Post on 14-Jan-2016
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Ice hydrometeor microphysical parameterisations in NWP
Amy Doherty
T. R. Sreerekha, Una O’Keeffe, Stephen English
October 2006
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Outline
Motivation
Background
Model and data
Case study results
Summary and Future work
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Motivation
Currently precipitation and ice affected microwave radiances are not assimilated at the Met Office
Information in these conditions is sparse, so use of these data would be beneficial
For direct assimilation of radiances a scattering RTM is required – RTTOVSCATT
Testing of RTTOVSCATT before operational implementation revealed questions about ice microphysical assumptions
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Background
Ice scattering causes TB depression at AMSU-B frequencies
Strength of depression depends on microphysics of ice particles: size, shape, density
No prognostic a priori information is available about the microphysics so assumptions have to be made
Different methods of solving the scattering RTE perform to similar standard
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RTTOV 8.7
Simple two stream scattering solution (Eddington)
Fast geometric optics ocean surface emissivity model
Marshall-Palmer/Modified Gamma Drop Size Distribution
Ice particle diameter up to 100 microns, snow 100-20000 microns
Density of ice particles 0.9 g/cm3
Density of snow particles 0.1 g/cm3
Permittivity dependent on ice/water/air mixture of hydrometeors (Maxwell-Garnet mixing formula)
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Met Office Model Fields
Pressure, temperature and moisture profiles available from forecast model
Frozen hydrometeor, rain and liquid cloud content profiles available
Smooth transition between different types of frozen hydrometeor
Ice particle density inversely proportional to diameter and exponential size distribution dependant on temperature
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Case study simulations
AMSU Ch 20 (183.3±7 GHz)
NOAA-16 Observations
RTTOV Simulated TBs
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Experiments
Experiment Density Size distribution
1 0.1 g/cm3 Modified Gamma
2 0.5 g/cm3 Modified Gamma
3 0.5 g/cm3 Field*
4 8.74 x 10-4 exp{-0.625D2} + 4.5 x 10-5 ($)
Modified Gamma
5 0.132 D-1 Modified Gamma
6 0.132 D-1 Field*
*Field et al 2005 $Jones 1995
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Same PSD different density
Density = 0.5 g/cm3
(exp 2)
Density = 0.874 exp{-625*D2} + 0.045
(exp 4)
Modified gamma distribution
183.3±7 GHz
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Same density different PSD
Density = 0.132*D-1
Modified gamma distribution
(exp 5)
Paul Field size distribution
(exp 6)
183.3±7 GHz
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ComparisonsAMSU Channel 20: 183 ± 7 GHz
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Results for Experiment 6
Observation Experiment 6
183.3±7 GHz
PSD = Field et al.,2005 (based on T and IWC)
Density = 0.132 D-1 (Wilson and Ballard, 1999)
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Summary
Comparisons of TB observations with RTTOV8 simulations using Met Office forecast model inputs have highlighted strong sensitivity to ice microphysical assumptions at microwave frequencies affected by scattering
Interface between forecast and RT models is very important
Parameterisations of PSD based on T and IWC of cloud are better supported by simulations than more general ones
Parameterisations of density based on size of ice particles are better supported by TB simulations than constant density
Best parameterisation may depend on cloud type/latitude band, only tested so far with UK case studies
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Future Work
Option to use Experiment 6 microphysics will be available with RTTOV9
Test parameterisations in other conditions and areas
Investigate other available parameterisations
Implement best set of assumptions operationally at the Met Office
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References
Bauer et al., 2006, QJRMS, 132, 1259-1281
Doherty et al., 2006? Submitted to QJRMS
Field et al., 2005, QJRMS, 131, 1997-2017
Jones, 1995, PhD Thesis, University of Reading
Wilson and Ballard, 1999, QJRMS, 125, 1607-1636
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Questions?
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