Standard names: Propose new additions to the standard names table (MODIS output from COSP simulator)

[brandonduran]

CF-Convention Discussion Proposal: Proposer's name: Brandon Duran Date: 25-6-2024

I would like to propose standard names for MODIS output from the COSP satellite simulator package, including new joint histograms of cloud droplet effective radius (CER) and cloud water path (CWP). These diagnostics are similar to those from the ISCCP satellite simulator, but feature distinction by cloud thermodynamic phase. All of the proposed names below are for joint histograms summarizing the co-variability of different cloud properties (cloud top pressure, cloud optical depth, cloud water path, cloud droplet effective radius).

Naming is guided by conventions for the ISCCP satellite simulator; specifically, clisccp, which is a 7x7 (cloud top pressure x optical depth) matrix. As such, I propose that all MODIS cloud top pressure by optical depth histograms follow this naming, with the base of clmodis and any additional modifiers. To distinguish the new CER-CWP histograms, I propose the modifier ‘cwpr’, such that the base 6x7 (CER x CWP) joint histogram is named _clmodiscwpr.

The MODIS optical depth (tau) bounds are as follows: 0-0.3, 0.3-1.3, 1.3-3.6, 3.6-9.4, 9.4-23, 23-60, >60. The MODIS cloud top pressure (CTP) bounds are as follows [hPa]: 800 and higher, 800-680, 680-560, 560-440, 440-310, 310-180, 180-0. CTP and tau bounds match bounds from the ISCCP clisccp output.

The MODIS cloud liquid water path (LWP) bounds are as follows [g/m2]: 0-10, 10-30, 30-60, 60-100, 100-150, 150-250, >250. The MODIS cloud ice water path (IWP) bounds are as follows [g/m2]: 0-20, 20-50, 50-100, 100-200, 200-400, 400-1000, >1000. The MODIS liquid cloud droplet effective radius (CER) bounds are as follows [𝜇m]: 4-8, 8-10, 10-12.5, 12.5-15, 15-20, >20. The MODIS ice cloud ice-crystal effective radius bounds are as follows [𝜇m]: 5-10, 10-20, 20-30, 30-40, 40-50, >50.

The proposed name for the joint histogram diagnostics are: Term Long Name Units

1. clmodis _modis_cloud_areafraction 1 The MODIS cloud area fraction is diagnosed from atmosphere model output by the MODIS simulator software in such a way as to be comparable with the observational diagnostics of MODIS (Moderate Resolution Imaging Spectroradiometer). Cloud area fraction is also called “cloud amount” and “cloud cover.” As seen from above, mean fraction of grid column occupied by cloud of optical depths and heights specified by the tau and pressure intervals given above. Dimensions of the histogram are cloud top pressure and cloud optical depth (7x7).

2. clmodis_liquid _modis_cloud_area_fractionliquid 1 Liquid means liquid-topped clouds, as seen by the MODIS simulator. Dimensions of the histogram are cloud top pressure and cloud optical depth (7x7).

3. clmodis_ice _modis_cloud_area_fractionice 1 Ice means ice-topped clouds, as seen by the MODIS simulator. Dimensions of the histogram are cloud top pressure and cloud optical depth (7x7).

4. clmodis_cwpr_liquid _modis_cloud_area_fraction_cloud_water_path_effective_radiusliquid 1 Liquid means liquid-topped clouds, as seen by the MODIS simulator. Dimensions of the histogram are cloud liquid water path and cloud droplet effective radius (7x6).

5. clmodis_cwpr_ice _modis_cloud_area_fraction_cloud_water_path_effective_radiusice 1 Ice means ice-topped clouds, as seen by the MODIS simulator. Dimensions of the histogram are cloud ice water path and cloud ice-crystal effective radius (7x6).

Thank you!

[github-actions[bot]]

Thank you for your proposal. These terms will be added to the cfeditor (http://cfeditor.ceda.ac.uk/proposals/1) shortly. Your proposal will then be reviewed and commented on by the community and Standard Names moderator.

