Dear Alison,
thank you for another excellent analysis.
1.14 Proposed as tendency_of_surface_snow_amount_due_to_drifting [kg m-2 s-1]
I have a comment on this term based on my memories of an exchange with Dirk a year or two ago: if I remember correctly, this term relates to the tendency of snow amount due to snow drifting off the sea ice and into the sea within a grid cell. As it is written it could also include horizontal transport between grid cells, but that is a slightly different process. Before Dirk drew this to my attention, I had assumed that it was mainly associated with transport between cells. It would, I think, be useful to have some reference to the importance of drifting into the sea. This also makes the concept for sea ice rather different from wind driven snow transport on land: on land snow might blow into a warmer area and melt, but the melting would not be considered part of the tendency due to drifting. Here, because the snow is considered to melt as soon as it drifts into the sea, there is no separation between the two. I suggest:
tendency_of_surface_snow_amount_due_to_drifting_into_sea
Mass fluxes and transports.
I note that there are also 15 terms referring to "water_flux" and one using "liquid_water_mass_flux". We also 12 "precipitation_flux" terms which are mass fluxes (and no precipitation_mass_flux terms). I would hold off on this change for now, and try to look at the issue in more detail. It is a good idea to rationalise the usage here, but I'd hesitate to make so many changes without more time for consideration.
regards,
Martin
________________________________
From: Pamment, Alison (STFC,RAL,RALSP)
Sent: 14 June 2018 18:38
To: CF-metadata (cf-metadata at cgd.ucar.edu); Juckes, Martin (STFC,RAL,RALSP)
Subject: RE: [CF-metadata] SIMIP: 5 standard names and one area type for CMIP6
Dear Martin, All,
Thank you, Martin, for proposing the new standard names and area type for SIMIP.
Firstly as regards the area_type for ridged sea ice, this was added to the area type table in Version 7 as sea_ice_ridges.
Regarding the standard name proposals, there are actually two sets to consider. Dirk Notz proposed 29 names in 2016 (see
http://mailman.cgd.ucar.edu/pipermail/cf-metadata/2016/058889.html). These were subsequently discussed, and largely agreed. However, I had not finished processing them through the standard names editor, for which, my apologies. I have now reviewed the complete list of Dirk's proposals and the outcome of each is listed below, numbered 1.1 - 1.29. The only points remaining to be resolved are regarding some aliases arising from 1.28 and the units of 1.29. I hope we can agree these remaining points very quickly.
My comments on Martin's much more recent proposals are listed in points 2.1 - 2.3c.
On a general point, I note that in the area type table we define sea_ice as follows:
'The area occupied by type "sea_ice" within a grid cell is the sum of the areas of types "sea_ice_melt_pond" and "melt_pond_free_sea_ice". Melt ponds occur on top of the existing sea ice. The area occupied by type "sea" is equal to the sum of the areas of types "ice_free_sea" and "sea_ice".'
This is useful for explaining the relationship between a number of different area types but does not fully define which ice areas are included in sea ice. None of the existing sea ice standard names define this either. We have just had a careful discussion of land_ice and ice_and_snow_on_land regarding which ice areas, e.g. glaciers and lake_ice, are included and excluded in each case. It would be good to have a similar definition of sea_ice, therefore I am suggesting the following for addition to all sea_ice standard names:
' "Sea_ice" means all ice floating on the sea with the exception of floating ice shelves, which are regarded as land ice in models.' Is this an accurate description? As you will see, I have already added this definition to names that I have accepted. However, I can adjust the definitions before publishing the names if we need to make any changes to the suggested text.
1.1 proportion_of_model_time_steps_with_sea_ice_fraction_above_threshold
Dirk and Jonathan agreed (
http://mailman.cgd.ucar.edu/pipermail/cf-metadata/2016/058902.html) that this one should be:
fraction_of_time_with_sea_ice_area_fraction_above_threshold (1)
' "X_area_fraction" means the fraction of horizontal area occupied by X. Sea ice area fraction is area of the sea surface occupied by sea ice. The area threshold value must be specified by supplying a coordinate variable or scalar coordinate variable with the standard name of sea_ice_area_fraction. "Sea_ice" means all ice floating on the sea with the exception of floating ice shelves, which are regarded as land ice in models.'
This name is accepted for publication in the standard name table and will be added in the July 2nd update.
1.2 sea_ice_mass_content [kg m-2]
describes the total mass of sea ice divided by grid-cell area.
Dirk and Jonathan agreed that this quantity could be represented by the existing name sea_ice_amount.
1.3 sea_ice_basal_temperature (K)
' "Sea_ice" means all ice floating on the sea with the exception of floating ice shelves, which are regarded as land ice in models. The standard name sea_ice_basal_temperature means the temperature of the land ice at its lower boundary.'
This proposal is directly analogous to the existing name land_ice_basal_temperature, therefore it is a straight forward addition to the standard name table. This name is accepted and will be published in the July 2nd update.
1.4 Proposed as sea_ice_salt_content [kg m-2]
'the total mass of all salt in sea ice divided by grid cell area'
By analogy with soil mass content names, I suggest this one should be:
sea_ice_mass_content_of_salt (kg m-2)
' "Sea_ice" means all ice floating on the sea with the exception of floating ice shelves, which are regarded as land ice in models. "Content" indicates a quantity per unit area. The "sea_ice content" of a quantity refers to the vertical integral from the surface down to the bottom of the sea ice.'
I think this one is a straight forward addition to the standard name table, so it is accepted and will be published in the July 2nd update.
1.5 sea_ice_melt_pond_thickness [m]
' "Thickness" means the vertical extent of a layer. The quantity with standard name sea_ice_melt_pond_thickness is the thickness of the melt pond occurring on top of existing sea ice. The cell_methods attribute should be used to indicate whether the data variable represents a local thickness, an area-mean thickness or some other statistic. "Sea_ice" means all ice floating on the sea with the exception of floating ice shelves, which are regarded as land ice in models.'
