* Fix for issue #782

- Validity checks with new kwargs min_pw and max_pw
- Note added in docstring of epw.read_epw() concerning TMY3.epw files from energyplus.net
- corresponding tests

Author: tylunel

docs/sphinx/source/whatsnew/v0.7.0.rst

@@ -83,6 +83,12 @@ API Changes
       - Methods `PVSystem.ashraeiam`, `PVSystem.physicaliam` and
        `PVSystem.sapm_aoi_loss` are deprecated and will be removed in v0.8.
 
+* Changes related to spectral modifier (:issue:`782`):
+   * Changes to functions
+      - Added the argument `pw_min` and `pw_max`, default values 0.1 and 8 resp., 
+	to `atmosphere.first_solar_spectral_correction`. This function now returns NaN
+	if pw value higher than `pw_max`.
+
 * Calling :py:func:`pvlib.pvsystem.retrieve_sam` with no parameters will raise
   an exception instead of displaying a dialog.
 * The `times` keyword argument has been deprecated in the
@@ -145,6 +151,8 @@ Documentation
 * Edited docstring for `pvsystem.sapm` to remove DataFrame option for input
   `module`. The DataFrame option was never tested and would cause an error if
   used. (:issue:`785`)
+* Note warning about _TMY3.epw files retrieved from energyplus.net in docstring 
+  of `epw.read_epw`
 
 Removal of prior version deprecations
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

pvlib/atmosphere.py

@@ -320,8 +320,9 @@ def gueymard94_pw(temp_air, relative_humidity):
     return pw
 
 
-def first_solar_spectral_correction(pw, airmass_absolute, module_type=None,
-                                    coefficients=None):
+def first_solar_spectral_correction(pw, airmass_absolute,
+                                    module_type=None, coefficients=None,
+                                    min_pw=0.1, max_pw=8):
     r"""
     Spectral mismatch modifier based on precipitable water and absolute
     (pressure corrected) airmass.
@@ -364,6 +365,14 @@ def first_solar_spectral_correction(pw, airmass_absolute, module_type=None,
     airmass_absolute : array-like
         absolute (pressure corrected) airmass.
 
+    min_pw : float, default 0.1
+        minimum atmospheric precipitable water (cm). A lower pw value will be
+        automatically set to this minimum value to avoid model divergence.
+
+    max_pw : float, default 8
+        maximum atmospheric precipitable water (cm). If a higher value is
+        encountered it will be set to np.nan to avoid model divergence.
+
     module_type : None or string, default None
         a string specifying a cell type. Can be lower or upper case
         letters. Admits values of 'cdte', 'monosi', 'xsi', 'multisi',
@@ -422,16 +431,18 @@ def first_solar_spectral_correction(pw, airmass_absolute, module_type=None,
     # *** Pwat ***
     # Replace Pwat Values below 0.1 cm with 0.1 cm to prevent model from
     # diverging"
-
-    if np.min(pw) < 0.1:
-        pw = np.maximum(pw, 0.1)
-        warn('Exceptionally low Pwat values replaced with 0.1 cm to prevent' +
-             ' model divergence')
+    pw = np.atleast_1d(pw)
+    pw = pw.astype('float64')
+    if np.min(pw) < min_pw:
+        pw = np.maximum(pw, min_pw)
+        warn('Exceptionally low pw values replaced with {0} cm to prevent '
+             'model divergence'.format(min_pw))
 
     # Warn user about Pwat data that is exceptionally high
-    if np.max(pw) > 8:
-        warn('Exceptionally high Pwat values. Check input data:' +
-             ' model may diverge in this range')
+    if np.max(pw) > max_pw:
+        pw[pw > max_pw] = np.nan
+        warn('Exceptionally high pw values replaced by np.nan: '
+             'check input data.')
 
     # *** AMa ***
     # Replace Extremely High AM with AM 10 to prevent model divergence

pvlib/iotools/epw.py

@@ -67,9 +67,9 @@ def read_epw(filename, coerce_year=None):
     ===============   ======  =========================================
 
 
-    =============================       ==============================================================================================================================================================
+    =============================       ==================================================================================================================================================================================================================  # noqa: E501
     EPWData field                       description
-    =============================       ==============================================================================================================================================================
+    =============================       ==================================================================================================================================================================================================================
     index                               A pandas datetime index. NOTE, times are set to local standard time (daylight savings is not included). Days run from 0-23h to comply with PVLIB's convention
     year                                Year, from original EPW file. Can be overwritten using coerce function.
     month                               Month, from original EPW file
@@ -99,14 +99,14 @@ def read_epw(filename, coerce_year=None):
     ceiling_height                      Height of cloud base above local terrain (7777=unlimited), meter
     present_weather_observation         Indicator for remaining fields: If 0, then the observed weather codes are taken from the following field. If 9, then missing weather is assumed.
     present_weather_codes               Present weather code, see [1], chapter 2.9.1.28
-    precipitable_water                  Total precipitable water contained in a column of unit cross section from earth to top of atmosphere, cm
+    precipitable_water                  Total precipitable water contained in a column of unit cross section from earth to top of atmosphere, cm. Note that some old *_TMY3.epw files may have incorrect unit if it was retrieved from www.energyplus.net.
     aerosol_optical_depth               The broadband aerosol optical depth per unit of air mass due to extinction by aerosol component of atmosphere, unitless
     snow_depth                          Snow depth in centimeters on the day indicated, (999 = missing data)
     days_since_last_snowfall            Number of days since last snowfall (maximum value of 88, where 88 = 88 or greater days; 99 = missing data)
     albedo                              The ratio of reflected solar irradiance to global horizontal irradiance, unitless
     liquid_precipitation_depth          The amount of liquid precipitation observed at indicated time for the period indicated in the liquid precipitation quantity field, millimeter
     liquid_precipitation_quantity       The period of accumulation for the liquid precipitation depth field, hour
-    =============================       ==============================================================================================================================================================
+    =============================       ==================================================================================================================================================================================================================
 
     References
     ----------

pvlib/test/test_atmosphere.py

@@ -128,6 +128,27 @@ def test_first_solar_spectral_correction_ambiguous():
         atmosphere.first_solar_spectral_correction(1, 1)
 
 
+def test_first_solar_spectral_correction_range():
+    with pytest.warns(UserWarning, match='Exceptionally high pw values'):
+        out = atmosphere.first_solar_spectral_correction(np.array([.1, 3, 10]),
+                                                         np.array([1, 3, 5]),
+                                                         module_type='monosi')
+    expected = np.array([0.96080878, 1.03055092,        nan])
+    assert_allclose(out, expected, atol=1e-3)
+    with pytest.warns(UserWarning, match='Exceptionally high pw values'):
+        out = atmosphere.first_solar_spectral_correction(6, 1.5, max_pw=5,
+                                                         module_type='monosi')
+    with pytest.warns(UserWarning, match='Exceptionally low pw values'):
+        out = atmosphere.first_solar_spectral_correction(np.array([0, 3, 8]),
+                                                         np.array([1, 3, 5]),
+                                                         module_type='monosi')
+    expected = np.array([0.96080878, 1.03055092, 1.04932727])
+    assert_allclose(out, expected, atol=1e-3)
+    with pytest.warns(UserWarning, match='Exceptionally low pw values'):
+        out = atmosphere.first_solar_spectral_correction(0.2, 1.5, min_pw=1,
+                                                         module_type='monosi')
+
+
 def test_kasten96_lt():
     """Test Linke turbidity factor calculated from AOD, Pwat and AM"""
     amp = np.array([1, 3, 5])