Characterization of ice-particle concentration, size,

Transcription

1 Small Ice Particles in Tropospheric Clouds: Fact or Artifact? Airborne Icing Instrumentation Evaluation Experiment by A. V. Ko r o l e v, E. F. Em e ry, J. W. Str app, S. G. Co b e r, G. A. Is a a c, M. Wa s e y, a n d D. Mar c o t t e Characterization of ice-particle concentration, size, and habit is one of the primary objectives of experimental microphysical studies of cold clouds. Ice particles are effective sinks of water vapor, and they can quickly grow into precipitation-sized particles. Most liquid precipitation on the Earth goes through the ice phase before melting into rain or drizzle. The number concentration of ice particles is an important parameter determining the rate of conversion of liquid into ice in mixed-phase clouds. Knowledge of the size distribution of ice particles is of significant importance for numerical weather prediction and climate models. Airborne microphysical measurements of ice clouds are routinely used for verification of remote-sensing instruments such as radars and lidars. Most observations of the size and concentration of cloud ice particles have been obtained using airborne optical particle size spectrometers, which became widely available in the mid-1970s. Since that time, the amount of data describing the microphysics of ice clouds has been progressively increasing. One striking observation about ice clouds has been the omnipresence of small ice crystals with characteristic size less than 100 µm. It has been observed that the ice crystal number concentration is essentially always dominated by small ice particles. Small ice Affiliations: Ko r o l e v, St r a p p, Co b e r, Is a a c, a n d Wa s e y Cloud Physics and Severe Weather Research Section, Environment Canada, Toronto, Ontario, Canada; Em e ry NASA Glenn Research Center, Cleveland, Ohio; Ma r c o t t e National Research Council Canada, Ottawa, Ontario, Canada Corresponding author: Alexei Korolev, Environment Canada, 4905 Dufferin St., Toronto, ON M3H 5T4, Canada Alexei.Korolev@ec.gc.ca DOI: /2010BAMS American Meteorological Society particles in high concentrations have been observed in environments undersaturated and supersaturated with respect to ice. Numerical simulations suggest that small ice particles should quickly grow to larger sizes in a supersaturated environment, or completely evaporate in a subsaturated environment, keeping the concentration of ice crystals D < 100 µm relatively low. In order to explain this high concentration of small ice particles, a number of mechanisms of ice multiplication in nature have been hypothesized. However, there is no experimental confirmation of any of these mechanisms, with the exception of that established by Hallett and Mossop in their 1974 Nature article. The Hallett Mossop mechanism is active only at relatively warm temperatures (between -5 and -8 C), and it cannot explain the high concentration of small ice particles observed at lower temperatures. One possible explanation for the dominating concentrations of small ice particles is related to instrument-induced particle shattering. Prior to entering the instrument sample volume, an ice particle may impact the probe s upstream tips or inlet and shatter into small fragments, which may cause multiple artificial counts of small ice. There have been a number of attempts to investigate the effect of shattering from in situ measurements; a review of these works can be found in Jensen et al. (2009). The subsequent sections will discuss new evidence for crystal bouncing, and shattering is presented from wind tunnel studies and aircraft measurements using modified probe geometries. Wind Tunnel Documentation. Direct experimental evidence for the shattering hypothesis has been obtained with high-speed video recording led by the NASA Glenn Research Center, with support from Environment Canada () at the Cox and Co. wind tunnel. Figure 1 shows images of ice particles AMERICAN METEOROLOGICAL SOCIETY August

