Midlatitude Cirrus

The imagery shown in the figure below was collected during the FIRE-II (First ISCCP Regional Experiment (FIRE; ISCCP refers to the International Satellite Cloud Climatology Experiment) field campaign held in Coffeyville, KS in the fall of 1991. The cirrus were at fairly cold temperatures, with cloud tops colder than -55oC. Note how the pristine particles in the central region of the cirrus tends to be composed primarily of cylindrical crystals, with fewer platelike crystals and bullet rosettes.

Midlatitude cirrus often display 3 distinct layers:

  • small particles in “generating region” near cloud top
  • growth region containing pristine ice crystals in middle region
  • sublimation layer near cloud base, with largest particles

Size sorting occurs because of the relatively low vertical velocities; the situation may be completely different for cirrus generated near centers of strong convection such as anvils.

Contrast the data collected during FIRE-II with the following example collected during an ARM campaign held in March, 2000 in the vicinity of the ARM Southern Great Plains (SGP) site in Oklahoma. These data were recorded in a dyamically active band of cirrus, which had embedded generating cells and trails (known as cirrus uncinus). Cloud top temperatures were about -50oC. The predominant habit for the larger crystals is the bullet rosette. Further discussion may be found in Heymsfield et al. (J. Atmos. Sci., 59, 3-29, 2002).

We have developed an approach to developing scattering properties for individual cirrus particles and also distributions of particles. In pictorial form, we approximate relatively common cirrus habits as plates, columns, bullet rosettes, aggregrates, and droxtals.

Based on in-situ data, we are developing cirrus scattering models in which the habits used in the calculations may change as a function of particle size. Small particles may be droxtals, for example, and pristine habits such as columns and bullet rosettes may comprise the moderate crystal sizes, and aggregrates may best depict the largest crystal sizes. This is an area of active investigation.

What is a droxtal...?

The reference for the calculation of scattering properties for droxtals is the subject of a recent study by our team:

Yang, P., B. A. Baum, A. J. Heymsfield, Y.-X. Hu, H.-L. Huang, S.-C. Tsay, and S. A. Ackerman, 2003: Single scattering properties of droxtals. J. Quant. Spectros. Rad. Transfer, 79-80, 1159-1169.

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