ICE INITIATION IN MIXED-PHASE OROGRAPHIC WAVE CLOUDS

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1 ICE INITIATION IN MIXED-PHASE OROGRAPHIC WAVE CLOUDS Andrew J. Heymsfield, P. R. Field 2, D. C. Rogers 1, J. Stith 1, J. Jensen 1 Z. Wang 3, J. French 3, S. Haimov 3, P. J. DeMott 4, and C. Twohy 5 1. NCAR, 3450 Mitchell Lane, Boulder Co U. K. Met. Office, Devon EX1 3PB United Kingdom 3. Department of Atmospheric Sciences, University of Wyoming, Laramie, WY Department of Atmospheric Science, Colorado State University, Fort Collins, CO College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis OR INTRODUCTION More than 50% of the earth s precipitation originates in the ice phase. Ice nucleation, therefore, is one of the most basic processes that lead to precipitation. The poorly understood processes of ice initiation and secondary ice multiplication in clouds result in large uncertainties in the ability to model precipitation production and to predict climate changes. Therefore, progress in modeling precipitation accurately requires a better understanding of ice formation processes. Recent advances in observational tools, laboratory cloud simulation chambers, numerical models, and computer hardware provide new capabilities to understand and model ice initiation processes. In recognition of these new capabilities, a field program was conducted in November and December 2007 in clouds that are amenable to studying ice nucleation processes. The Ice in Clouds Experiment, referred to as ICE-L, made use of the NCAR C130 to study ice production in lenticular and stratiform layer clouds, focusing on the following scientific goal: TO SHOW THAT UNDER GIVEN CONDITIONS, DIRECT ICE NUCLEATION MEASUREMENT(S), OR OTHER SPECIFIC MEASURABLE CHARACTERISTICS OF THE AEROSOL, CAN BE USED TO PREDICT THE NUMBER OF ICE PARTICLES FORMING BY NUCLEATION MECHANISMS IN SELECTED CLOUDS. WITH THESE NEW CAPABILITIES, WE ATTEMPT TO IMPROVE THE QUANTITATIVE UNDERSTANDING OF THE ROLES OF THERMODYNAMIC PATHWAY, LOCATION WITHIN THE CLOUD, AND TEMPORAL DEPENDENCY ON ICE NUCLEATION PROCESSES. This article will highlight observations from three of the ICE-L lenticular wave cloud flights. In Section 2, we will provide an overview of the instruments and the flights investigated here. Section 3 will present the observations and Section 4 will provide a summary of the main observations and draw conclusions. 2. FIELD CAMPAIGN The NCAR C130 was equipped with a wide range of state of the art particle probes, CCN spectrometers, and measurements of IN concentration and composition (Table 1). The completeness of the data set was further enhanced by remote sensing measurements from an upward and downward viewing cloud Doppler radar and an upward viewing lidar. There were twelve research flights during the six week field campaign. Eight were directed to lenticular clouds and four to stratiform, upslope clouds. In-cloud temperatures for the wave cloud studies, the focus of this paper, spanned the range from -7 to -37C. Cloud depths ranged from 100 m to >1 km. A typical flight pattern involved a series of penetrations along, against, and across the

2 Table 1 Probes on NCAR C130 during ICE-L INSTRUMENT MEASUREMENT INVESTIGATOR COMMENTS -Particle Probes UHSAS Aerosols 55 nm-1µm Facility (RAF) FSSP PSD 3-45 µm Facility CDP PSD 3-45 µm Facility CAS PSD 3-45 µm DMT Part of CAPS Probe SID-2 PSD 1-60 µm Facility FAST FSSP PSD 3-45 µm SPEC Only available for RF06-08 FAST 2D-C PSD µm Facility 2D Grey PSD µm DMT 2D-S PSD µm SPEC Only available for RF01-03, 08 -Condensed water CVI LWC, Facility IWC>0.01g/m 3 King Probe LWC Facility RICE Supercooled LW Facility presence/amount -CCN DRI CCN-S spectra Hudson DMT CCN-S spectra DMT -IN Ice nucleus conc. DeMott -Mass Spectrometer ctof-ams IN composition Seinfeld A-ATOFMS IN composition Prather CVI CCN/IN composition Twohy -Black Carbon SP2 spectrometer DMT -Remote Sensing 95 GHz radar Reflectivity, Doppler particle French/Haimov speed Lidar Backscatter, Wang asymmetry param. -Wind/Thermo. 3D winds U, V, W Facility T, RH Temp. RH Facility Upward and downward viewing Upward viewing

wind direction to map out the thermodynamic, wind, and microphysical structures in the vertical. The first penetration sampled the upwind thermodynamics and aerosol properties.

layer. The second leg was usually under cloud base, parallel to the wind, to remotely sense the cloud structure. Successive legs probed the cloud microphysics in-situ. 3.

