Dear Martin, Karl and Jonathan, I wrote most of this email late on Friday evening, before Martin and Karl's comments over the weekend. However, I think it is largely consistent with the later discussion. I have amended my comments on the evaporation (2.1, 2.2), evapotranspiration (2.3) and heat flux into snow pack (3) names to reflect the latest position.
Thanks all for the very quick and helpful responses. I think we are making good progress on these names. Jonathan, I'm afraid I haven't seen your comments from earlier last week - they don't seem to have appeared on the mailing list. These proposals have raised a number of important questions. I'll try to address the broad issues first before going through the names again in turn. 1. 'Canopy' versus 'Vegetation' In the context of discussing canopy evaporation/sublimation, Karl wrote the following: > Is there a precise definition of canopy?How does it differ from > "vegetation"?A definition I found says "The canopy refers to the highest > layer of > vegetation in a forest or woodland, made up of the crowns of its tallest > trees." I would be surprised if modelers are using the term in this sense. If > "canopy" is restricted to "trees" (or tallest trees), how are they defined > since some shrubs seem to have similar characteristics as trees.Why > would modelers want to isolate water evaporation from the canopy, rather than > considering water evaporation from all the vegetation?If > "canopy" includes all vegetation that "hides" the surface soil from above, > wouldn't all plants do that?If so, why not just say "vegetation" > (meaning surface vegetation) and avoid use of "canopy" in standard names. The AMS Glossary entry for 'canopy' says: 'The vegetative covering over a surface. The canopy is often considered to be the outer surfaces of the vegetation. Plant height and the distribution, orientation, and shape of plant leaves within a canopy influence the atmospheric environment and many plant processes within the canopy.' I think this is consistent with Karl's interpretation. Certainly I think the existing names don't mean only the tops of trees, rather 'canopy' has indeed been used as short hand for 'the parts of plants that protrude into the air and prevent light and precipitation from reaching the surface directly'. We use 'vegetation' in standard names such as carbon_mass_flux_into_soil_from_vegetation_excluding_litter where it can mean any part of the plant, including roots. 'Vegetation' is also used as an area type. Thus I think canopy does have a more specific meaning. I suggest that we replace the current not very helpful definition of canopy: '"Canopy" means the plant or vegetation canopy' with the one from the AMS Glossary, but that we retain it as a separate term. 2. Albedo Jonathan has pointed out that an albedo of a particular surface type such as 'ice' is not the same as an albedo assuming_condition because the former is an observable surface characteristic while the latter is purely a model construct. This is a good point - thank you. I agree that we should retain surface_albedo_assuming_deep_snow and surface_albedo_assuming_no_snow as separate quantities. However, I think we are agreed that we should have a single surface_albedo name and use area_types in the circumstances where the surface type truly is different and no additional conditions are being assumed. 3. Evaporation Karl made some important points about the definition of evaporation, evapotranspiration and sublimation. Clearly we need to remain consistent with previous usage of these terms - I have taken this into account in my comments on some of the individual names. > 1.1 albc Canopy Albedo > canopy_albedo (1) > "Canopy" means the plant or vegetation canopy. Albedo is the ratio of > outgoing to incoming shortwave irradiance. I think we are all agreed that canopy_albedo should be a separate standard name and certainly it is different from a surface albedo. Using the updated definition of canopy we would now have: 'Albedo is the ratio of outgoing to incoming shortwave irradiance. "Canopy" means the vegetative covering over a surface. The canopy is often considered to be the outer surfaces of the vegetation. Plant height and the distribution, orientation, and shape of plant leaves within a canopy influence the atmospheric environment and many plant processes within the canopy. Reference: AMS Glossary http://glossary.ametsoc.org/wiki/Canopy.' Okay? This name is still under discussion. > 1.2 albsn Snow and Ice Albedo [1] > Albedo of snow and ice covered surface. > snow_and_ice_albedo > Snow and ice albedo. > -- OR -- > This could be handled with existing name surface_albedo and a new area type > "snow_or_ice". [Note that "landice" appears to exclude ice on lakes, which > may be a significant component of this albedo] I think we are agreed now to use the existing surface_albedo name for this quantity. The definition