This one was agreed in discussion between Dirk and Jonathan:
http://mailman.cgd.ucar.edu/pipermail/cf-metadata/2016/058901.html. This name is accepted for publication in the standard name table and will be added in the July 2nd update.
1.6 Proposed as thickness_of_sea_ice_melt_pond_refrozen_ice [m]
the volume of refrozen ice in meltponds divided by meltpond covered area
Dirk, Jonathan and Martin agreed this one:
thickness_of_ice_on_sea_ice_melt_pond (m)
' "Thickness" means the vertical extent of a layer. Melt ponds occur on top of the existing sea ice. The water in melt ponds can refreeze at the surface, giving rise to a layer of ice on the melt pond, which is turn resting on the sea_ice below. "Sea_ice" means all ice floating on the sea with the exception of floating ice shelves, which are regarded as land ice in models.'
This name is accepted for publication in the standard name table and will be added in the 2nd July update. N.B. It was also agreed to add sea_ice_melt_pond as an area type - this has already been done.
1.7 ridged_sea_ice_thickness [m]
total volume of ridged sea ice divided by area of ridges
It was agreed that this quantity could be represented by the existing name sea_ice_thickness and the introduction of a new area_type of sea_ice_ridges. This area type has since been added to the area type table.
1.8 tendency_of_sea_ice_amount_due_to_thermodynamics [kg m-2 s-1]
total change in sea-ice mass from thermodynamic processes divided by
grid-cell area
For consistency with existing names I suggest this one should be:
tendency_of_sea_ice_amount_due_to_sea_ice_thermodynamics (kg m-2 s-1)
'The phrase "tendency_of_X" means derivative of X with respect to time. "Amount" means mass per unit area. "Sea_ice" means all ice floating on the sea with the exception of floating ice shelves, which are regarded as land ice in models. 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. "Sea ice thermodynamics" refers to the addition or subtraction of mass due to surface and basal fluxes, i.e., due to melting, sublimation and fusion.'
This is similar to other sea ice tendencies. The phrase 'due_to_sea_ice_thermodynamics' is used in existing names. This name is accepted for publication in the standard name table and will be added in the July 2nd update.
1.9 Proposed as tendency_of_sea_ice_amount_due_to_dynamics [kg m-2 s-1]
total change in sea-ice mass from dynamic processes divided by grid-cell
area
For consistency with the thermodynamics name (1.8) and existing names I suggest this one should be:
tendency_of_sea_ice_amount_due_to_sea_ice_dynamics (kg m-2 s-1)
'The phrase "tendency_of_X" means derivative of X with respect to time. "Amount" means mass per unit area. "Sea_ice" means all ice floating on the sea with the exception of floating ice shelves, which are regarded as land ice in models. 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. "Sea ice dynamics" refers to advection of sea ice.'
This is similar to other sea ice tendencies. The phrase 'due_to_sea_ice_dynamics' is used in existing names. This name is accepted for publication in the standard name table and will be added in the July 2nd update.
1.10 Proposed as tendency_of_sea_ice_amount_due_to_lateral_melting [kg m-2 s-1]
total change in sea-ice mass from lateral melting divided by grid-cell area
tendency_of_sea_ice_amount_due_to_lateral_melting (kg m-2 s-2)
'The phrase "tendency_of_X" means derivative of X with respect to time. "Amount" means mass per unit area. "Sea_ice" means all ice floating on the sea with the exception of floating ice shelves, which are regarded as land ice in models. 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.'
This name did not receive any comments during the original discussion. We have an existing name tendency_of_sea_ice_amount_due_to_lateral_growth_of_ice_floes. Since 'lateral_growth' is an established phrase I don't see a problem with introducing 'lateral_melting' and the rest of the name follows existing patterns. This name is accepted for publication in the standard name table and will be added in the July 2nd update.
1.11 Proposed as surface_snow_sublimation_flux [kg m-2 s-1]
the rate of change of snow mass through sublimation and evaporation
divided by grid-cell area
Dirk and Jonathan agreed that this quantity could be represented by the existing name surface_snow_and_ice_sublimation_flux. (N.B. This is now an alias of tendency_of_surface_snow_and_ice_amount_due_to_sublimation).
1.12 Proposed as tendency_of_surface_snow_amount_due_to_sea_ice_dynamics [kg m-2 s-1]
the rate of change of snow mass through advection with sea ice divided
by grid-cell area
The name and units look fine. I have added a definition as follows:
tendency_of_surface_snow_amount_due_to_sea_ice_dynamics (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 phrase "surface_snow" means snow lying on the surface. The quantity with standard name tendency_of_surface_snow_amount_due_to_sea_ice_dynamics is the rate of change of snow amount caused by advection of the sea ice upon which the snow is lying. 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. "Sea_ice" means all ice floating on the sea with the exception of floating ice shelves, which are regarded as land ice in models. "Sea ice dynamics" refers to advection of sea ice.
This name is based on existing syntax: 'surface_snow' and 'due_to_sea_ice_dynamics' are used in existing names. This name is accepted for publication in the standard name table and will be added in the 2nd of July update.
1.13 Proposed as tendency_of_surface_snow_amount_due_to_ice_conversion [kg m-2 s-1]
the rate of change of snow mass due to transformation of snow to sea ice
divided by grid-cell area
Dirk and Jonathan agreed (
http://mailman.cgd.ucar.edu/pipermail/cf-metadata/2016/058902.html) that this one should be:
tendency_of_surface_snow_amount_due_to_conversion_of_snow_to_sea_ice (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 phrase "surface_snow" means snow lying on the surface. 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. "Conversion of snow to sea ice" occurs when the mass of snow accumulated on an area of sea ice is sufficient to cause the ice to become mostly submerged. Waves can then wash over the ice and snow surface and freeze into a layer that becomes "snow ice". "Sea_ice" means all ice floating on the sea with the exception of floating ice shelves, which are regarded as land ice in models.'
