Supercooled large drop detection with NASA's Icing Remote Sensing System

Transcription

1 Supercooled large drop detection with NASA's Icing Remote Sensing System David J. Serke NCAR, Research Applications Laboratory, Boulder, CO Andrew L. Reehorst NASA Glenn Research Center, Cleveland, OH Marcia K. Politovich NCAR, Research Applications Laboratory, Boulder, CO ABSTRACT In-flight icing occurs when aircraft impact supercooled liquid drops. The supercooled liquid freezes on contact and the accreted ice changes a plane's aerodynamic characteristics, which can lead to dangerous loss of control. NASA's Icing Remote Sensing System consists of a multi-channel radiometer, a laser ceilometer and a vertically-pointing Kaband radar, whos fields are merged with internal software logic to arrive at a hazard classification for in-flight icing. The radiometer is used to derive atmospheric temperature soundings and integrated liquid water and the ceilometer and radar are used to define cloud boundaries. The integrated liquid is then distributed within the determined cloud boundaries and layers to arrive at liquid water content profiles, which if present below freezing are categorized as icing hazards. This work outlines how the derived liquid water content and measured Ka-band reflectivity factor profiles can be used to derive a vertical profile of radar-estimated particle size. This is only possible because NASA's system arrives at independent and non-correlated measures of liquid water and reflectivity factor for a given range volume. The size of the drops significantly effect the drop collection efficiency and the location that icing accretion occurs on the craft's superstructure and thus how a vehicle's performance is altered. Large drops, generally defined as over 50 μm in diameter, tend to accrete behind the normal ice protected areas of the leading edge of the wing and other control surfaces. The NASA Icing Remote Sensing System was operated near Montreal, Canada for the Alliance Icing Research Study II in 2003 and near Cleveland, Ohio from 2006 onward. In this study, we present case studies to show how NASA's Icing Remote Sensing System can detect and differentiate between no icing, small drop and large drop in-flight icing hazards to aircraft. This new product provides crucial realtime hazard detection capabilities which improve avaiation safety in the near-airport environment with cost-effective, existing instrumentation technologies. KEYWORDS NASA, remote sensing, in-flight icing, liquid water content, Ka-band radar, radiometer, radar-estimated size