3 wind direction to map out the thermodynamic, wind, and microphysical structures in the vertical. The first penetration sampled the upwind thermodynamics and aerosol properties. This leg usually provided data below the visible cloud base to map out the equivalent potential temperature (qe) and relative humidity (with respect to water, RHw) structure flowing into the cloud layer. The second leg was usually under cloud base, parallel to the wind, to remotely sense the cloud structure. Successive legs probed the cloud microphysics in-situ. 3. DATA We present observations from three flights, on 16 December [RF12], 29 November [RF06], and 13 December [RF11]. We highlight the thermodynamic, dynamic and microphysical observations. Figure 2 shows the track of the C130 during the 11 penetrations of RF12, wave cloud 2. The x axis falls along the mean wind direction 290 +/- 6o, with the C130 positions derived from a coordinate transformation to that axis. The (0, 0) point of the transformed system is at about the leading cloud edge (x axis) and N-S cloud extent. The region of cloud is approximately 3 km in length along the wind direction, with a temperature close to -22C. Cloud vertical depth is only a few hundred meters. The mean wind direction for these penetrations was and the wind speed was /- 1.7 m/sec. Therefore, the transit time for particles through the cloud is only about 140 sec. Vertical motions through the cloud layer were strong, exceeding 4 m/sec in places (Fig. 3), with downward motion noted upwind and downwind of these peaks. Parcels would have only ascended about 200 m across the cloud layer, assuming an average 3 m/s updraft across the 2km wide updrafts. Fig. 1: Track of NCAR C130 aircraft during five penetrations (different colored traces) for RF12 (16 December 2007) during ICE-L. In the right panel, black squares are projection of locations of cloud liquid water noted during the penetrations.

4 Fig. 2: Track of the C130 during the penetrations for RF12, second cloud sampled. Locations of cloud are shown with black squares. The emphasis will be placed on Pens. 3, 9, 8, 4, and 7, discussed in that order to reflect increasing height. The tracks of the C130 for these penetrations are shown on the Google Earth maps in Fig. 1. Pens. 3 and 9 just were largely below cloud base and penetrations 8, 4 and 7 were in liquid cloud, profiling from near base to top. In Figs. 4 and 5, we show the upward and downward viewing 95 GHz radar observations (top panel), and the lidar backscatter and depolarization ratio above the aircraft (middle and lower panels) for Pens. 3 and 8, respectively. Note the arched-shaped cloud base in Pen. 3, (Fig. 4) and the penetration of the ice grown within the wave cloud on the downwind side, which is also evident in the radar observations. Both (and all other) penetrations show the absence of ice upwind of cloud and ice virga streaming downwind of and below the aircraft level. Figure 6 shows some of the microphysical observations for these penetrations, plotted along the x (parcel time) axis, stacked according to increasing penetration height. Orange and blue horizontal bars in the panels in the left two columns show where the vertical velocity is above 0.5 m/s and below -0.5 m/s, respectively. Heights, temperatures, mean aircraft heading (negative into the wind) and winds are shown in the titles for each penetration. Consider first Pen. 3. There is an absence of condensate (left panel) except the upwind edge (near 0 km), at the upwind cloud edge, and on the downwind side, where the aircraft crossed the ice virga in the downdraft (see Fig. 4). The condensed water contents, in this case all ice (no RICE LW), were less than 0.01 g/m 3,

5 below the detection level of the CVI. Below the panels for this penetration are the concentrations (N) of particles (when detected) from the CDP and FSSP and CPI (>30 µm) and 2D probes (>50 µm). All but the CPI showed N of about 10 L -1. Because drizzle was noted in higher penetrations and cloud droplets extended into the downdrafts (from RICE), we attribute most of these particles to drizzle drops. Particles imaged with the CPI (>30 µm) were all ice, with N~1 L -1. During Pen. 9, liquid water was detected on the upwind and downwind portions of the cloud, again because the cloud base was arched. (This was noted in the lidar imagery for this penetration, not shown here). Penetration 9 sampled liquid cloud in both the upwind updraft and the downwind downdraft, but the arched cloud base was above the aircraft during part of this penetration. FSSP and CDP concentrations, presumably droplets reached 200 cm -3. Concentrations >30 µm were ~10 L -1, presumably drizzle droplets. The particles sampled by the CPI and fast 2D probe were located on the east side of the cloud, in the downdraft region, in concentrations ~1 L -1 or below. Penetration 8 sampled above cloud base throughout (Figs. 5 and 6). Water contents were only of order several hundredths gram per cubic meter. Drizzle was evidently detected from the FSSP and the CPI and 2D-C suggest N~1 L -1. Fig. 3: Profile showing vertical wind speeds along each of the tracks of RF12, with the track numbers shown at the beginning and end of each penetration. Regions of updraft are shown with the thicker lines in green through orange, downdrafts are turquoise through blue. The height of topography at each point below the aircraft are shown.

