Add Rm, Cm, Lv, Lf, Lm

Calculate gas constant and specific heat capacity of moist air. Add temperature-dependent latent heat quantity calculations.

src/metpy/calc/thermo.py

@@ -29,6 +29,203 @@
 exporter = Exporter(globals())
 
 
+@exporter.export
+@preprocess_and_wrap(wrap_like='specific_humidity')
+@check_units('[dimensionless]')
+def moist_air_gas_constant(specific_humidity):
+    r"""Calculate R_m, the specific gas constant for a parcel of moist air.
+
+    Parameters
+    ----------
+    specific_humidity : `pint.Quantity`
+
+    Returns
+    -------
+    `pint.Quantity`
+        Specific gas constant
+
+    Examples
+    --------
+    >>> from metpy.calc import moist_air_gas_constant
+    >>> from metpy.units import units
+    >>> moist_air_gas_constant(11 * units('g/kg'))
+    <Quantity(288.966723, 'joule / kelvin / kilogram')>
+
+    See Also
+    --------
+    moist_air_specific_heat_pressure
+
+    Notes
+    -----
+    .. math:: R_m = (1 - q_v) R_a + q_v R_v
+
+    Eq 16, [Romps2017]_ using MetPy-defined constants in place of cited values.
+
+    """
+    return ((1 - specific_humidity) * mpconsts.dry_air_gas_constant
+            + specific_humidity * mpconsts.water_gas_constant)
+
+
+@exporter.export
+@preprocess_and_wrap(wrap_like='specific_humidity')
+@check_units('[dimensionless]')
+def moist_air_specific_heat_pressure(specific_humidity):
+    r"""Calculate C_pm, the specific heat at constant pressure for a moist air parcel.
+
+    Parameters
+    ----------
+    specific_humidity : `pint.Quantity`
+
+    Returns
+    -------
+    `pint.Quantity`
+        Specific heat capacity of air at constant pressure
+
+    Examples
+    --------
+    >>> from metpy.calc import moist_air_specific_heat_pressure
+    >>> from metpy.units import units
+    >>> moist_air_specific_heat_pressure(11 * units('g/kg'))
+    <Quantity(1014.07575, 'joule / kelvin / kilogram')>
+
+    See Also
+    --------
+    moist_air_gas_constant
+
+    Notes
+    -----
+    .. math:: c_{pm} = (1 - q_v) c_{pa} + q_v c_{pv}
+
+    Eq 17, [Romps2017]_ using MetPy-defined constants in place of cited values.
+
+    """
+    return ((1 - specific_humidity) * mpconsts.dry_air_spec_heat_press
+            + specific_humidity * mpconsts.wv_specific_heat_press)
+
+
+@exporter.export
+@preprocess_and_wrap(wrap_like='temperature')
+@check_units('[temperature]')
+def water_latent_heat_vaporization(temperature):
+    r"""Calculate the latent heat of vaporization for water.
+
+    Accounts for variations in latent heat across valid temperature range.
+
+    Parameters
+    ----------
+    temperature : `pint.Quantity`
+
+    Returns
+    -------
+    `pint.Quantity`
+        Latent heat of vaporization
+
+    Examples
+    --------
+    >>> from metpy.calc import water_latent_heat_vaporization
+    >>> from metpy.units import units
+    >>> water_latent_heat_vaporization(20 * units.degC)
+    <Quantity(2453677.15, 'joule / kilogram')>
+
+    See Also
+    --------
+    water_latent_heat_sublimation, water_latent_heat_melting
+
+    Notes
+    -----
+    Assumption of constant :math:`C_pv` limits validity to :math:`0 \deg \en 100 \deg C` range.
+
+    .. math:: L = L_0 - (c_{pl} - c_{pv}) (T - T_0)
+
+    Eq 15, [Ambaum2020]_, using MetPy-defined constants in place of cited values.
+
+    """
+    return (mpconsts.water_heat_vaporization
+            - (mpconsts.water_specific_heat - mpconsts.wv_specific_heat_press)
+            * (temperature - mpconsts.water_triple_point_temperature))
+
+
+@exporter.export
+@preprocess_and_wrap(wrap_like='temperature')
+@check_units('[temperature]')
+def water_latent_heat_sublimation(temperature):
+    r"""Calculate the latent heat of sublimation for water.
+
+    Accounts for variations in latent heat across valid temperature range.
+
+    Parameters
+    ----------
+    temperature : `pint.Quantity`
+
+    Returns
+    -------
+    `pint.Quantity`
+        Latent heat of vaporization
+
+    Examples
+    --------
+    >>> from metpy.calc import water_latent_heat_sublimation
+    >>> from metpy.units import units
+    >>> water_latent_heat_sublimation(-15 * units.degC)
+    <Quantity(2837991.13, 'joule / kilogram')>
+
+    See Also
+    --------
+    water_latent_heat_vaporization, water_latent_heat_melting
+
+    Notes
+    -----
+    .. math:: L_s = L_{s0} - (c_{pl} - c_{pv}) (T - T_0)
+
+    Eq 18, [Ambaum2020]_, using MetPy-defined constants in place of cited values.
+
+    """
+    return (mpconsts.water_heat_sublimation
+            - (mpconsts.ice_specific_heat - mpconsts.wv_specific_heat_press)
+            * (temperature - mpconsts.water_triple_point_temperature))
+
+
+@exporter.export
+@preprocess_and_wrap(wrap_like='temperature')
+@check_units('[temperature]')
+def water_latent_heat_melting(temperature):
+    r"""Calculate the latent heat of melting for water.
+
+    Accounts for variations in latent heat across valid temperature range.
+
+    Parameters
+    ----------
+    temperature : `pint.Quantity`
+
+    Returns
+    -------
+    `pint.Quantity`
+        Latent heat of vaporization
+
+    Examples
+    --------
+    >>> from metpy.calc import water_latent_heat_melting
+    >>> from metpy.units import units
+    >>> water_latent_heat_melting(-15 * units.degC)
+    <Quantity(365662.294, 'joule / kilogram')>
+
+    See Also
+    --------
+    water_latent_heat_vaporization, water_latent_heat_sublimation
+
+    Notes
+    -----
+    .. math:: L_m = L_{m0} + (c_{pl} - c_{pi}) (T - T_0)
+
+    Body text below Eq 20, [Ambaum2020]_, derived from Eq 15, Eq 18.
+    Uses MetPy-defined constants in place of cited values.
+
+    """
+    return (mpconsts.water_heat_fusion
+            - (mpconsts.water_specific_heat - mpconsts.ice_specific_heat)
+            * (temperature - mpconsts.water_triple_point_temperature))
+
+
 @exporter.export
 @preprocess_and_wrap(wrap_like='temperature', broadcast=('temperature', 'dewpoint'))
 @check_units('[temperature]', '[temperature]')