would be amended to read as follows: 'The surface called "surface" means the lower boundary of the atmosphere. Albedo is the ratio of outgoing to incoming shortwave irradiance. To specify the nature of the surface a cell_methods attribute should be supplied as described in Chapter 7.3.3 of the CF Conventions.' Okay? In addition we would introduce two new area_types as suggested by Martin: > ice_on_land 'The area type ice_on_land means Ice in glaciers, ice caps, ice sheets & shelves, river and lake ice, and any other ice on a land surface, such as frozen flood water. This is distinct from the area type 'land ice' which has a narrower definition.' > The combined term could be ice_and_snow_on_land 'The area type ice_and_snow_on_land means ice in glaciers, ice caps, ice sheets & shelves, river and lake ice, any other ice on a land surface, such as frozen flood water, and snow lying on such ice or on the land surface.' Are these definitions okay? This name is still under discussion. > 2. Evaporation and transpiration fluxes [3] Variations on the theme of the > existing water_evaporation_flux term. > 2.1 ec water_evaporation_flux_from_canopy_due_to_interception (kg m-2 s-1) > ' "Water" means water in all phases. Evaporation is the conversion of liquid > or solid into vapor. (The conversion of solid alone into vapor is called > > "sublimation".) In accordance with common usage in geophysical disciplines, > "flux" implies per unit area, called "flux density" in physics. The > specification of a physical process by the phrase "due_to_" process means > that the quantity named is a single term in a sum of terms which > together compose the general quantity named by omitting the phrase. "Canopy" > means the plant or vegetation canopy. "Canopy interception" is > the precipitation, including snow, that is intercepted by the canopy of a > tree and then evaporates from the leaves.' Karl wrote: > a) This seems quite obscure."interception of what?"[snow fall, > yes?]Would water_evaporation_flux_from_canopy_snow work? > > b) How else does snow accumulate on the canopy except by falling as snow? > Naively, I would have thought that both rain and snow could be intercepted. Some of the rain will drip from the leaves onto the ground, while some will sit on the leaves as water droplets and eventually evaporate into the air. Certainly some snowfall would also be intercepted. > c) It appears that "evaporation" here must surely include sublimation > (as discussed in 1 above), which I think is good. Certainly the definition as given above does include sublimation. Karl makes the point that in CMIP5 > a) "water_evaporation_flux" has included "transpiration" and included > water_sublimation_flux from all parts of the surface (soil and vegetation) and that it is important we do not alter or confuse this interpretation in the current set of proposals. I understand the proposed quantity to mean the evaporation (including sublimation) of any precipitation that falls and comes to rest on the canopy. I think it doesn't include transpiration, nor does it include evaporation from the ground. We do have existing names canopy_water_amount and change_over_time_in_canopy_water_amount. Following this pattern, perhaps we could call the proposed quantity: tendency_of_canopy_water_amount_due_to_evaporation_of_intercepted_precipitation (kg m-2 s-1) 'The phrase "tendency_of_X" means derivative of X with respect to time. "Amount" means mass per unit area. "Water" means water in all phases. "Canopy" means the vegetative covering over a surface. The canopy is often considered to be the outer surfaces of the vegetation. Plant height and the distribution, orientation, and shape of plant leaves within a canopy influence the atmospheric environment and many plant processes within the canopy. Reference: AMS Glossary http://glossary.ametsoc.org/wiki/Canopy.' The specification of a physical process by the phrase "due_to_" process means that the quantity named is a single term in a sum of terms which together compose the general quantity named by omitting the phrase. "Intercepted precipitation" is the precipitation, including snow, that is intercepted by the vegetation canopy and then evaporates without ever reaching the surface or entering the plant through its roots. Evaporation is the conversion of liquid or solid into vapor. (The conversion of solid alone into vapor is called "sublimation".) Evaporation of intercepted precipitation excludes plant transpiration and evaporation from the surface beneath the canopy.' This avoids calling it a 'water_evaporation_flux' while (I hope) still accurately representing the quantity. Is this an acceptable compromise? This name is still under discussion. > 2.2 eow Open Water Evaporation [kg m-2 s-1] > > A new term "water_evaporation_flux_from_open_water" would work here, but it > might make more sense to define an area type for open water > and use the existing