2 1. OVERVIEW In-flight icing hazards occur when an airframe impacts with supercooled liquid water (SLW). The liquid instantly freezes to the airframe, which changes the aerodynamic characteristics of the vehicle. Accreted ice effects an aircraft by altering the drag and lift over the control surfaces and the overall weight of the vehicle to the point of reaching a dangerous loss of control. The size of the encountered SLW drops are very important to how an aircraft is effected by in-flight icing. SLW is generally classified as small drop (smaller than 50 μm) or large drop (larger than 50 μm, termed Supercooled Large Drop, SLD). Large drop cases are often referred to with terms like 'drizzle' or 'freezing rain'. Politovich et al1 found that only 10 to 15 minutes of exposure to low concentrations of large supercooled drops led to substantial loss in rate of climb capability with King-Air research aircraft. These low concentrations of large drops cause icing to accrete beyond the leading edges of the wings (Fig. 1). Bragg2 discussed the aerodynamic loss of lift due to accretion of SLD aft of deicing mechanisms on modern aircraft. For these reasons, the ability to differentiate between small and large drop icing events is crucial for aviation safety. The Federal Aviation Administration in the United States recognizes this importance, and will soon enact new rules for aircraft to fly in SLD. Several products have been developed to provide in-flight icing hazard warnings to the aviation community. NCAR's Current Icing Product3 (CIP) was developed to provide a near-realtime assessment of the hazard presented by SLW aloft in an algorithm that combines data from satellites, the Rapid Update Cycle model, the national 2-D composite of S-band NEXRAD radar reflectivity, surface observations and pilot reports (PIREPs). One of the end products of CIP is an icing severity, or aircraft-independent estimate of how severe the icing conditions are. It is known that the Rapid Update Cycle model is not very adept at discerning the size and phase of condensate population4. For this reason, CIP does not attempt to describe the mean size of SLW encounters. The NASA Icing Remote Sensing System (NIRSS)5 (Fig. 2) was developed by the National Aeronautics and Space Administration (NASA) to provide a ground-based, qualitative in-flight icing hazard severity assessment in the near-airport environment with commercially available instrumentation. The system utilizes a multichannel microwave radiometer6, built by Radiometrics Corporation, to derive the temperature profile and integrated liquid water (ILW), or amount of liquid water along the instrument viewing path. The system's radar is a METEK Ka-band cloud radar7. The ceilometer used is a standard Vaisala CT25K Laser Ceilometer. Data from the vertically pointing ceilometer and radar are used to define the cloud bases and tops. The ILW is then distributed within the detected cloud layer(s) based on internal fuzzy logic so that profiles of liquid water content (LWC) are produced. A qualitative icing severity profile is produced where the temperature profile is between 0 and -20oC and there is measurable LWC. NIRSS is currently considered in the testbed phase by NASA, and so modifications to the instrument fusion and hazard detection software are ongoing. Some studies have derived estimates for mean particle size or particle size distribution in cases where SLW are present aloft. Fabry et al8 and Zawadzki et al9 looked at data from vertically pointing doppler radars to explore the relation of fall velocity spectra to particle size. Vivekanandan et al10 demonstrated that a reasonable estimate of drop size and LWC was possible with a dual-wavelength radar system, but no operational system is yet available due to the extensive associated hardware costsof their employed systems. Other efforts by Frisch et al11, Zhang et al12 and Serke et al13 to detect radar estimated size and LWC with a radar and multi-channel radiometer have been demonstrated with research instrument systems. With this work, we show how LWC derived from NIRSS can be combined with reflectivity profiles from the NIRSS Ka-band radar to arrive at a realtime profile of radar-estimated size. This calculation is only valid under Rayleigh radar conditions and thus should not be attempted in conditions such as snow. The following section describes how a mean particle size similar to 'radar estimated size' is derived from NIRSS LWC and Ka-band reflectivity profiles. A case study from the AIRS-II field campaign in 2003 is presented, followed by two case studies from the winter of 2009/2010 over Cleveland, Ohio. Time versus height profiles of this new radar estimated size field are shown alongside the derived NIRSS icing hazard and pilot reported icing severity for the latter two cases. This effort is important because it represents the first real-time operational capability to detect and differentiate SLD from small drop icing and non-icing conditions.

3 Figure 1. The accumulated icing on the wing of the Twin Otter research aircraft due to supercooled large drops. Figure 2. NASA Icing Remote Sensing System hardware as currently installed at NASA Glenn Research Center in Cleveland, Ohio. 2. LIQUID WATER CONTENT AND RADAR ESTIMATED SIZE ILW [units gm-2] is derived from the multichannel radiometer as described in Solheim et al6. NIRSS's software then distributes the liquid into vertical profiles of liquid water content [units gm-3] within the detected cloud layer with fuzzy logic5. NIRSS defines a profile as being 'all liquid' or 'mixed phase' based on the cloud base temperature and reflectivity. These homogeneous and heterogeneous phase scenarios utilize different weighting functions for liquid that are vertically distributed as a function of radar reflectivity, ambient temperature, wedge-shaped profiles and uniformly distributed liquid profiles. The LWC profile derived by NIRSS is related to the radiometer derived ILW as follows (1) where n is the total number of sample radar volumes and dhj is the depth of the sample radar volume. Due to the nature of the system's measurement techniques, NIRSS produces independent measurements of radar reflectivity factor (z) and liquid water content (LWC) for any given range volume. In Rayleigh conditions, where the wavelength of the radar is much larger than the mean particle size, (2) where N is the drop concentration [#/m3] and D is the drop diameter [m]. If we consider z for each individual radar volume and assume a simplified mono-disperse drop size distribution for these wintertime stratiform icing events, then the reflectivity factor within each volume is simplified to z = N D6 (3) where N can vary from volume to volume but is fixed within each individual volume, and LWC = ρw π N D3 / 6 (4) where ρw is the density of water [g/m3]. Solving (3) for N and combining with (4) and given a measured z from the Kaband radar and a derived LWC from NIRSS, we can solve for D, such that