[efisher008]

Dear Brandon,

Thank you for your proposal. I have now added the names to the CF editor. Thanks for your patience as the editor had been experiencing some technical issues, which are now resolved. I have used the long names as the "interim" names (as abbreviations are not generally accepted in names aside from where the usage is well-defined) , but there will need to be some discussion on the format of these, in particular the longer names to ensure they are comprehensible and consistent with existing names.

You can view the entries here:

1. modis_cloud_area_fraction - https://cfeditor.ceda.ac.uk/proposal/5355/edit
2. modis_cloud_area_fraction_liquid - https://cfeditor.ceda.ac.uk/proposal/5357/edit
3. modis_cloud_area_fraction_ice - https://cfeditor.ceda.ac.uk/proposal/5354/edit
4. modis_cloud_area_fraction_cloud_water_path_effective_radius_liquid - https://cfeditor.ceda.ac.uk/proposal/5356/edit
5. modis_cloud_area_fraction_cloud_water_path_effective_radius_ice - https://cfeditor.ceda.ac.uk/proposal/5353/edit

Should the matrix dimensions (i.e. numbers in brackets at the end of the description) be included in the editor entries?

Best regards, Ellie

[brandonduran]

Hi Ellie,

No worries about the delay. I think it is fine to omit the dimensions. Although the MODIS output differs from ISCCP output in that not all histograms share the same (7x7) shape, this would simply be reflected in the output of these proposed variables and probably does not need to be included in the editor entries.

Thanks! -Brandon

[taylor13]

Without a careful analysis, it would be nice to make the names a little easier for humans to parse. For example, can somehow "liquid cloud top" be worked into the name? (I must confess I can't come up with anything that works.)

[brandonduran]

Perhaps the names could be modified as such, although they do become a mouthful:

1. modis_cloud_area_fraction_liquid --> modis_cloud_area_fraction_liquid_topped
2. modis_cloud_area_fraction_ice --> modis_cloud_area_fraction_ice_topped
3. modis_cloud_area_fraction_cloud_water_path_effective_radius_liquid --> modis_cloud_area_fraction_cloud_water_path_effective_radius_liquid_topped
4. modis_cloud_area_fraction_cloud_water_path_effective_radius_ice --> modis_cloud_area_fraction_cloud_water_path_effective_radius_ice_topped

Another variation of this convention could be:

1. modis_cloud_area_fraction_liquid --> modis_liquid_topped_cloud_area_fraction etc., which retains the cloud area fraction expression, but rearranges the position of 'ice' and 'liquid' to more directly specify that we are referring to liquid- and ice-topped clouds separately.

I would strongly suggest leaving modis_cloud_area_fraction unchanged, as this diagnostic is not partitioned by cloud-top phase. At most, it could be modified to modis_total_cloud_area_fraction

[taylor13]

I liked your suggestions and agree with your last remark. Let's see what others think.

[efisher008]

Hi @brandonduran and @taylor13,

I agree with the variation stated in @brandonduran's post: having the liquid/ice-topped component earlier in the term e.g. modis_liquid_topped_cloud_area_fraction seems sensible as it is more understandable that we are distinguishing between liquid- and ice-topped clouds with the name variations. There are currently no names in the CF standard names table with modis as a component, so this will be an opportunity to establish a new precedent for these sort of variables going forward.

As a compromise which does not break up the modis_cloud_area_fraction string but still has a more intelligible order, how does the format {ice_topped/liquid_topped}_cloud_area_fraction{_etc.} sound?

Best, Ellie

[JonathanGregory]

Dear @brandonduran

Thanks for working on this.

