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Ice water content from radar reflectivity factor and temperature

Ice water content from radar reflectivity factor and temperature. Robin Hogan Anthony Illingworth Marion Mittermaier. Overview. Use of mass-size relationships in calculating Z from aircraft size spectra in ice clouds Radar-aircraft comparisons of Z

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Ice water content from radar reflectivity factor and temperature

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  1. Ice water content from radar reflectivity factorand temperature Robin Hogan Anthony Illingworth Marion Mittermaier

  2. Overview • Use of mass-size relationships in calculating Z from aircraft size spectra in ice clouds • Radar-aircraft comparisons of Z • Derivation of IWC(Z,T): Rayleigh scattering • Evaluation of model IWC in precipitating cases using 3 GHz radar data • The problem of non-Rayleigh scattering • Derivation of IWC(Z,T): non-Rayleigh scattering • The effect of rain on IWC statistics from cloud radars

  3. Interpretation of aircraft size spectra • To use aircraft size distributions to derive IWC(Z,T), need to be confident of mass-size relationship • Brown and Francis used m=0.0185D1.9 (SI units) • It produced the best agreement between IWC from size spectra and from independent bulk measurement • But can we use it for calculating radar reflectivity factor? • Use scanning 3 GHz data from Chilbolton during the Clouds, Water Vapour and Climate (CWVC) and Cloud Lidar and Radar Experiment (CLARE’98) • Rayleigh-scattering Z prop. to mass squared • Error in mass-size relationship of factor of 2 would lead to a 6 dB disagreement in radar-measured and aircraft-calculated values!

  4. Comparisons from CLARE’98 T=-32ºC, Z=-0.7dB, m=-8% T=-15ºC, Z=-1.0dB, m=-11%

  5. Comparisons from CWVC T=-21ºC, Z=+0.3dB, m=+3% T=-10ºC, Z=+0.3dB, m=+4%

  6. Another CLARE case But this case was mixed-phase: liquid water leads to riming and depositional growth rather than aggregation: higher density T=-7ºC, Z=+3.7dB, m=+54% Implies particle mass/density is up to factor 2 too small

  7. 3 GHz Mean slope: IWC~Z0.6

  8. Relationship for Rayleigh scattering • Relationship derived for Rayleigh-scattering radars: • log10(IWC) = 0.06Z – 0.0197T – 1.70 • Can also derive relationship from assumptions made in Met Office model (Wilson and Ballard 1999) • log10(IWC) = 0.06Z – 0.0212T – 1.92 • Similar in form; main difference is due to Met Office assuming density twice that of Brown & Francis (1995) • The IWC~Z0.6 form arises only if T term is assumed due to T-dependence of number concentration parameter N0 (or N0*) rather than D0 • Aircraft calculations from Field et al. (2004) confirm this

  9. IWC evaluation using 3 GHz radar • Now evaluate Met Office mesoscale model in raining events using Chilbolton 3 GHz radar • Advantages over cloud radar: • Rayleigh scattering: Z easier to interpret • Very low attenuation: retrievals possible above rain/melting ice • Radar calibration to 0.5 dB using Goddard et al. (1994) technique • Scanning capability allows representative sample of gridbox • 39 hours of data from 8 frontal events in 2000 • Apply IWC(Z,T) relationship and average data in horizontal scans to model grid • Threshold observations & model at 0.2 mm/h • Need to be aware of radar sensitivity; only use data closer than 36 km where minimum detectable reflectivity is –11 dBZ

  10. Comparison of mean IWC • Results: • Accurate to 10% between –10ºC and -30ºC • Factor of 2 too low between -30ºC and -45ºC • Results at colder temperatures unreliable due to sensitivity sensitivity at 10 km sensitivity at 36 km

  11. Comparison of IWC distribution • Distribution generally too narrow in model, problem worse at warmer temperatures • Malcolm Brooks’ cloud radar results also show model too broad

  12. Non-Rayleigh scattering Mie-scattering using equivalent area diameter Mie-scattering using mean of max dimensions • Representation of Mie scattering has large effect… Equivalent-area diameter Mean of max dimensions Typical aircraft crystal image

  13. 35 GHz log10(IWC) = 0.000242 ZT + 0.0699 Z – 0.0186T – 1.63 Non-Rayleigh scattering

  14. 94 GHz log10(IWC) = 0.000580 ZT + 0.0923 Z – 0.00706T – 0.992 Non-Rayleigh scattering

  15. Ice water Observations Met Office Mesoscale Model ECMWF Global Model Meteo-France ARPEGE Model KNMI RACMO Model Swedish RCA Model

  16. Rain in cloud radar IWC comparisons • Cloud radars can’t retrieve reliable IWC in rain • But around half ice mass in Met Office model occurs over rain • Implies comparisons of mean IWC are not very useful • Possible solution: PDFs

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