2 Fig. 1. High-speed video images of the trajectories of ice particles bouncing from (a) the arm tips of CIP, (b) OAP-2DC, and (c) FSSP inlet tube. Frames are from high-speed videos that were taken in ice sprays in the Cox Wind Tunnel at an airspeed of 80 m s -1. Red areas in (a) and (b) highlight the sample volumes of (a) CIP and (b) OAP-2DC probes, respectively. Particles unaffected by bouncing and shattering appear as horizontal lines. Particles bounced inside the FSSP inlet in (c) are not visible due to the lack of illumination. bouncing from the optical array probe (OAP)-2DC and cloud imaging probe (CIP) arm tips and forward scattering spectrometer probe (FSSP) inlet tube. The high-speed videos associated with these snapshots can be downloaded from (ftp://depot.cmc.ec.gc.ca/ upload/hsvideo). Analysis of high-speed video recording showed that ice particles bouncing from the surface of cloud probes may travel up to 10 cm across the airflow at 80 m s -1. It was also found that, after bouncing, ice particles may move against the airflow up to 1 cm. Airflow simulations showed that the travel distance depends on the air density, particle speed, and particle elasticity. The video technique provides only a qualitative characterization of the possible flow of shattered fragments from a larger particle breakup. However, for the first time, it has been documented that at typical aircraft speeds, ice particles bouncing off forward surfaces can travel several centimeters across the airflow before passing through the sample volume. Airborne Icing Instrumentation Evaluation (AIIE) Flight Campaign. The AIIE flight campaign focused on quantifying the effect of shattering on ice measurements, and better understanding the problem of existence of small ice particles in clouds. To meet these objectives the following strategy was used: Probe tips and inlets were modified to mitigate the effects of shattering. Each probe type was simultaneously flown in its modified and standard 1 configuration side-by-side to quantify the effect of shattering by comparing the results from the modified and standard probes (Fig. 2). The arm tips of the OAP-2DC and CIP imaging probes were modified by Environment Canada to deflect bouncing particles and shedding water away from the sample volume and optical field apertures. The inlet tube of the modified FSSP was removed, and 1 Hereafter, the term standard refers to unmodified probes housing configurations as they were built by manufacturers. 968 August 2011

3 the original hemispherical tips were replaced with new designs (Fig. 2b). The suite of instruments installed on the aircraft is listed in Table 1. Thirteen research flights were conducted with modified/unmodified pairs in the vicinity of Ottawa during March April A number of different sets of modified arms and tips for the OAP-2DC (8 sets), CIP (3 sets), and FSSP (3 sets) were designed and fabricated for the AIIE project. The goal of each research flight was to evaluate the performance of each set in a variety of cloud conditions with different ice particle habits, sizes, concentrations, and ice-water contents. For this reason, the flights were deliberately conducted in deep precipitating glaciated frontal cloud systems. Typically, in such clouds the characteristic size of ice particles gradually changes from a minimum near the cloud top to a maximum in the precipitation below the cloud base. Cloud sampling was conducted during spiral or porpoise ascent to maximum altitude (7.5 km), and then descent to the minimum allowed altitude (usually km). In order to estimate the effect of air speed and angle of attack on the performance of the standard and modified tips, a horizontal leg followed by pitch-up and pitch-down maneuvers were carried out on each flight. Shattering by the FSSP and OAP-2DC. Figure 3a shows comparisons of the particle concentration in ice and mixed-phase clouds measured by standard and modified FSSPs. Regions of mixedphase and supercooled liquid clouds are identified by the increase of the Rosemount Icing Detector ramp signal (not shown here). In these regions, the total particle number concentration measured by both FSSPs are in close agreement, whereas in ice clouds the difference reaches two orders of magnitude. The analysis of the CIP imagery indicates that the liquid phase in this particular mixed-phase cloud is composed mainly of supercooled large drops (SLD). The total number concentrations from the FSSPs in the mixed-phase/liquid regions in Fig.3a are quite low, varying in the 2 10 cm -3 range. Such concentrations are often observed for SLD clouds, and are comparable to the apparent concentrations of ice particles outside these regions measured by the standard FSSP in Fig.3a. In this particular case, the standard FSSP is unable to distinguish mixed-phase/liquid cloud regions from ice regions due to the shattering