Robin Hogan Anthony Illingworth Marion Mittermaier Ice water content from radar reflectivity factor...
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Transcript of Robin Hogan Anthony Illingworth Marion Mittermaier Ice water content from radar reflectivity factor...
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