Subsetting unstructured grid

gistfile1.py

import datetime
import time

import iris


def time_coord(cube):
    """Return the variable attached to time axis and rename it to time."""
    try:
        cube.coord(axis='T').rename('time')
    except CoordinateNotFoundError:
        pass
    timevar = cube.coord('time')
    return timevar


def time_near(cube, datetime):
    """Return the nearest index to a `datetime`."""
    timevar = time_coord(cube)
    try:
        time = timevar.units.date2num(datetime)
        idx = timevar.nearest_neighbour_index(time)
    except IndexError:
        idx = -1
    return idx


def time_slice(cube, start, stop=None):
    """TODO: Re-write to use `iris.FUTURE.cell_datetime_objects`."""
    istart = time_near(cube, start)
    if stop:
        istop = time_near(cube, stop)
        if istart == istop:
            raise ValueError('istart must be different from istop!'
                             'Got istart {!r} and '
                             ' istop {!r}'.format(istart, istop))
        return cube[istart:istop, ...]
    else:
        return cube[istart, ...]


iris.FUTURE.netcdf_promote = True


print 'Initial load...'
t = time.time()
url = ('http://www.smast.umassd.edu:8080/thredds/dodsC/FVCOM/NECOFS/'
       'Forecasts/NECOFS_FVCOM_OCEAN_MASSBAY_FORECAST.nc')
cube = iris.load_cube(url, 'sea_water_potential_temperature')
print '   ...done in {} s'.format(time.time() - t)
print `cube`

print
print 'Slicing...'
t = time.time()
start = datetime.datetime.utcnow() - datetime.timedelta(days=7)
cube = time_slice(cube, start, None)
print '   ...done in {} s'.format(time.time() - t)
print `cube`

print
print 'Converting units...'
t = time.time()
units = iris.unit.Unit('Kelvin')
cube.convert_units(units)
print '   ...done in {} s'.format(time.time() - t)
print `cube`

print
print 'Intersecting...'
t = time.time()
bbox = [-70.8, 41.4, -69.9, 42.3]
cube = cube.intersection(longitude=(bbox[0], bbox[2]),
                         latitude=(bbox[1], bbox[3]))
print '   ...done in {} s'.format(time.time() - t)
print `cube`

gistfile2.txt

Initial load...
   ...done in 134.799810886 s
<iris 'Cube' of sea_water_potential_temperature / (degrees_C) (time: 145; -- : 10; -- : 98432)>

Slicing...
   ...done in 0.0312080383301 s
<iris 'Cube' of sea_water_potential_temperature / (degrees_C) (-- : 10; -- : 98432)>

Converting units...
   ...done in 8.565335989 s
<iris 'Cube' of sea_water_potential_temperature / (Kelvin) (-- : 10; -- : 98432)>

Intersecting...
   ...done in 0.0781300067902 s
<iris 'Cube' of sea_water_potential_temperature / (Kelvin) (-- : 10; -- : 44575)>