Add time_shift_mali_geometry_from_dhdt.py

landice/mesh_tools_li/time_shift_mali_geometry_from_dhdt.py

@@ -0,0 +1,284 @@
+#!/usr/bin/env python
+'''
+Time-shift the ice thickness of a MALI mesh between two years using observed
+surface-elevation-change datasets from the NASA MEaSUREs ITS_LIVE project.
+The mesh can be shifted backward in time (rewound) or forward in time
+(fast-forwarded) depending on the choice of start and end years.
+
+Two complementary datasets are used, both provided on the same EPSG:3031
+Antarctic Polar Stereographic grid (2916 x 2916, 1920 m posting):
+
+  * Grounded ice (NSIDC-0782, ANT_G1920_GroundedIceHeight_v01.nc)
+      variable ``height_change(time, y, x)`` = change in ice-sheet surface
+      elevation [m] relative to a fixed reference date.  For grounded ice the
+      bed is assumed fixed, so surface-elevation change equals thickness
+      change.
+
+  * Floating ice (NSIDC-0792, NSIDC-0792_19920317-20171216_V01.0.nc)
+      variable ``height_change(time, y, x)`` = change in ice-shelf surface
+      (freeboard) elevation [m].  Assuming hydrostatic equilibrium, the
+      corresponding thickness change is the surface change multiplied by the
+      flotation factor rhosw / (rhosw - rhoi).
+
+The thickness change applied to the mesh is the cumulative height change at the
+requested ``--endYear`` minus that at ``--startYear`` (each sampled at the
+nearest available time step in the respective dataset):
+
+    dThickness = height_change(endYear) - height_change(startYear)
+
+To shift a mesh that represents ``startYear`` (e.g. 2015) back to an earlier
+``endYear`` (e.g. 2000), this difference is negative where the ice thinned over
+that interval, so the resulting mesh is correspondingly thicker in the past.
+The same tool can also advance a mesh forward in time simply by choosing
+``endYear > startYear``.
+
+Because the grounded and floating datasets do not cover exactly the same
+footprint, small gaps (notably near grounding lines) can occur between them.
+These gaps are filled by linear interpolation from surrounding valid mesh
+cells.  By default the thickness change is applied only to cells that already
+contain ice; ice-free cells are left unchanged so that interpolation artifacts
+do not appear in the open ocean or on bare land.  Pass ``--allowExtentChange``
+to let the ice extent change between the two years: new ice may then appear,
+but only where a dataset directly measured a change (the observed ice
+footprint) and where the resulting thickness is positive, so the result is
+clipped to the ice extent at the target time and artifacts cannot spread to
+the mesh edge.  Mesh cells that fall entirely outside the combined data
+coverage are left unchanged.  Resulting thicknesses are clamped to be
+non-negative.
+
+The input mesh is copied to a new file (``*_shifted_<endYear>.nc`` by default)
+before its ``thickness`` field is overwritten; the original mesh is not
+modified.  A diagnostic integer field ``dThicknessSource`` is also written to
+the output, flagging for each cell how its thickness change was obtained
+(0 = unchanged, 1 = directly measured, 2 = gap-filled by interpolation).
+
+NOTE: the MALI mesh must use the same EPSG:3031 projected coordinates as the
+ITS_LIVE datasets (xCell / yCell in metres).  A warning is printed if the mesh
+cell centres fall largely outside the dataset extent.
+
+Trevor Hillebrand, 2026
+'''
+
+import shutil
+import numpy as np
+import xarray as xr
+import cftime
+from netCDF4 import Dataset
+from scipy.interpolate import RegularGridInterpolator, griddata
+from argparse import ArgumentParser
+
+
+def parse_args():
+    parser = ArgumentParser(
+        prog='time_shift_mali_geometry_from_dhdt.py',
+        description=__doc__)
+    parser.add_argument("-m", "--mesh", dest="meshFile", required=True,
+                        metavar="FILENAME",
+                        help="MALI mesh file to time-shift (EPSG:3031 "
+                             "coordinates)")
+    parser.add_argument("-g", "--grounded", dest="groundedFile", required=True,
+                        metavar="FILENAME",
+                        help="Grounded-ice height-change dataset "
+                             "(ANT_G1920_GroundedIceHeight_v01.nc)")
+    parser.add_argument("-f", "--floating", dest="floatingFile", required=True,
+                        metavar="FILENAME",
+                        help="Floating-ice height-change dataset "
+                             "(NSIDC-0792_19920317-20171216_V01.0.nc)")
+    parser.add_argument("-s", "--startYear", dest="startYear", type=float,
+                        required=True,
+                        help="Nominal year the mesh currently represents "
+                             "(e.g. 2015)")
+    parser.add_argument("-e", "--endYear", dest="endYear", type=float,
+                        required=True,
+                        help="Year to shift the mesh to (e.g. 2000)")
+    parser.add_argument("-o", "--outFile", dest="outFile", default=None,
+                        metavar="FILENAME",
+                        help="Output mesh filename. Default: input name with "
+                             "'_shifted_<endYear>.nc' suffix")
+    parser.add_argument("--rhoi", dest="rhoi", type=float, default=910.0,
+                        help="Ice density [kg m-3] used for the floating "
+                             "flotation factor (default: 910)")
+    parser.add_argument("--rhosw", dest="rhosw", type=float, default=1028.0,
+                        help="Sea-water density [kg m-3] used for the floating "
+                             "flotation factor (default: 1028)")
+    parser.add_argument("--allowExtentChange", dest="allowExtentChange",
+                        action="store_true",
+                        help="Allow the ice extent to change between the two "
+                             "years. New ice may appear only where a dataset "
+                             "directly measured a change and where the "
+                             "resulting thickness is positive, clipping the "
+                             "result to the ice extent at the target time. By "
+                             "default only cells that currently contain ice "
+                             "are modified.")
