Klaus Gierens

Global Distribution of Contrail Radiative Forcing

The global distribution of radiative forcing by persistent linear contrails has been estimated for 1992 and 2050 using global contrail cover computed for aircraft fuel consumption scenarios for the two periods, a detailed prescription of the radiative properties of the Earths surface and the cloudy atmosphere, and flux computations with an established radiative transfer model. The computed global mean radiative forcing by line-shaped contrails is ∼0.02 Wm−2 in 1992 and ∼0.1 Wm−2 in 2050. At northern mid-latitudes, the zonal mean forcing is five times larger than the global mean. Diffuse contrails and indirect effects of aircraft emissions on natural cirrus are not included in this study. Thus, the results are considered a minimum estimate of contrail effects.

Water vapour measurements inside cirrus clouds in Northern and Southern hemispheres during INCA

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.

Ice supersaturation in the tropopause region over Lindenberg, Germany

The occurrence of ice-supersaturation layers (in either clear air or in cirrus) over the Meteorological Observatory Lindenberg is investigated for the period February 2000 to April 2001 by means of the humidity, temperature, and pressure reports obtained from the Lindenberg corrected RS80A routine radiosonde. The RS80A routine sonde data are corrected on the basis of weekly comparison ascents with Lindenberg research radiosonde humidity data. This research sonde applies the Lindenberg measuring and evaluation technique of standardized frequencies. We study the frequency of occurrence of ice supersaturation in the tropopause region over Lindenberg, the vertical distribution of ice-supersaturation layers in the upper troposphere and lowermost stratosphere, their situation relative to the tropopause, their vertical dimensions, their temperatures and the statistical distribution of relative humidities. The mean frequency of occurrence of ice-supersaturation layers is about 28%. Most of them occur within a broad layer extending 200 hPa down from the tropopause. Most events occur in cold air with temperature below -40°C. Their vertical extensions can be fitted by a pair of Weibull distributions with mean 560 ± 610 m. The results are compared with findings from the MOZAIC project, from the SAGE II satellite instrument, and with various results from Lidar measurements.

Ice‐supersaturated regions and subvisible cirrus in the northern midlatitude upper troposphere

Humidity and temperature data from the Measurement of Ozone by Airbus in-service Aircraft (MOZAIC) project have been used to produce maps of probability for ice supersaturation in two 50 hPa thick layers centered around 200 and 250 hPa. As the MOZAIC data cover only international air routes, the resulting maps cover mainly the northern midlatitudes. The data of ice supersaturation have then been correlated with data of frequency of occurrence of subvisible cirrus from the Stratospheric Aerosol and Gas Experiment (SAGE II) satellite instrument. The correlation analysis provided strong indications that subvisible cirrus (SVC) is associated to ice-supersaturated regions (ISSRs), although processes are possible that can decouple SVC from ISSRs. A first trial to derive a global picture of ice supersaturation near the tropopause was performed using a measure of cirrus fractional coverage constructed from meteorological analyses of European Centre for Medium-Range Weather Forecasts and to correlate this with the supersaturation data. The correlation was only moderate (although significant), leading to the tentative conjecture that regions of frequent ice supersaturation are to be expected over the Indonesian archipelago, over the Amazonas basin, and over the northern Pacific between Japan and Canada. A final correlation analysis between the meteorological analysis data and the SVC data indicated that the formation of SVC is generally thermodynamically controlled, with the exception of the northern midlatitude SVC. The composition of the aerosol at the northern midlatitude tropopause is probably variable due to industrial emissions and air traffic. Hence the freezing properties of these particles may become important, which results in a weaker thermodynamic control of SVC formation in the northern midlatitudes.

Numerical Simulations of Persistent Contrails

Abstract Numerical (2D) simulations of persistent contrails have been performed. The simulations begin in the vortex phase (i.e., when the aircraft wake dynamics are dominated by the pair of downward travelling vortices) and pursue the evolution for half an hour. Particular emphasis was laid on the mechanisms by which contrails expand to reach the large lateral dimensions often observed and on the ice production in these artificial clouds. Cross-sectional spreading rates are found to range from 120 to 290 m2 s−1. The expansion is mainly caused by the secondary and higher-order vortices that develop from the reaction of the atmosphere with the downward travelling pair of primary vortices that are themselves produced by the aircraft. An additional driving force for contrail expansion is the gravitational collapse that results from differences between the potential temperature profiles inside and outside the contrails. Humidity and temperature of the ambient air control the growth of the ice particles and, t...