artifacts. However, the modified FSSP shows only a very small signal in ice clouds, and can clearly identify the liquid-containing cloud. This illustrates a potentially Fig. 2. Cloud particle probes installed on the NRC Convair-580 during the AIIE flight campaign. Pairs of the (a) OAP-2DC, (b) FSSP, and (b) CIP with modified and standard tips or inlets were mounted side-by-side on the same pylons. This enabled direct comparisons of the measurements of standard and modified probes and quantification of the effect of the ice shattering on the measurements. significant improvement of FSSP measurements in mixed-phase clouds. The comparison in Fig. 3b shows the difference in the total number concentrations measured in an ice cloud by standard and modified OAP-2DCs. It appears that the difference depends on particle size and it varies from a few times, when particles are less than 1 mm, to two orders of magnitude, when the maximum size of particles exceeds 5 mm. Visual analysis of the images sampled in this cloud clearly indicates that the probe with the standard tips detects a large number of small particles not observed on the probe with modified tips (Fig.4). Since we can conceive of no mechanism by which these modifications AMERICAN METEOROLOGICAL SOCIETY August

4 Table 1. List of instruments installed on the NRC Convair-580 during the AIIE project. Instrument Type Measured parameter Owner FSSP (PMS) Standard Droplet size distribution; 2 47 µm; 15 size bins FSSP (PMS) Modified Droplet size distribution; 2 47 µm; 15 size bins NASA OAP-2DC (PMS) Standard µm; particle shadow-images at 25-µm OAP-2DC (PMS) Modified µm; particle shadow-images at 25-µm NCAR/ CIP (DMT) Standard µm; particle shadow-images at 15-µm CIP (DMT) Modified µm; particle shadow-images at 15-µm NOAA OAP-2DP (PMS) Standard µm; particle shadow-images at 200-µm 2D-S (SP) Standard or modified µm; particle shadow-images at 10-µm Nevzorov (SkyTech) Standard Liquid and total water content; g m -3 Nevzorov (SkyTech) Modified Liquid and total water content; g m -3 SkyTech King probe (CSIRO) Standard Liquid water content; g m -3 Cloud Spectrometer and Impactor (DMT) Standard Total water content; g m -3 Extinction probe () Standard Extinction coefficient km -1 Rosemount Icing Detector (Goodrich) Standard Rate of icing in supercooled liquid clouds LI-6262 (LI-COR) Standard Absolute humidity State parameters P, T, TAS, latitude, longitude NRC Ka-band radar (SEA) Radar reflectivity DAS M200 (SEA) Data acquisition system DAS M300 (SEA) Data acquisition system 970 August 2011

5 Fig. 3. Time series of (a) cloud-particle concentration measured by the modified and standard FSSPs in transit through a sequence of ice and mixed-phase clouds on 4 Apr 2009; (b) uncorrected ice-particle concentration measured in fully glaciated clouds by two OAP-2DCs with the standard and modified arm tips, measured on 8 Apr Both measurements (a) and (b) were conducted northwest of Ottawa, in frontal cirrostratus (Cs) nimbostratus (Ns) cloud systems. to the probes could have eliminated real particles, the contention here is that these additional small particles must be artifacts resulting from shattering. Modified and standard probe size distributions are shown in Fig. 5 for a 10-minute period containing large particles from the ice cloud shown in Fig. 3b. The shattering effect has a large impact on the small part of the size distribution, and it appears to be quite subtle above 500 µm. It should be noted that due to large uncertainties in sizing and concentration calculations of particles less than four pixels in imaging probes, the measured OAP-2DC and OAP-2DP size distributions with D < 100 and 800 µm, respectively, should be considered with caution. Size distributions corrected for the shattering events, based on an interarrival time algorithm and on fragmented image identification are also presented in Fig. 5. At small sizes < 500 µm, the number concentrations of corrected size distributions for standard probes are still higher than those with the modified arms and tips. This suggests that for the OAP- 2DC, the algorithms for filtering shattering events used here for cases as in Fig. 5 were insufficient to eliminate all shattering artifacts. The response of the CIP to ice shattering was found to be similar to that of the OAP- 2DC (not shown here), and the conclusions obtained for the OAP-2DC are equally valid for the CIP. Concluding Comments. The results of this study