Title: Robin Hogan
1Ice water content from radar reflectivity
factorand temperature
- Robin Hogan
- Anthony Illingworth
- Marion Mittermaier
2Overview
- 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
3Interpretation 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!
4Comparisons from CLARE98
- T-32ºC, ?Z-0.7dB, ?m-8 T-15ºC, ?Z-1.0dB,
?m-11
5Comparisons from CWVC
T-21ºC, ?Z0.3dB, ?m3 T-10ºC, ?Z0.3dB,
?m4
6Another CLARE case
- T-7ºC, ?Z3.7dB, ?m54
- Implies particle mass/density is up to factor 2
too small
73 GHz
Mean slope IWCZ0.6
8Relationship 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!
9Relationship 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
10IWC 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
11Comparison 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
12Comparison of IWC distribution
- Distribution generally too narrow in model,
problem worse at warmer temperatures
13Non-Rayleigh scattering
- Representation of Mie scattering has large effect
Equivalent-area diameter
Mean of max dimensions
Typical aircraft crystal image
1435 GHz
log10(IWC) 0.000242 ZT 0.0699 Z 0.0186T
1.63
Non-Rayleigh scattering
1594 GHz
log10(IWC) 0.000580 ZT 0.0923 Z
0.00706T 0.992
Non-Rayleigh scattering
16Ice water
Observations Met Office Mesoscale
Model ECMWF Global Model Meteo-France ARPEGE
Model KNMI RACMO Model Swedish RCA Model
17Rain 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
18Comparison 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
19Influence 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
20IWC 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
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