Lidar and numerical studies on the different evolution of vortex pair and secondary wake in young contrails

Vortex-regime evolution of contrails is investigated by focusing on the role of ambient humidity. Lidar cross-section measurements and observational analysis are combined with numerical simulations of fluid dynamics and microphysics. Contrail evolution behind four-turbofan aircraft is classified into three different scenarios. In the case of ice-subsaturated air, a visible pair of wingtip vortices is formed that disappears at the end of the vortex regime. In case of ice supersaturation, a diffuse secondary wake evolves above the wingtip vortices. It is due to detrainment of ice particles growing by sublimation of ambient humidity. A vertical wake-gap opens between the wingtip vortices and the secondary wake. It is due to subsaturated air moving upward along the outer edges of the sinking vortex tubes accumulating around the upper stagnation point of the vortex system. The vertical wake-gap preferably occurs in the wake of heavy (four turbofans) aircraft, since the vortices behind light aircraft migrate down too slowly. The secondary wake is composed of nonspherical particles larger than the ones in the wingtip vortices which are spherical particles and/or particles smaller than ≈0.5 μm. In most cases the secondary wake is the only part of a contrail that persists after vortex breakdown. This is because the ice in the vortex tubes evaporates due to adiabatic heating as the vortices travel downward. Only in the rare case of higher ambient ice supersaturation (>2%) do both parts of a contrail contribute to the persistent ice cloud. The number of ice crystals initially formed is typically reduced by a factor of 200 by evaporation (60% ambient humidity). This leads to a high population of interstitial particles. The results imply that formation of persistent contrails can be minimized by technical means.

A review of various strategies for contrail avoidance

A review is given of various contrail avoidance strategies that have been developed since the publication of the Intergovernmental Panel on Climate Change (IPCC) special report on Aviation and the Global Atmosphere. The goal of this review is to provide an overview of the various options for contrail mitigation, to describe the state of the art, and to indicate future directions for research.

The evolution of contrail microphysics in the vortex phase

We investigate the evolution of contrails during the vortex phase using numerical simulations. Emphasis is placed on microphysical properties and on the vertical distribution of ice mass and number concentration at the end of the vortex phase. Instead of using a 3D model which would be preferable but computationally too costly, we use a 2D model equipped with a special tool for controlling vortex decay. We conduct a great number of sensitivity studies for one aircraft type. It turns out that atmospheric parameters, namely supersaturation, temperature, stability and turbulence level have the biggest impact on the number of ice crystals and on the ice mass that survives until vortex breakup and that therefore makes up the persistent contrail in supersaturated air. The initial ice crystal number density and its distribution in the vortex, are of minor importance.

Effective Radius of Ice Particles in Cirrus and Contrails

Abstract This paper discusses the ratio C between the volume mean radius and the effective radius of ice particles in cirrus and contrails. The volume mean radius is proportional to the third root of the ratio between ice water content and number of ice particles, and the effective radius measures the ratio between ice particle volume and projected cross-sectional area. For given ice water content and number concentration of ice particles, the optical depth scales linearly with C. Hence, C is an important input parameter for radiative forcing estimates. The ratio C in general depends strongly on the particle size distribution (PSD) and on the particle habits. For constant habits, C can be factored into a PSD and a habit factor. The PSD factor is generally less than one, while the habit factor is larger than one for convex or concave ice particles with random orientation. The value of C may get very small for power-law PSDs with exponent n between −4 and 0, which is often observed. For such PSDs, most of t...

The Statistical Distribution Law of Relative Humidity in the Global Tropopause Region

Cloud cleared Microwave Limb Sounder (MLS) data of upper tropospheric humidity are evaluated in order to determine the global statistics of relative humidity with respect to ice, RHi. The evaluation is performed for the 215 hPa level, in order to compare the results with earlier results from the Measurement of ozone by Airbus in-service aircraft (MOZAIC) project. In agreement with the earlier study we find that in the lowermost stratosphere the probability to get a certain value of relative humidity decreases exponentially with the relative humidity. In the Antarctic data class (data south of 55°S, mainly winter data) we also find an exponential distribution for RHi but with less steep slope. There is no change in the slope of the exponential distribution function at ice saturation. In the upper troposphere there are corresponding exponential distributions for RHi in ice-supersaturated regions and in subsaturated regions (for 20% 150%, and the exponential distributions extend without change of slope up to 180-200%. Such extreme humidity events occur mostly in the tropics and at the edge of Antarctica, and could result from incidental lack of aerosol.