Update pvl_perez.m (#24)

* Update pvl_perez.m

These correlations are not numerically stable; i.e. you can get physically unreasonable values for inputs that are reasonable (I'll send figure separately to Cliff). And unreasonable inputs that generate unreasonable outputs are not flagged. This change warns user of unreasonable outputs and converts them to NaNs so unreasonable values are not propagated.

* Update pvl_perez.m

Added comments. Thanks.

* Update pvl_perez.m

Author: joallen6

pvl_perez.m

@@ -61,6 +61,9 @@
 %     reflected irradiance or the irradiance due to the beam.
 %     SkyDiffuse is a column vector vector with a number of elements equal to
 %     the input vector(s).
+%     User is warned if calculated SkyDiffuse is negative or greater than 
+%     extraterrestrial radiation; all outputs are NaN for unreasonable
+%     calculated SkyDiffuse.
 %   SkyDiffuse_Iso - the isotropic diffuse component of the solar radiation on an
 %     arbitrarily tilted surface defined by the Perez model as given in
 %     reference [3].
@@ -89,10 +92,10 @@
 %   Solar Energy vol. 81. pp. 254-267.
 %   [2] Perez, R., Seals, R., Ineichen, P., Stewart, R., Menicucci, D., 1987. A new
 %   simplified version of the Perez diffuse irradiance model for tilted
-%   surfaces. Solar Energy 39 (3), 221–232.
+%   surfaces. Solar Energy 39 (3), 221–232.
 %   [3] Perez, R., Ineichen, P., Seals, R., Michalsky, J., Stewart, R., 1990.
 %   Modeling daylight availability and irradiance components from direct
-%   and global irradiance. Solar Energy 44 (5), 271–289.
+%   and global irradiance. Solar Energy 44 (5), 271–289.
 %   [4] Perez, R. et. al 1988. The Development and Verification of the
 %   Perez Diffuse Radiation Model,.SAND88-7030, Sandia National
 %   Laboratories.
@@ -246,11 +249,21 @@
 
 % SkyDiffuse(ebinfilter) = DHI(ebinfilter).* 0.5.* (1-F1(ebinfilter)).*(1+cosd(SurfTilt)) +...
 %     F1(ebinfilter) .* A(ebinfilter)./ B(ebinfilter) + F2(ebinfilter).* sind(SurfTilt);
-con = (SkyDiffuse <= 0);
-SkyDiffuse(con) = 0;
-SkyDiffuse_Iso(con) = 0;
-SkyDiffuse_Cir(con) = 0;
-SkyDiffuse_Hor(con) = 0;
+% con = (SkyDiffuse <= 0);
+% SkyDiffuse(con) = 0;
+% SkyDiffuse_Iso(con) = 0;
+% SkyDiffuse_Cir(con) = 0;
+% SkyDiffuse_Hor(con) = 0;
+
+% screen invalid SkyDiffuse and warn user
+idx = SkyDiffuse < 0 | SkyDiffuse > max(HExtra);
+if any(idx)
+    SkyDiffuse(idx) = NaN;
+    SkyDiffuse_Iso(idx) = NaN;
+    SkyDiffuse_Cir(idx) = NaN;
+    SkyDiffuse_Hor(idx) = NaN;
+    warning('NaNs returned for SkyDiffuse < 0 or > max(HExtra).');
+end
 
 SkyDiffuse = SkyDiffuse(:);
 SkyDiffuse_Iso = SkyDiffuse_Iso(:);
@@ -390,4 +403,4 @@
 
     F1coeffs = PerezCoeffs(:,1:3);
     F2coeffs = PerezCoeffs(:,4:6);
-end
+end