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--- pmip3:design:pi:final:icesheet [2009/12/02 17:51]
pasb
+++ pmip3:design:pi:final:icesheet [2013/10/16 10:45] (current)
jypeter [Northern Hemisphere (GLAC-1)] Added ref to EPSL 2012 paper
@@ Line 4: / Line 4: @@
 ===== Introduction =====
+//Note: use paragraph below...
+The ice sheet provided for PMIP3/CMIP5 LGM experiments is a blended product obtained by averaging three different ice sheets:
+  * ICE6G provided by Dick Peltier
+  * MOCA provided by Lev Tarasov
+  * ANU provided by Kurt Lambeck
+A short description and references for the different ice sheets is provided under the link below:
+[[pmip3:design:pi:final:icesheet|LGM icesheet description]]
+This solution was proposed by PMIP bureau in light of a community checking. Given the uncertainties that still exist on the reconstruction of the ice-sheet, resulting from uncertainties in datation for the data used for global or regional constraints, climate intput from ice core temperature reconstructions or climate model simulations, etc... it sounds reasonable to consider that the average is a best estimate of the LGM ice-sheet.//
 Several PMIP participants were in favour of using a new ice-sheet reconstruction for PMIP3/CMIP5 that better matches the geomorphologic and glaciological constraints than the ICE-5G reconstruction used in PMIP2.
@@ Line 12: / Line 26: @@
 Several documents and figures, as well as discussion and comments from the community can be found on the PMIP ice-sheet discussion page [[pmip3:design:21k:icesheet:index|Ice Sheet discussion]]
+===== Ice-sheet reconstruction to be used for PMIP3/CMIP5 simulations =====
+ Procedure to make the averages.
+. Sources:
+   - <H, RSL>  ANU ice model (2009) incl. British area (*1)
+   - <s, mask> ICE-6G v1.02
+   - <H, s>    GLAC-1 nn454 model for North America, and ne8234 for Eurasia
+   H:    thickness
+   s:    surface topography
+   mask: ice or not
+ *1 Only the ice thickness at 20ka is provided for the British region
+    at the moment.  RSL is computed with assumption of mantle density
+kg m-3.
+    Surface elevation at LGM is computed as follows:
+      s[LGM,ANU] = s[present,ICE-6G] + H[LGM,ANU] - RSL[LGM,ANU]
+. Interpolation on to ICE-6G domain
+  /ICE-6G domain/
+    lon [0, 1, ..., 359]
+    lat [-89.5, -88.5, ..., 89.5]
+  Gridpoints out of the sources are set `Undefined'
+  *Resolution Notes*
+    ANU    Antarctica         [-179,180]      x [-89.5,-61.5]   1 deg
+           Eurasia            [0,115.5]       x [50.25,83]      0.5 deg
+           North AMerica      [-139,-7]       x [38,84.5]       0.5 deg
+           British            [-10,-2]        x [52,59]         1 deg
+    GLAC-1 North America      [187.5,354.5]   x [34.75,84.25]   1 x 0.5 deg
+           Eurasia            [347.25,479.25] x [48.125,83.125] 0.5 x 0.25 deg
+. Average
+  The average is computed as follows:
+    X(ave) = [X(ANU) + X (6G) + X(GLAC-1)] / <Number of `defined' grids>
+  X(ANU or GLAC-1) = X (if defined)
+                or = 0 (if undefined)
+. Plots
+{{:pmip3:design:pi:final:antarctica21ka.pdf|}}
+  In the surface topography plots, light green indicates that the
+  elevation at the gridpoint is below zero;  brown is above zero but
+  not covered by ice; grey or blank region are `undefined' gridpoints.
+  Number of `defined` grids are plotted besides the average in the
+  NH figure.  Red = 1, Blue = 2, Magenta = 3.
+  By this configuration; Magenta corresponds to the ANU domain,
+  magenta and blue to GLAC-1 domain, and all region to the ICE-6G
+  domain, respectively.
