The Tenth Cloud Layer EXperiment: an Overview

The Cloud Layer EXperiments (CLEX) are part of an ongoing effort funded by the Department of Defense's Center for Geosciences/Atmospheric Research to observe and characterize the microphysical properties, dynamics and morphology of non-precipitating, mid-level, mixed-phase clouds. Primarily, CLEX has focused on non-frontal, non-orographic, altocumulus, altostratus, and "altostratocumulus" clouds.

Mid-level clouds (i.e. altocumulus, altostratus, and "altostratocumulus") are the clouds that form between 2 km and 7 km (6500 ft - 23000 ft) above sea level. Studies have shown that they cover approximately 22% of the Earth. They typically have temperatures between 0° and -30° C. Since these clouds have temperatures below freezing, they are likely to (and often do) contain supercooled liquid droplets in addition to ice particles.

Scientifically, these clouds are of interest since they provide a test-bed for cloud microphysical theories (e.g. how ice forms in the presence of liquid droplets and how ice and liquid cloud particles interact). They also push (or, in many cases, exceed) the limits of commonly used forecast models in their ability to be predicted. Typical "CLEX clouds" form in humid layers of weak vertical motion (~1 m/s or less) in the mid-troposphere and consist of a thin layer (~300 to ~500 m thick) of supercooled liquid with light to moderate ice virga precipitating from that. With the liquid droplets occurring at/near cloud top, where temperatures are the lowest, these clouds have a microphysical profile opposite of the common assumption that the colder the cloud parcel, the more likely it is to be made of ice. General forecast models also typically have the poorest vertical resolution in the mid-troposphere and a difficult time accurately predicting the weak vertical motions.

Practically, the clouds are important to civilian and military aviation. Supercooled liquid droplets cause aircraft icing, which is hazardous to light aircraft and Unmanned Aerial Vehicles (UAVs). (Icing is important for all aircraft, but rest assured that our thin mid-level clouds pose a minimal threat to most commercial planes.) Altocumulus clouds are also a significant source of turbulence.

Mid-level clouds also have the potential to be significant players in the global radiation budget - meaning they may have an impact on global warming. The extent of this is not well known, since their radiative properties depend largely on their microphysical properties, which are not well known. One goal of CLEX is to fill in these knowledge gaps.

Previous CLEX experiments conducted at Colorado State University have examined mid-level clouds over the Great Plains of the United States through a combination of direct aircraft measurements and remote measurements from satellites and ground based lidar among other things. Results have been (and are currently being) published. See our list of publications.

The next experiment - CLEX-10 - will be the first to step outside the Great Plains and will explore mixed-phase clouds over Southern Ontario. This will be done in conjunction with the Canadian CloudSat/CALIPSO Validation Project (C3VP). CLEX-10 and C3VP will have the most extensive suite of microphysical probes ever collected on one aircraft (click here to see for yourself). While CLEX-10 will continue the tradition of observing the microphysical properties and morphology of mid-level, mixed-phase clouds, we will have our first opportunity to thoroughly explore the processes of ice particle formation and investigate why some mid-level clouds remain primarily liquid, some glaciate and others remain mixed-phase over long periods of time (i.e. hours). Our measurements will also be used to validate or improve satellite detection and retrieval methods of mid-level clouds, specificaly cloud phase.

Want to know more? Details of the science plan can be found here.