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