Polarimetric Radar Observations of Hydrometeor Refreezing in Winter Storms Matthew R. Kumjian 1,2, Alexander V. Ryzhkov 1,2, Terry J. Schuur 1,2, and Heather D. Reeves 2 1.Cooperative Institute for Mesoscale Meteorological Studies, University of Oklahoma, Norman, Oklahoma, USA 2.NOAA/OAR/National Severe Storms Laboratory, Norman, Oklahoma, USA US – Coratian Workshop on Mesometeorology 19 June 2012 Kraš, Croatia

Motivation

temperature height 0 °C warm layer cold layer Total or partial melting? Refreezing?

Dual-Polarization Radar Radar reflectivity factor at horizontal polarization: Z H - Measure of returned power; strongly depends on particle size Differential reflectivity factor: Z DR - Measure of the shape of particles; independent of concentration Differential propagation phase shift: Φ DP Specific differential phase shift: K DP - Measure of the difference in phase acquired during propagation through anisotropic media Co-polar cross-correlation coefficient: ρ hv - Measure of the diversity of particle shapes, orientations, and compositions within the radar sampling volume; independent of concentration. Transmits and receives electromagnetic radiation at orthogonal polarizations (H, V). Comparing the difference in the power and phase of the received signal provides information about the bulk characteristics of precipitation.

Freezing of Hydrometeors - Should result in a decrease in Z H (6-7 dBZ), a decrease in Z DR, a decrease in K DP, and a decrease in ρ hv. Instead, an increase in Z DR and K DP was observed. Looked at data collected during heavy ice pellet storm on January 2009; the storm dropped 3 – 5 cm of ice pellets over parts of central and SW Oklahoma, with some places receiving over 10 cm. ?

27 January 2009 at 2005 UTC, elevation angle 3.4°

Refreezing Signature How repeatable is it? Looked at data collected during prolonged ice pellet storms on 30 November 2006 (featured lightning during the event!) 24 December 2009 (“The Christmas Eve Blizzard”) 1 February 2011 (featured hail briefly during the event!)

30 Nov. 2006, 1406z, 4.3°, KOUN

24 Dec. 2009, 1700z, 4.3° (OU-PRIME)

1 Feb. 2011, 0406z, 4.5° (OU-PRIME)

1 Feb. 2011, 0420z, 4.5° (OU-PRIME)

Interpolated T at 0400z = -8 °C at 600 m AGL

Refreezing Signature One case (27 January 2009) offers a larger sample to quantify some important characteristics of the refreezing signature and look for ties to the physics.

Normalized to height of maximum Z DR

Refreezing Signature What does it mean? Increases in Z DR and K DP indicate the production of anisotropic ice crystals within the refreezing layer. Conditions at level of Z DR maximum are prime for rapid depositional growth of needle- or column-like ice crystals. What initiates the ice crystals? DIRECT Hypothesis I:Shattering of freezing drops produce splinters INDIRECT Hypothesis:Crystals are initiated on ice nuclei when temperatures are sufficiently cold -> Precipitation is necessary for signature to appear -> Precipitation is not necessary – would occur anyway DIRECT Hypothesis II:Crystal growth begins from tiny frozen raindrops (micron sized) -> Precipitation is necessary for signature to appear

Refreezing Signature If the indirect hypothesis is true, the signature would appear in the absence of precipitation. - No observations to support this - yet! Awaiting more observations from radars across the nation (world?) for further analysis. Quantify ice production (Ryzhkov et al. 1998) to test direct hypotheses. Modify/combine existing microphysics models (e.g., Kumjian et al. 2013; Ryzhkov et al. 2008) to simulate hydrometeor refreezing and ice production.

Questions? Beautiful artwork by Thierry Poncelet