This name is accepted for publication in the standard name table and will be added in the 2nd July update.
We have an existing name tendency_of_sea_ice_amount_due_to_snow_conversion which is complementary to the proposed term. It was agreed during the SIMIP discussion that 'conversion_of_snow_to_sea_ice' is a more informative phrase than simply 'ice_conversion' or 'snow_conversion'. Therefore I will introduce an alias for the existing name:
tendency_of_sea_ice_amount_due_to_snow_conversion -> tendency_of_sea_ice_amount_due_to_conversion_of_snow_to_sea_ice.
This will also be added in the 2nd of July update.
1.14 Proposed as tendency_of_surface_snow_amount_due_to_drifting [kg m-2 s-1]
the rate of change of snow mass through wind drift of snow divided by
grid-cell area
The name and units look fine. I have added a definition as follows:
tendency_of_surface_snow_amount_due_to_drifting (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 phrase "surface_snow" means snow lying on the surface. 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 quantity with standard name tendency_of_surface_snow_amount_due_to_drifting is the rate of change of snow amount caused by wind drift of snow.'
This name did not receive any comments in the original discussion. However, a definition has been provided for 'drifting' and the name otherwise follows the standard pattern. This name is accepted for publication in the standard name table and will be added in the 2nd July update.
1.15 sea_ice_basal_downwelling_shortwave_flux_in_sea_water [W m-2]
the downwelling shortwave flux underneath sea ice (always positive)
Dirk and Jonathan agreed (
http://mailman.cgd.ucar.edu/pipermail/cf-metadata/2016/058902.html) that this one should be:
downwelling_shortwave_flux_in_sea_water_at_sea_ice_base (Wm-2)
'Downwelling radiation is radiation from above. It does not mean "net downward". The term "shortwave" means shortwave radiation. When thought of as being incident on a surface, a radiative flux is sometimes called "irradiance". In addition, it is identical with the quantity measured by a cosine-collector light-meter and sometimes called "vector irradiance". In accordance with common usage in geophysical disciplines, "flux" implies per unit area, called "flux density" in physics. "Sea_ice" means all ice floating on the sea with the exception of floating ice shelves, which are regarded as land ice in models.'
This name is accepted for publication in the standard name table and will be added in the 2nd July update.
1.16 sea_ice_basal_net_downward_sensible_heat_flux [W m-2]
the net sensible heat flux under sea ice from the ocean
For this quantity it was agreed to use the existing name upward_sea_ice_basal_heat_flux.
1.17 sea_ice_surface_net_downward_conductive_heat_flux [W m-2]
the net heat conduction flux at the ice surface
1.18 sea_ice_basal_net_downward_conductive_heat_flux [W m-2]
the net heat conduction flux at the ice base
Dirk and Jonathan agreed (
http://mailman.cgd.ucar.edu/pipermail/cf-metadata/2016/058902.html) that these two names should appear as follows:
surface_downward_heat_flux_in_sea_ice (W m-2)
' "Downward" indicates a vector component which is positive when directed downward (negative upward). In accordance with common usage in geophysical disciplines, "flux" implies per unit area, called "flux density" in physics. "Sea_ice" means all ice floating on the sea with the exception of floating ice shelves, which are regarded as land ice in models.'
basal_downward_heat_flux_in_sea_ice (W m-2)
' "Downward" indicates a vector component which is positive when directed downward (negative upward). In accordance with common usage in geophysical disciplines, "flux" implies per unit area, called "flux density" in physics. "Sea_ice" means all ice floating on the sea with the exception of floating ice shelves, which are regarded as land ice in models.'
These names are both accepted for publication in the standard name table and will be added in the 2nd July update.
1.19 salt_flux_into_sea_water_from_sea_ice [kg m-2 s-1]
Total flux of salt from water into sea ice divided by grid-cell area;
salt flux is upward (negative) during ice growth when salt is embedded
into the ice and downward (positive) during melt when salt from sea ice
is again released to the ocean
Does this one represent salt fluxes at the sea ice base and due to lateral growth and melting? Assuming so, then the name and units are okay and I have added a definition as follows:
salt_flux_into_sea_water_from_sea_ice (kg m-2 s-1)
'In accordance with common usage in geophysical disciplines, "flux" implies per unit area, called "flux density" in physics. The quantity with standard name salt_flux_into_sea_water_from_sea_ice is negative during ice growth when salt becomes embedded into the ice and positive during ice melting when salt is released into the ocean. "Sea_ice" means all ice floating on the sea with the exception of floating ice shelves, which are regarded as land ice in models.'
Dirk and Jonathan agreed (
http://mailman.cgd.ucar.edu/pipermail/cf-metadata/2016/058902.html) that this quantity is the same as the one described by the existing name virtual_salt_flux_into_sea_water_due_to_sea_ice_thermodynamics, except that in this case the flux is not virtual. Therefore, this one should be written as:
salt_flux_into_sea_water_due_to_sea_ice_thermodynamics (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. "Sea ice thermodynamics" refers to the addition or subtraction of ice mass due to surface and basal fluxes, i.e., due to melting, sublimation and fusion. The quantity with standard name salt_flux_into_sea_water_due_to_sea_ice_thermodynamics is negative during ice growth when salt becomes embedded into the ice and positive during ice melting when salt is released into the ocean. "Sea_ice" means all ice floating on the sea with the exception of floating ice shelves, which are regarded as land ice in models.'
This name is accepted for publication in the standard name table and will be added in the 2nd July update.
