page 1© crown copyright 2006 ice hydrometeor microphysical parameterisations in nwp amy doherty t....

© Crown copyright 2006

Ice hydrometeor microphysical parameterisations in NWP

Amy Doherty

T. R. Sreerekha, Una O’Keeffe, Stephen English

October 2006


Outline

Motivation

Background

Model and data

Case study results

Summary and Future work


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


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


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)


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


Case study simulations

AMSU Ch 20 (183.3±7 GHz)

NOAA-16 Observations

RTTOV Simulated TBs


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


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


Same density different PSD

Density = 0.132*D-1

Modified gamma distribution

(exp 5)

Paul Field size distribution

(exp 6)

183.3±7 GHz


ComparisonsAMSU Channel 20: 183 ± 7 GHz


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)


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


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


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


Questions?


IPWG to work more closely with NWP centresScale matching – degrading the resolution of obs to make comparison with models agree better

Climatology, Hydrology, Nowcasting and Operational forecasts

Beam filling, justification for 3DVAR

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