4 D = z π ρw /(6 LWC)1/3 (5) This exercise is only valid for measurements of z and LWC that are independent. If a reflectivity-based correlation is used to determine LWC, then this technique will fail by producing the assumed drop size from the correlation. For realistic cases with a drop size distribution, D = Radar Estimated Size (RES). The RES will typically not be the same as median volume diameter, but rather be weighted towards larger drop size. However, as a flight safety threshold indicator, RES may be more appropriate than median volume diameter since the aircraft icing hazard is also weighted towards larger drops. The relation described in (5) will next be compared to icing research flight data. 3. CASE STUDIES 3.1 Verification against research flight The Alliance Icing Research Studies II (AIRS II) was conducted near Montreal, Canada during the winters of The primary research aircraft were dispatched from Ottawa and ground-based remote sensing instrumentation, including NIRSS8, were based at Mirabel Airport. One of the icing research aircraft was a DeHavilland DHC-6 Twin Otter which was modified for sustained flight and data collection in icing environments. The outside air temperature was measured with a Rosemount model 102AU1P probe. Microphysical probes mounted on the aircraft collected data specific to icing research. LWC was measured with a CSIRO King probe and a Nevzorov LWC probe. Particle size was measured with a Forward Scattering Spectrometer Probe (FSSP-100) and an Optical Array Probe 2DC-Gray (2DG). The November 18, 2003 case had warm advection at 850 mb over Mirabel Airport, ahead of a surface warm front positioned from Illinois to Kentucky and low pressure trough over from central Canada to Oklahoma (not shown). The temperature was below freezing at the surface but ranged between +2 and +6 oc in the vertical flight maneuver profile due to an inversion. Even though icing conditions could not exist at this time due to above freezing temperatures, Figure 3. Twin Otter LWC (blue) and NIRSS LWC (black) (left, [gm-3] ) and Twin Otter MVD (blue) and NIRSS-derived RES (black) (right, [m x 10-6]) at 12:40 UTC on November 18, 2003.

5 it is still valid to explore the microphysical characteristics of this all-liquid small drop case. A wedge-shaped LWC profile with a maximum value above 0.4 gm-3 were detected by the aircraft (Fig. 3a). NIRSS spread out the same ILW above the temperature inversion at 1.1 km, since radiometers do poorly discerning temperature inversions14. The mean NIRSS RES (Fig. 3b) is reasonable when compared to the the Twin Otter aircraft's insitu median volume diameter over the altitude range of the sampled cloud layer for this small drop case. In this case with all liquid, the NIRSS LWC and RES values are not highly accurate but are consistent with the in situ values from research aircraft. This is especially true when the known deficiencies with the radiometer and real world differences between MVD quantity and radar-derived estimated size are fully considered. It should also be noted that the NIRSS operated during AIRS II utilized X-band instead of the current Ka-band radar. 3.2 Supercooled large drops aloft and positive PIREPs For the following three case studies, we rely on surface obsevations of drizzle as our best in situ proof of large drop size, and PIREPs as our in situ proof of SLW. During the weather event from November 23rd through November 24th of 2009, Cleveland International Airport reported drizzle from 20:00 UTC through 1:00 UTC on the 24th (Fig 4 top). A trace measurable amount of precipitation was recorded during this period, and the visibility was reduced from 10 miles before the precipitation down to 3 miles by 18 UTC. The hourly surface weather observations shown in figure 4a are temporally matched with the RES derived from NIRSS in figure 4b and the NIRSS icing and PIREP severity in figure 4c. In the time versus height cross section of NIRSS derived RES shown in figure 4 (middle), the K-band reflectivity Figure 4. Cleveland Hopkins Intl. Airport reported weather (top), NIRSS derived RES (middle, [μm]) and NIRSS derived icing hazard severity (bottom) and coded PIREP severity (red numbers) from 12:00 UTC on November 23, 2009 to 6:00 UTC on November 24, 2009.