1.20 water_flux_into_sea_water_from_sea_ice [kg m-2 s-1]
Total flux of fresh water from water into sea ice divided by grid-cell
area; This flux is negative during ice growth (liquid water mass
decreases, hence upward flux of freshwater), positive during ice melt
(liquid water mass increases, hence downward flux of freshwater)
Dirk and Jonathan agreed (
http://mailman.cgd.ucar.edu/pipermail/cf-metadata/2016/058902.html) that this quantity can be described using the existing name
water_flux_into_sea_water_due_to_sea_ice_thermodynamics (kg m-2 s-1)
1.21 surface_drag_coefficient_for_momentum_in_sea_water [1]
Oceanic drag coefficient that is used to calculate the oceanic momentum
drag on sea ice
Dirk and Jonathan agreed (
http://mailman.cgd.ucar.edu/pipermail/cf-metadata/2016/058902.html) that this quantity should be
sea_ice_basal_drag_coefficient_for_momentum_in_sea_water (1)
'The quantity with standard name sea_ice_basal_drag_coefficient_for_momentum_in_sea_water is used to calculate the oceanic momentum drag on sea ice movement. Basal drag is a resistive stress opposing ice flow at the boundary between sea ice and sea water. "Sea_ice" means all ice floating on the sea with the exception of floating ice shelves, which are regarded as land ice in models.'
This name is accepted for publication in the standard name table and will be added in the 2nd July update.
22. sea_ice_specific_x_force_due_to_sea_surface_tilt [N m-2]
X-component of force on sea ice caused by sea-surface tilt
23. sea_ice_specific_y_force_due_to_sea_surface_tilt [N m-2]
Y-component of force on sea ice caused by sea-surface tilt
Dirk and Jonathan agreed (
http://mailman.cgd.ucar.edu/pipermail/cf-metadata/2016/058901.html) that these quantities should be
sea_ice_x_force_per_unit_area_due_to_sea_surface_tilt
' "x" indicates a vector component along the grid x-axis, positive with increasing x. 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. Several factors contribute to differences in the ocean surface level, including uneven heating, salinity variations, and currents, especially near coastal regions or ice shelves. Differences in surface level result in sea-surface tilt, a force that influences the ice motion. Reference: National Snow and Ice Data Center
https://nsidc.org/cryosphere/seaice/processes/dynamics.html. "Sea_ice" means all ice floating on the sea with the exception of floating ice shelves, which are regarded as land ice in models.'
sea_ice_y_force_per_unit_area_due_to_sea_surface_tilt
' "y" indicates a vector component along the grid y-axis, positive with increasing y. 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. Several factors contribute to differences in the ocean surface level, including uneven heating, salinity variations, and currents, especially near coastal regions or ice shelves. Differences in surface level result in sea-surface tilt, a force that influences the ice motion. Reference: National Snow and Ice Data Center
https://nsidc.org/cryosphere/seaice/processes/dynamics.html. "Sea_ice" means all ice floating on the sea with the exception of floating ice shelves, which are regarded as land ice in models.'
Since units of N m-2 are the same as Pascals, the canonical unit will appear in the standard name table as Pa for consistency with other names. These names are accepted for publication in the standard name table and will be added in the 2nd July update.
24. sea_ice_specific_x_force_due_to_coriolis_term [N m-2]
X-component of force on sea ice caused by coriolis force
25. sea_ice_spcecific_y_force_due_to_coriolis_term [N m-2]
Y-component of force on sea ice caused by coriolis force
Dirk and Jonathan agreed (
http://mailman.cgd.ucar.edu/pipermail/cf-metadata/2016/058901.html) that these quantities should be
24. sea_ice_x_force_per_unit_area_due_to_coriolis_effect
' "x" indicates a vector component along the grid x-axis, positive with increasing x. 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. In meteorology and oceanography, the Coriolis force per unit mass arises solely from the earth's rotation and acts as a deflecting force, normal to the velocity, to the right of the motion in the Northern Hemisphere and to the left in the Southern Hemisphere. Reference: American Meteorological Society Glossary
http://glossary.ametsoc.org/wiki/Coriolis_force. "Sea_ice" means all ice floating on the sea with the exception of floating ice shelves, which are regarded as land ice in models.'
25. sea_ice_y_force_per_unit_area_due_to_coriolis_effect
' "y" indicates a vector component along the grid y-axis, positive with increasing y. 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. In meteorology and oceanography, the Coriolis effect per unit mass arises solely from the earth's rotation and acts as a deflecting force, normal to the velocity, to the right of the motion in the Northern Hemisphere and to the left in the Southern Hemisphere. Reference: American Meteorological Society Glossary
http://glossary.ametsoc.org/wiki/Coriolis_force. "Sea_ice" means all ice floating on the sea with the exception of floating ice shelves, which are regarded as land ice in models.'
Since units of N m-2 are the same as Pascals, the canonical unit will appear in the standard name table as Pa for consistency with other names. These names are accepted for publication in the standard name table and will be added in the 2nd July update.
26. sea_ice_specific_x_force_due_to_internal_forces [N m-2]
X-component of force on sea ice caused by internal stress (divergence of
sigma)
27. sea_ice_specific_y_force_due_to_internal_forces [N m-2]
Y-component of force on sea ice caused by internal stress (divergence of
sigma)
Dirk and Jonathan agreed (
http://mailman.cgd.ucar.edu/pipermail/cf-metadata/2016/058901.html) that these quantities should be
26. sea_ice_x_internal_stress
' "x" indicates a vector component along the grid x-axis, positive with increasing x. Internal ice stress is a measure of the compactness, or strength, of the ice. Internal ice stress usually acts as a resistance to the motion caused by the wind force. Reference: National Snow and Ice Data Center
https://nsidc.org/cryosphere/seaice/processes/dynamics.html. "Sea_ice" means all ice floating on the sea with the exception of floating ice shelves, which are regarded as land ice in models.'
27. sea_ice_y_internal_stress
' "y" indicates a vector component along the grid y-axis, positive with increasing y. Internal ice stress is a measure of the compactness, or strength, of the ice. Internal ice stress usually acts as a resistance to the motion caused by the wind force. Reference: National Snow and Ice Data Center
https://nsidc.org/cryosphere/seaice/processes/dynamics.html. "Sea_ice" means all ice floating on the sea with the exception of floating ice shelves, which are regarded as land ice in models.'