6 Fig. 4. Remote sensing data acquired from the NCAR C130 across Pen. 3 for RF12. Top panel: Upward and downward viewing 95 GHz radar observations, with the aircraft positioned at a height of 0 km. Middle and lower panels: upward viewing lidar relative backscattered power and depolarization ratio, respectively.

7 Fig. 5: Same as Fig. 4, except for Penetration 8 for RF12.

8 Fig. 6. Microphysical observations obtained during five of the penetrations for RF12. The title for each panel shows the altitude, mean temperature, penetration number, aircraft heading (a minus indicates a penetration into the wind), and the mean wind direction at the flight level. The left panels show the condensed water content, the middle panels the particle concentrations, and the right panels the vertical velocities. The legend for the left and center panels are shown at the bottom of the figure. Average concentrations for ice taken to be where the diameters are above 30 microns for the FSSP, CDP and CPI, and above microns for the 2D probes, are averaged in 5-sec intervals and are plotted and listed between panels. Penetrations 4 and 7, near cloud top, detected a continuous band of cloud liquid cloud was observed both in the updraft and extending into the downdraft. Concentrations from the CPI and 2D-C were of order 1 L -1. For the cloud studied during the RF12 penetrations, the CFDC IN concentrations were L -1 out of cloud, comparable to those measured here. On 29 November, the NCAR C130 flew a mission over Wyoming, sampling a wave cloud at heights from 4.6 to 5.6 km and temperatures from -26 to -29C. Vertical velocities were again strong, with a broad and well-defined updraft region approximately 4 km wide (Fig. 7). The remote sensing data indicates that there was little if any ice generated in the layer nor streaming downwind of the cloud. However, at a height of about 1 km above the C130 flight altitude, corresponding to a temperature of about -35C from earlier C130 sampling, a ice trail streamed downwind of the wave cloud. This height

9 Fig. 7: Same as Fig. 3, except for RF06, 29 November, 2007.

10 Fig. 8: Same as Fig. 4, except for Penetration 2, RF06. The penetration is from downwind to upwind.

Fig. 9: Same as Fig. 4, except for Pen. 4, RF06. The penetration was from downwind to upwind. and above is where homogeneous ice nucleation was likely to be operative in the cloud layer.

11 Fig. 9: Same as Fig. 4, except for Pen. 4, RF06. The penetration was from downwind to upwind. and above is where homogeneous ice nucleation was likely to be operative in the cloud layer. Just below the wave cloud, an orographic wave cloud developed precipitation and convective elements that we observed to be feeding into the base of the wave cloud. These were noted in the lidar imagery at the base of our cloud (Fig. 9). Liquid water but significant ice (2D-C and CPI) is noted for the lowest two penetrations (4.6 and 5 km), where the lower cloud layer visually supplied copious ice into the base of the wave cloud. Higher up, the ice was either too small or in concentrations too low for the probes to detect it. The latter is an unlikely explanation because ample growth time would have been available to produce detectable ice as evidenced by the streamers advecting downwind of the cloud in the homogeneous ice nucleation zone. Furthermore, a careful examination of the CPI imagery indicates that there were only a few particles that were ice and these were of sizes of about 150 microns. Copious micron drizzle drops were observed on this day. For RF06 (Fig. 10), there is minimal ice noted in the downwind regions of each of the cloud penetrations, indicating that sufficient time was available for drizzle growth but ice production was suppressed. Therefore, we would conclude on this basis alone that ice nuclei were present in minimal concentrations, even at temperatures close to -30C. The IN measurements (see later) confirm that ice nuclei were detectable above a sensitive threshold of 0.4 L -1 at the cloud

