Wolken – die großen Unbekannten im Klimasystem

Lange Zeit hatten uns die Klimawissenschaftler vorgegaukelt, es gäbe nur noch i-Punkte in den Klimagleichungen zu setzen. Heute wissen wir, dass dies ein schwerer Fehler war. Noch immer gibt es viele große Unbekannte in den Klimamodellen. Im Februar 2015 räumte das Deutsche Klimakonsortium (DKK) ein:

Wolken – die großen Unbekannten im Klimasystem
[...] Wolken sind für systematische Messungen nicht nur schwer zugänglich, sie unterliegen auch ständiger und schneller Veränderung, sind in Form, Entstehung, Zusammensetzung und Höhenvorkommen unterschiedlich und haben deshalb verschiedene Auswirkung auf die Energiebilanz in der Atmosphäre. So wirken Wolken in tieferen Atmosphärenschichten kühlend, weil sie Sonnenstrahlen reflektieren und weniger Energie die Erdoberfläche erreicht. Hohe Eiswolken, Zirren genannt, wirken hingegen eher wärmend, da sie die Wärmeabstrahlung der Erde nicht in die Atmosphäre entlassen, sondern wieder Richtung Erdoberfläche reflektieren. [...] Frank Stratmann untersucht im weltweit einzigartigen Leipziger Wolkensimulator (Leipzig Aerosol Cloud Interaction Simulator, LACIS) die Wechselwirkungen zwischen Aerosolpartikeln und Wolkentropfen: Im Kleinen wird die Wolkenbildung simuliert, die sich normalerweise bis zu einer Höhe von 15 km über der Erdoberfläche abspielt. [...]

In Stratmanns Vortrag heißt es:

Was ist der Stand:

Wolken in Klimamodellen werden in der Regel nicht im Detail modelliert weil:

a) es teilweise an grundlegendem physikalischem Verständnis mangelt
b) eine Prozessmodellierung numerisch zu aufwändig ist
c) es ein grundsätzliches Skalenproblem gibt
(Wolken – Kilometerskala, Modellauflösung – Hundertkilometerskala)
d) Wolken werden in parametrisierter Form behandelt

Klimamodelle müssen anhand von Messwerten validiert werden

Im März 2015 wiesen auch Sandrine Bony und Kollegen in Nature Geoscience auf die großen Fragezeichen bei der klimatischen Rolle der Wolken hin:

Clouds, circulation and climate sensitivity
Fundamental puzzles of climate science remain unsolved because of our limited understanding of how clouds, circulation and climate interact. One example is our inability to provide robust assessments of future global and regional climate changes. However, ongoing advances in our capacity to observe, simulate and conceptualize the climate system now make it possible to fill gaps in our knowledge. We argue that progress can be accelerated by focusing research on a handful of important scientific questions that have become tractable as a result of recent advances. We propose four such questions below; they involve understanding the role of cloud feedbacks and convective organization in climate, and the factors that control the position, the strength and the variability of the tropical rain belts and the extratropical storm tracks.

Im Mai 2015 brachte auch proplanta.de Klimarealimus:

Forscher nehmen Wolken ins Visier
Wie beeinflussen die Wolken den weltweiten Klimawandel? Forscher haben darauf noch keine umfassenden Antworten.
Klar sei nur eines, sagt Professor Thomas Leisner, Klimaforscher am Karlsruher Institut für Technologie (KIT): «Wenn sich das Klima ändert, ändern sich auch die Wolken – und umgekehrt». Doch in welchem Maße? Und in welche Richtung? In der deutschlandweit einzigartigen Wolkenkammer «Aida» versuchen Leisner und seine Mitarbeiter, Antworten zu finden. Zwei zentrale Fragen stellen sich: Werden die Wolken im Klimawandel mehr kühlend oder mehr erwärmend wirken?

Weiterlesen auf proplanta.de

In der ZDF-Fernsehdoku “Abenteuer Wolkenforschung” kann man sich über das Thema etwas genauer informieren:

 

Im Oktober 2015 legte das Institute of Physical Chemistry of the Polish Academy of Sciences per Pressemiteilung nach:

Evaporation for review — and with it global warming

The process of evaporation, one of the most widespread on our planet, takes place differently than we once thought – this has been shown by new computer simulations carried out at the Institute of Physical Chemistry of the Polish Academy of Sciences in Warsaw. The discovery has far-reaching consequences for, among others, current global climate models, where a key role is played by evaporation of the oceans.