Since units of N m-2 are the same as Pascals, the canonical unit will appear in the standard name table as Pa for consistency with other names. These names are accepted for publication in the standard name table and will be added in the 2nd July update.
28. sea_ice_mass_transport_across_line [kg s-1]
"net (sum of transport in all directions) sea ice mass transport through
given passages, positive into the Arctic Ocean
For this quantity it was agreed to use the existing name sea_ice_transport_across_line. However, in the same way that we normally qualify the term 'flux' in standard names, e.g. mass_flux, heat_flux, longwave_flux, etc., I think it would be preferable to describe the existing transport names as mass_transport when that is what is intended. We already have volume_transport and heat_transport names and 8 existing ocean mass_transport names. Thus I suggest creating the alias sea_ice_transport_across_line -> sea_ice_mass_transport_across_line.
In addition I suggest creating the following 21 aliases for other existing names:
eastward_atmosphere_water_transport_across_unit_distance -> eastward_atmosphere_water_mass_transport_across_unit_distance
eastward_atmosphere_water_vapor_transport_across_unit_distance -> eastward_atmosphere_water_vapor_mass_transport_across_unit_distance
eastward_water_vapor_transport_across_unit_distance_in_atmosphere_layer -> eastward_water_vapor_mass_transport_across_unit_distance_in_atmosphere_layer
northward_atmosphere_water_transport_across_unit_distance -> northward_atmosphere_water_mass_transport_across_unit_distance
northward_atmosphere_water_vapor_transport_across_unit_distance -> northward_atmosphere_water_vapor_mass_transport_across_unit_distance
northward_ocean_freshwater_transport -> northward_ocean_freshwater_mass_transport
northward_ocean_freshwater_transport_due_to_diffusion -> northward_ocean_freshwater_mass_transport_due_to_diffusion
northward_ocean_freshwater_transport_due_to_gyre -> northward_ocean_freshwater_mass_transport_due_to_gyre
northward_ocean_freshwater_transport_due_to_overturning -> northward_ocean_freshwater_mass_transport_due_to_overturning
northward_ocean_freshwater_transport_due_to_parameterized_eddy_advection -> northward_ocean_freshwater_mass_transport_due_to_parameterized_eddy_advection
northward_ocean_salt_transport -> northward_ocean_salt_mass_transport
northward_ocean_salt_transport_due_to_diffusion -> northward_ocean_salt_mass_transport_due_to_diffusion
northward_ocean_salt_transport_due_to_gyre -> northward_ocean_salt_mass_transport_due_to_gyre
northward_ocean_salt_transport_due_to_overturning -> northward_ocean_salt_mass_transport_due_to_overturning
northward_ocean_salt_transport_due_to_parameterized_eddy_advection -> northward_ocean_salt_mass_transport_due_to_parameterized_eddy_advection
northward_water_vapor_transport_across_unit_distance_in_atmosphere_layer -> northward_water_vapor_mass_transport_across_unit_distance_in_atmosphere_layer
ocean_salt_x_transport -> ocean_salt_mass_x_transport
ocean_salt_y_transport -> ocean_salt_mass_y_transport
sea_water_transport_across_line -> sea_water_mass_transport_across_line
sea_ice_x_transport -> sea_ice_mass_x_transport
sea_ice_y_transport -> sea_ice_mass_y_transport
Do others support this change?
29. sea_ice_area_transport_across_line [kg s-1]
"net (sum of transport in all directions) sea ice area transport through
given passages, positive into the Arctic Ocean
This is a new quantity to standard names and this particular proposal did not receive any comments during the original discussion. I think it would be okay to introduce area_transport for a quantity such as sea_ice, but the units should then surely be m2 s-1. Then we would have:
sea_ice_area_transport_across_line (m2 s-1)
'Transport "across_line" means that which crosses a particular line on the Earth's surface; formally this means the integral along the line of the normal component of the transport. "Sea_ice" means all ice floating on the sea with the exception of floating ice shelves, which are regarded as land ice in models.'
Okay?
Now I'd like to move on to Martin's recent proposals.
> (1) siflfwdrain Freshwater flux from sea-ice surface [kg m-2 s-1]
>
> I think this variable was added later than the others. We already have, in the CF editor: water_flux_into_sea_water_from_sea_ice
>
> The variable name refers to the process of drainage, so I suggest we use the standard name construction referring to a specific process:
> water_flux_into_sea_water_from_sea_ice_due_to_surface_drainage
>
The water_flux_into_sea_water_from_sea_ice name you refer to in the editor was Dirk's proposal 1.20, for which it was eventually agreed the existing name water_flux_into_sea_water_due_to_sea_ice_thermodynamics should be used. Using a similar due_to_process pattern would be appropriate for the proposed quantity and I wonder whether it would be useful to be even more explicit about the source of the water being drained. I suggest something like
water_flux_into_sea_water_due_to_drainage_from_melt_ponds (kg m-2 s-1)
'"Water flux into sea water" means the freshwater entering the sea water as a result of precipitation, evaporation, river inflow, sea ice effects and water flux correction (if applied). 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. Melt ponds occur on top of the existing sea ice.'
We already use the term "melt pond" in area types, I think we could also use it in standard names. What do you think?
> (2) snmassacrossline Snow mass flux through straits (kg s-1)
>
> For sea ice we have sea_ice_transport_across_line. We might try snow_transport_across_line, but that would probably be interpreted as including transport of snow in the atmosphere, hence:
> snow_transport_across_line_due_to_sea_ice_dynamics
>
For Dirk's proposal 1.12 we have agreed to use tendency_of_surface_snow_amount_due_to_sea_ice_dynamics, so I think your suggestion fits very well with this and existing names. The definition would be based on agreed and existing names as follows:
'Transport "across_line" means that which crosses a particular line on the Earth's surface; formally this means the integral along the line of the normal component of the transport. 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. "Sea ice dynamics" refers to advection of sea ice. "Sea_ice" means all ice floating on the sea with the exception of floating ice shelves, which are regarded as land ice in models.'
