Merge pull request Unidata#739 from jrleeman/ageostrophic_wind

Ageostrophic wind

Author: dopplershift

metpy/calc/kinematics.py

@@ -500,6 +500,40 @@ def geostrophic_wind(heights, f, dx, dy):
     return -norm_factor * dhdy, norm_factor * dhdx
 
 
+@exporter.export
+@ensure_yx_order
+def ageostrophic_wind(heights, f, dx, dy, u, v, dim_order='yx'):
+    r"""Calculate the ageostrophic wind given from the heights or geopotential.
+
+    Parameters
+    ----------
+    heights : (M, N) ndarray
+        The height field, with either leading dimensions of (x, y) or trailing dimensions
+        of (y, x), depending on the value of ``dim_order``.
+    f : array_like
+        The coriolis parameter.  This can be a scalar to be applied
+        everywhere or an array of values.
+    dx : scalar
+        The grid spacing in the x-direction
+    dy : scalar
+        The grid spacing in the y-direction
+    u : (M, N) ndarray
+        The u wind field, with either leading dimensions of (x, y) or trailing dimensions
+        of (y, x), depending on the value of ``dim_order``.
+    v : (M, N) ndarray
+        The u wind field, with either leading dimensions of (x, y) or trailing dimensions
+        of (y, x), depending on the value of ``dim_order``.
+
+    Returns
+    -------
+    A 2-item tuple of arrays
+        A tuple of the u-component and v-component of the ageostrophic wind.
+
+    """
+    u_geostrophic, v_geostrophic = geostrophic_wind(heights, f, dx, dy, dim_order=dim_order)
+    return u - u_geostrophic, v - v_geostrophic
+
+
 @exporter.export
 @check_units('[length]', '[temperature]')
 def montgomery_streamfunction(height, temperature):

metpy/calc/tests/test_kinematics.py

@@ -6,7 +6,7 @@
 import numpy as np
 import pytest
 
-from metpy.calc import (advection, convergence_vorticity, divergence,
+from metpy.calc import (advection, ageostrophic_wind, convergence_vorticity, divergence,
                         frontogenesis, geostrophic_wind, get_wind_components, h_convergence,
                         lat_lon_grid_deltas, lat_lon_grid_spacing, montgomery_streamfunction,
                         shearing_deformation, shearing_stretching_deformation,
@@ -399,6 +399,32 @@ def test_geostrophic_gempak():
     assert_almost_equal(vg[1, 1], true_v[1, 1] * units('m/s'), 2)
 
 
+def test_no_ageostrophic_geopotential():
+    """Test ageostrophic wind calculation with geopotential and no ageostrophic wind."""
+    z = np.array([[48, 49, 48], [49, 50, 49], [48, 49, 48]]) * 100. * units('m^2/s^2')
+    u = np.array([[-2, 0, 2]] * 3) * units('m/s')
+    v = -u.T
+    uag, vag = ageostrophic_wind(z, 1 / units.sec, 100. * units.meter, 100. * units.meter,
+                                 u, v, dim_order='xy')
+    true = np.array([[0, 0, 0]] * 3) * units('m/s')
+    assert_array_equal(uag, true)
+    assert_array_equal(vag, true)
+
+
+def test_ageostrophic_geopotential():
+    """Test ageostrophic wind calculation with geopotential and ageostrophic wind."""
+    z = np.array([[48, 49, 48], [49, 50, 49], [48, 49, 48]]) * 100. * units('m^2/s^2')
+    u = v = np.array([[0, 0, 0]] * 3) * units('m/s')
+    uag, vag = ageostrophic_wind(z, 1 / units.sec, 100. * units.meter, 100. * units.meter,
+                                 u, v, dim_order='xy')
+
+    u_true = np.array([[2, 0, -2]] * 3) * units('m/s')
+    v_true = -u_true.T
+
+    assert_array_equal(uag, u_true)
+    assert_array_equal(vag, v_true)
+
+
 def test_streamfunc():
     """Test of Montgomery Streamfunction calculation."""
     t = 287. * units.kelvin