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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