They all evaporate: oceans and seas, microdroplets of fuel in engines and the sweat on our own skin. For every one of us evaporation is of paramount importance: it shapes the climate of the planet, it affects the cost of car travel, and is one of the most important factors controlling the temperature of the human body. So common is it that it seemed that evaporation was a phenomenon that had been stripped of any more secrets. In the renowned scientific journal Soft Matter physicists from the Institute of Physical Chemistry of the Polish Academy of Sciences (IPC PAS) in Warsaw, Poland, prove that this belief was erroneous and the mechanism of evaporation must operate differently than had previously been assumed.

“Science copes badly with descriptions of processes occurring in nature. We are perfectly able to describe the states at the beginning of the process and at its end. But what happens in between? How does the given process really take place? For so many years we have been asking ourselves this question in relation to the phenomenon of evaporation – and we are coming to ever more interesting conclusions,” says Prof. Robert Holyst (IPC PAS).

In scientific and technical deliberations we use the Hertz-Knudsen equation, known for over a hundred years, to describe the evaporation rate. What follows from it is quite an intuitive prediction: that at a given temperature the rate of evaporation of the liquid depends on how different the actual pressure at the surface is from the pressure which would be present if the evaporating liquid were to be in thermodynamic equilibrium with its environment.

“The further the system is from equilibrium, the more dynamically it should return to it. It’s so intuitive! So we checked the Hertz-Knudsen equation – because we like to check. In order to do this we prepared exceptionally accurate computer simulations which allowed us for the first time to take a closer look at the process of evaporation,” explains Dr. Marek Litniewski (IPC PAS).

Advanced computer simulations carried out using molecular dynamics showed that the values of some parameters describing evaporation are even several times larger than those predicted by the Hertz-Knudsen equation. However, an even more interesting effect was noted: the stream of gas being liberated from the surface of the liquid during evaporation changed very little despite significant fluctuations in pressure.

“There could only be one conclusion from this observation: the rate of evaporation and the vapour pressure, that is, the physical quantities that were previously considered to be closely related, were not so. For more than a century we had all been making a serious error in the theoretical description of the phenomenon of evaporation!,” says Dr. Litniewski.

The hitherto model of evaporation was based on the principle of conservation of mass: the mass of molecules released from the surface of a liquid had to respectively increase the mass of the gas in its surroundings. Physicists from the IPC PAS noticed, however, that since the particles released from the surface have a certain velocity, in order to describe this phenomenon what should be applied is the principle of conservation of momentum.

“We realized that to some extent evaporation resembles shooting from a cannon: the missile flies in one direction, but the overall momentum of the system must be maintained, so the gun recoils in the opposite direction. The same happens with the molecules of evaporating liquid. Since there is an increase in momentum, there must be recoil, and if there is recoil, the pressure felt by the molecules on the surface of the liquid will be different,” says Prof. Holyst.

The new computer simulations were also used to measure the velocities of the molecules released from the liquid surface. They proved to be small, of the order of hundreds of micrometres per second, which corresponds to only a few kilometres per hour. This fact means that practically any naturally occurring flow over the surface of the liquid has to strongly interfere with the evaporation process. The evaporation cannot thus be described by an equation derived for a very specific case, for liquid that is in thermodynamic equilibrium with the environment.

The discovery of the IPC PAS researchers is of the utmost importance for, among others, the understanding of the real mechanisms responsible for global warming. Contrary to common belief, the most abundant greenhouse gas in the atmosphere of our planet is not carbon dioxide but water vapour. At the same time, it is known that the speed of flow of air masses over the oceans can significantly exceed one hundred kilometres per hour and therefore they will certainly affect the rate of evaporation. The hitherto evaluation of the rate of evaporation of the oceans must therefore be subject to error, which will certainly affect the accuracy of the predictions of contemporary models of the Earth’s climate.