+    return parser.parse_args()
+
+
+def nearest_time_index(ds, year):
+    '''Return the index of the time step nearest to a (fractional) calendar
+    year, along with the actual fractional year of that step.'''
+    units = ds['time'].attrs.get('units', 'days since 1950-01-01')
+    calendar = ds['time'].attrs.get('calendar', 'standard')
+    dates = cftime.num2date(ds['time'].values, units, calendar=calendar)
+    years = np.array([d.year + (d.dayofyr - 1) / 365.0 for d in dates])
+    idx = int(np.argmin(np.abs(years - year)))
+    return idx, years[idx]
+
+
+def height_change_difference(ds, startYear, endYear):
+    '''Cumulative height-change difference height_change(endYear) -
+    height_change(startYear), sampled at the nearest time step to each year.
+    Returned as a 2D array (y, x) with NaN where either slice is missing.'''
+    iStart, yStart = nearest_time_index(ds, startYear)
+    iEnd, yEnd = nearest_time_index(ds, endYear)
+    hcStart = ds['height_change'].isel(time=iStart).values.astype(np.float64)
+    hcEnd = ds['height_change'].isel(time=iEnd).values.astype(np.float64)
+    print(f"    startYear {startYear:g} -> nearest step {iStart} "
+          f"({yStart:.2f}); endYear {endYear:g} -> nearest step {iEnd} "
+          f"({yEnd:.2f})")
+    return hcEnd - hcStart
+
+
+def interp_grid_to_mesh(field2d, xGrid, yGrid, xCell, yCell):
+    '''Bilinearly interpolate a 2D grid field (dims y, x) onto mesh cell
+    centres.  Handles a descending y coordinate.  Returns NaN for mesh cells
+    outside the grid or adjacent to missing (NaN) grid values.'''
+    # RegularGridInterpolator requires strictly increasing coordinates.
+    if yGrid[0] > yGrid[-1]:
+        yGrid = yGrid[::-1]
+        field2d = field2d[::-1, :]
+    if xGrid[0] > xGrid[-1]:
+        xGrid = xGrid[::-1]
+        field2d = field2d[:, ::-1]
+    interp = RegularGridInterpolator(
+        (yGrid, xGrid), field2d,
+        method='linear', bounds_error=False, fill_value=np.nan)
+    return interp(np.column_stack((yCell, xCell)))
+
+
+def main():
+    args = parse_args()
+    flotationFactor = args.rhosw / (args.rhosw - args.rhoi)
+
+    outFile = args.outFile
+    if outFile is None:
+        outFile = f"{args.meshFile.rsplit('.nc', 1)[0]}" \
+                  f"_shifted_{args.endYear:g}.nc"
+    print(f"Copying '{args.meshFile}' to '{outFile}'")
+    shutil.copy(args.meshFile, outFile)
+
+    # --- Read mesh geometry ---
+    mesh = Dataset(outFile, 'r+')
+    xCell = mesh.variables['xCell'][:]
+    yCell = mesh.variables['yCell'][:]
+    thickness = mesh.variables['thickness'][0, :].astype(np.float64)
+    nCells = len(xCell)
+
+    # --- Grounded dataset: thickness change = surface-elevation change ---
+    print("Reading grounded-ice dataset:")
+    with xr.open_dataset(args.groundedFile, decode_times=False) as gds:
+        xGrid = gds['x'].values
+        yGrid = gds['y'].values
+        dThkGrounded = height_change_difference(
+            gds, args.startYear, args.endYear)
+    dGrounded = interp_grid_to_mesh(dThkGrounded, xGrid, yGrid, xCell, yCell)
+
+    # Sanity check that the mesh overlaps the dataset extent.
+    inExtent = ((xCell >= xGrid.min()) & (xCell <= xGrid.max()) &
+                (yCell >= yGrid.min()) & (yCell <= yGrid.max()))
+    if inExtent.mean() < 0.5:
+        print("WARNING: fewer than half of the mesh cell centres fall within "
+              "the ITS_LIVE grid extent. Check that the mesh uses EPSG:3031 "
+              "(Antarctic Polar Stereographic) coordinates in metres.")