This name is accepted for publication in the standard name table and will be added in the 2nd July update.
> (3a) sishevel Maximum shear strain rate of sea-ice velocity field (s-1)
>
> Maximum shear strain rate of sea-ice velocity field (second shear strain invariant: maximum is taken over coordinate rotations)
>
> sea_ice_horizontal_shear_strain_rate_maximum_over_coordinate_rotation
> Help text: "Stress is the symmetric component of the tensor representing the gradient of internal forces (e.g. in ice). Shear stress refers to off-diagonal element(s) of the stress tensor (a single element for horizontal shear stress). The
> maximum over coordinate rotations of the shear strain rate, often referred to as the maximum shear strain [rate], represents the second invariant of strain rate."
I'm afraid I'm not expert enough to comment very usefully on these quantities. I note that the names sea_ice_x_internal_stress and sea_ice_y_internal_stress were agreed for Dirk's proposals 1.26 and 1.27. Would it be correct to describe the horizontal shear strain as also being 'internal'? If so, perhaps we should add it to the name. I'm not sure what 'horizontal' means here - is it parallel to the sea-surface or some other surface such as the sea ice base perhaps? Or does it mean it's in the same plane as the x and y coordinates in Dirk's names? I note that the definition of 3c says 'Horizontal stress refers to the stress in the horizontal plane' but that still doesn't really explain what is considered to be horizontal in this context. If you are including 'maximum' in the name, does that mean it's an intrinsic part of how the variable is calculated by the model, rather than a statistic representative of an area of sea ice? I'd welcome comments from anyone more knowledgeable than me about these quantities
!
This name is still under discussion.
> (3b) sistremax Maximum shear stress in sea ice (N m-1)
> Maximum shear stress in sea ice (second stress invariant)
>
> sea_ice_horizontal_shear_stress_maximum_over_coordinate_rotation
>
> Help text: "Stress is the symmetric component of the tensor representing the gradient of internal forces (e.g. in ice). Shear stress refers to off-diagonal element(s) of the stress tensor (a single element for horizontal shear stress). The
> maximum over coordinate rotations of the shear stress, often referred to as the maximum shear stress, represents the second invariant of stress."
This name appears to be consistent with 3a and my comments and questions would be the same for this one.
This name is still under discussion.
> (3c) sistresave Average normal stress in sea ice (N m-1)
> Average normal stress in sea ice (first stress invariant: average of diagonal elements of the stress tensor)
>
> sea_ice_average_normal_horizontal_stress
> Help text: "Stress is the symmetric component of the tensor representing the gradient of internal forces (e.g. in ice). Horizontal stress refers to the stress in the horizontal plane. Average normal stress refers to the average of the
> diagonal elements of the stress tensor and represents the first invariant of stress."
Again I'd ask similar questions to those for 3a and 3b. By 'average' here are we referring to the mean? If so, then I think we should say that in the name. By 'normal' is this one referring to the 'normal' to the horizontal? Am I being na?ve in thinking we should call that 'vertical'?
This name is still under discussion.
Best wishes,
Alison
------
Alison Pamment Tel: +44 1235 778065
NCAS/Centre for Environmental Data Archival Email: alison.pamment at stfc.ac.uk
STFC Rutherford Appleton Laboratory
R25, 2.22
Harwell Oxford, Didcot, OX11 0QX, U.K.
-----Original Message-----
From: CF-metadata <cf-metadata-bounces at cgd.ucar.edu> On Behalf Of Martin Juckes - UKRI STFC
Sent: 31 May 2018 09:55
To: cf-metadata at cgd.ucar.edu
Subject: [CF-metadata] SIMIP: 5 standard names and one area type for CMIP6
Dear All,
following some feedback from Bruno Tremblay, on behalf of SIMIP, I'd like to modify the standard names proposed for sea ice stress and strain invariants last month (25th April).
The proposed terms are refer to the first and second invariants of the horizontal stress and rate of strain tensors in the sea ice. I had not appreciated that the terms "first invariant" and "second invariant" are rather more flexible than we want for a standard name. The fist invariant, for instance, is sometimes represented by the trace of the tensor, and sometime by half the trace. These two quantities clearly carry the same information, but we need to know which one. In the SIMIP request they want half the trace of the stress tensor, because that is the form most commonly used in the sea ice community. This quantity is generally referred to as the "average normal stress" (the normal stress terms being the two diagonal elements of the stress tensor).
Similarly, the second invariants requested are not, as I had assumed, the effective stress/rate of strain (which is the square root of the determinant) but the "maximum shear stress/rate of strain", which is the maximum value of the off-diagonal tensor element, maximized over all coordinate rotations. At first I thought this was a completely different quantity, but, following Bruno's patient explanations, I now understand that it is related to the first invariant and the determinant by a simple algebraic equation, and hence represents the 2nd invariant just as well as the determinant or effective stress.
We only have requests for terms related to the horizontal stress/rate of strain, but I feel that the standard names should be explicit about this restriction to the horizontal plane, to avoid ambiguity.
Taking this into account, I'd like to propose the following modified names:
(3a) sishevel Maximum shear strain rate of sea-ice velocity field (s-1)
Maximum shear strain rate of sea-ice velocity field (second shear strain invariant: maximum is taken over coordinate rotations)
sea_ice_horizontal_shear_strain_rate_maximum_over_coordinate_rotation
Help text: "Stress is the symmetric component of the tensor representing the gradient of internal forces (e.g. in ice). Shear stress refers to off-diagonal element(s) of the stress tensor (a single element for horizontal shear stress). The maximum over coordinate rotations of the shear strain rate, often referred to as the maximum shear strain [rate], represents the second invariant of strain rate."
