Backscatter differential phase - estimation and variability. S. - PowerPoint PPT Presentation

�� Backscatter differential phase - estimation and variability. S. Trömel (1) , M. Kumjian (2) , A. Ryzhkov (2) , C. Simmer (3) , A. Tokay (4) , J.-B. Schroer (3) (1) Hans-Ertel-Centre for Weather Research, Atmospheric Dynamics and Predictability Branch, University Bonn, Germany (2) NOAA’s National Severe Storms Laboratory, Norman, USA (3) Meteorological Institute of the University of Bonn, Germany (4) Joint Center for Earth Systems Technology (JCET), University of Maryland Baltimore County (UMBC), USA

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Outline 1. Introduction 2. 2. 2. 2. δ δ δ δ in rain − Estimation of δ δ δ δ − DSD analysis with respect to Z DR - δ − DSD analysis with respect to Z DR - δ δ -relation δ -relation δ δ δ δ 3. δ 3. 3. 3. δ δ δ within the melting layer − Estimation of δ δ δ δ − The impact of non-uniform beam filling − Variability of δ δ δ at X, C, and S-bands δ 4. Conclusions Introduction δ in rain δ within the melting layer Conclusions ERAD 2012

Introduction The measured total differential phase Φ Φ DP shift consists of the 2 components: Φ Φ r ∫ 2 ( ) Φ = δ + = δ + ϕ K s ds DP DP DP 0 where Φ Φ Φ Φ DP = total differential phase, ϕ ϕ DP = differential propagation phase, ϕ ϕ K DP = specific differential phase, K = specific differential phase, δ = backscatter differential phase. δ δ δ For accurate rainfall estimation using K DP backscattered and propagation components of Φ DP need to be separated before specific differential phase K DP is estimated from the range derivative of Φ DP. Perturbations of the Φ Φ DP profile through the melting layer can be attributed either to δ δ or Φ Φ δ δ effects of nonuniform beam filling (NBF). Benefits of using δ δ δ is its direct relation to the prevalent size of hydrometeors δ δ can be used for more accurate retrieval of hydrometeor size distributions δ δ δ δ δ should be generally correlated with Z DR , can serve as a proxy for Z DR δ δ δ in rain δ within the melting layer Conclusions Introduction ERAD 2012

Backscatter differential phase δ δ of raindrops δ δ Diameter (mm) Temperature = 0 o C Temperature = 20 o C 30 30 S band S band C band C band X band X band 20 20 δ (degrees) (degrees) δ (degrees) (degrees) 10 10 10 10 0 0 −10 −10 0 2 4 6 8 0 2 4 6 8 Diameter (mm) Diameter (mm) Fig. Simulated δ δ δ as a function of equivolume raindrop diameter for different wavelengths δ o and temperatures. δ in rain depends on λ δ δ δ λ λ λ and T and increases with raindrop size. Introduction δ δ δ δ in rain δ within the melting layer Conclusions ERAD 2012

Estimation of δ δ δ in rain δ Application of the ZPHI-method (Testud et al., 2000) and the slightly modified self-consistent method proposed by Bringi et al. (2001): External constraint: ∆φ ∆φ ∆φ ∆φ DP = φ φ DP (r 2 )- φ φ φ φ φ φ DP (r 1 ) with ranges r 1 and r 2 from the radar • A = b A = β Z α K 2 relationships and with β β β , α β α =fkt(drop shape, T) α α • h h h DP b ∆ φ [Z (r)] f( ) r 2 = A (r) a DP ∫ 0 46 , = b I(r;r ) . b [Z (s)] ds where h + φ I(r r ) f( � ( )I(r;r ) 2 a 1 2 2 ; DP r r 2 ∫ 0 46 = b I(r ;r ) . b [Z (s)] ds 1 2 a r 1 ( ) 10 0 . 1 α ∆ DP φ 1 ∆ φ = b − f DP The selfconsistent method (Bringi et al., 2001) , slightly modified, searches for optimal α α α α and b by comparing calculated and measured Φ Φ DP : Φ Φ where Introduction δ δ δ δ in rain δ within the melting layer Conclusions ERAD 2012

Estimation of δ δ in rain δ δ Application of the ZPHI-method (Testud et al., 2000) and the slightly modified self-consistent method proposed by Bringi et al. (2001). Differences between Φ Φ DP and ϕ ϕ cal Φ Φ ϕ ϕ DP calculated via the ZPHI-method reveal statistical fluctuations and δ δ . δ δ ρ HV >0.9 ρ ρ ρ δ δ δ δ is used as criterion for seperating perturbations and the ones caused by noise. Pro: Method provides reasonably robust estimates of δ δ δ and K DP in pure rain outside areas δ affected by NBF or low S/N ratios. Spatial and δ δ δ δ temporal coherency of retrieved can be demonstrated. Con: Less suitable for areas with high K DP . Fig: PPIs of δ δ for the BoXPol observations δ δ on June 22, 2011 between 11:11UTC and 11:26 UTC. Introduction δ δ in rain δ δ δ within the melting layer Conclusions ERAD 2012

