Radiative Treatment of Ice
Crystals in UM
John M Edwards, Stephan Havemann, JeanClaude Thelen, Anthony Baran
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Overview
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Does non-sphericity matter?
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Crystal shape and crystal size in a GCM
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The new scheme
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Experimental set-up
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Impact
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Future directions
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Does Non-sphericity matter?
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Crystal Shape and Crystal Size
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Irregular ice crystals are the most common
habit; pristine crystals are much less
common.
– Processes determining shape are not well
understood
– No attempt to use multiple shapes
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Crystal size is crucial for parametrizations
– Various relationships to environment proposed
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Crystal Size
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Simple relationships based on temperature have been
used in GCMs – Kristjánsson et al. (2000):
Dl = 719.89 exp(0.05522(T − Tm ))
– Source: Analysis of observational data from CEPEX, FIRE and
other sources by D. L. Mitchell
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Crystal Size
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Other proposals involve temperature and ice
water content – Donovan and Van Lammeren
(2002)
Variation in real situations is enormous
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Crystal Size
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New Parametrization in UM
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Radiative treatment is based on the ensemble
model of ice crystals (Baran (2001, 2003))
Single scattering properties depend on ice
water contents and effective dimension
Effective dimension predicted from
temperature, consistently with previous
relationship to temperature
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Experimental Set-up
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Assess the new scheme in parallel ten-year
climate runs using prototype version of new
Met Office climate model (HadGEM1)
– Control: Treatment of single scattering properties
follows Kristjánsson et al. (2000)
– Experiment: New ensemble scheme
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Radiation is fully interactive
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Impact on TOA Fluxes
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Impact on Heating Rates
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Impact on Zonal Mean
Temperatures
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Conclusions and Future
Developments
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Given crystal size, shape and IWC, radiative
fluxes can be predicted with some confidence
Considerable uncertainties still remain on the
microphysical side: what determines the size?
If radiation is well-characterized, it should be
used as a tool to exploit satellite data to
reduce other uncertainties.
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Potential Impact of Crystal Size
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Forward Modelling: SEVIRI 0.78µm channel
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