(3b) sistremax Maximum shear stress in sea ice (N m-1)
Maximum shear stress in sea ice (second stress invariant)
sea_ice_horizontal_shear_stress_maximum_over_coordinate_rotation
Help text: "Stress is the symmetric component of the tensor representing the gradient of internal forces (e.g. in ice). Shear stress refers to off-diagonal element(s) of the stress tensor (a single element for horizontal shear stress). The maximum over coordinate rotations of the shear stress, often referred to as the maximum shear stress, represents the second invariant of stress."
(3c) sistresave Average normal stress in sea ice (N m-1)
Average normal stress in sea ice (first stress invariant: average of diagonal elements of the stress tensor)
sea_ice_average_normal_horizontal_stress
Help text: "Stress is the symmetric component of the tensor representing the gradient of internal forces (e.g. in ice). Horizontal stress refers to the stress in the horizontal plane. Average normal stress refers to the average of the diagonal elements of the stress tensor and represents the first invariant of stress."
regards,
Martin
________________________________
From: Bruno Tremblay <bruno.tremblay at mcgill.ca>
Sent: 30 May 2018 17:27
To: Juckes, Martin (STFC,RAL,RALSP)
Cc: Dirk Notz; Pamment, Alison (STFC,RAL,RALSP)
Subject: Re: SIMIP -- a few remaining issues
Hi Martin
What you write is entirely correct. There is a factor of 2 difference. But since a constant factor in front of the invariant does not change the invariant nature we opt for a definition that is directly linked with the the way we plot the yield curve and write the constitutive relation.
I am glad it helped
Bruno
On Wed, May 30, 2018 at 11:37 AM, Martin Juckes - UKRI STFC <martin.juckes at stfc.ac.uk<mailto:martin.juckes at stfc.ac.uk>> wrote:
Dear Bruno,
thanks, that clears up a lot.
I'm afraid I'm still struggling with the relation between the "maximum shear stress" and the determinant of the stress tensor. You point out that the coordinates can be rotated to make the horizontal shear stress diagonal -- lets call the components A11 and A22. The determinant is then clearly det=A11*A22. If we rotate the coordinates by 45 degrees, I would expect the off-diagonal elements to be x=0.5*(A22-A11). If y=0.5*(A11+A22), then y**2 - x**2 = 2*det, so value maximum shear stress x is related to the determinant, but not exactly equal to it.
If these are just different, inter-changeable, approaches to representing invariant information, then we just need to be clear about this in the definitions.
regards,
Martin
________________________________
From: Bruno Tremblay <bruno.tremblay at mcgill.ca<mailto:bruno.tremblay at mcgill.ca>>
Sent: 30 May 2018 15:38
To: Juckes, Martin (STFC,RAL,RALSP)
Cc: Dirk Notz; Pamment, Alison (STFC,RAL,RALSP)
Subject: Re: SIMIP -- a few remaining issues
Hi Martin et al
Please see below after BT>> for comments
I hope this helps
Bruno
On Wed, May 30, 2018 at 5:59 AM, Martin Juckes - UKRI STFC <martin.juckes at stfc.ac.uk<mailto:martin.juckes at stfc.ac.uk><mailto:martin.juckes at stfc.ac.uk<mailto:martin.juckes at stfc.ac.uk>>> wrote:
Dear Dirk, Bruno,
thanks for the feedback.
Firstly, we do need to be clear about which variables are requested. At the moment we have sidivvel, sishevel, sistremax and sistresave requested.
'sidivvel' has a corresponding CF standard name "divergence_of_sea_ice_velocity" and a data request description "Divergence of sea-ice velocity field (first shear strain invariant)".
BT>> There is an inconsistency here. Strain has units of m/m (or unitless) and is the % elongation of a linear elastic material when compressed, pulled or sheared. This is used in linear elastic solid sea ice models such as the Elasto Brittle model but even there, people take the derivative of sigma with respect to time so that the d(sigma)/dt is expressed in term of the strain rates rather than the strain.
Bottom line:
1- We are speaking here of the first shear strain rate (m/s / m or sec^-1) invariant, not the first strain invariant. Many refer to it as the strain invariant but this is incorrect.
2- I said yesterday, we did not request to have the divergence of the sea ice velocity. Please scratch that. The first strain rate invariant is eps_11 + eps_22 which is the divergence of the sea ice velocity since eps_11 = du/dx and eps_22 = dv/dy.
The variable list provided by Dirk pairs sidivvel with sishevel, "Maximum shear of sea-ice velocity field (second shear strain invariant)". Both variables are requested with units of "s-1", which perhaps implies that they are related to the rate of strain, rather than strain itself, which would be dimensionless? If sishevel is not related to the rate of strain of the sea-ice velocity field (i.e. the symmetric component of the gradient of the velocity field), please clarify what it is.
BT>> Correct, everything in sea ice models is expressed in terms of strain rates rather than strain. Many use the term strain, when they really mean strain rate. It is the maximum shear of sea ice velocity field expressed in terms of the strain rate.
There appears to be some relation between sidivvel, which is referred to as the "first shear strain invariant", and sistreave, which is referred to as the "first stress invariant". And yet, if we have a tensor with elements a11, a12, a21, a22, then it is clear that "divergence" would be a11 + a22, and "average normal ...." would be 0.5*(a11+a22). We need a more precise definition -- do you want the average of the diagonal elements of the tensor or the sum?
BT>>
sidivvel is the first strain rate invariant, not the first shear strain invariant. The second strain rate invariant that is the maximum shear strain rate.
We want the sum for the first strain rate invariant (du/dv + dv/dy) and the average for the first stress invariant because the bulk viscosity in the sea ice constitutive relation is multiplied by the divergence of sea ice velocity (i.e. the sum) and because when we plot the stress in stress invariant space (to get the yield curve), we use the average normal stress (not the sum of the normal stresses). We could give the sum for both as long as we are clear in the definition with words.
