ENH: Add cumulative integration function.

docs/_templates/overrides/metpy.calc.rst

@@ -164,6 +164,7 @@ Mathematical Functions
       tangential_component
       unit_vectors_from_cross_section
       vector_derivative
+      cumulative_integrate
 
 
 Apparent Temperature

src/metpy/calc/tools.py

@@ -17,6 +17,7 @@
 
 import numpy.ma as ma
 from pyproj import CRS, Geod, Proj
+from scipy.integrate import cumulative_trapezoid
 from scipy.spatial import cKDTree
 import xarray as xr
 
@@ -1942,3 +1943,44 @@ def _remove_nans(*variables):
     for v in variables:
         ret.append(v[~mask])
     return ret
+
+
+@exporter.export
+@xarray_derivative_wrap
+def cumulative_integrate(field, axis=None, x=None, delta=None):
+    """Return cumulative integral of field along axis.
+
+    Uses trapezoidal rule for integration.
+
+    Parameters
+    ----------
+    field : array-like
+        Array of values for which to calculate the integral
+    axis : int or str
+        The axis along which to integrate.  If `field` is an
+        `np.ndarray` or `pint.Quantity`, must be an integer.  Defaults
+        to zero.
+    x : array-like, optional
+        The coordinate values along which to integrate
+    delta : array-like, optional
+        Spacing between grid points in `field`.
+
+    Examples
+    --------
+    >>> cumulative_integrate(units.Quantity(np.arange(5), 'm'), delta=units('1 m'))
+    <Quantity([0.  0.5 2.  4.5 8. ], 'meter ** 2')>
+    """
+    n, axis, delta = _process_deriv_args(field, axis, x, delta)
+    take = make_take(n, axis)
+    right = np.cumsum(delta, axis=axis)
+    left = np.zeros_like(right[take(slice(1))])
+    x = concatenate([left, right], axis=axis)
+    try:
+        result = cumulative_trapezoid(field.magnitude, x=x.magnitude, axis=axis, initial=0)
+        return units.Quantity(result, field.units * delta.units)
+    except AttributeError:
+        # NumPy arrays without units
+        raise TypeError(
+            'cumulative_integrate called with unitless arguments\n'
+            'Either add units or use scipy.integrate.cumulative_trapezoid'
+        ) from None

tests/calc/test_calc_tools.py

@@ -12,7 +12,8 @@
 import pytest
 import xarray as xr
 
-from metpy.calc import (angle_to_direction, find_bounding_indices, find_intersections,
+from metpy.calc import (angle_to_direction, cumulative_integrate,
+                        find_bounding_indices, find_intersections,
                         first_derivative, geospatial_gradient, get_layer, get_layer_heights,
                         gradient, laplacian, lat_lon_grid_deltas, nearest_intersection_idx,
                         parse_angle, pressure_to_height_std, reduce_point_density,
@@ -1557,3 +1558,53 @@ def test_vector_derivative_return_subset(return_only, length):
         u, v, longitude=lons, latitude=lats, crs=crs, return_only=return_only)
 
     assert len(ddx) == length
+
+
+def test_cumulative_integrate_numpy():
+    """Test that cumulative_integrate works with numpy arrays."""
+    field = np.arange(5)
+    with pytest.raises(
+            TypeError, match='cumulative_integrate called with unitless arguments'
+    ):
+        cumulative_integrate(field, delta=1)
+
+
+def test_cumulative_integrate_pint():
+    """Test that cumulative_integrate works with pint Quantities."""
+    field = np.arange(6) * units('kg/m^3')
+    delta = np.array([1, 2, 3, 2, 1]) * units('cm')
+    integral = cumulative_integrate(field, delta=delta)
+    assert integral.magnitude == pytest.approx(
+        np.array([0, 0.5, 3.5, 11, 18, 22.5])
+    )
+    assert units.Quantity(1, integral.units).to('dag/m^2').magnitude == 1
+
+
+def test_cumulative_integrate_xarray():
+    """Test that cumulative_integrate works with XArray DataArrays."""
+    field = xr.DataArray(
+        np.arange(10) / 100,
+        {'x': (('x',), np.arange(100, 1001, 100), {'units': 'hPa'})},
+        attrs={'units': 'g/kg'}
+    )
+    integral = cumulative_integrate(field, axis='x')
+    assert integral.metpy.magnitude == pytest.approx(
+        np.array([0, 0.5, 2, 4.5, 8, 12.5, 18, 24.5, 32, 40.5])
+    )
+    assert units.Quantity(1, integral.metpy.units).to('hg/(m s^2)').magnitude == 1
+
+
+def test_cumulative_integrate_xr_2d():
+    """Test that cumulative_integrate works with 2D DataArrays."""
+    arr = np.arange(5)
+    data_xr = xr.DataArray(
+        np.ones((5, 5)),
+        {'y': ('y', arr, {'units': 'm'}), 'x': ('x', arr, {'units': 'm'})},
+        ('y', 'x'),
+        'height',
+        {'units': 'm'}
+    )
+    integral = cumulative_integrate(data_xr, axis='x')
+    assert integral.dims == data_xr.dims
+    assert integral.coords.keys() == data_xr.coords.keys()
+    assert units.Quantity(1, integral.metpy.units).to('m^2').magnitude == 1