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Robin Hogan

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Marion Mittermaier. Ice water content from radar reflectivity factor ... 0.000580 ZT 0.0923 Z. 0.00706T. 0.992. 94 GHz. Non-Rayleigh scattering. Ice water ... – PowerPoint PPT presentation

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Title: Robin Hogan


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

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 m0.0185D1.9 (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 (CLARE98)
  • 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 CLARE98
  • T-32ºC, ?Z-0.7dB, ?m-8 T-15ºC, ?Z-1.0dB,
    ?m-11

5
Comparisons from CWVC
T-21ºC, ?Z0.3dB, ?m3 T-10ºC, ?Z0.3dB,
?m4
6
Another CLARE case
  • T-7ºC, ?Z3.7dB, ?m54
  • Implies particle mass/density is up to factor 2
    too small

7
3 GHz
Mean slope IWCZ0.6
8
Relationship for Rayleigh scattering
  • Relationship derived for Rayleigh-scattering
    radars
  • log10(IWC) 0.06Z 0.0197T 1.70 i.e.
    IWC ? Z 0.6?f(T )
  • What is the origin of the temperature
    relationship?
  • For an exponential distribution with density ?
    D-1
  • IWC ? N0D03 and Z ? N0D05
  • If T is a proxy for D0 then eliminate N0
  • IWC ? Z D0-2 ? Z f(T )
  • Not observed!
  • If T is a proxy for N0 then eliminate D0
  • IWC ? Z 0.6N00.4 ? Z 0.6f(T )
  • Correct!

9
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 IWCZ0.6 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

10
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

11
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
12
Comparison of IWC distribution
  • Distribution generally too narrow in model,
    problem worse at warmer temperatures

13
Non-Rayleigh scattering
  • Representation of Mie scattering has large effect

Equivalent-area diameter
Mean of max dimensions
Typical aircraft crystal image
14
35 GHz
log10(IWC) 0.000242 ZT 0.0699 Z 0.0186T
1.63
Non-Rayleigh scattering
15
94 GHz
log10(IWC) 0.000580 ZT 0.0923 Z
0.00706T 0.992
Non-Rayleigh scattering
16
Ice water
Observations Met Office Mesoscale
Model ECMWF Global Model Meteo-France ARPEGE
Model KNMI RACMO Model Swedish RCA Model
17
Rain in cloud radar IWC comparisons
  • Cloud radars cant 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

18
Comparison of the IWC products (lidar/radar vs.
Z,T) retrieved from Chilbolton
data (2003)
IWCZT IWC
Linear regression
  • The linear regression fit in log-space of all
    data is close to the 1 to 1 line.
  • The distribution is wide and not symmetric

19
Influence of Radar reflectivity and T on the IWC
ratio
  • The IWC/IWCZT ratio is correlated with the Radar
    reflectivity
  • The IWCZT overestimates the lidar/radar IWC by a
    factor 2-3 for all T

20
IWC to IWC ratio
  • The IWCZT parameterization has a different radar
    reflectivity dependence as suggested by the
    IWC(lidar/radar) results.
  • There is a small temperature (x2-3) offset
    between the two methods

21
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