heights of TBOT and Tair_from_atm

[KeerZ]

Dear all,

I am running a case with I2000Clm50SpGs compset, and I would like to output air temperature at the first grid height above ground from forcing data.

There are two outputs called "TBOT" (longname: atmospheric air temperature received from atmosphere (pre-downscaling)', & ptr_gcell=this%forc_t_not_downscaled_grc) and "Tair_from_atm" (longname: atmospheric air temperature (downscaled to columns in glacier regions)', & ptr_col=this%forc_t_downscaled_col))

So I checked the script atm2lndMod.f90 and found that the TBOT and Tair_from_atm represent air temperature at gridcell-level elevation and column-level elevation respectivltly. Could anyone tell me what are the definitions of gridcell-level elevation and column-level elevation? Can I regard TBOT as air temperature from forcing data at first grid height above ground or air temperature at atmospheric reference height (30m)?

Thank you!

 

[oleson]

TBOT is air temperature at the forcing height or atmospheric reference height (30m; not downscaled over glacier columns). Tair_from_atm is the air temperature at forcing height but downscaled to the elevation of glacier columns in gridcells that have glaciers.
The gridcell elevation is provided by the topography file specified in datm.streams.txt.topo.observed.
Regarding column elevation, the following is from the CLM5 technical note:

"The percentage glacier mask was derived from vector data of global glacier and ice sheet spatial coverage. Vector data for glaciers (ice caps, icefields and mountain glaciers) were taken from the first globally complete glacier inventory, the Randolph Glacier Inventory version 1.0 (RGIv1.0: Arendt et al. 2012). Vector data for the Greenland Ice Sheet were provided by Frank Paul and Tobias Bolch (University of Zurich: Rastner et al. 2012). Antarctic Ice Sheet data were provided by Andrew Bliss (University of Alaska) and were extracted from the Scientific Committee on Antarctic Research (SCAR) Antarctic Digital Database version 5.0. Floating ice is only provided for the Antarctic and does not include the small area of Arctic ice shelves. High spatial resolution vector data were then processed to determine the area of glacier, ice sheet and floating ice within 30-second grid cells globally. The 30-second glacier, ice sheet and Antarctic ice shelf masks were subsequently draped over equivalent-resolution GLOBE topography (Global Land One-km Base Elevation Project, Hastings et al. 1999) to extract approximate ice-covered elevations of ice-covered regions. Grid cells flagged as land-ice in the mask but ocean in GLOBE (typically, around ice sheets at high latitudes) were designated land-ice with an elevation of 0 meters. Finally, the high-resolution mask/topography datasets were aggregated and processed into three 3-minute datasets: 3-minute fractional areal land ice coverage (including both glaciers and ice sheets); 3-minute distributions of areal glacier fractional coverage by elevation and areal ice sheet fractional coverage by elevation. Ice fractions were binned at 100 meter intervals, with bin edges defined from 0 to 6000 meters (plus one top bin encompassing all remaining high-elevation ice, primarily in the Himalaya). These distributions by elevation are used to divide each glacier land unit into columns based on elevation class.

When running with the CISM ice sheet model, CISM dictates glacier areas and elevations in its domain, overriding the values specified by CLM’s datasets. In typical CLM5 configurations, this means that CISM dictates glacier areas and elevations over Greenland."

See also this for further information on downscaling:

2.13. Glaciers — ctsm release-clm5.0 documentation

Note that in later versions of CLM, the definitions of TBOT and Tair_from_atm are reversed (not sure what version of CLM you are using).

 

[KeerZ]

TBOT is air temperature at the forcing height or atmospheric reference height (30m; not downscaled over glacier columns). Tair_from_atm is the air temperature at forcing height but downscaled to the elevation of glacier columns in gridcells that have glaciers.
The gridcell elevation is provided by the topography file specified in datm.streams.txt.topo.observed.
Regarding column elevation, the following is from the CLM5 technical note:

"The percentage glacier mask was derived from vector data of global glacier and ice sheet spatial coverage. Vector data for glaciers (ice caps, icefields and mountain glaciers) were taken from the first globally complete glacier inventory, the Randolph Glacier Inventory version 1.0 (RGIv1.0: Arendt et al. 2012). Vector data for the Greenland Ice Sheet were provided by Frank Paul and Tobias Bolch (University of Zurich: Rastner et al. 2012). Antarctic Ice Sheet data were provided by Andrew Bliss (University of Alaska) and were extracted from the Scientific Committee on Antarctic Research (SCAR) Antarctic Digital Database version 5.0. Floating ice is only provided for the Antarctic and does not include the small area of Arctic ice shelves. High spatial resolution vector data were then processed to determine the area of glacier, ice sheet and floating ice within 30-second grid cells globally. The 30-second glacier, ice sheet and Antarctic ice shelf masks were subsequently draped over equivalent-resolution GLOBE topography (Global Land One-km Base Elevation Project, Hastings et al. 1999) to extract approximate ice-covered elevations of ice-covered regions. Grid cells flagged as land-ice in the mask but ocean in GLOBE (typically, around ice sheets at high latitudes) were designated land-ice with an elevation of 0 meters. Finally, the high-resolution mask/topography datasets were aggregated and processed into three 3-minute datasets: 3-minute fractional areal land ice coverage (including both glaciers and ice sheets); 3-minute distributions of areal glacier fractional coverage by elevation and areal ice sheet fractional coverage by elevation. Ice fractions were binned at 100 meter intervals, with bin edges defined from 0 to 6000 meters (plus one top bin encompassing all remaining high-elevation ice, primarily in the Himalaya). These distributions by elevation are used to divide each glacier land unit into columns based on elevation class.

