Observing Characteristics of Wintertime Cold-Air Pools Using Laser Ceilometers

The Persistent Cold-Air Pool Study (PCAPS) investigated the formation of persistent high stability events in the complex terrain of the Salt Lake Valley during the winter of 2010 - 2011. Major goals of this project were to observe, understand and model the evolution of persistent wintertime cold-air pools. As part of this deployment, a Vaisala CL31 lidar ceilometer was placed near the center of the valley. Over the 2-month observing period, this vertically pointing remote sensing instrument was able to measure the depth of the aerosol layer within the valley and the relative distribution of particulates within the layer.

Our analyses of ceilometer data have tested a variety of numerical methods for determining mixing layer and aerosol depths under the high stability conditions seen in persistent cold-air pools. By combining the lidar backscatter data with other datasets, the lifecycle of a cold air pool could be evaluated, emphasizing the additional information that can be gleaned from ceilometer data. The usefulness of low-cost lidar ceilometers for research in high-particulate, complex terrain environments is remarkable, and presents opportunities for new research to gain understanding of cold-air pool and other events.

In this presentation we evaluate various published methods for objectively determining aerosol layer depths and find that the most successful method uses a simple constant backscatter ratio to contour the top of the aerosol layer. A time-height cross-section of backscatter ratio, when used as an analysis aid with rawinsonde temperature, dew point temperature, and wind vector soundings plotted over it has been found to be a very useful analysis technique, elucidating the relationship between aerosol layers and atmospheric structure layers. Meteorological case studies will be presented to illustrate the techniques.