Robin HoganAnthony IllingworthMarion Mittermaier
Ice water content from radar reflectivity factor
and temperature
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
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!
Comparisons from CLARE’98
T=-32ºC, Z=-0.7dB, m=-8% T=-15ºC, Z=-1.0dB, m=-11%
Comparisons from CWVC
T=-21ºC, Z=+0.3dB, m=+3% T=-10ºC, Z=+0.3dB, m=+4%
Another CLARE case
T=-7ºC, Z=+3.7dB, m=+54%Implies particle mass/density is
up to factor 2 too small
But this case was mixed-phase: liquid water leads to riming and depositional growth rather than aggregation: higher density
3 GHz
Mean slope: IWC~Z0.6
Relationship for Rayleigh scattering
• Relationship derived for Rayleigh-scattering radars:– log10(IWC) = 0.06Z – 0.0197T – 1.70 i.e. IWC Z 0.6f(T )
• What is the origin of the temperature relationship?• For an exponential distribution with density D-1:
– IWC N0D03 and Z N0D0
5
• 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!
Observations by Field et al. (2004) demonstrate the T dependence of N0
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
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
sensitivityat 10 km
sensitivity
at 36 km
Comparison of IWC distribution
• Distribution generally too narrow in model, problem worse at warmer temperatures
Non-Rayleigh scattering• Representation of Mie scattering has large effect…
Mie-scattering using equivalent area diameterMie-scattering using mean of max dimensions
Equivalent-area diameter
Mean of max dimensions
Typical aircraft crystal image
35 GHz
Non-Rayleigh scattering
log10(IWC) =
0.000242 ZT + 0.0699 Z– 0.0186T– 1.63
log10(IWC) =
0.000580 ZT + 0.0923 Z– 0.00706T– 0.992
94 GHz
Non-Rayleigh scattering
Ice water
Observations
Met Office
Mesoscale Model
ECMWF
Global Model
Meteo-France
ARPEGE Model
KNMI
RACMO Model
Swedish
RCA Model
•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
Comparison of the IWC products (lidar/radar vs. Z,T) Comparison of the IWC products (lidar/radar vs. Z,T) retrieved from Chilbolton data (2003) retrieved from Chilbolton data (2003)
IWCZT =IWC
Linear regression
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