From: David Rogers Date: Mon, 19 Oct 1998 14:47:14 -0600 (MDT) Subject: comments on W4 Marcia's list Dear IN-WG members, Contributions to the discussion - Here are a few comments on Marcia's list of measurements for topic W4, "What additional aerosol and cloud parameters need to be determined and with what accuracies in order to perform the wave cloud experiment? Are we ready for this, or when will we be ready? " Dividing the topic into the 3 major divisions seems a reasonable way to go. I especially like the approach of distinguishing between particles found in ice clouds and those that are active in an instrument, even though we think they represent the same things. As Marcia said, the list of measurements was long. I think it may be too general. A "general characterization of the system" is a good thing to shoot for, but I think we must be more specific and include only those parameters known to be important for ice nucleation (or thought to be important). Remote sensing measurements were not mentioned. Would it be useful to have high resolution spectral images of wave clouds? Given the right assortment of wavelengths, an average nucleation/phase change could probably be deduced. IR emission would give temperature. Perhaps a survey of cloud phase could be generated over a large region, either in support of or independent of a research aircraft. What about radar? Gabor and Bob Kelly showed some highly detailed structure of wave clouds, derived from mm-wave radar. [ Specific comments follow, with original text between dash lines and indented ] ---------------------------------------------------------------- 1. Molecular composition of (a) gas phase ---------------------------------------------------------------- The potential breadth of gas phase measurements has me a bit concerned. For example, a list of organic trace gases would be huge, made even bigger by different isomers. It has been suggested that organic coatings will affect surface kinetics, and therefore perhaps ice nucleation, so maybe we should measure those. Certainly, measurements should be obtained for specific gases known/suspected to be important, but what are those gases? If we don't know which ones, is it reasonable to try measuring all of them? Is it sufficient to go after only those gases that have a mature measurement technology, like fast response spectral absorption techniques that give information on a wide assortment of gases (NO, CO2, CO, H2O, CH4, N20, HOx, CHCO, O3, ...)? Would it be practicable? I fear that if we tried to measure everything, a fleet of C5-A Galaxy aircraft would be needed to carry all the hardware and researchers. Are there some lab experiments that could be done to provide guidance here, or would they expand rather than focus the research? ---------------------------------------------------------------- (b) particulates (sorted by size) ---------------------------------------------------------------- The capabilities available now are truly amazing -- composition of molecular fragments from particles 0.2-2um, in real time. EM samples extend the size range and provide morphological information as well, although they are time-integrated collections. CCN spectra should also be on the list. How about CCN chemistry? ---------------------------------------------------------------- 4. Electric charge on particles. Total charge is easy to measure: surface charge is not. The net charge distribution can be measured concurrently with the number-size distribution to give charge as a function of size. ---------------------------------------------------------------- I'm not sure how you do this measurement. The electrical DMA measures the size from about 0.01 to 1um but imposes its own charge distribution; a size distribution takes a few minutes. Optical probes are fast and go from ~0.12 to 3um but give no charge information. ---------------------------------------------------------------- 6. Thermodynamic and dynamic parameters, of course---T, p, RH, w. ---------------------------------------------------------------- Add some estimate of turbulence intensity and structural mixing scale. ---------------------------------------------------------------- B. Measurements made on particles on which ice forms (which may or may not be nuclei) and those on which it does not. This set of measurements assumes the relevant agents are aerosol particles/small solution droplets. The two classes of particles can be separated either by sending them through a diffusion chamber (which introduces its own problems) or a Counterflow Virtual Impactor, if in cloud. ---------------------------------------------------------------- Sounds great and do-able. ---------------------------------------------------------------- C. In-cloud parameters ---------------------------------------------------------------- Add some estimate of turbulence intensity and structural mixing scale. Another piece of information needed for a wave cloud experiment has to do with aircraft position relative to the cloud boundaries and upwind and downwind air trajectories. It is needed in real time to conduct the experiment, and in post hoc analysis, to construct the history of air parcels. ..dave..