6 area is shown in red and the 'no-return' area is shown in blue. NIRSS RES values above zero are shown in the rainbow colorscale shown at the right. Pockets of RES values above 50 μm can be seen from 19:00 UTC to 0:00 UTC on the 24th, but at no other times during the two day weather event. Figure 4 (bottom) shows NIRSS icing hazard severity on a scale of 0 to 8, with 1 and 2 being categorized as 'trace icing', 3 through 5 as 'moderate icing' and 6 through 8 as 'severe icing'. PIREP icing severity is reported on the same 0 through 8 scale and are plotted as red text numbers at the reported altitudes. Moderate PIREPs are recorded at the time and height of the SLD during this case. It is also important to note that the areal extent and magnitudes of NIRSS in-flight icing hazard matches up well with the positive and negative icing PIREPs for this event. The lower NIRSS hazard boundary at about 6 to 7 kilofeet corresponds to the freezing temperature height as derived from the radiometer V-band channels6,. A significant portion of RES > 50μm exists above this freezing altitude and thus are SLD. In this case, a non-zero NIRSS icing hazard value colocated with RES greater than 50 μm could be used to provide a SLD flag on the NIRSS display. 3.3 Supercooled large and small drops near surface and positive PIREPs During the weather event from november 19th of 2010, Cleveland International Airport reported fog through 4:00 UTC, freezing drizzle from 5:00 to 6:00 UTC and then drizzle from 7:00 to 14:00 UTC (Fig 4 top). No measurable precipitation amount was recorded during this period, and the visibility improved from 2 to 5 miles to 8 to 10 miles after significant surface weather stopped being reported after 14:00 UTC. The hourly surface weather observations shown in Figure 5. Cleveland Hopkins Intl. Airport reported weather, precipitation amount and visibility (top), NIRSS derived RES (middle, [μm]) and NIRSS derived icing hazard severity (bottom) and coded PIREP severity (red numbers) from 00:00 to 24:00 UTC on January 19, 2010.

7 figure 4 (top) are once again temporally matched with the RES derived from NIRSS in figure 4 (middle) and the NIRSS icing and PIREP severity in figure 4 (bottom). Pockets of RES values above 50 μm can be seen from 4:00 UTC to 8:00 UTC at low altitudes above the surface, but at no other times during the weather event. Two cloud layers are evident from 0:00 to 7:00 UTC. Moderate PIREPs are recorded at many times during this day at altitudes associated with the lower cloud deck. No icing PIREPs occur at the times when NIRSS detects SLD, but this could just mean that no flights occurred during this timespan. This is likely the case since the SLD times correspond to 10pm to 2am local time. Once again, the areal extent and magnitudes of NIRSS in-flight icing hazard matches up reasonably well with the positive and negative icing PIREPs for this event. All portions of the RES > 50μm exist at temperatures between 0 and -20oC ( a requisite for positive NIRSS icing hazards) and thus are SLD. In this case, a non-zero NIRSS icing hazard value colocated with RES greater than 50 μm could be used to provide a SLD flag on the NIRSS display. 3.4 Supercooled small drops near surface and positive PIREPs During the event on January 16th of 2010, Cleveland International Airport reported fog from 1:00 to 2:00 UTC and again from 10:00 to 20:00 UTC (Fig 4 top). No measurable precipitation amount was recorded at the surface during this case. RES values never climbed above 50 μm at any time during this small drop event (Fig 4 middle). Light to moderate PIREPs are reported sporadically from 1:00 to 6:00 UTC (Fig. 4 bottom), and a few negative reports of icing exist from 13:00 UTC through the end of the day. Once again, the areal extent and magnitudes of NIRSS in-flight icing hazard matches up reasonably well with the positive and negative icing PIREPs for this event. All portions of the RES < 50μm exist at temperatures between 0 and -20oC (a requisite for positive NIRSS icing hazards) and thus are in-flight icing due to relatively small drops. In this case, a non-zero NIRSS icing hazard value colocated with RES less than 50 μm could be used to qualify the detected in-flight icing as originating from small drops. Figure 6. Cleveland Hopkins Intl. Airport reported weather, precipitation amount and visibility (top), NIRSS derived RES(middle, [μm]) and NIRSS derived icing hazard severity (bottom) and coded PIREP severity (red numbers) from 00:00 to 24:00 UTC on January 16, 2010.