standard name "water_evaporation_flux". The definition of the existing water_evaporation_flux says: 'Water means water in all phases. Evaporation is the conversion of liquid or solid into vapor. (The conversion of solid alone into vapor is called "sublimation".) In accordance with common usage in geophysical disciplines, "flux" implies per unit area, called "flux density" in physics. Unless indicated in the cell_methods attribute, a quantity is assumed to apply to the whole area of each horizontal grid box. Previously, the qualifier where_type was used to specify that the quantity applies only to the part of the grid box of the named type. Names containing the where_type qualifier are deprecated and newly created data should use the cell_methods attribute to indicate the horizontal area to which the quantity applies.' Regarding "open water" Martin wrote: > The phrase "open water" appears to have different meanings for oceans and > freshwater. For freshwater it means areas that are free of > vegetation or other obstacles. I'm not sure it is really right for an area > type: when used in the context of swimming, "open water" means water > some distance from the shore or banks, but for evaporation it just means > evaporation from the water/air interface as opposed to evaporation of > water which is in contact with a solid boundary. We could, perhaps, use > "fresh_free_water_surface" (Areas where there is a free interface > between freshwater and the atmosphere without vegetation or other > obstructions). I think perhaps we could just say "fresh_free_water" and the definition would be 'An area type of fresh_free_water means a free interface between freshwater and the atmosphere without vegetation or other obstructions'. The standard name definition doesn't mention anything about transpiration and, for the open water quantity, no vegetation is involved. To avoid confusion with the total evaporation/evapotranspiration quantity, I suggest we introduce a new standard name of surface_water_evaporation_flux. It would be defined exactly as above, except for one additional sentence saying that transpiration is NOT included in this name. This name is still under discussion. > 2.3 et Total Evapotranspiration [kg m-2 s-1] > "Evapotranspiration" is a new concept to the CF standard names, but appears > to be clearly defined. > evapotranspiration_flux (kg m-2 s-1) > "Evapotranspiration refers to the flux of water into the atmosphere from a > combination of transpiration by plants and evaporation from soil and > other land surfaces." Karl wrote: > If you want to introduce "evapotranspiration_flux", you would have to > make "water_evaporation_flux" an alias. It would be o.k. to define > "transpiration_flux" as a new name identifying the portion of > "water_evaporation" (i.e., "evapotranspiration") that is due to > transpiration.I think it would be a mistake to change the meaning of > water_evaporation_flux so that it excludes the portion due to > transpiration.water_evaporation_flux has been in use too long with the > original meaning."evapotranspiration" is *not* a new concept in CF, it > is a suggested new name for "evaporation".My vote would be *not* to > replace "evaporation" with "evapotranspiration"; the atmosphere doesn't > care that a portion of the evaporating water traveled through the plant > roots and exited through stomata in the leaves; it still ends up in the > atmosphere as vapor. In view of this, Martin has suggested that we should make the existing water_evaporation_flux name into an alias of water_evapotranspiration_flux and adding to the definition. I agree this would be a very good step. So then we would have: water_evapotranspiration_flux 'Water means water in all phases. "Evapotranspiration" means all water vapor fluxes from the surface: liquid evaporation, sublimation and transpiration. Evaporation is the conversion of liquid or solid into vapor. (The conversion of solid alone into vapor is called "sublimation".) Transpiration is the process by which water is carried from the roots of plants and evaporates from the stomata. In accordance with common usage in geophysical disciplines, "flux" implies per unit area, called "flux density" in physics. Unless indicated in the cell_methods attribute, a quantity is assumed to apply to the whole area of each horizontal grid box.' Keeping the reference to cell_methods in the definition is useful because it potentially allows the use of area type to describe evapotranspiration from different types of vegetation in a single grid box, even if there is no requirement to do that at the moment. I have also added a definition for 'transpiration' (which we haven't had before). Taking this approach of having separate surface evaporation and evapotranspiration fluxes in proposals 2.2 and 2.3 seems to me the tidiest and least confusing way of accommodating the CMIP6 quantities while not introducing