12 pass temperature at this time. On 13 December 2007, RF11 profiled a cloud centered over Elk Mountain, Wyoming. Two stacked layers of wave clouds were located over and to the east of the mountains (Fig. 11). Vertical motions were strong, with parcels moving upwards at up to 6 m/s. The lower cloud ranged in heights from 4.3 to 4.8 km, with temperatures from -14 to -18C, and the upper one from 5.2 to 5.6 km, with temperatures in the -21 to -23C range. On the upwind side of the wave cloud, ice is evident in the radar imagery above and below the aircraft (Fig. 12). The cloud is detectable by radar throughout, suggesting that ice particles from upwind are entering cloud. This conjecture is supported by the remote sensing observations. In Fig. 13, the radar data clearly shows Elk Mountain, first going from downwind to upwind, then reciprocally from upwind to downwind. There is clear evidence of detectable wave cloud below the aircraft, over the mountain. It is entirely possible that low concentrations of ice were picked up from the surface (which was covered with snow) and then carried in the winds into the cloud layer. In Fig. 14, we show that in the wave cloud layer, the concentrations measured by the fast 2D probe were considerably less than

13 Fig. 10: Same as Fig. 6, but for RF06, 29 November observations during the upwind leg (leg 10, 1 L -1. There was obviously a problem with the 2D grey probe at the next to lowest sampling level and the CPI malfunctioned, rendering the concentrations shown as unreliable. In summary, is noted that the concentrations of ice crystals generated in this cloud layer were low. IN measurements suggest no ice nuclei detectable at a resolution of 1 per liter at this time, although cloud residuals were being processed for water subsaturated conditions at this time to determine deposition nuclei concentrations. 4. SUMMARY AND CONCLUSIONS This study uses an unprecedented set of in-situ and remote sensing observations to examine whether under given conditions, direct ice nucleation measurement(s), or other specific measurable characteristics of the aerosol, can be used to predict the number of ice particles forming by nucleation mechanisms in selected clouds. The three wave cloud flights focused on here span the temperature range from about -15 to -30C. The concentrations of ice crystals sampled with many probes in Fig. 11: Same as Fig. 3, except for RF11, 13 December 2007.

14 the critical size range from about 30 to 200 microns reveals little or no discrepancy with the measurements of ice nuclei concentrations. The following conclusions were also drawn from the observations: 1: There is no evidence for enhanced ice production in the downdrafts of the wave studied. This mechanism has been proposed from earlier wave cloud observations. 2: The concentrations of ice crystals push the limits of the sampling volumes of the various probes. That is, the scattering-type probes and the 2D imaging probes have insufficient sampling volume to detect the low concentrations of ice observed in the cloud layers. clouds studied. Such a mechanism that 3. The remote sensing observations were crucial in unraveling the processes of ice nucleation. Clear instances of ice streaming from sources upwind that entered the wave clouds could have been construed as the result of deposition nucleation. Further synthesis of this relatively complete data set, including the observations of the composition of the ice nuclei is ongoing. Fig. 12: Same as Fig. 4, except for Pen. 3, RF11, 13 December Profile shown from downwind to upwind.

15 Fig. 13: Same as Fig. 4, except for Pen. 10, RF11, 13 December The C130 first flew upwind, then turned and sampled downwind to upwind. Elk Mountain is clearly visible, as is cloud overlying it.

Fig. 13: Same as Fig. 6, except for RF11, 13 December 2007. ACKNOWLEDGEMENTS The authors are indebted to the National Science Foundation for funding this project.

16 Fig. 13: Same as Fig. 6, except for RF11, 13 December ACKNOWLEDGEMENTS The authors are indebted to the National Science Foundation for funding this project. The National Center for Atmospheric Research contributors are funded by base funding from NCAR 6. We wish to thank the RAF Aviation Facility, especially Henry Boynton, Ed Ringleman, Bob Olson and Bill Irwin, and to the EOL group including Jim Moore and Steve Williams, for their support of this project. Specific grants from NSF to ICE-L investigators, including DeMott (ATM ), Twohy (ATM ) and Wang (NSF ATM ), contributed substantially to the success of this project. Support for Rogers ( NASA NRA-01-OES-02) is also acknowleged. 6. The National Center for Atmospheric Research is sponsored by the National Science Foundation. Any opinions, findings, and conclusions or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

P2.17 OBSERVATIONS OF STRONG MOUNTAIN WAVES IN THE LEE OF THE MEDICINE BOW MOUNTAINS OF SOUTHEAST WYOMING

P2.17 OBSERVATIONS OF STRONG MOUNTAIN WAVES IN THE LEE OF THE MEDICINE BOW MOUNTAINS OF SOUTHEAST WYOMING Larry D. Oolman 1, Jeffrey R. French 1, Samuel Haimov 1, David Leon 1, and Vanda Grubišić 2 1 University