Robin Hogan Anthony Illingworth Marion 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=0.0185D 1.9 (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~Z 0.6

Relationship for Rayleigh scattering Relationship derived for Rayleigh-scattering radars: –log 10 (IWC) = 0.06Z – T – 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  N 0 D 0 3 andZ  N 0 D 0 5 If T is a proxy for D 0 then eliminate N 0 : – IWC  Z D 0 -2  Z f(T ) – Not observed! If T is a proxy for N 0 then eliminate D 0 : – IWC  Z 0.6 N  Z 0.6 f(T ) – Correct! Observations by Field et al. (2004) demonstrate the T dependence of N 0

Relationship for Rayleigh scattering Relationship derived for Rayleigh-scattering radars: –log 10 (IWC) = 0.06Z – T – 1.70 Can also derive relationship from assumptions made in Met Office model (Wilson and Ballard 1999) –log 10 (IWC) = 0.06Z – T – 1.92 –Similar in form; main difference is due to Met Office assuming density twice that of Brown & Francis (1995) –The IWC~Z 0.6 form arises only if T term is assumed due to T-dependence of number concentration parameter N 0 (or N 0 *) rather than D 0 –Aircraft calculations from Field et al. (2004) confirm this

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 sensitivity at 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 diameter Mie-scattering using mean of max dimensions Equivalent-area diameter Mean of max dimensions Typical aircraft crystal image

35 GHz Non-Rayleigh scattering log 10 (IWC) = ZT Z – T – 1.63

log 10 (IWC) = ZT Z – T – GHz Non-Rayleigh scattering

Ice water Observations Met Office Mesoscale Model ECMWF Global Model Meteo-France ARPEGE Model KNMI RACMO Model Swedish RCA Model

Rain in cloud radar IWC comparisons Cloud radars can’t 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

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) retrieved from Chilbolton data (2003) IWC ZT =IWC Linear regression

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

IWC to IWC ratio The IWC ZT 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