Hi Mark,
concerning question 3.: the methodology used to calculate the different
cloud-types from model-layers: Many models use a method called
maximum-random overlap which seems to have several implementations.
Roughly, for adjacent levels, the max is taken, while for non-adjacent
levels, a random overlap is taken (e.g. 50% + 50% -> 75%)
Heiko
On 2014-06-11 19:53, Heiko Klein wrote:
> Hi Mark,
>
> I've been the original requestor for the
> low/middle/high_type_cloud_area_fraction.
>
> WMO has introduced the low/middle/high clouds already before models
> became as famous as they are today, and they still seem to be a
> excellent simplification and forecasters still use these 3 types (+fog).
>
> Model output can come in many levels, currently in Europe between 60 and
> 200. This is definitely too much to transfer and to handle for a
> forecaster and a reduction to 3 (4) cloud-types is done at all known
> centres (Norway/ECMWF/SMHI). You are right that this reduction is often
> just a summation of certain levels, but it doesn't need to - I've seen
> at least one model which calculates fog different than just taking
> clouds in ground-level.
>
> Examples:
>
>
> # using correct standard_name
> http://thredds.met.no/thredds/dodsC/arome25/arome_norway_default2_5km_latest.nc.html
>
>
>
> # layer named e.g. High cloud cover
> http://wrep.ecmwf.int/wms/?token=MetOceanIE&request=GetCapabilities&version=1.1.1&service=WMS
>
>
>
> Concerning your questions:
>
> 1. Forecasts are never accurate, but the definitions are at least well
> established by WMO, and it is the models task to translate them as best
> as possible
>
> 2. No, the cloud types predate models and model levels and eventually
> accurate height measurements. If a model-level - cloud type assumption
> is used, this is just a guess.
>
> 3. Really good question, I'll ask our modellers how they calculate the
> different types.
>
> Heiko
>
> On 2014-06-11 15:41, Hedley, Mark wrote:
>> Hello CF
>>
>> I have been having an interesting conversation with some modelling
>> colleagues regarding the standard names and descriptions for cloud
>> amount:
>>
>> cloud_area_fraction:
>> 'X_area_fraction' means the fraction of horizontal area occupied by X.
>> 'X_area' means the horizontal area occupied by X within the grid cell.
>> Cloud area fraction is also called 'cloud amount' and 'cloud cover'. The
>> cloud area fraction is for the whole atmosphere column, as seen from the
>> surface or the top of the atmosphere. The cloud area fraction in a layer
>> of the atmosphere has the standard name
>> cloud_area_fraction_in_atmosphere_layer.
>>
>> low_type_cloud_area_fraction:
>> Low type clouds are: Stratus, Stratocumulus, Cumulus, Cumulonimbus.
>> "X_area_fraction" means the fraction of horizontal area occupied by X.
>> Cloud area fraction is also called "cloud amount" and "cloud cover".
>> X_type_cloud_area_fraction is determined on the basis of cloud type and
>> not on the vertical location of the cloud.
>>
>> middle_type_cloud_area_fraction:
>> Middle type clouds are: Altostratus, Altocumulus, Nimbostratus.
>> "X_area_fraction" means the fraction of horizontal area occupied by X.
>> Cloud area fraction is also called "cloud amount" and "cloud cover".
>> X_type_cloud_area_fraction is determined on the basis of cloud type and
>> not on the vertical location of the cloud.
>>
>> high_type_cloud_area_fraction:
>> High type clouds are: Cirrus, Cirrostratus, Cirrocumulus.
>> "X_area_fraction" means the fraction of horizontal area occupied by X.
>> Cloud area fraction is also called "cloud amount" and "cloud cover".
>> X_type_cloud_area_fraction is determined on the basis of cloud type and
>> not on the vertical location of the cloud.
>>
>> In our local models, we have diagnostics labelled:
>> TOTAL CLOUD AMOUNT - RANDOM OVERLAP
>> LOW CLOUD AMOUNT
>> MEDIUM CLOUD AMOUNT
>> HIGH CLOUD AMOUNT
>>
>> The model calculates LOW CLOUD AMOUNT by finding the maximum amount of
>> cloud cover in a model level which exists within a range defined as
>> low. The model has no inclination about cloud types and makes no
>> evaluation of overlap for these diagnostics.
>>
>> The model calculates TOTAL CLOUD AMOUNT - RANDOM OVERLAP by evaluating
>> the cloud over all vertical levels and its spatial displacement.
>>
>> Base on this, it seems reasonable to use the CF standard name
>> cloud_area_fraction
>> for output fields but based on the descriptive text then I do not see
>> how we could ever be able to use
>> low_type_cloud_area_fraction
>> for data output from our model.
>>
>> I would be really interested to hear from people who use these standard
>> names regularly on the applicability to our case; specifically:
>>
>> 1. Are these definitions explicit, complete and accurate?
>>
>> 2. Should all uses of these standard names be sure that they are based
>> on typing the cloud correctly, not on the level of the cloud within the
>> model?
>>
>> 3. Do these high/medium/low cloud type standard names also assume that
>> spatially displaced levels are summed, or are they aiming to report the
>> maximum in any one level within the range?
>>
>> many thanks
>> mark
>>
>>
>> _______________________________________________
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>> CF-metadata at cgd.ucar.edu
>> http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
>>
>
--
Dr. Heiko Klein Tel. + 47 22 96 32 58
Development Section / IT Department Fax. + 47 22 69 63 55
Norwegian Meteorological Institute http://www.met.no
P.O. Box 43 Blindern 0313 Oslo NORWAY
Received on Thu Jun 12 2014 - 00:37:58 BST
