From: David Rogers Date: Mon, 19 Oct 1998 14:45:54 -0600 (MDT) Subject: W3 airborne IN detector Dear IN-WG members, Here is a draft discussion for ICE Initiative topic W3, "What are the development steps necessary toward an airborne IN detector in wave cloud inflow air?". The integration between this topic and others, is not present. The integration is needed to tie together the separate pieces and show consistency in our group's thinking. For example, parts of this topic are clearly connected to Marcia's list of measurements for W4 and to W1 (dominant nucleation modes). Presumably, the integration parts happen in the discussions and next go-around. ..dave.. ---------------------------------------------------------------------- ICE - Nucleation Working Group ****Discussion Topics**** WAVE CLOUD EXPERIMENT. W3. What are the development steps necessary toward an airborne IN detector in wave cloud inflow air? This section identifies the ice nuclei measurements needed for an airborne study of ice formation in wave clouds. It then lists current instrumentation and makes recommendations for developing the necessary measurement capabilities where none exist. For studying the ice formation processes, there is a distinction between clouds with dominant heterogeneous freezing processes and those with primarily homogeneous freezing. Earlier measurements in wave clouds showed that most ice formation occurred near the visible leading edge, that is, within a region a few hundred meters wide. Air parcel residence times in this region are a few to tens of seconds; research aircraft flying along the streamlines will traverse this region in a few seconds. For clouds warmer than about -40C, the dominant heterogeneous ice processes were shown in earlier studies to be either condensation freezing or contact freezing by very small aerosol particles. Serious questions remain about the deposition mode (formation of ice below water saturation), viz., does it exist and if so, what contribution to the total ice can be attributed to deposition? Ice nucleation occurs in response to two primary and independent thermodynamic factors, temperature and humidity. In addition, time and the presence of supercooled drops are important factors. With these factors in mind, airborne IN detector(s) should have the following capabilities: - Fast response (~1 second) for accurate temporal and spatial resolution. If the inflow air is steady and has uniform properties, this requirement can be relaxed to allow prolonged sampling in the inflow air upwind of the cloud. - Measurement of ice nucleation spectra of the aerosol, to describe the IN response to both temperature and supersaturation within the ranges observed during aircraft penetrations of the cloud. - Sensitivity to the dominant nucleation mode(s), to include condensation freezing, deposition (upwind of water cloud), contact freezing, and immersion freezing. Homogeneous freezing is a distinctly separate mode; its measurement is needed for clouds colder than about -35C and should be closely coupled to simultaneous measurements of CCN activity and chemistry. - Sensitivity to a hypothesized evaporation ice nuclei (EIN) process, if this process is observed in the field studies. - Sample particles over a wide size range, from fine particles (<0.1um) to giant aerosols (>10um). - Characterize the physico-chemical properties of IN. - Capture cloud crystals in-situ for physico-chemical studies of the non- volatile components (including the ice nuclei). Airborne counter virtual flow impactor (CVI) instruments provide this capability. At this time, there are no IN instruments capable of providing all these measurements at the same time, but there are instruments with strengths in several areas. A wide variety of different measurement techniques have been used; each one usually favors one or two nucleation mechanisms at the expense of being able to detect others. Thus, while the continuous flow diffusion (CFD) chamber technique has shown promise in recent field campaigns, it has well recognized limitations: no sensitivity to contact nucleation; sample rate of only about 1 liter per minute; and sample residence time in the chamber of only a few seconds. The Rogers (1988) version CFD does not sample aerosol particles larger than 2um diameter, and it measures at one temperature and supersaturation at a time. Mixing chambers (Langer 1973) are sensitive to contact nucleation because they produce very high concentrations of supercooled water droplets; they have, however, strong and uncharacterized transient supersaturations which can be of primary importance for nucleation. Membrane filters (Bigg 1996) sample large volumes of air (few hundred liters). The filters can be processed in different ways to simulate all four nucleation modes, but temporal resolution is usually no better than 20-30 minutes. Complicating factors include possible chemical and diffusion interference from the filter substrate and high concentrations of hygroscopic particles. Attention is needed on the following items for developing capabilities to support IN measurements in the ICE initiative experiments: - Greater air sample rates, in order to improve temporal resolution and sampling statistics. - New approaches to measuring IN that emphasize or isolate particular nucleation mechanisms. For example, perhaps the time element of contact freezing could be overcome by accelerating droplet-particle interactions by electrostatic charging or an acoustic field. Another example would be segregating particles and testing a subset with the IN instrument, e.g., droplets from CVI, CCN, non-CCN, etc. - An assessment of the effects that warming and drying the sample air before measurement with the IN instrument. Is it important to maintain the sample at ambient conditions? - Reference standard IN aerosol particles or procedures, in order to compare different measurement methods. - Comparison of IN measurement techniques, probably in a laboratory-based intercomparison workshop. A large expansion type cloud chamber can provide the most accurate simulation of the natural processes and should be part of the laboratory study. The ultimate IN detector is, of course, the cloud itself, and it is the ultimate judge of how ice forms. If it were possible to build an instrument to sample air and subject it to the same thermodynamic forcing as the cloud environment, then water drops and ice crystals would form and dissipate in the same manner as in the real cloud. This exercise might demonstrate our ability to imitate nature, but it would not improve our knowledge of how the nucleation processes work or how to describe them in terms of the physics and chemistry. An ice nuclei detector that only imitates the natural process is not sufficient for these studies. --- add some references ---