If these quantities are histograms, the standard name should be histogram_of_X, where X is the variable that has been histogrammed. This pattern is in the guidelines, and there are a couple of existing standard names which use it. The reason for this is that a histogram of X isn't the same geophysical quantity as X itself. The histogram is a dimensionless (in the sense of being a pure number) number of counts in the bin, whereas X could have any dimension (i.e. any canonical unit) e.g. a histogram of temperature has units of 1, not K. Area fraction is dimensionless anyway, so the unit is not affected in your case, but a histogram of cloud area as a function of cloud-top pressure and cloud optional depth is not the same quantity as cloud area as a function of the same two variables. The former is a count, which could be zero or any positive integer, while the latter is a floating-point number between 0 and 1.

If these are all histograms, I suppose that (1) is histogram_of_cloud_area_fraction, (2) and (4) are histogram_of_liquid_water_topped_cloud_area_fraction, and (3) and (5) histogram_of_ice_topped_cloud_area_fraction. (Other standard names use the liquid_water, not just liquid, to describe clouds.) I'm mot sure I've understood this correctly, but it looks like (2) and (4) are distinguished only by the dimensions of the histogram (array dimensions, that is, not the same sense of "dimension" as above), likewise (3) and (5). Those pairs can each have the same standard name, because the physical quantity is the same. That is perfectly fine for CF, since the coordinate variables are also metadata.

Alternatively, the standard names could say what the dimensions are. The guideline allows for that as well with _over_Y; these are two-dimensional histograms, so they'd be _over_Y_and_Z, I suppose. If these were probability density functions, rather than histograms, it would be necessary to identify the dimensions, because they affect the unit. For example, the units of probability density of cloud area fraction as a function of liquid water path and effective radius are (kg m-2)-1 m-1 = kg-1 m.

Best wishes

Jonathan

[brandonduran]

Hi @JonathanGregory , thanks for your very thorough review. I will attempt to clarify in what follows.

These variables can all be thought of variations of the CMIP6 clisccp variable, or what is often called FISCCP1_COSP. As such, their units are really percentages, such that summing up the histogram over all bins yields the total grid-box cloud fraction (as a percentage). In this sense, these are not true 'histograms' and are more accurately described as _cloud area percentages as a function of different cloud _properties;__ I simply have adopted the traditional nomenclature of referring to these as ISCCP or MODIS joint histograms. This artifact is a result of the observational version of these quantities, which indeed are pixel counts of clouds, representing a true histogram. Your clarification is important though, as the method in which these diagnostics have currently been implemented in the COSP code package results in them being reported in percentages, not fractions. Therefore, I will modify all that follows to represent these as cloud area percentages %, rather than cloud area fractions 1. I'm not sure if this is a necessary change, as my understanding is that isccp_cloud_area_fraction is still reported in units of %, rather than 1, so please advise regarding this. Apologies for the confusion and mistake on my end.

In light of this, it seems like the histogram_of_ prefix would not be an accurate description of these variables. Your second point, however, is correct. (2) and (4), and (3) and (5) are only distinguished by the array dimensions of the 'histogram.' They represent the same physical quantity, but differ in how they are partitioned (cloud-top pressure x optical depth, cloud water path x effective radius).

Incorporating your thoughts and @efisher008 suggestions:

1. modis_cloud_area_percentage or modis_total_cloud_area_percentage
2. modis_ice_topped_cloud_area_percentage , coordinate variables: (cloud-top pressure x optical depth)
3. modis_liquid_topped_cloud_area_percentage, coordinate variables: (cloud-top pressure x optical depth)
4. modis_ice_topped_cloud_area_percentage, coordinate variables: (cloud water path x effective radius)
5. modis_liquid_topped_cloud_area_percentage, coordinate variables: (cloud water path x effective radius)

Note that now (2) and (4), and (3) and (5) share the same standard name due to their representation of the same physical quantity. For CF, they would be distinguished by their differing coordinate variables in the metadata. I think that we should retain modis at the front of the standard names, following the standard for other COSP output, where the name of the simulator precedes the geophysical quantity (ie, isccp_cloud_area_fraction). I am not averse to rearranging, though, given that this is the first instance of the modis qualifier .

Thanks for the discussion and happy to clarify the above further.