So, what's so special about Arctic clouds?

Michael Tjernström

Professor of Meteorology
Stockholm University
CIRES Visiting Fellow

The Climate in the Arctic appears to be changing faster than at any other region on Earth. Every year we are told that the minimum ice cover, always appearing naturally in late summer, is the smallest on record and recently the Arctic Climate Impacts Assessment concluded that since the 1960's the Arctic near-surface warming is twice that of the Earth as a whole. Although there are several viable hypotheses as to why the Arctic climate is so sensitive, there is now real understanding to what the physical processes are that contribute to magnifying climate trends in this particular region. Much of our understanding of the climate - in the Arctic and elsewhere - stems from studies using global numerical climate models, so called GCM's. A problem here is that they perform so poorly in the Arctic. This is also one reason why the inter-model spread between different climate models projecting future climate is larger in the Arctic than for any other region on Earth.

All climate models have problems with the way they describe clouds. In this seminar I will discuss this problem with special emphasis on the Arctic. Clouds consist of liquid or frozen (ice) water that condense or sublimate to particles from gas as the air becomes supersaturated. But supersaturation is not sufficient for clouds to form. Surface tension goes to infinity as particle size goes to zero, so in principle droplets or ice particles cannot form unless they have a suitable "platform" on which to start forming. We call these cloud condensation nuclei (CCN) for liquid water and ice nuclei (IN) for ice particles. The availability of such particles - that may be natural and/or anthropogenic - governs the distribution of the cloud water between the liquid and solid phase and also the optical properties of the resulting clouds.

So, are clouds special in the Arctic? Evidence from several experiments seem to suggest that they are. And this may be one reason for modeling problems in the Arctic. A low concentration of IN in winter, compared to for example the mid-latitudes, allow liquid water to be present in clouds down to quite low temperatures; liquid water clouds have dramatically different optical properties than to ice clouds. In summer, similarly low concentration of CCN tend to make Arctic low clouds "grayer" than their mid-latitude "cousins", and they thus reflect less solar radiation. And is there a possible feedback hidden here? Well, several have been proposed, and I will touch upon two: the "indirect effect of biogenic aerosol" and the "Arctic dehydration feedback".

About the Lecturer

For more information about Michael Tjernström, visit www.misu.su.se/~michaelt/peop_michael.html.

Michael Tjernström