Polarimetric Radar Properties of Hail: Size, Shape, Composition, and Fall Mode, Study notes of Meteorology

Download Polarimetric Radar Properties of Hail: Size, Shape, Composition, and Fall Mode and more Study notes Meteorology in PDF only on Docsity! ATMO 689 Lecture #10 (11-16-04) Polarimetric Radar Properties of Hail* (BC Ch. 7.2) • Hailstone properties – Size, shape, composition, fall mode • Hail Signatures in – Differential Reflectivity (Zdr) • Rain line in Zh-Zdr space: Hdr – Linear Depolarization Ratio (LDR) – Correlation Coefficient (ρHV) – Specific Differential Phase (Kdp) * Some slides adapted with permission from W. A. Petersen (UAH) Hailstone Properties – Size and Shape • Hailstones range in size from Deq = 5 mm to 55 mm and larger – Record is now 7” (17.78 cm) over Aurora, NE on 22 June 2003 • Hailstones tend to have oblate spheroidal shapes but can also have very irregular shapes (e.g. lumps and protuberances) depending on growth mode. Hubbert et al. (1998) Pruppacher and Klett (1997) Modeled Zdr for vertically oriented hailstones Mie scattering effects, D ~ 0.5λ V Ha b • S-band •Oblate spheroid with major axis in the vertical. • Varied axis ratio: a/b = 0.8, 0.6 • Varied water/ice arrangement and hence dielectric: dry, wet, spongy • Varied size: diameter up to 6 cm BC2001 from Balakrishnan and Zrnic (1990) “Hail Hole”: Large Zh and near zero or negative Zdr Doviak and Zrnic (1993) adapted From Bringi et al. (1986) Hubbert et al. (1998) Zh Zdr Zh Zdr Herzegh and Jameson (1992) Zh Zdr HDR (Aydin et al., 1986): The rain-line idea • Similar to using ZDP to delineate regions of rain and ice (e.g., recall fice) • Combine Zh and ZDR pairs into scatter plot. ZDR on horizontal axis and ZH on vertical axis. Define HDR as: HDR = Zh – f(ZDR) dB And, f(ZDR) = 27, ZDR ≤ 0 (dB) aZDR + 27, 0 ≤ ZDR ≤ b (dB) 60, ZDR > 1.74 (dB) a, b are functions of axis ratio model used in scattering calculations to obtain rain lines from disdrometer DSD data as input. Equilibrium axis ratio model: a= 16.5, b=2 dB Oscillations included: a ≈ 19, b ≈ 1.7 Dots are disdrometer-measured ZH and ZDR (S-band assumed). Lines are modeled “rain-lines” with differerent drop axis ratio assumptions. HDR ≥ 10 dB or so, indicates hail (conservatively) Potential ambiguity if wet (e.g., melting) hail is oblate enough to yield a ZDR similar to raindrops. hail rain Linear Depolarization Ratio in Hail Brief Review: Svh Shh 2 2 10LogLDR =Recall from the Covariance Matrix, LDR is written as: 1. LDR is similar to ZDR in terms of its mathematical geometric dependence on particle shape (increases with decreased axis ratio), its reflectivity weighting, and dependence on particle phase (dielectric strength). 2. Orientation dependence is a bit different though. For measurable LDR, particles must be canting, wobbling, tumbling and/or irregularly shaped so that some fraction of the incident horizontally polarized wave is depolarized upon backscatter. Send out a horizontally polarized wave and listen for a depolarized return in cross-pol channel (e.g., Svh) in addition to co-pol returned power (e.g., Shh) Svh 2 > 0 Shh 2 > 0 LDR Finite < 0 Canting or tumbling about z-axis z-axis LDR in Large Hail (STEPS, 29 June 2000)LDR examples in hailstorms Courtesy K. Weins, CSU LDR (colors) dBZ contoured dBZ (colors) ZDR contoured NCAR Hydrometeor ID • Region of high Zh (> 60 dBZ), elevated LDR ( -26<LDR<-21 dB) aloft indicates tumbling/wobbling hail in wet growth. • Note large melting hail signatures near ground – Large Zh (Zh > 60 dBZ) – Zdr hole (Zdr < 0 dB) – -21 dB < LDR < -18 dB – (Note: LDR < -26 dB in rain) • Consistent with output from NCAR “fuzzy logic” hydrometeor ID output at the bottom. – LH: Large hail • LDR tends to increase with hail size. – SH: Small hail – LH+r: Large hail and rain mixed LDR example in hailstorms Zh Zdr LDR Recall: Herzegh and Jameson (1992). • Recall “hail hole” as surface indication of hail. However, Zdr gives little or no information aloft. • Enhanced LDR (-20 dB < LDR < -15 dB) aloft co-located with elevated Zh (> 50 dBZ) is an indication of wet growth hail. • Even larger LDR (-20 dB < LDR < -12 dB) near surface (with near zero Zdr and elevated Zh) are indications of melting hail. See also Hubbert et al. (1998), which was reviewed in class, for excellent LDR hail signatures. Specific Differential Phase - Hail • The differential propagation phase (φdp) and specific differential phase (Kdp) are immune to the presence of hail. – Notice in the figure to the right that Kdp is only a function of rain rate and is essentially independent of the hail rate (i.e., Kdp are nearly vertical lines). – Because Kdp ~ 0 deg/km in hail. • Could look for “Kdp holes” near surface to differentiate heavy rain and hail. • This property makes Kdp a desirable parameter for estimating rain rates in the presence of hail (more later). Relationship between rain rate (Rr), hail rate (Rh), horizontal reflectivity (Zh), and specific differential phase (Kdp). From Fig.4b in Balakrishnan and Zrnic 1990b, JAS, 47,565-583. Polarimetric Measurements in the Hail Cascade of a Supercell – put it all together. Carey and Rutledge (1998) or see BC2001 Figs. 7.56 and 7.57 ρHV Kdp Zdr LDR 0°C