demonstrated that contamination of particle size distribution caused by shattering of ice particles on probe tips and inlets is a significant problem for airborne microphysical characterization of ice clouds. Further analysis of a variety of cloud cases from the AIIE flights yields the following important conclusions: 1) shattering can be significantly mitigated by using modified tips; 2) flying modified and standard probes in pairs enables quantification of shattering effect on the measurements; 3) the portion of the particle size range with diameters larger than ~ 500 µm is much less affected by shattering, whereas the smaller size range can be strongly contaminated in ice clouds; 4) existing shattering algorithms are unable to filter out all shattering events from the measurements of OAP-2DC and CIP, and in many cases they result in disregarding intact ice particles. Large ice particles were found to produce a higher level of contamination with their artifacts than small ice particles. For size distributions with the maximum particle size of several millimeters (e.g., Fig. 5), the total number concentration was greatly enhanced by shattering for the OAP-2DC and CIP. However, when maximum particle sizes were < 500 µm, the effect was significantly reduced. AMERICAN METEOROLOGICAL SOCIETY August

6 Since the extinction coefficient, mass, and radar reflectivity of typical size distributions measured in all AIIE datasets are normally dominated by the larger particles, the above parameters estimated from 2D particle images are significantly less affected by shattering than the number concentration. In most cases, the estimated error related to the shattering artifacts in the extinction and IWC calculations does not exceed a factor of two and 20% for the radar reflectivity. Despite that, by modifying the probe inlets and by applying interarrival time algorithms, the effects of shattering can be significantly reduced; the remaining detections at the smallest sizes cannot at the present time be definitively stated to be real. Numerical simulation of bouncing, analysis of high-speed videos from the wind tunnel tests, and the in situ measurements collected during the AIIE project suggest that shattering, in a complex way, depends on air density, airspeed, angle of attack, shape and temperature of the tips, ice-particle size and habit, and Summary of our current knowledge about shattering After impact with a solid surface, an ice particle may shatter into small fragments. The number of fragments may reach the order of The number of fragments that intersect the probe s sample volume may reach a few hundred per shattered particle. The number of fragments is expected to depend on the particle velocity, particle size, particle density, particle habit, angle of impact, and surface temperature. At aircraft speeds, the size of particle fragments has been observed to be as small as 10 µm. Shattered particles often form a cluster of closely spaced fragments. The dimension of the clusters depends of the shape of the surface, angle of impact, particle properties, and airspeed, and typically does not exceed 10 cm along the direction of the airflow. Fig. 4. Comparisons of the ice particle images measured by two OAP-2DCs with the (left) standard and (right) modified tips. The images on the left have many more small particles relative to those on the right. The majority of the small particles on the left result from ice crystal shattering on probe tips. The of both OAP-2DCs is 25 µm. The images were collected in a frontal Cs Ns cloud system in the vicinity of Ottawa, 8 Apr 2009, at P = 690 mb and T = -15 C. particle orientation at the moment of impact. Taking all of these effects into account in numerical models of shattering is a challenging problem. It is hindered by the lack of information about the rebound coefficient of ice, number and size of shattered fragments, angle of bouncing of shattered fragments, and other factors. Large sets of OAP-2DC data collected by the community over the past 30 years have been used for parameterization of cloud microphysics for numerical weather and climate models and validation of remote-sensing instruments. Much of these data are likely to have been contaminated by shattering artifacts and should be reexamined. This raises an important question: Can the historical data be reanalyzed to filter out the shattering effect? To address this issue, the cloud physics community should undertake efforts to determine the limiting factors in using the historical data and identify possible ways of its retrieval. In this regard, a series of dedicated flight campaigns to study the effect of shattering and other problems related to the accuracy of ice measurement should be considered as one of the high-priority issues for the cloud physics community. Acknowledgments. This work was funded by Environment Canada, Transport Canada, the Federal Aviation Administration, and NASA. Special thanks to Sara Lance (NOAA), Jorge Delgado (NOAA), and Dave Rogers (NCAR) 972 August 2011