+
+    # --- Floating dataset: apply flotation factor to surface change ---
+    print("Reading floating-ice dataset:")
+    with xr.open_dataset(args.floatingFile, decode_times=False) as fds:
+        xGridF = fds['x'].values
+        yGridF = fds['y'].values
+        dSurfFloating = height_change_difference(
+            fds, args.startYear, args.endYear)
+    dThkFloating = dSurfFloating * flotationFactor
+    print(f"    applied flotation factor rhosw/(rhosw-rhoi) = "
+          f"{flotationFactor:.3f}")
+    dFloating = interp_grid_to_mesh(dThkFloating, xGridF, yGridF, xCell, yCell)
+
+    # --- Combine (grounded takes precedence; footprints do not overlap) ---
+    # directMask flags cells where at least one dataset *directly* provides a
+    # height change (i.e. the cell falls within the observed ice footprint,
+    # not an extrapolated value).
+    directMask = np.isfinite(dGrounded) | np.isfinite(dFloating)
+    dThk = np.where(np.isfinite(dGrounded), dGrounded, dFloating)
+
+    nCovered = directMask.sum()
+    print(f"Thickness change defined on {nCovered} of {nCells} mesh cells "
+          f"before gap filling")
+
+    # --- Determine which cells may be modified ---
+    # By default only cells that currently contain ice are changed, which
+    # avoids interpolation artifacts in ice-free ocean/bare-land cells.
+    # With --allowExtentChange the ice extent is permitted to change, but new
+    # ice may appear only where a dataset directly measured a change (the
+    # observed ice footprint); combined with the non-negative clamp on
+    # (thickness + dThk) below, this clips the result to the ice extent at the
+    # target time and prevents artifacts reaching the mesh edge.
+    iceMask = thickness > 0.0
+    if args.allowExtentChange:
+        modifyMask = inExtent & (iceMask | directMask)
+    else:
+        modifyMask = inExtent & iceMask
+
+    # --- Fill gaps between the two datasets by linear interpolation from
+    #     surrounding directly-measured mesh cells (within the convex hull
+    #     only). Only gaps inside modifyMask are filled, so extrapolation
+    #     cannot spread beyond the observed/existing ice. ---
+    gapFilled = np.zeros(nCells, dtype=bool)
+    gaps = (~directMask) & modifyMask
+    if gaps.any() and directMask.sum() >= 3:
+        filled = griddata(
+            np.column_stack((xCell[directMask], yCell[directMask])),
+            dThk[directMask],
+            np.column_stack((xCell[gaps], yCell[gaps])), method='linear')
+        dThk[gaps] = filled
+        gapFilled[gaps] = np.isfinite(filled)
+        nFilled = gapFilled.sum()
+        print(f"Filled {nFilled} gap cells by linear interpolation")
+
+    # Cells without a value (outside coverage) are left unchanged.
+    dThk = np.where(np.isfinite(dThk), dThk, 0.0)
+
+    # Restrict the change to the cells we are allowed to modify (removes
+    # interpolation artifacts outside the ice).
+    dThk = np.where(modifyMask, dThk, 0.0)
+
+    # --- Classify the source of each cell's thickness change (diagnostic) ---
+    # 0 = unchanged, 1 = directly measured, 2 = gap-filled by interpolation.
+    dThkSource = np.zeros(nCells, dtype=np.int32)
+    dThkSource[directMask & modifyMask] = 1
+    dThkSource[gapFilled & modifyMask] = 2
+
+    # --- Apply and write ---
+    newThickness = np.maximum(thickness + dThk, 0.0)
+    nChanged = np.count_nonzero(dThk != 0.0)
+    print(f"Updated thickness on {nChanged} cells "
+          f"(min dThk {dThk.min():.2f} m, max dThk {dThk.max():.2f} m)")
+
+    mesh.variables['thickness'][0, :] = newThickness
+
+    # Write the diagnostic source field, creating it if necessary.
+    if 'dThicknessSource' not in mesh.variables:
+        src = mesh.createVariable('dThicknessSource', 'i4', ('Time', 'nCells'))
+        src.long_name = ('source of the applied thickness change from '
+                         'time_shift_mali_geometry_from_dhdt.py')
+        src.flag_values = np.array([0, 1, 2], dtype=np.int32)
+        src.flag_meanings = 'unchanged directly_measured gap_filled'
+    mesh.variables['dThicknessSource'][0, :] = dThkSource
+    print(f"Wrote dThicknessSource diagnostic field "
+          f"(directly_measured {np.count_nonzero(dThkSource == 1)}, "
+          f"gap_filled {np.count_nonzero(dThkSource == 2)})")
+
+    mesh.close()
+    print(f"Wrote time-shifted mesh to '{outFile}'")
+
+
+if __name__ == '__main__':
+    main()