Midlatitude cirrus cloud climatology from the Facility for Atmospheric Remote Sensing. IV. Optical displays

Area of Science:

• Atmospheric Science
• Cloud Physics
• Optical Meteorology

Background:

• Long-term cloud research at the Facility for Atmospheric Remote Sensing in Salt Lake City.
• Analysis of a 10-year record of polarization lidar and photographic observations.

Purpose of the Study:

• Characterize the occurrence of optical displays in midlatitude cirrus clouds.
• Evaluate frequencies and longevity of halo, arc, and corona displays.
• Determine meteorological influences on cirrus optical phenomena.

Main Methods:

• Analysis of polarization lidar and photographic data over 10 years.
• Categorization of optical displays (halo, arc, corona) by type, appearance, and duration.
• Correlation of display occurrence with cirrus cloud meteorological sources and ice crystal orientation.

Main Results:

• Complex halo-arc displays are rare; even 22-degree halos are infrequent as complete rings (6% of periods).
• Common displays include 22-degree upper tangent arcs (8.6%), parhelia (8.5%), and solar coronas (7.2%).
• Most displays (83.6%) are refraction-based, with few reflection-based (1.9%) and notable diffraction-based (15.4%) phenomena.

Conclusions:

• Cirrus optical display frequency and type are influenced by ice crystal habits and orientation, with complex displays linked to tropical/subtropical airflows.
• Lidar depolarization ratios differentiate halo-producing (randomly oriented planar crystals) and corona-producing (small, randomly oriented crystals) cirrus.
• Geographical variations in cirrus optical displays can be explained by fundamental cirrus cloud formation processes.