Fall Speeds of Cirrus Crystals Faster than Expected
Fridlind, Ann M. — NASA - Goddard Institute for Space Studies
Area of research:
The fall speeds of high cirrus cloud crystals depend upon crystal mass, for which no direct measurements exist. Based on new measurements of crystal images, calculated mass yields fall speeds as a function of maximum particle dimension that are, surprisingly, roughly twice as large as derived in the most commonly used past studies.
High ice clouds cover large parts of the Earth and the extent to which they warm or cool the planet has been found to be sensitive to assumed fall speeds in past studies.
We embarked on this study in order to prepare internally consistent ice physical and optical properties, with an emphasis on the latter. We expected to corroborate past derivations of bullet rosette mass as a function of maximum dimension (a physical property of the ice), but instead we found substantially greater crystal masses. However, perhaps not surprisingly, we found generally similar projected areas, which are directly measured. We identified several likely sources of error. First, virtually no direct measurements of individual crystal mass exist for cirrus particles. Second, the maximum dimension commonly used for projected area is randomly oriented, whereas that used for idealized calculations of mass is true maximum dimension; we find that randomly oriented maximum dimension is substantially smaller. Finally, large uncertainties are expected in particle mass derived from measured particle size distributions and total ice mass, and such measurement uncertainties (in both bin-wise number concentration and total ice mass) have remained essentially uncharacterized in literature to date.