When running with the CISM ice sheet model, CISM dictates glacier areas and elevations in its domain, overriding the values specified by CLM’s datasets. In typical CLM5 configurations, this means that CISM dictates glacier areas and elevations over Greenland."

See also this for further information on downscaling:

2.13. Glaciers — ctsm release-clm5.0 documentation

Note that in later versions of CLM, the definitions of TBOT and Tair_from_atm are reversed (not sure what version of CLM you are using).

Thank you, Keith! Your explanation is very clear.

I am using CLM5.0. I just confirmed that in this version Tair_from_atm is pre-downscaling air temperature at the forcing height and TBOT has been downscaled to columns in glacier regions. So as you said, they are reversed in CLM5.

 

[QINKONG]

TBOT is air temperature at the forcing height or atmospheric reference height (30m; not downscaled over glacier columns). Tair_from_atm is the air temperature at forcing height but downscaled to the elevation of glacier columns in gridcells that have glaciers.
The gridcell elevation is provided by the topography file specified in datm.streams.txt.topo.observed.
Regarding column elevation, the following is from the CLM5 technical note:

"The percentage glacier mask was derived from vector data of global glacier and ice sheet spatial coverage. Vector data for glaciers (ice caps, icefields and mountain glaciers) were taken from the first globally complete glacier inventory, the Randolph Glacier Inventory version 1.0 (RGIv1.0: Arendt et al. 2012). Vector data for the Greenland Ice Sheet were provided by Frank Paul and Tobias Bolch (University of Zurich: Rastner et al. 2012). Antarctic Ice Sheet data were provided by Andrew Bliss (University of Alaska) and were extracted from the Scientific Committee on Antarctic Research (SCAR) Antarctic Digital Database version 5.0. Floating ice is only provided for the Antarctic and does not include the small area of Arctic ice shelves. High spatial resolution vector data were then processed to determine the area of glacier, ice sheet and floating ice within 30-second grid cells globally. The 30-second glacier, ice sheet and Antarctic ice shelf masks were subsequently draped over equivalent-resolution GLOBE topography (Global Land One-km Base Elevation Project, Hastings et al. 1999) to extract approximate ice-covered elevations of ice-covered regions. Grid cells flagged as land-ice in the mask but ocean in GLOBE (typically, around ice sheets at high latitudes) were designated land-ice with an elevation of 0 meters. Finally, the high-resolution mask/topography datasets were aggregated and processed into three 3-minute datasets: 3-minute fractional areal land ice coverage (including both glaciers and ice sheets); 3-minute distributions of areal glacier fractional coverage by elevation and areal ice sheet fractional coverage by elevation. Ice fractions were binned at 100 meter intervals, with bin edges defined from 0 to 6000 meters (plus one top bin encompassing all remaining high-elevation ice, primarily in the Himalaya). These distributions by elevation are used to divide each glacier land unit into columns based on elevation class.

When running with the CISM ice sheet model, CISM dictates glacier areas and elevations in its domain, overriding the values specified by CLM’s datasets. In typical CLM5 configurations, this means that CISM dictates glacier areas and elevations over Greenland."

See also this for further information on downscaling:

2.13. Glaciers — ctsm release-clm5.0 documentation

Note that in later versions of CLM, the definitions of TBOT and Tair_from_atm are reversed (not sure what version of CLM you are using).

Hi Keith. The explanation is really helpful. But I found the concept of reference height confusing across CLM and CAM. In the technical note of CLM5.0, atmospheric reference height is input from CAM to CLM (as shown in the snapshot below). I suspect that this Zatm is the lowest model level height of CAM and correspond to the 'forcing height' in your reply? Meanwhile the 'TREFHT' (reference height temperature) in CAM history fields refers to 2-meter temperature which seems to be different definition of reference height.
Also, in CLM5.0 technical notes, 2-meter temperature and humidity are CLM output to CAM. So, can I assume that the TREFHT history fields in CAM are actually calculated in CLM and passed to CAM? Moreover, are all the near-surface variables (2-meter temperature, humidity and 10-meter wind speed) in CMIP6 archive are output of the land component?

Thanks a lot!

 

[oleson]

Yes, Zatm (prime) is the lowest model level height of CAM, also described as the forcing height.
The 2-m temperature and humidity history fields in CLM should be the same as the ones in CAM for gridcells that are 100% land.
A10-m wind speed calculated in CLM is also passed through the coupler to CAM. I believe that the CLM and CAM 10-m winds are the same, but I've never verified this.
One could verify these fields are the same by comparing CLM and CAM history outputs.

 

[QINKONG]

Yes, Zatm (prime) is the lowest model level height of CAM, also described as the forcing height.
The 2-m temperature and humidity history fields in CLM should be the same as the ones in CAM for gridcells that are 100% land.
A10-m wind speed calculated in CLM is also passed through the coupler to CAM. I believe that the CLM and CAM 10-m winds are the same, but I've never verified this.
One could verify these fields are the same by comparing CLM and CAM history outputs.

Thanks! It's helpful.