inconsistencies with CMIP5. I note that this does raise an interesting wrinkle as regards aliases: currently water_evaporation_flux has an alias of water_evaporation_flux_where_sea_ice which clearly would not include vegetated areas and is therefore more akin to the surface_water_evaporation_flux name I am now suggesting for 2.2. I think it makes sense to list water_evaporation_flux_where_sea_ice as an alias of surface_water_evaporation_flux, rather than water_evapotranspiration_flux. We have never 'transferred' an alias in this way before, but I think it is the best approach to preserving the meaning of all the terms, past and present. Do others agree? This name is still under discussion. > 3. Heat fluxes [2] > 3.1 hfrs Heat transferred to snowpack by rainfall [W m-2] > A variation of > "temperature_flux_due_to_rainfall_expressed_as_heat_flux_into_sea_water". > > temperature_flux_due_to_rainfall_expressed_as_heat_flux_into_snow_and_ice (W > m-2) Karl wrote: > I must have missed the discussion about this, but does > temperature_flux_due_to_rainfall_expressed_as_heat_flux_into_sea_water > make sense? Our use of flux as a short-hand for "flux density" (as > formally used in physics)wouldn't seem to apply to "temperature", which > is a property of a material not something that can flow from one place > to another.Does it mean > "heat_flux_into_sea_water_due_to_rainfall_added_at_a_different_temperature"?Or > > is it the "tendancy_of_sea_water_temperature_due_to_rainfall"? Or > something else? This name was introduced some years ago (possibly for CMIP5, but I can't quite remember). I do recall it being discussed for quite a while on the list before being agreed. I'm not keen on having 'temperature_fluxes' either! It is defined as: 'The quantity with standard name temperature_flux_due_to_rainfall_expressed_as_heat_flux_into_sea_water is the heat energy carried by rainfall entering the sea at the sea surface. It is calculated relative to the heat that would be carried by rainfall entering the sea at zero degrees Celsius. It is calculated as the product QrainCpTrain, where Qrain is the mass flux of rainfall entering the sea (kg m-2 s-1), Cp is the specific heat capacity of water and Train is the temperature in degrees Celsius of the rain water entering the sea surface. In accordance with common usage in geophysical disciplines, "flux" implies per unit area, called "flux density" in physics. The specification of a physical process by the phrase due_to_process means that the quantity named is a single term in a sum of terms which together compose the general quantity named by omitting the phrase.' So it is a heat flux, but a relative one. It has been suggested that we might call this one: heat_flux_into_snow_and_ice_on_land_due_to_rainfall Martin wrote: I believe that the heat flux here depends on the temperature of the target medium (or, rather, the difference in temperature between the target and source) So adapting the definition of the existing name would give us something like: 'The quantity with standard name heat_flux_into_snow_and_ice_due_to_rainfall is the heat energy carried by rain falling onto the snow and ice. It is calculated as the product QrainCpTdiff, where Qrain is the mass flux of rainfall reaching the snow and ice surface, Cp is the specific heat capacity of water and Tdiff is the difference in temperature of the rain water and the frozen surface. In accordance with common usage in geophysical disciplines, "flux" implies per unit area, called "flux density" in physics. The specification of a physical process by the phrase due_to_process means that the quantity named is a single term in a sum of terms which together compose the general quantity named by omitting the phrase.' However, I see that none of us is currently very clear about what this quantity really means and Martin is seeking further clarification. I will await the outcome of that before proceeding any further with this name. This name is still under discussion. > 3.2 hfsbl Energy of sublimation [W m-2] > Variation of: surface_upward_latent_heat_flux The definitions of latent heat > flux state that the latent heat flux includes sublimation heat flux, so it > makes sense to use the same pattern: > surface_upward_sublimation_heat_flux (W m-2) 'The surface called "surface" means the lower boundary of the atmosphere. "Upward" indicates a vector component which is positive when directed upward (negative downward). Sublimation is the conversion of solid into vapor. In accordance with common usage in geophysical disciplines, "flux" implies per unit area, called "flux density" in physics.' Karl wrote: > I've never seen the term "sublimation heat flux" used, although I > guess its meaning is clear enough.Does anyone know of a different term > meaning "heat flux from the surface to the atmosphere due to sublimation > of surface ice, frost, and snow? Martin has now suggested we call