Within the data request we have variable labels, titles, descriptions and standard names. The titles and descriptions often reflect the wording in the standard name, but sometimes the titles and descriptions are adjusted to reflect more of the usage in the community requesting the diagnostic, while the standard name needs to follow usage patterns established within the CF convention. The breadth of the CF convention means that these patterns of usage may look odd, but that is an unavoidable consequence of using such a standard: the advantage of having a common approach are huge.
The terms "maximum shear stress" and "maximum shear strain rate" may be unambiguous within the sea ice community, but there is an element of jargon here. The phrase "maximum shear strain" appears to be more widely used to refer to a critical value of shear strain which a material can withstand before breaking, but I think that a different interpretation is wanted here. The
BT>> No. The yield curve is expressed in terms of the stresses not strain rate. I.e. in your sentence above, you should write " ...to refer to a critical value of shear stress that a material can withstand before breaking.
maximum is, I believe, being taken over rotations of the horizontal stress and strain rate tensors. Is that correct?
BT>> Yes correct. A 2D symmetric tensor can always be rotated in such a way that the off-diagonal components will be zero. The normal components are then the principal stresses (the maximum and minimum axial stresses) and they are also the eigen-values of the 2D stress tensor. The amount of rotation that one must impose to get to the principal axes of stress is found from the rotation matrix composed of the two eigen-vectors associated with the two eigen-values. If we rotate the principal axis system by 45 degrees, then the off-diagonal components of the 2D stress tensor is the 2nd stress invariant (or the maximum shear stress) and the diagonal components are equal to each other and they are the average nornal stress (or the first stress invariant). These are two sets of stress invariants thatt are used in the community, one is called the principal stress and the other are called the stress invariants.
So a well formed standard name might be "sea_ice_velocity_horizontal_shear_strain_rate_maximum_over_coordinate_rotations". i.e. this is a diagnostic based on the horizontal strain rate tensor of the sea ice velocity field, and it is defined as the maximum value of the horizontal shear strain rate wrt. coordinate rotations. Besides commenting on the choice of CF terminology, could you also comment on the definition.
BT>> Yes this is a wordy definition but if you have the space then it is entirely correct.
I'm puzzled by the connection between the "2nd invariant of stress" and "maximum shear stress": I've found multiple sources identifying the 2nd invariant of a rank two tensor with the determinant, but none that relate it to a maximum over coordinate rotations.
BT>> They are exactly the same. When we rotate a 2D symmetric matrix, we can write the off-diagonal component and normal component using parametric equations of a circle. This circle is called the Mohr-Coulomb circle of stress. The center of the circle is the average normal stress (or first stress invariant ) and the radius of the circle is the maximum shear stress (or the second stress invariant).
BT>>
regards,
Martin
________________________________
From: Dirk Notz <dirk.notz at mpimet.mpg.de<mailto:dirk.notz at mpimet.mpg.de><mailto:dirk.notz at mpimet.mpg.de<mailto:dirk.notz at mpimet.mpg.de>>>
Sent: 30 May 2018 08:02
To: Juckes, Martin (STFC,RAL,RALSP); Pamment, Alison (STFC,RAL,RALSP)
Subject: Re: SIMIP -- a few remaining issues
Dear Martin,
please find below, after the excerpt from your initial email, some thoughts from our sea-ice dynamics expert Bruno Tremblay. I generally share his view, but I know that naming conventions are a difficult and sometimes non-intuitive issue. I believe that you and Alison are better qualified than us to judge the relevance of these thoughts.
Thanks,
Dirk
> (3) Stresses and strains
>
> --------------------------------
>
>
>
> There are 3 stress/strain variables (see below) for which I think we need a bit more information. Presumably these refer to the horizontal stress and strain fields? Since these are rank two tensors, the first invariant will be the trace, or sum of eigenvalues, and the 2nd will be the determinant, or product of eigenvalues. Given the units, it looks as though you are interpreting the 2nd invariant as the square root of the determinant? I found one source which suggest that the square root of the 2nd invariant is often used instead of the invariant itself, and referred to as the "effective stress" (e.g. http://www.engr.colostate.edu/~thompson/hPage/CourseMat/Tutorials/Solid_Mechanics/J2.pdf -- effective stress = square root [ 3 * 2nd invariant] ), but I don't know how wide spread this usage is.
>
>
> We don't have any standard names referring to the invariants of tensors, so we need some new terminology here.
>
>
> (3a) sishevel Maximum shear of sea-ice velocity field (s-1)
>
> Maximum shear of sea-ice velocity field (second shear strain
> invariant)
>
>
> sea_ice_second_invariant_of_horizontal_strain_tensor[_expressed_as_eff
> ective_strain]
>
> The last part, "_expressed_as_effective_stress" may be needed to deal with the fact that the second invariant would usually have units s-2.
>
>
> (3b) sistremax Maximum shear stress in sea ice (N m-1)
> Maximum shear stress in sea ice (second stress invariant)
>
> sea_ice_second_invariant_of_horizontal_stress_tensor[_expressed_as_eff
> ective_stress]
>
> (3c) sistresave Average normal stress in sea ice (N m-1)
> Average normal stress in sea ice (first stress invariant)
>
> sea_ice_trace_of_horizontal_stress_tensor
> Here we can just refer to the trace, but the help text could also refer to the "first invariant".
Reply from Bruno Tremblay
[...]
There are two strain invariants and two stress invariants. I am not sure the comment about maximum shear in the sea ice velocity field applies to us. I don't recall requesting this.
The naming convention proposed by Martin are more mathematical. I lean more towards names that refer to their physical meaning.
1st invariant of stress: AverageNormalStress 2nd invariant of stress: MaximumShearStress
1st invariant of strain: Divergence
2st invariant of strain: MaximumSheaStrainRate
This is the way I would call them.
I would stay away from the more mathematical definition since the trace and the determinant (1st and 2nd invariants) or any other linear combination of the trace and determinant are also invariant. The reason we use the one defined above is because they have physical meaning. I.e.
ice will fail in shear when the maximum shear stress reaches the critical shear strength of the ice, etc.
I hope this helps
Bruno
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