Dear Tomoo,
I am open to making the distinction between slow and fast falling
precipitation. But let me push back just a little.
I see no physical distinction between the fast and slow falling, other than
characteristic timescales. It is a continuum, and both can evaporate on the
way down.
The bigger difference is that the fast and slow processes could well be handled
in separate parts of a model: rain produced from a convective parameterization
is often assumed to fall to the ground instantaneously, I can also imagine that
there could be a sedimentation parameterization for the clouds themselves (and
aerosols).
Part of the distinction in models is presumably because the fall time for large
drops is less than a typical GCM timestep. But at the timestep of regional
models (a few minutes) that may not be true.
Are these distinctions big enough to warrant having a separate CF std_name?
Perhaps.
One would certainly need to have a criteria for distinguishing the two regimes
that works across all the groups interested in CF, including observations.
CF would also need to consider whether the existing names need to be modified.
Do the 'precipitation' terms include all aerosol-droplet sizes (including the
sedimenting ones), or just the droplets that are too large to sediment?
Either way, the descriptions of those precipitation names will need to be
updated. If the new definition of the precipitation names changes, then CF
will need to be careful about backwards compatibility.
In this case I suspect that the issues above could be resolved without
excessive pain, but it would be good to get the input from people in the
relevant communities.
Yours truly,
Philip
-----------------------------------------------------------------------
Dr Philip Cameron-Smith, p...@llnl.gov, Lawrence Livermore National Lab.
-----------------------------------------------------------------------
> -----Original Message-----
> From: Tomoo Ogura [mailto:og...@nies.go.jp]
> Sent: Monday, February 28, 2011 5:26 PM
> To: Jonathan Gregory; Cameron-smith, Philip
> Cc: Jennifer Kay; Yoko Tsushima; cf-metadata@cgd.ucar.edu; Tomoo Ogura
> Subject: Re: [CF-metadata] Proposal for new standard names
>
> Dear Jonathan and Philip Cc: Jen,
>
> Many thanks for your comments.
> My understanding of the issue is as follows (please correct me if I'm
> wrong).
>
> The tendencies of cloud liquid (condensed) water represented by the
> proposed
> standard names refer to the slow falling process of cloud liquid water.
> They have typical terminal fall speeds of less than 1 [m/s].
> The fallen cloud liquid doesn't always reach the ground
> ; it often evaporates in the air.
>
> There are fast falling processes of cloud liquid water, but they are
> represented
> by other existing standard names. For example,
>
> (a) tendency_of_mass_fraction_of_stratiform_cloud_liquid_water_in_air
> _due_to_autoconversion
> and
> (b) tendency_of_mass_fraction_of_stratiform_cloud_liquid_water_in_air
> _due_to_accretion_to_rain
>
> both convert the slow-falling "cloud liquid" to fast-falling "rain" in
> GCMs.
>
> In this case, it appears to me that we need some distinction on the
> basis of fall speed.
> If "precipitation" implies that something falls to the earth's
> surface,
> then the slow falling process of cloud liquid may be represented by
> other words
> since it doesn't necessarily reach the surface
> - does this make sense?
>
> Best wishes
>
> Tomoo
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