Remapper for “online” remapping of data sets¶
Xylar Asay-Davis
date: 2017/04/15
Summary
This document describes the design and implementation of a Remapper
class
for performing either “online” (in memory) or “offline” (through files
via ncremap
) remapping of horizontal data sets. The Remapper
is needed in
order to support remapping to and from grids grids not currently supported by
ncremap
such as polar stereographic grids commonly used for polar data sets.
Requirements
Requirement: Support for remapping to and from stereographic grids
Date last modified: 2017/04/15
Contributors: Xylar Asay-Davis
There should exist a method for interpolating from stereographic grids to the comparison grid used in MPAS-Analysis. This is needed to support observations that are stored on stereographic grids.
It would often be more efficient (in terms of the size of data sets) and more practical to perform analysis of polar data sets on a stereographic grid centered at that pole. Support for mapping to stereographic grids should be included, if feasible.
Algorithmic Formulations
Design solution: Support for remapping to and from stereographic grids
Date last modified: 2017/04/15
Contributors: Xylar Asay-Davis
The design solution is somewhat complex and will be described in multiple sections.
MeshDescriptor classes
To support mapping to and from MPAS meshes, lat/lon grid and stereographic
grids (as well as future grids we might want to support), I propose defining a
“mesh descriptor” that defines the mesh either by reading it from a file or by
creating it from simple numpy ndarrays. Each MeshDescriptor
class defines
enough information (such as the locations of cell centers and corners) about
the mesh or grid to allow remapping between meshes.
An MpasMeshDescriptor
class will define MPAS meshes read from a file.
A LatLonGridDescriptor
class will define global lat/lon grids such as the
existing comparison grid.
A ProjectionGridDescriptor
class will define any grid that can be described
by a logically rectangular grid with pyproj
projection. In particular, such
a projection grid could be used to support both polar stereographic grids and
regional lat/lon grids.
Remapper class
Remapping between meshes described by MeshDescriptor
classes will be performed
by a Remapper
class. This class will support both “online” mapping in memory
and “offline” mapping with ncremap
. Only “online” mapping will be supported
for grids defined with the ProjectionGridDescriptor
, as these are not
supported by ncremap
. A Remapper
object will be created by giving it source
and destintion MeshDescriptor
objects and an optional mapping file name.
(If the mapping file name is not given, it is assumed that the source and
destination grids are the same, and no remapping is needed.)
If remapping is performed “online”, it supports renormalization of masked
arrays. If a data sets includes NaN
s in a given data array, both the data
array and a mask are remapped, and renormalization is performed anywhere the
remapped mask exceeds a given threshold.
Design and Implementation
Implementation: Support for remapping to and from stereographic grids
Date last modified: 2017/04/15
Contributors: Xylar Asay-Davis
The implementation is on the branch xylar/MPAS-Analysis/add_polar_stereographic_interp
MeshDescriptor classes
Each MeshDescriptor
subclass includes the following member variables or
methods:
meshName
a name of the mesh or grid, used for naming mapping files andclimatologies
regional
: whether the mesh is regional or globalcoords
anddims
dictionaries defining the coordinates and dimensionsof this mesh, used to update a data set following remapping
to_scrip
method: used to write out a SCRIP file defining the mesh.
Remapper class
Below is a skeleton of the Remapper
public API.
class Remapper(object):
def __init__(self, sourceDescriptor, destinationDescriptor,
mappingFileName=None):
'''
Create the remapper and read weights and indices from the given file
for later used in remapping fields.
'''
def build_mapping_file(self, method='bilinear',
additionalArgs=None):
'''
Given a source file defining either an MPAS mesh or a lat-lon grid and
a destination file or set of arrays defining a lat-lon grid, constructs
a mapping file used for interpolation between the source and
destination grids.
'''
def remap_file(self, inFileName, outFileName,
variableList=None, overwrite=False):
'''
Given a source file defining either an MPAS mesh or a lat-lon grid and
a destination file or set of arrays defining a lat-lon grid, constructs
a mapping file used for interpolation between the source and
destination grids.
'''
def remap(self, ds, renormalizationThreshold=None):
'''
Given a source data set, returns a remapped version of the data set,
possibly masked and renormalized.
'''
Testing and Validation: Support for remapping to and from stereographic
grids
Date last modified: 2017/04/15
Contributors: Xylar Asay-Davis
On the branch xylar/MPAS-Analysis/add_polar_stereographic_interp,
climatologies have ben updated to use Remapper
objects. Analysis has been
run on both QU240
and EC60to30
beta0 ACME results, and results have been
compared by eye. Results from ncremap
are identical, as expected. Because of
renormalization, results with “online” remapping differ from those from
ncremap
, typically with less severe masking of missing data.
Continuous integration unit tests for climatology and interpolation have both
been updated to make use of the Remapper
class. New tests have been added to
perform remapping with stereographic grids.