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"""Plugin to calculate probabilities of occurrence between specified thresholds
"""
from typing import List
import iris
import numpy as np
from cf_units import Unit
from iris.cube import Cube
from iris.exceptions import CoordinateNotFoundError
from improver import PostProcessingPlugin
from improver.metadata.probabilistic import (
find_threshold_coordinate,
probability_is_above_or_below,
)
[docs]class OccurrenceBetweenThresholds(PostProcessingPlugin):
"""Calculate the probability of occurrence between thresholds"""
[docs] def __init__(
self, threshold_ranges: List[List[float]], threshold_units: str
) -> None:
"""
Initialise the class. Threshold ranges must be specified in a unit
that is NOT sensitive to differences at the 1e-5 (float32) precision
level.
Args:
threshold_ranges:
List of 2-item iterables specifying thresholds between which
probabilities should be calculated
threshold_units:
Units in which the thresholds are specified
Raises:
ValueError:
If any of the specified thresholds are indistinguishable at the
1e-5 (float32) precision level
"""
threshold_diffs = np.diff(threshold_ranges)
if any(diff < 1e-5 for diff in threshold_diffs):
raise ValueError(
"Plugin cannot distinguish between thresholds at "
"{} {}".format(threshold_ranges, threshold_units)
)
self.threshold_ranges = threshold_ranges
self.threshold_units = threshold_units
[docs] def _slice_cube(self) -> List[List[Cube]]:
"""
Extract required slices from input cube
Returns:
List of 2-item lists containing lower and upper
threshold cubes
Raises:
ValueError:
If any of the required constraints returns None
"""
thresh_coord = self.cube.coord(self.thresh_coord.name())
error_string = (
f"{thresh_coord.name()} threshold {{}} {self.threshold_units} "
"is not available\n"
)
error_msg = ""
cubes = []
for t_range in self.threshold_ranges:
t_range.sort()
lower_constraint = iris.Constraint(
coord_values={
thresh_coord: lambda t: np.isclose(t.point, t_range[0], atol=1e-5)
}
)
lower_cube = self.cube.extract(lower_constraint)
if lower_cube is None:
error_msg += error_string.format(t_range[0])
upper_constraint = iris.Constraint(
coord_values={
thresh_coord: lambda t: np.isclose(t.point, t_range[1], atol=1e-5)
}
)
upper_cube = self.cube.extract(upper_constraint)
if upper_cube is None:
error_msg += error_string.format(t_range[1])
cubes.append([lower_cube, upper_cube])
if error_msg:
# if any thresholds were unavailable, raise errors together here
raise ValueError(error_msg)
return cubes
[docs] def _get_multiplier(self) -> float:
"""
Check whether the cube contains "above" or "below" threshold
probabilities. For "above", the probability of occurrence between
thresholds is the difference between probabilities at the lower
and higher thresholds: P(lower) - P(higher). For "below" it is the
inverse of this: P(higher) - P(lower), which is implemented by
multiplying the difference by -1.
Returns:
1. or -1.
Raises:
ValueError: If the spp__relative_to_threshold attribute is
not recognised
"""
relative_to_threshold = probability_is_above_or_below(self.cube)
if relative_to_threshold == "above":
multiplier = 1.0
elif relative_to_threshold == "below":
multiplier = -1.0
else:
raise ValueError(
"Input cube must contain probabilities of "
"occurrence above or below threshold"
)
return multiplier
[docs] def _calculate_probabilities(self) -> Cube:
"""
Calculate between_threshold probabilities cube
Returns:
Merged cube containing recalculated probabilities
"""
multiplier = self._get_multiplier()
thresh_name = self.thresh_coord.name()
cubelist = iris.cube.CubeList([])
for (lower_cube, upper_cube) in self.cube_slices:
# construct difference cube
between_thresholds_data = (lower_cube.data - upper_cube.data) * multiplier
between_thresholds_cube = upper_cube.copy(between_thresholds_data)
# add threshold coordinate bounds
lower_threshold = lower_cube.coord(thresh_name).points[0]
upper_threshold = upper_cube.coord(thresh_name).points[0]
between_thresholds_cube.coord(thresh_name).bounds = [
lower_threshold,
upper_threshold,
]
cubelist.append(between_thresholds_cube)
return cubelist.merge_cube()
[docs] def process(self, cube: Cube) -> Cube:
"""
Calculate probabilities between thresholds for the input cube
Args:
cube:
Probability cube containing thresholded data (above or below)
Returns:
Cube containing probability of occurrence between thresholds
"""
# if cube has no threshold-type coordinate, raise an error
try:
self.thresh_coord = find_threshold_coordinate(cube)
except CoordinateNotFoundError:
raise ValueError(
"Input is not a probability cube " "(has no threshold-type coordinate)"
)
self.cube = cube.copy()
# check input cube units and convert if needed
original_units = self.thresh_coord.units
if original_units != self.threshold_units:
self.cube.coord(self.thresh_coord).convert_units(self.threshold_units)
# extract suitable cube slices
self.cube_slices = self._slice_cube()
# generate "between thresholds" probabilities
output_cube = self._calculate_probabilities()
self._update_metadata(output_cube, original_units)
return output_cube