Reliability of the method for δ δ δ δ detection Example: PPIs of δ for the BoXPol observations on June 22 between 11:11UTC and 11:26UTC. Pro: Spatial and temporal coherency of retrieved δ can be demonstrated. Con: Less suitable for areas with high K DP . Introduction δ δ δ δ in rain δ within the melting layer Conclusions ERAD 2012

Z DR - δ δ relationships δ δ Simulations for X-band based on.. 2DVideo measurements in Oklahoma, USA Parsivel measurements in Bonn, Germany ● at 0° C ● at 15° C ● at 30° δ =Z DR C 1.8 δ =Z δ =Z DR 1.8 1.8 The overwhelming part of variability can be related to the temperature of raindrops. The impact of differences in DSDs seems to be small. Introduction δ δ δ δ in rain δ within the melting layer Conclusions ERAD 2012

Backscatter differential phase δ δ δ δ - another parameter for characterizing dropsizes - Simulations for X-band at 15 ° C based on.. 2DVideo measurements in Oklahoma, USA Parsivel measurements in Bonn, Germany Introduction δ δ δ δ in rain δ within the melting layer Conclusions ERAD 2012

Backscatter differential phase δ δ δ in the melting layer δ Observed bumbs in differential phase Φ Φ Φ Φ DP may be associated either with backscatter differential phase δ δ δ δ 1. Φ d dZ 2 ∆Φ = 0.02 Ω DP (Ryzhkov et al., 2007) 2. nonuniform beamfilling (NBF): DP θ θ d d Method for reliable δ δ δ δ δ -estimation in the melting layer: δ δ δ Calculate azimuthally averaged radial profiles of Φ DP from measurements at higher elevation angles - forward propagation contribution is minimized - suppress fluctuations of Φ DP caused by reduction of ρ HV within the melting layer, - impact of NBF is minimized Introduction δ in rain δ δ δ δ within the melting layer Conclusions ERAD 2012

Backscatter differential phase δ δ δ δ in the melting layer Observed bumbs in differential phase Φ Φ Φ Φ DP may be associated either with backscatter differential phase δ δ δ δ 1. Φ d dZ 2 ∆Φ = 0.02 Ω DP (Ryzhkov et al., 2007) 2. nonuniform beamfilling (NBF) DP θ θ d d Method for reliable δ δ δ δ -estimation in the melting layer: δ δ δ δ Fig. Azimuthally averaged quasi-vertical profiles from the polarimetric X-band radar in Bonn (BoXPol), Germany, obtained on 04 December 2011, at 20:51 UTC, from the PPI at elevation 7° . Introduction δ in rain δ δ δ δ within the melting layer Conclusions ERAD 2012

Backscatter differential phase δ δ δ in the melting layer δ Observed bumbs in differential phase Φ Φ Φ Φ DP may be associated either with δ δ obs,X = 3° δ δ backscatter differential phase δ δ δ δ 1. ∆Φ ∆Φ DP = 0.11° ∆Φ ∆Φ 2. nonuniform beamfilling (NBF) Method for reliable δ Method for reliable δ δ -estimation in the melting layer: δ -estimation in the melting layer: δ δ δ δ Fig. Azimuthally averaged quasi-vertical profiles from the polarimetric X-band radar in Bonn (BoXPol), Germany, obtained on 04 December 2011, at 20:51 UTC, from the PPI at elevation 7° . Introduction δ in rain δ δ δ δ within the melting layer Conclusions ERAD 2012

Variability of δ δ within the melting layer at X, C, and S bands δ δ -Observations at X-band (BoXPol), δ δ obs,X ≈ 7° δ δ - Fig. Magnitudes of the extremes of Z DR , ρ ρ HV , and δ ρ ρ δ δ δ in the melting layer observed with BoXPol at 7° elevation on December 04, 2011 between19:36UTC and 22:29UTC. Introduction δ in rain δ δ δ δ within the melting layer Conclusions ERAD 2012

Variability of δ δ within the melting layer at X, C, and S bands δ δ -Observations at X-band (BoXPol), δ δ obs,X ≈ 7° δ δ - Fig. Relative heights of the extremes of Z DR , ρ ρ HV , and δ ρ ρ δ δ δ in the melting layer observed with BoXPol at 7°elevation on December 04, 2011 between19:36UTC and 22:29UTC. Introduction δ in rain δ δ δ δ within the melting layer Conclusions ERAD 2012

Variability of δ δ within the melting layer at X, C, and S bands δ δ - Simulations - Z Z DR δ Fig. Simulated vertical profiles of Z, Z DR , and δ within the melting layer at S, C, and X bands. Freezing level is at 1 km, temperature lapse rate is 6.5 ° /k m, relative humidity is 100%, and rain rate near the surface is 5 mm/h. Introduction δ in rain δ δ δ δ within the melting layer Conclusions ERAD 2012