TRACE-P Cloud Index 22 May 2002 Carolyn Jordan NASA Langley Research Center 757-864-3787 c.e.jordan@larc.nasa.gov Cloud droplet sizes range from a few microns up to 50 µm, with average diameters in the 10-20 µm range (Seinfeld and Pandis, 1998). Given this, observations made by Bruce Anderson's group of particles in the 10-20 µm size range may be used to determine when the DC-8 was flying through clouds. Anderson's 1 second measurements of 10-20 µm volumes were used in conjunction with Henry Fuelberg's cloud observations and videotapes made on the DC-8 flights to determine an appropriate threshold indicative of passing through a cloud. The 1 second data was averaged into 1 minute intervals to match that of the TRACE-P merge files. Three classifications were defined aside from clear air. When the 1 s volume of 10-20 µm particles exceeded 100 µm2/cm3, the appropriate 1 minute interval was classified as haze. Similarly, when the volume exceeded 800 or 2000 µm2/cm3, it was called "inter" (for intermediate) or "cloud", respectively. Typically, passage into a cloud from a non-cloudy area results in a volume increase of 3 - 5 orders of magnitude, making this classification approach fairly straightforward. Bear in mind, however, that this index only indicates the air the DC-8 passed through. That is, "clear" does not mean an absence of clouds, but rather the air being sampled was not within a cloud. Clouds may, in fact, be above or below the aircraft, or all around it at the time "clear" air samples are being collected. No attempt has been made to classify the type of cloud (cirrus, cumulus, etc.). However, one can look at altitude and the depth (on ascents and descents) as well as Henry Fuelberg's meteorological notes to get an idea of the likely type of cloud encountered. While the cloud category was clear cut based on the videotapes, the haze and intermediate categories were a little murkier. Sometimes intermediate seemed to be a dense area within low altitude haze, perhaps bordering on fog. Sometimes, it was a thin area within a mid-altitude cloud. At high altitudes, intermediate can be indicative of high cirrus clouds, which are drier (hence lower particle volumes are recorded) than for lower altitude clouds. So some care should be applied when using this index. Also, sometimes the instrument would saturate within a cloud, registering a zero measurement, when there is clear evidence of being within a cloud. Those few occasions have been corrected by hand, thus there may be occasional discrepancies between the cloud index and the reported volumes in the final data. This index should be considered a guide, a place to start, when looking at data in the context of being within clouds. However, users are encouraged to look more carefully at individual case studies, should the index not appear to be quite right. Feel free to contact me, if you have any questions or need more detailed information. If there is interest, an index could be prepared on a 1 second time frame rather than 1 minute. At this time, the index is available for DC-8 flights 7 - 17. If anyone is interested in the transit flights, let me know and I'll produce an index for those as well (given any data limitations on those flights). Reference: Seinfeld, J. H., and S. N. Pandis, Atmospheric Chemistry and Physics: From Air Pollution to Climate Change, John Wiley & Sons, Inc., New York, NY, p. 339, 1998.