tests/calc/test_thermo.py

@@ -20,7 +20,8 @@
                         isentropic_interpolation, isentropic_interpolation_as_dataset, k_index,
                         lcl, lfc, lifted_index, mixed_layer, mixed_layer_cape_cin,
                         mixed_parcel, mixing_ratio, mixing_ratio_from_relative_humidity,
-                        mixing_ratio_from_specific_humidity, moist_lapse, moist_static_energy,
+                        mixing_ratio_from_specific_humidity, moist_air_gas_constant,
+                        moist_air_specific_heat_pressure, moist_lapse, moist_static_energy,
                         most_unstable_cape_cin, most_unstable_parcel, parcel_profile,
                         parcel_profile_with_lcl, parcel_profile_with_lcl_as_dataset,
                         potential_temperature, psychrometric_vapor_pressure_wet,
@@ -36,13 +37,62 @@
                         vapor_pressure, vertical_totals, vertical_velocity,
                         vertical_velocity_pressure, virtual_potential_temperature,
                         virtual_temperature, virtual_temperature_from_dewpoint,
-                        wet_bulb_potential_temperature, wet_bulb_temperature)
+                        water_latent_heat_melting, water_latent_heat_sublimation,
+                        water_latent_heat_vaporization, wet_bulb_potential_temperature,
+                        wet_bulb_temperature)
 from metpy.calc.thermo import _find_append_zero_crossings, galvez_davison_index
+from metpy.constants import (dry_air_gas_constant, dry_air_spec_heat_press, water_heat_fusion,
+                             water_heat_sublimation, water_heat_vaporization,
+                             water_triple_point_temperature)
 from metpy.testing import (assert_almost_equal, assert_array_almost_equal, assert_nan,
                            version_check)
 from metpy.units import is_quantity, masked_array, units
 
 
+def test_moist_air_gas_constant():
+    """Test calculation of gas constant for moist air."""
+    q = 9 * units('g/kg')
+    assert_almost_equal(moist_air_gas_constant(q), 288.62 * units('J / kg / K'), 2)
+    assert_almost_equal(moist_air_gas_constant(0), dry_air_gas_constant)
+
+
+def test_moist_air_specific_heat_pressure():
+    """Test calculation of specific heat for moist air."""
+    q = 9 * units('g/kg')
+    assert_almost_equal(moist_air_specific_heat_pressure(q), 1012.36 * units('J / kg /K'), 2)
+    assert_almost_equal(moist_air_specific_heat_pressure(0), dry_air_spec_heat_press)
+
+
+def test_water_latent_heat_vaporization():
+    """Test temperature-dependent calculation of latent heat of vaporization for water."""
+    temperature = 300 * units.K
+    # Divide out sig figs in results for decimal comparison
+    assert_almost_equal(water_latent_heat_vaporization(temperature) / 10**6,
+                        2.4375 * units('J / kg'), 4)
+    assert_almost_equal(water_latent_heat_vaporization(water_triple_point_temperature),
+                        water_heat_vaporization)
+
+
+def test_water_latent_heat_sublimation():
+    """Test temperature-dependent calculation of latent heat of sublimation for water."""
+    temperature = 233 * units.K
+    # Divide out sig figs in results for decimal comparison
+    assert_almost_equal(water_latent_heat_sublimation(temperature) / 10**6,
+                        2.8438 * units('J / kg'), 4)
+    assert_almost_equal(water_latent_heat_sublimation(water_triple_point_temperature),
+                        water_heat_sublimation)
+
+
+def test_water_latent_heat_melting():
+    """Test temperature-dependent calculation of latent heat of melting for water."""
+    temperature = 233 * units.K
+    # Divide out sig figs in results for decimal comparison
+    assert_almost_equal(water_latent_heat_melting(temperature) / 10**6,
+                        0.4192 * units('J / kg'), 4)
+    assert_almost_equal(water_latent_heat_melting(water_triple_point_temperature),
+                        water_heat_fusion)
+
+
 def test_relative_humidity_from_dewpoint():
     """Test Relative Humidity calculation."""
     assert_almost_equal(relative_humidity_from_dewpoint(25. * units.degC, 15. * units.degC),