85th AMS Annual Meeting

Tuesday, 11 January 2005
Observations of ice crystals at the South Pole using a CPI and polar nephelometer
R. Paul Lawson, SPEC Inc., Boulder, CO; and Q. Mo
Poster PDF (1.8 MB)
Results are presented from analysis of over 1 million high-resolution digital images of ice crystals collected at the South Pole Station (SPS) in Antarctica during the Austral summers of 2001 and 2002. This is the first time a statistically large data set of high-definition images recorded by a Cloud Particle Imager (CPI) has been collected in Antarctica and analyzed. In 2001, diamond dust accounted for about 30% of the number of particles, but only about 15% of the mass, while bullet rosettes accounted for only about 25% of the number of ice particles, they comprised about 70% of the particle mass. In 2002 data were also collected using a polar nephelometer that measures scattering phase function, so that CPI digital images could be compared with measured phase functions and visual observations of atmospheric optics. CPI data collected at SPS in 2001 show a consistent trend indicating that bullet rosettes, budding rosettes and solid columns are almost always observed when there is a relatively low overcast or broken cloud cover that is likely to contain supercooled liquid water. On the other hand, diamond dust (i.e., plates, small hexagonal crystals and clear columns) are mostly observed when there are higher, thinner clouds. Geometric relationships between the length, width, area and perimeter of SPS columns and bullet rosettes were compared with a data set of over 250,000 CPI images collected in mid-latitude cirrus by a Learjet. A very strong similarity was found in the geometry of SPS and mid-latitude cirrus ice crystals, suggesting that the light-scattering properties of (rosette and columnar) ice particles observed at the SPS during the Austral summer can be used as surrogates for those observed in mid-latitude cirrus. Visual observations and measurements of scattering phase function made in conjunction with CPI images in 2002 show that 22 and 46 halos were associated with diamond dust, but not precipitating rosettes and columns.

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