+  There is a region of isolated red at the north of Alaska.  This is
+  because of the `undefined' grids in the GLAC-1 source.  I have no
+  idea why this isolated region is undefined in the GLAC-1.
+  Average surface topography shows straight coastline at the Bering
+  strait on the Alaska part.  This reflects the original GLAC-1 data.
+  I think this region should be skipped when averaging, but this is
+  just a quick-look.
+  Number of `defined` grids are NOT plotted in the SH figure.
+  But, this is easily deduced by other two plots.
 ===== Documentation of the different ice-sheet reconstructions =====
+For those interested in sensitivy experiments below is more details of the different ice-sheets anf the acess to the corresponding files.
 ==== ICE-6G ====
@@ Line 35: / Line 129: @@
   * Get in touch with [[pmip2web@lsce.ipsl.fr|Jean-Yves Peterschmitt]] if you want to download the netcdf files
-<note tip>You should download the {{:pmip3:design:21k:icesheet:ice-ng_compare_polar.pdf}} file if you want to look at detailed graphics comparing the different versions of ICE-nG (56 Mb, 77 pages)</note>
+A comparison between the different versions of ICE-nG is provided on the pdf document {{:pmip3:design:21k:icesheet:ice-ng_compare_polar.pdf}}(56 Mb, 77 pages)
 ==== Northern Hemisphere (GLAC-1) ====
@@ Line 64: / Line 160: @@
 (B3), 2143, doi:10.1029/2001JB001731, 2003]]
   * [[http://www.atmosp.physics.utoronto.ca/people/lev/g5jpub.pdf|Tarasov, L., and W. R. Peltier, Greenland glacial history and local geodynamic consequences, Geophysical Journal International, Volume 150, Pages 198-229, doi:10.1046/j.1365-246X.2002.01702.x, 2002]]
+  * [[http://www.sciencedirect.com/science/article/pii/S0012821X11005243|Tarasov, Lev and Arthur S. Dyke and Radford M. Neal and W. R. Peltier, A data-calibrated distribution of deglacial chronologies for the North American ice complex from glaciological modeling, Earth and Planetary Science Letters, Volumes 315–316, 15 January 2012, Pages 30-40]]
 === Data ===
-<note warning>Note that the North Am model is calibrated with a stub Greenland by Ellesmere. When a final model is chosen, the North American and Greenland models can be easily melded.\\ \\ Also, please note, that none of the above have yet been published, so these files are provided for discussion and consideration. Any fields and data not selected as a PMIP boundary condition are not to be used for research without prior consent.</note>
   * Get in touch with [[lev@mun.ca|Lev Tarasov]] or [[pmip2web@lsce.ipsl.fr|Jean-Yves Peterschmitt]] if you want to download the following NetCDF files:
-    * NetCDF of 21 to 10ka surface elevation, thickness, basal velocities for the nn9021 North Am model and nn8234 Eurasian model: **NAnn9021.nc**, **EAnn8234.nc**
+    * NetCDF of 21 to 10ka surface elevation, thickness, basal velocities for the nn454 North Am model and nn8234 Eurasian model: **NAnn454.nc**, **EAnn8234.nc**
     * NetCDF of 21 to 10ka for nn445  ensemble means and 1.5 sigma range: **NAnn445.nc**
   * Other data available upon request from [[lev@mun.ca|Lev Tarasov]]
@@ Line 85: / Line 181: @@
 Broadly, the first iterations are based on the analyses of far-field data where the sea-level signal is predominantly a measure of the changes in total ice volume (the ice-volume equivalent sea level or esl) with the principal isostatic component often being the water-load term and a function of the rate at which water is added into or removed from the oceans.  Simple models are initially used for the ice sheets.  The separation of mantle rheology from the esl function is achieved by using the spatial variability of the far-field sea-level signals (Nakada and Lambeck, 1990 #127 see http://rses.anu.edu.au/people/lambeck_k/index.php?p=pubs for references).  The resulting ice function is then redistributed between the ice sheets by using simple scaling relations in the first place and the process is iterated to ensure some convergence (Lambeck, Yokoyama and Purcell, 2002 # 228).