8 4. CONCLUSION In this work, a new method for providing an estimate of the radar-estimated size of water particles from NASA's Icing Remote Sensing System is presented. The method relies on combining reflectivity profiles from the NIRSS Kaband radar with the LWC profiles derived from the NIRSS algorithm. The need for such a product arises from the fact that SLD can be more dangerous to aircraft, as the larger drops can freeze to surfaces beyond the ice protected leading edges of airframes. It was shown that the LWC profiles and the new RES product derived from NIRSS correspond reasonably well with LWC detected by sensors on icing research flights during the winter 2003 AIRS-II field campaign over Montreal, Canada. Three more cases from the winter of 2009/2010 were presented. In two of these cases, large drops were detected by a nearby automated surface weather station and numerous pilot reports of in-flight icing were recorded. These in situ SLD verifications were temporally correlated with NIRSS RES values over 50 μm. In the other case, no precipitation of any size was reported at the surface and the RES indicated smaller supercooled liquid hazard colocated with numerous positive PIREPs. In this fashion, the detection of liquid aloft whos mean RES exceeds 50 μm can be used to provide a SLD flag by NIRSS. NIRSS has already shown skill in differentiating between no in-flight icing, light, moderate and severe icing severity. The initial analyses from this research shows great promise for the differentiation between small drop and large drop in-flight icing hazards in all-liquid clouds. In the future, it would be very useful to collect more in situ microphysical research flight data over the NIRSS site to provide verification for this new NIRSS drop size product. 5. ACKNOWLEDGMENTS This research is in response to requirements and funding by the NASA Aviation Safety Program. The views expressed in this document represents the opinion of the authors and do not necessarily represent the official policy or position of NASA. 6. REFERENCES [1] Politovich, M. K., Aircraft icing caused by large supercooled droplets, J. Appl. Meteor., 28, (1989). [2] Bragg, M.B., Aircraft aerodynamic effects due to large-droplet ice accretions AIAA Paper , Jan. (1996). [3] Bernstein, B., McDonough, F., Politovich, M., Brown, B., Ratvasky, T., Miller, D., Wolff, C., and Cunning G., Current Icing Potential: Algorithm description and comparison to aircraft observations, Journal of Applied Meteorology, 44, (2005). [4] Wolff, C. A., and McDonough, F., A comparison of WRF-RR and RUC forecasts of aircraft icing conditions, AMS 11th Conference on Aviation, Range, and Aerospace Meteorology, January 17-21, Atlanta, GA. (2010). [5] Reehorst, A., Politovich, M., Zednik, S., Isaac, G., and Cober, S., Progress in the development of practical remote detection of icing conditions, NASA/TM (2006). [6] Solheim, F., Godwin, J., Westwater, E., Han, Y., Keihm, S., Marsh, K., and Ware, R., Radiometric profiling of temperature, water vapor and cloud liquid water using various inversion methods, Radio Science, 33, (1998). [7] METEK MIRA Meteorological Ka-Band Cloud RADAR System Description Document, see: (2008). [8] Fabry, F., Zawadzki, I., Bell, C., and Cote, C., Doppler radar signatures of in-flight icing conditions, 30th International Conference on Radar Meteorology, Munich, Germany, Jul (2001). [9] Zawadzki, I., Fabry F., and Szyrmer, W., Observations of supercooled water and of secondary ice generation by a vertically pointing x-band doppler radar, Atmospheric Research, 59-60, (2001).

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