Altocumulus (Ac)

This series of images documents the temporal evolution of a lee wave cloud named altocumulus lenticularis undulatus. The envelope of the cloud resembles a lenticularis type cloud, however the inner structure sometimes was wave-like. The event was observed between 7:39 a.m. and 10.15 a.m. when the clouds disappeared again. The images were all taken from the same location in Plaka, Crete. The file names contain time and the digit 1 or 2, if several pictures were taken within the same minute. Hereby the second picture is mostly a zoom into a part of the first image or represents another line of vision.

In connection with a low pressure system in the Balkan region a strong north-west flow was guided over Crete. It created lee-wave clouds when passing the mountains of 300 to 900 m altitude at the north-east coast of Crete. Those clouds were formed at the medium level and were certainly stationary. On the lower level there was a layer which consisted of cumulus mediocris clouds moving fast from north-west to south-east. These lower clouds are displayed in ACUnd_0739:01.jpg in the lower left corner and also in ACUnd_0851_2.jpg at the horizon.

Those clouds were generated at the top of the marine boundary layer when passing the lifting condensation level and were advected to Crete later.
The cloud consists of liquid water droplets despite of the stranded border indicating ice particles. This can be derived from temperature and humidity profiles measured by a research airplane in the region. During its lifetime the cloud varied significantly in thickness and appearance.

S. Borrmann; Crete, MINOS field campaign, Plaka, Crete, 3 August 2001 between 7:39 a.m. and 10:15 a.m.