+In parallel inversions are attempted for the individual ice sheets using data from within and close to the ice margins.  These observations are most sensitive to the ice models and mantle rheology.  For the northern hemisphere these analyses are carried out separately for Scandinavia (Lambeck, Smither, and Johnston, 1998 #187, Lambeck et al., in press), Barents-Kara (Lambeck, 1995, 1996 #166, 170), Greenland (Fleming and Lambeck, 2004 #238), British Isles (Lambeck, 1993; 1995 #164, 156) and North America (as yet unpublished).  In all cases new compilations of the field data have been made.  These separate solutions allow for lateral variability in mantle viscosity.  Some interactions between the ice sheets occur and the solutions are therefore iterated.
+The Antarctic field data is insufficient for a similar analysis for the southern hemisphere and we use the difference between the global esl and the northern hemisphere esl to estimate the volume changes for Antarctica eslant (allowing for mountain deglaciation in both hemispheres, Lambeck and Purcell, 2005 #247).  The ice in Antarctica is then distributed according to the LGM ice margins proposed by Anderson et al. (2002) and on the assumption that the ice profiles followed the quasi-parabolic function proposed by Paterson.  The retreat history is determined by the eslant function.  These models are not meant to be accurate reflections of the Antarctic ice history but as a convenient way of disposing of the ice volume that cannot be attributed to the northern hemisphere, in a way that will not impact in a major way on the far-field and northern hemisphere analyses.
+With the new ice models the far-field analysis is repeated and the individual ice sheet analyses are also repeated.  Several such iterations have now been carried out but the successive results have not yet been published.  The LGM results provided here represent the most recent (2009) solution.  The full solutions for some of the ice sheets extend back to MIS-6 (Lambeck et al., 2006 # 252).
+The rebound inversions result in the changes in ice thickness compared to the present day ice volumes.  Thus the LGM ice thickness is obtained by adding the present-day ice thickness.  The LGM ice elevation, with respect to sea level at the LGM is obtained by subtracting the sea-level change (geoid change beneath the ice sheet) from the palaeo ice thickness.
+The esl function as used in the ANU solutions is defined as all land ice and grounded ice on the shelves and the ocean margin at the LGM is defined by the ice grounding line (Lambeck et al., 2003 #233).
+=== References ===
+  * Lambeck & Johnston, 1998: The viscosity of the mantle: evidence from analyses of glacial rebound phenomena. "The Earth's Mantle" (ed. I. Jackson). Cambridge University Press, Cambridge. pp 461-502
+  * Lambeck, Purcell, Johnston, Nakada & Yokoyama, 2003: Water-load definition in the glacio-hydro-isostatic sea-level equation. Quaternary Science Reviews, vol. 22, pp 309-318
+  * Lambeck, & Chappell, 2001: Sea level change through the last glacial cycle. Science, 292, 679-686.
+  * Lambeck, Yokoyama & Purcell, 2002: Into and out of the Last Glacial Maximum: sea-level change during Oxygen Isotope Stages 3 & 2. Quaternary Science Reviews, vol. 21, pp 343-360
+==== Plots of the different ice-sheets ====
+The following plots have been prepared by [[abeouchi@ccsr.u-tokyo.ac.jp|Ayako Abe-Ouchi]] and [[saitofuyuki@jamstec.go.jp|Saito Fuyuki]]:
+  * {{:pmip3:design:21k:icesheet:gaa5.pdf|Antarctica}} plots
+  * {{:pmip3:design:21k:icesheet:gnh5.pdf|Northern Hemisphere}} plots
+The plots show the combined //Surface Altitude// and //Fraction of Grid Cell Covered with Glacier//, and use data from:
+  * for ANU ice model (2009)
+  * for ICE-6G v1.02
+  * for GLAC-1 nn454 model for North America, and ne8234 for Eurasia

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