7 Fig. 5. Comparisons of size distributions before and after interarrival time corrections, as measured by (blue) standard and (red) modified OAP-2DCs. The results are for the large-particle region of Fig. 3b. The size spectra were averaged over the time interval 1409: :00 UTC. for loaning their cloud particle probes for the AIIE project. We express our sincere gratitude to the National Research Council (NRC) pilots Anthony Brown, John Aitken, and Tim Leslie for their outstanding support during the flight operations. The efforts of the NRC technicians in preparing and organizing the Convair-580 flights, and of the participants from DMT Inc. for their willingness and attempts to support the AIIE project, are greatly appreciated. A special thank you to the Cox and Co. personnel and, in particular, to Adam Lawrence for such a high level of cooperation and support in operating the Cox wind tunnel facility. The NASA Glenn Research Center video group Vince Reich, Chris Lynch, and Quentin Schwinn did an excellent job capturing highspeed videos during the Cox wind tunnel tests. The authors express their gratitude to three anonymous reviewers and Cindy Twohy for thoughtful comments. For Further Reading Cooper, W. A., 1977: Cloud physics investigation by the University of Wyoming in HIPLEX Bureau of Reclamation Report AS 119, 321 pp. Field, P. R., R. Wood, P. R. A. Brown, P. H. Kaye, E. Hirst, R. Greenaway, and J. A. Smith, 2003: Ice particle interarrival times measured with a Fast FSSP. J. Atmos. Oceanic Technol., 20, , A. J. Heymsfield, and A. Bansemer, 2006: Shattering and particle interarrival times measured by optical array probes in ice clouds. J. Atmos. Oceanic Technol., 23, Fugal, J. P., and R. A. Shaw, 2009: Cloud particle size distributions measured with an airborne digital inline holographic instrument. Atmos. Meas. Tech., 2, Gardiner, B. A., and J. Hallett, 1985: Degradation of in-cloud forward scattering spectrometer probe measurements in the presence of ice particles. J. Atmos. Oceanic Technol., 2, Gayet, J. F., G. Febvre, and H. Larsen, 1996: The reliability of the PMS FSSP in the presence of small ice crystals. J. Atmos. Oceanic Technol., 13, Hallett, J., and S. C. Mossop, 1974: Production of secondary ice particles during the riming process. Nature, 249, Heymsfield, A. J., 2007: On measurements of small ice particles in clouds. Geophys. Res. Lett., 34, L23812, doi: /2007gl , and C. M. R. Platt, 1984: A parameterization of the particle size spectrum of ice clouds in terms of the ambient temperature and ice water content. J. Atmos. Sci., 41, Jensen, E. J., and Coauthors, 2009: On the importance of small ice crystals in tropical anvil cirrus. Atmos. Chem. Phys., 9, Knollenberg, R. G. 1976: Three new instruments for cloud physics measurements: The 2D-spectrometer probe, the forward scattering spectrometer probe and the active scattering spectrometer probe. Preprints, International Conf. on Cloud Physics, Boulder, CO, Amer. Meteor. Soc., Korolev, A. V., and G. A. Isaac, 2005: Shattering during sampling by OAPs and HVPS. Part 1: Snow particles. J. Atmos. Oceanic Technol., 22, , J. W. Strapp, and G. A. Isaac, 1998: Evaluation of the accuracy of PMS optical array probes. J. Atmos. Oceanic Technol., 15, McFarquhar, G. M., J. Um, M. Freer, D. Baumgardner, G. L. Kok, and G. Mace, 2007: The importance of small ice crystals to cirrus properties: Observations from the Tropical Warm Pool International cloud Experiment (TWP-ICE). Geophys. Res. Lett., 57, L13803, doi: /2007gl Patent Application, 2009: Probe tips for airborne instruments used to measure cloud microphysical parameters. U.S. Patent Application No. 12/415,314; Canadian Patent Application No Vidaurre, G., and J. Hallett, 2009: Particle impact and breakup in aircraft measurement. J. Atmos. Oceanic Technol., 26, AMERICAN METEOROLOGICAL SOCIETY August