horizontal drifts between the probed volumes. A NAT particle sedimentation velocity of 69 ± 14 m/h is derived by cross-covariance analysis of corresponding pairs of two-dimensional backscatter sections of the layers. This implies a particle size of rp ≈ 7–8 μm and a mixing ratio of condensed HNO3 near 2 ppbv. The downward NOy flux below 19 km altitude amounts to j ≈ 3 ppbv km/d. Based on particle back trajectory analysis, this flux, derived from the observations in different regions of the Northern European Ice Sea, has a significant denitrification potential.

Validation of contour advection simulations with airborne lidar measurements of filaments during the Second European Stratospheric Arctic and Midlatitude Experiment (SESAME)

Airborne lidar measurements of vertical sections of ozone and aerosol across the lower stratospheric vortex boundary during the Arctic winter 1994/1995 are used to study the permeability of this dynamical barrier. Both constituents exhibit strong gradients in the lower levels of the vortex edge and substantial structure at subsynoptic scales in the surrounding surf zone. These measurements provide validation of two-dimensional (2-D) Contour Advection (CA) calculations which use potential vorticity (PV) as a proxy tracer. The CA calculations are initialized from gridded U.K. Meteorological Office analyses and thus may smooth subgrid scale structures. Contour Advection reproduces most of the observed filaments with an accuracy of few tenths of a degree, but neglect of diabatic effects causes small errors. The CA calculations show that the arctic stratosphere was dynamically disturbed over extended periods by breaking planetary waves as indicated by numerous spiral-like PV filaments which surrounded the polar vortex. The transport rate across the vortex boundary due to filamentation and subsequent detachment of the filaments off the vortex can be inferred from the model calculations. In winter 1994/1995, two wave breaking events eroded the area of the Arctic vortex by 8% and 40% at 450 K and by 22% and 60% at 600 K. A third event triggered off the final warming. Since the vortex air partly was chemically disturbed, this cross vortex boundary transport, together with effective catalytic ozone destruction cycles, has considerable potential for reducing ozone concentrations at midlatitudes.

Erosion and mixing of filaments in the arctic lower stratosphere revealed by airborne lidar measurements

[1]xa0The extent of ozone depletion in the Arctic stratosphere is limited by the dynamical instability of the winter circulation. During January 1999, transport across the polar vortex boundary occurred with intrusions of midlatitude air into the vortex and via peeling of filaments off the vortex edge, followed by intensive stirring. Contour advection simulations follow the reduction of the scales. The structures are evident as heterogeneities in two-dimensional (2-D) airborne lidar sections of aerosol backscatter ratio, along flight paths inside the vortex during January/February 1999. Large gradients of the particle backscatter ratio indicate that microscopic mixing is slow in the presence of continuous intensive macroscopic stirring. The gradients persist several weeks after the vortex recovered and isentropic stirring started. The filament boundary layers typically extend over 100 m vertically and a few kilometers in the horizontal. Under typical stratospheric conditions, it takes about two weeks until 3-D turbulence takes over and effectively mixes air masses at these scales. In late January 1999 only about 10–13% of the air probed during the flights inside the vortex are mixed microscopically, which, however, is a prerequisite for their chemical and microphysical interaction. As chemical transport models cannot distinguish subgrid-scale stirring from molecular mixing, the incomplete mixing implies that homogenization would occur prematurely in a model run and interaction rates would be overestimated.

A New Airborne DIAL System for Tropospheric Ozone Measurements

Test measurements with an airborne ozone DIAL, based on a Nd:YAG pumped KTP-OPO are reported from a campaign over southern Germany in late August 2002. The ability of the instrument to measure tropospheric ozone concentrations with high spatial resolution has been proven. In the near future, it is planned to extend the set-up for SO2 measurement and add some channels to the existing system for aerosol back-scattering.