From: David Rogers Date: Wed, 28 Oct 1998 15:27:54 -0700 (MST) Subject: draft W2 estimating ice conc Dear IN-WG members, Here are a few comments on ICE Initiative topic W2, "How serious is the doubt that ice crystal concentrations were seriously underestimated by past measurements? This question arises from Lawson's data with the CPI and from the nearly ubiquitous exponential size distributions of ice crystals. The focus should still remain on wave clouds to the extent possible." I'd like to rephrase the question slightly - "Is it likely that past measurements of ice crystal concentrations were seriously underestimated in wave clouds, due to instrumental limitations?" Note that these comments also bear on topic W1 (dominant modes of nucleation). The general conclusion is that condensation-freezing or contact-freezing are the dominant heterogeneous processes. There are also observations showin rapid formation of ice during cloud evaporation (Cooper 1995). Suggestions have been made for possible mechanisms, but the observations have not been fully explained. ..dave.. ---------------------------------------------------------------------- ICE - Nucleation Working Group ****Discussion Topics**** WAVE CLOUD EXPERIMENT. W2. Ice concentrations underestimated In wave cloud studies over the past 30 years or so, instrument limitations (principally lack of sensitivity to particles <100um) made it difficult to detect and discriminate ice particles at the earliest stages of formation. This limitation occurs primarily near the cloud edge (upwind or downwind) and can be significant in at least two different cases where ice particle growth is limited. The first case is for relatively small wave clouds having correspondingly small air parcel residence times, and the second case is for any wave clouds having high crystal concentrations. In all other cases, the ice crystals will continue to grow larger than ~100um and can be detected with reasonable confidence. Some evidence is available from studies by Kelly, Cooper and Vali, Rogers and Vali. Aircraft passes were made through wave clouds along the direction of the wind. Cloud temperatures were generally in the range -10C to -30C, so that homogeneous nucleation processes were not active. The crystal observations were based on PMS 2D-C instruments and typically showed ice concentrations rising quickly from ~zero at the upwind liquid edge to a plateau value within ~100s parcel time. Liquid water was present throughout the clouds, and we can infer there were large ice supersaturations. In these circumstances, small crystals will grow to detectable size, given sufficient time. The observations showed crystal size distributions had a single broad mode, and the mode increased uniformly downwind. Calculations of crystal diffusion growth indicated that a major fraction of the crystals nucleated in the vicinity of the upstream liquid edge of the cloud. In these circumstances, it seems unlikely that ice concentrations were seriously underestimated. In colder wave clouds, however, where homogeneous freezing nucleation is the dominant ice formation process, or where there are other causes of high crystal concentration, it is more problematic to discriminate water and ice particles because particle growth rates are slower. In this situation, relatively high concentrations of particles ~10um can be detected with FSSP probes, and the particle phase can be determined from humidity measurements. In order to describe the details of liquid to ice phase transitions, a significant advantage is presented by new instruments that can identify the first appearance of ice at sizes ~15um. Of comparable advantage are new techniques for resolving the humidity field on small scales (~meters). --- some references --- Cooper, W.A. and Gabor Vali, 1981: The origin of ice in mountain cap clouds. J. Atmos. Sci., 38, 1244-1259. Cooper, W.A., 1995: Ice formation in wave clouds: Observed enhancement during evaporation. Cloud Phys. Conf., Amer. Meteor. Soc., Dallas, 147- 152. Kelly, R.D., 1978: Condensation-freezing ice nucleation in wintertime orographic clouds, M. S. Thesis, Dept. Atmospheric Science, Univ. Wyoming, 88pp. Rogers, D.C., 1982: Field and laboratory studies of ice nucleation in winter orographic clouds. Ph.D. Dissertation, Dept. Atmospheric Science, Univ. Wyoming, Laramie, 161pp. Vali, G., D.C. Rogers and T.L. Deshler, 1982: Ice nucleus and ice crystal measurements in cap clouds. Conf. Cloud Physics, Chicago, Illinois, 333-334.