this one surface_upward_latent_heat_flux_due_to_sublimation. I think that is fine. I had already accepted this name so I will amend it. It will still be included in the June update. > 4. Nudging increments [2] (4.1 nudging_increment_in_mass_content_of_water_in_soil (kg m-2) has already been accepted). > 4.2 nudgincswe Nudging Increment of Water in Snow [kg m-2] > nudging_increment_in_surface_snow_and_ice_amount The definition would be as follows: 'A "nudging increment" refers to an amount added to parts of a model system. The phrase "nudging_increment_in_X" refers to an increment in quantity X over a time period which should be defined in the bounds of the time coordinate. The surface called "surface" means the lower boundary of the atmosphere. "Amount" means mass per unit area.' I asked about the areas of snow and ice and Martin suggested: Can we use "nudging_increment_in_snow_and_ice_on_land_amount", following my suggestion for the area type above? I think that's good. The area type was 'ice_and_snow_on_land' so we should have them in the same order in the standard name. Also, I think ice_and_snow_amount_on_land is easier to understand, so we'd have: nudging_increment_in_ice_and_snow_amount_on_land (kg m-2) ''A "nudging increment" refers to an amount added to parts of a model system. The phrase "nudging_increment_in_X" refers to an increment in quantity X over a time period which should be defined in the bounds of the time coordinate. The surface called "surface" means the lower boundary of the atmosphere. "Amount" means mass per unit area. "Ice and snow on land" means ice in glaciers, ice caps, ice sheets & shelves, river and lake ice, any other ice on a land surface, such as frozen flood water, and snow lying on such ice or on the land surface.' This name is accepted for publication in the standard name table and will be added in the June update. > 5. River in- and out-flow [2] > water_volume_transport_in_river_channel and > water_volume_transport_into_sea_water_from_rivers exist. The new variables > represent cell averages of river > fluxes directed inwards and outwards respectively. > 5.1 rivi River Inflow Water flux from upstream [m3 s-1] > river_water_volume_transport_into_cell > '"Cell" refers to a model grid-cell. "River water" refers to the water > (liquid and solid) in the fluvial system (stream and floodplain).' ' "Cell" refers to a model grid-cell. The extent of an individual grid cell is defined by the horizontal coordinates and any associated coordinate bounds or by a string valued auxiliary coordinate variable with a standard name of "region". "Water" means water in all phases. "River" refers to water in the fluvial system (stream and floodplain).' Martin has confirmed that this name does include water in all phases, which was the only outstanding question. This name is accepted for publication in the standard name table and will be added in the June update. > 5.2 rivo River Discharge [m3 s-1] > river_water_volume_transport_out_of_cell ' "Cell" refers to a model grid-cell. The extent of an individual grid cell is defined by the horizontal coordinates and any associated coordinate bounds or by a string valued auxiliary coordinate variable with a standard name of "region". "Water" means water in all phases. "River" refers to water in the fluvial system (stream and floodplain).' Martin has confirmed that this name does include water in all phases, which was the only outstanding question. This name is accepted for publication in the standard name table and will be added in the June update. > 6. Roots [1] > > > 6.1 rzwc Root zone soil moisture [kg m-2] > > This is a variation on mass_content_of_water_in_soil_layer, but with a > > layer defined by the presence of roots rather than a coordinate range (cf. > > "stratosphere" in atmosphere). > > mass_content_of_water_in_soil_layer_defined_by_root_depth > ' "Content" indicates a quantity per unit area. The content of a soil layer > is the vertical integral of the specified quantity within the layer. The > quantity with > standard name mass_content_of_water_in_soil_layer_defined_by_root_depth is > the vertical integral between the surface and the depth to which plant roots > penetrate. A coordinate variable or scalar coordinate variable with standard > name root_depth can be used to specify the extent of the layer. "Water" means > water in all phases.' Martin has agreed this approach. This name is accepted for publication in the standard name table and will be added in the June update. > 7. Water fluxes [5] > 7.1 qgwr Groundwater recharge from soil layer [kg m-1 s-1] Currently this name is listed as follows: liquid_water_mass_flux_from_soil_to_groundwater (kg m-2 s-1) 'In accordance with common usage in geophysical disciplines, "flux" implies per unit area, called "flux density" in physics. Groundwater is subsurface water below the depth of the water table, including soil moisture and