WMO Aeronautical Meteorology Scientific Conference 2017

Transcription

1 Session 1 Science underpinning meteorological observations, forecasts, advisories and warnings 1.1 En route phenomena Ice crystal icing, and airframe icing research Signatures of supercooled liquid water drops in dual-polarization observations measured by ground-based radars Clotilde Augros, Météo France clotilde.augros@meteo.fr co-authors: Mathieu Lecocq, Sébastien Riette, Christine Le Bot, Nicolas Gaussiat speaker: Clotilde Augros 1. Introduction Dual-polarization observations can be particularly helpful in diagnosing ongoing microphysical processes in precipitation, as they provide valuable information about particle sizes, shapes, composition, and orientations. These measurements include reflectivity at horizontal polarization Z h (which can be linked to the intensity of precipitations), differential reflectivity Z dr (which is related to the hydrometeors oblateness), differential propagation phase shift Φ dp and half its range derivative specific differential phase shift K dp (sensitive to heavy precipitations but also to dense oblate hydrometeors), as well as the copolar correlation coefficient ρ hv (which decreases when different hydrometeors coexist). The focus of this study is to examine the potential of these observations for the detection of supercooled liquid water in clouds, which is of great interest for aircraft icing hazard monitoring. 2. Data and methodology Dual-polarization observations measured by the Plabennec operational C-band polarimetric radar were examined together with in-situ observations from an aircraft campaign where 35 flights sampled supercooled liquid water conditions during the winter The in-situ observations consisted of temperature, liquid water content (measured by the KING probes), and droplet mean volumetric diameters (measured by CDP and FSSP probes). Two cases are presented here, that were characterized by widespread and relatively homogeneous stratiform precipitation around the radar. They are referred as case 1 and case 2 in this study. For both cases, supercooled liquid water was present during all the flight, with liquid water contents ranging from 0.1 to 0.5 g m -3. The trajectory of the aircraft for case 1 is shown Figure 1, overlaid on the reflectivity image from elevation 0.8 at 0850 UTC. The analysis of the dual-polarization variables is explored using quasi-vertical profiles (QVPs: Ryzhkov et al. 2016) of radar observations. With the QVP technique, data from a given elevation angle scan are azimuthally averaged and the range coordinate is converted to height. This representation of the radar observations helps analysing the vertical distribution as well as the temporal evolution of the microphysical properties of hydrometeors in case of stratiform and relatively homogeneous precipitation. The temporal evolution of the radial velocity at 90 elevation is also examined as it is a proxy for the hydrometeor fall speed, which is a good indicator of the occurrence of riming. Unrimed crystals or aggregates rarely fall faster than 1.5

2 m s 1 while rimed particles usually fall at speeds from 1.5 to 2.5 m s 1 or faster (e. g. Vogel et al, 2015). 3. Results Figure 1: Reflectivity image (dbz) at elevation 0.8 measured by Plabennec radar at 0850 UTC on case 1. The aircraft trajectory from 0840 to 1105 UTC is overlaid as a black thick line. Circles denote distances of 100, 150 and 200 km from the radar. The quasi-vertical profiles of Zh, Kdp, Zdr and ρhv are shown Figure 2 (for case 1 from 1435 to 1650 UTC) and Figure 4 (for case 2 from 0840 to 1105 UTC) and the corresponding radial velocity at 90 profiles are shown Figure 3 and 5. Case 1 For this case, the aircraft remained most of the time at a constant altitude (around 3.5 km) corresponding to a temperature of about -5 to -4 C. KING and FSSP probes recorded Liquid Water Content (LWC) values larger than 0.1 g m -3 during most of the flight with maximum values (0.2 to 0.5 g m -3 ) between 70 and 125 min. The mean drop diameters measurements were in the range from 35 to 40 µm. In the quasi-vertical profiles of Zh, Kdp, Zdr and ρhv shown Figure 2, the transition between liquid and ice hydrometeors around 2.9 km can be clearly identified with most of the variables. Zh and Zdr peaks (30 dbz and 1.75 db), which are present just below the isotherm 0 C correspond to the bright band and are due to the increase of the dieletric constant of the melting hydrometeors in this region. The minimum of Kdp around the isotherm 0 C is probably due to the backscattering differential phase as shown by Trömel et al (2014). The transition between very high (about 0.99) and lower values of ρhv around 2 km corresponds to the bottom of the melting layer. In Figure 3, the transition between ice and liquid hydrometeors is

