Modelling of cirrus clouds – Part 1b: Structuring cirrus clouds by dynamics

A recently developed and validated bulk microphysics scheme for modelling cirrus clouds (Spichtinger and Gierens, 2009), implemented into the anelastic nonhydrostatic model EULAG is used for investigation of the impact of dynamics on the evolution of an arctic cirrostratus. Sensitivity studies are performed, using variation of large-scale updraughts as well as addition of small-scale temperature fluctuations and wind shear. The results show the importance of sedimentation of ice crystals on cloud evolution. Due to non-linear processes like homogeneous nucleation situations can arise where small changes in the outer parameters have large effects on the resulting cloud structure. In-cloud ice supersaturation is a common feature of all our simulations, and we show that dynamics is as least as important for its appearance than is microphysics

Meteorological Conditions That Promote Persistent Contrails

Climate-impacting contrails need ice (super-)saturation to persist longer than a few minutes. However, this simple criterion cannot be easily applied for the prediction of persistent contrails. The current weather forecast models, which lack humidity data for assimilation in the upper troposphere, have difficulties coping with the enormous variability and sharp gradients in the relative humidity field. Thus, ice supersaturation, which is an extremal state of relative humidity, is hard to forecast at a precise location and time to allow contrail-avoiding flight routing. In this paper, we investigate the possibility of using dynamical proxy variables for improved contrail prediction. This idea is guided by the fact that the probability of ice supersaturation differs in different dynamical regimes. Therefore, we determine probability distributions of temperature, water vapour concentration, vertical velocity, divergence, relative and potential vorticity, geopotential height, and lapse rate conditioned on three situations: (a) contrail persistence not possible; (b) contrail persistence possible; and (c) strongly warming persistent contrails possible. While the atmospheric variables are taken from reanalysis data, the conditions (a–c) are based on airborne measurement data and radiation quantities from the reanalysis. It turns out that the vorticity variables, and in particular geopotential and lapse rate, show quite distinct conditional probabilities, suggesting a possibility to base an improved forecast of persistent contrails not only on the traditional quantities of temperature and relative humidity, but on these dynamical proxies as well. Furthermore, we show the existence of long flight tracks with the formation of strongly warming contrails, which are probably embedded in larger ice-supersaturated regions with conditions that foster such contrails. For forecasting purposes, this is a beneficial property since the humidity forecast is easier on large, rather than small, spatial scales

Statistics of potential radiative forcing of persistent contrails

Contrails affect climate if they are persistent, that is, if they are located in an ice-supersaturated region (ISSR). They do this by reflecting sunlight back to space (cooling) and by blocking thermal radiation from the Earth surface and lower atmosphere (warming). During night, there is always net warming since sunlight and thus it’s possible reflection is absent. In most (daytime) cases there is substantial cancellation of the warming and cooling effects, but occasionally (in particular during night) the long-wave warming effect dominates such that the respective contrail has a particularly strong contribution to climate warming. This is a Big Hit, and such contrails should be avoided already in the flight planning phase. Such an avoidance strategy needs of course a reliable prediction of the conditions under which contrails actually are that strong climate warmers. The topic of this presentation is how situations with strong warming contrails can be characterised and whether and how reliably it is possible to predict them

Contrails, contrail prediction, contrail avoidance

Einführung in 1) Was sind Kondensstreifen, wie entstehen sie, wie entwicken sie sich 2) Strahungs- und Klimawirkung von Kondensstreifen 3) Vermeidung von Kondensstreifen und die dazu notwendige Verbesserung der 4) Vorhersage eisübersättigter Regionen (und damit der Möglichkeit persistenter Kondensstreifen

Theory of Contrail Formation for Fuel Cells

The theory of contrail formation for fuel cells is derived. It is a variant of the well-known Schmidt-Appleman theory. The contrail factor or G-factor for fuel cells is much larger than for jet engines, such that condensation of the exhaust water vapour can happen even at the Earth’s surface in sufficiently cold (a few degrees above zero) weather. Contrail formation from fuel cells will occur frequently in the lower troposphere and is unavoidable below moderate temperature limits, in the upper troposphere and in the stratosphere. Despite the high frequency of contrail formation from fuel cells, their climate impact is lower than that of contrails from jet engines. Most fuel cell contrails will be short and those persistent ones will be optically thinner and have on average a shorter lifetime than traditional persistent contrails. From a climate point of view, the introduction of fuel cells into aviation can be recommended

Contrail cirrus supporting areas in model and observations

Contrails form and persist dependent on the surrounding moisture, temperature and pressure fields and on fuel and aircraft specific variables. After formation, contrail persistence requires only supersaturation relative to ice. The fractional area in which contrails can form is called potential contrail coverage. We introduce a potential contrail cirrus coverage equivalent to the cloud free supersaturated area. This field, simulated by the ECHAM4 climate model, agrees fairly well with estimates of supersaturation frequency as inferred from aircraft and satellite measurements. In areas where the two potential coverages are different, especially at lower flight levels, potential contrail coverage is not a valid estimate of maximum attainable contrail cirrus coverage. We parameterize both potential coverages consistently with the ECHAM4 cloud cover parameterization. A comparison of the potential contrail coverage with an earlier estimate reveals substantial differences especially at upper height levels in the tropics

Technical note: On the intercalibration of HIRS channel 12 brightness temperatures following the transition from HIRS 2 to HIRS 3/4 for ice saturation studies

For studies of trends in ice supersaturation in the upper troposphere we need very long time series of upper tropospheric humidity. The set of HIRS channel 12 satellite data can be used for this purpose, since Shi and Bates (2011) had provided an intercalibrated time series of channel 12 brightness temperatures. In the current paper we improve the intercalibration at the low tail of brightness temperatures, which leads to a more homogeneous time series of upper-tropospheric humidities

Forecasting conditions for contrail development

Der Vortrag beschreibt 1) die aktuellen Probleme bei der Vorhersage persistenter Kondensstreifen zum Zwecke der umweltgerechten Flugplanung 2) Wo diese Probleme herrühren 3) Wie diese Probleme zu lösen sin