underground aquifers.' We are still awaiting clarification of the definition of groundwater and we will need to wait for the outcome of that before we can finalise this name. This name is still under discussion. > > 7.2 sblnosn Sublimation of the snow free area [kg m-2 s-1] > > > > This is a variation of the CMIP5 variable "sbl" which used the existing > > term surface_snow_and_ice_sublimation_flux. Here, we just drop "and_ice" > > > to get a > > term referring to the ice only. > > surface_ice_sublimation_flux Alison wrote: > Looking at existing names I see we also have surface_snow_sublimation_amount. > Rather than talking about sublimation amount in one name and sublimation > flux in others I think it would make sense to take a more uniform approach. I > think the primary purpose of the existing > surface_snow_and_ice_sublimation_flux > name is to describe changes at the surface, rather than a water vapour flux > into the atmosphere. Hence I suggest introducing aliases: > surface_snow_sublimation_amount -> > tendency_of_surface_snow_amount_due_to_sublimation > surface_snow_and_ice_sublimation_flux -> > tendency_of_surface_snow_and_ice_amount_due_to_sublimation > and calling the proposed quantity: > tendency_of_ice_amount_due_to_sublimation. > None of these names specify that the snow and ice are located on land, so as > with the albedo and evaporation names we probably need to use them in > > conjunction with an area type. Do you agree with this approach? Martin wrote: > Yes, this is good. Okay, thank you, these changes are agreed. I will create the aliases and add the new name as follows: tendency_of_surface_ice_amount_due_to_sublimation (kg m-2 s-1) 'The phrase "tendency_of_X" means derivative of X with respect to time. "Amount" means mass per unit area. The specification of a physical process by the phrase "due_to_" process means that the quantity named is a single term in a sum of terms which together compose the general quantity named by omitting the phrase. Sublimation is the conversion of solid into vapor. Unless indicated in the cell_methods attribute, a quantity is assumed to apply to the whole area of each horizontal grid box.' This name is accepted for publication in the standard name table and will be included in the June update. > > 7.3 snmsl Water flowing out of snowpack [kg m-2 s-1] > > This is a variation of snm [surface_snow_melt_flux], but considering only > > the component of the flux into soil: > > > > surface_snow_melt_flux_into_soil Alison wrote: > For this name I suggest a different form: > liquid_water_mass_flux_into_soil_due_to_surface_snow_melt (kg m-2 s-1) > 'In accordance with common usage in geophysical disciplines, "flux" implies > per unit area, called "flux density" in physics. The specification of a > physical process > by the phrase "due_to_" process means that the quantity > named is a single term in a sum of terms which together compose > the general quantity named by omitting the phrase. The phrase "surface_snow" > means snow lying on the surface.' Martin wrote: > Yes, this looks good to me. Longer, but the additional clarity justifies this. Okay, thank you. This name is accepted for publication in the standard name table and will be added in the June update. (7.4 canopy_snow_amount has already been accepted. > 7.5 sw Surface Water Storage (excluding snow) [kg m-2] > The existing term surface_water_amount refers to ".. the amount on the > ground, excluding that on the plant or vegetation canopy", and > "water" is assumed to refer to all phases. There is a term > "land_based_water_amount" under discussion for PMIP which refers to "This > quantity is often known as Terrestrial Water Storage. It includes surface > water (water in rivers, wetlands, lakes, snow, vegetation and > reservoirs) and subsurface water (soil moisture, groundwater)". > > Assuming that this term is intended to be a variation on the first, > surface_water_amount, and the amount includes only liquid phase water > (excluding snow and ice): > surface_liquid_water_amount This name is currently listed as: land_surface_liquid_water_amount (kg m-2) 'The surface called "surface" means the lower boundary of the atmosphere. "Amount" means mass per unit area. The quantity with standard name land_surface_liquid_water_amount includes water in rivers, wetlands, lakes, snow, vegetation and reservoirs.' We are still awaiting clarification of the definition of land water and we will need to wait for the outcome of that before we can finalise this name. This name is still under discussion. Best wishes, Alison ------ Alison Pamment Tel: +44 1235 778065 NCAS/Centre for Environmental Data Archival Email: [email protected] STFC Rutherford Appleton Laboratory R25, 2.22 Harwell Oxford, Didcot, OX11 0QX, U.K. _______________________________________________ CF-metadata mailing list [email protected] http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