3 also very clear, with fall speeds increasing suddenly up to more than 2.5 m s -1 below the isotherm 0 C. The analysis of the dual-polarization variables above the bright band reveals enhanced Kdp values (0.4 to 0.5 km -1 ) at the altitude of the aircraft, where supercooled liquid water is present. The layer of enhanced Kdp values is just above a layer with very low Zdr values (0 to -0.5 db). Such signatures have also been observed in recent studies (Sinclair et al, 2016; Kumjian and Lombardo, 2017), where the enhancement of Kdp was attributed to the presence of large concentrations of needles and to secondary ice production process (Hallet and Mossop, 1974). Between -3 C and -8 C, the growth of needles by vapor deposition is indeed favored in case of strong supersaturation. Concurrently to the growth of needles, the presence of supercooled water also favors the growth of graupel particles by riming, which tends to produce almost spherical particles (and even prolate in some cases) leading to Zdr close to 0 (or negative). The radial velocity at 90 elevation also infers the presence of ongoing riming as values larger than 1.5 m s -1 are present above the melting layer. These relatively large values are suspected to be due to the heavier graupel particles falling faster. Case 2 In case 2, the aircraft flew at a constant altitude around 1.6 km at a temperature of about - 5 C. Supercooled liquid water was present during all the flight with LWC around 0.2 g m -3 and maximum values up to 0.5 g m -3 at 80 min. The mean diameters of supercooled drops were around µm. In this case, the isotherm 0 C was around 800 m altitude (black dashed lines Figure 4 and 5) and the melting layer extended down to the ground. Like in the first case, the melting layer is characterized by higher values of Zh (up to 30 dbz) and Zdr (up to 2 db), and negative values of Kdp due to backscattering differential phase effects (Figure 4). In the region where supercooled liquid water was detected, the signatures of dual-polarization variables are not as clear as in the first case. A Kdp enhancement is observed between 1.4 and 2 km altitude but only between 40 and 60 min. Zdr values are relatively low above the bright band (0 to 0.25 db) but the minimum is not as pronounced as in the first case. Relatively large values of reflectivity are present just above the melting layer (Zh around 25 dbz) that could be due to the presence of rimed particles. Between 40 and 60 min and 100 and 120 min, fall speeds larger than 1.5 m/s were measured above the melting layer (Figure 5), which is also an indication of the presence of rimed particles.

4 Figure 2: Temporal evolution of quasi vertical profile of Zh, Kdp, Zdr and ρhv obtained by averaging all azimuths from elevation 2.8 of Plabennec radar. Case 1: 1435 to 1650 UTC. The black plan and dashed lines indicate, respectively, the altitude of the aircraft and the 0 C isotherm. Figure 3: Corresponding temporal evolution of the radial velocity measured at elevation 90 by Plabennec radar. Case 1: 1435 to 1650UTC

5 WMO Aeronautical Meteorology Scientific Conference 2017 Figure 4: Same as Figure 2 for case 2: 0840 to 1105 UTC. Figure 5: Same as Figure 3 for case 2: 0840 to 1105 UTC. 4. Conclusions and perspectives This study presented the preliminary results of the analysis of two stratiform cases with in-situ observations indicating the presence of supercooled liquid water at a temperature of about 5 C. The dual-polarization observations as well as the radial velocity measured at vertical incidence reveal important insights into the cloud microphysical processes that can be associated to the presence of supercooled liquid water. In both cases, a layer with very low values of Zdr was observed just above the melting layer (but less pronounced in the second case). These low values of Zdr are suspected to be due to the riming of supercooled drops resulting to rather spherical graupel particles. In the first case only, a layer with enhanced values of Kdp (up to 0.5 km-1) is present around -5 C, which could be due to secondary ice production and large concentrations of thin needles. Both cases were associated to hydrometeor fall speeds higher than 1.5 m s-1 above the melting layer. These first results suggest that the analysis of the vertical distribution of dual-polarization variables together with the vertical profile of radial velocity at 90 elevation can give clues about the presence of supercooled liquid water and thus the potential of aircraft icing hazard. More cases with icing and non icing conditions will be analyzed in the future, to better assess if the icing hazard can be reliably predicted thanks to these radar variables.

6 5. References Hallett, J., and S. C. Mossop, 1974: Production of secondary ice crystals during the riming process. Nature, 249, 26 28, doi: /249026a0 Kumjian, M.R. and K.A. Lombardo, 2017: Insights into the Evolving Microphysical and Kinematic Structure of Northeastern U.S. Winter Storms from Dual-Polarization Doppler Radar. Mon. Wea. Rev., 145, , Ryzhkov, A., P. Zhang, H. Reeves, M. Kumjian, T. Tschallener, S. Trömel, and C. Simmer, 2016: Quasi-Vertical Profiles A New Way to Look at Polarimetric Radar Data. J. Atmos. Oceanic Technol., 33, , Sinclair, V. A., D. Moisseev, and A. von Lerber, 2016: How dual-polarization radar observations can be used to verify model representation of secondary ice, J. Geophys. Res. Atmos., 121, 10,954 10,970, doi: /2016jd Trömel, S., A.V. Ryzhkov, P. Zhang, and C. Simmer, 2014: Investigations of Backscatter Differential Phase in the Melting Layer. J. Appl. Meteor. Climatol., 53, , Vogel, J., F. Fabry, and I. Zawadzki, 2015: Attempts to observe polarimetric signatures of riming in stratiform precipitation. 37th Conf. on Radar Meteorology, Norman, OK, Amer. Meteor. Soc., 6B.6.