Additional solar geometry

[chrisroadmap]

The current climateforcing.solar module calculates instantaneous and time-mean solar zenith angle, but of course we can be more generally useful:

• allow for array input for time in cos_mean_solar_zenith_angle
• calculate direct and diffuse (time-averaged) irradiance on any non-horizontal plane
  • calculate (time-averaged) solar azimuth angle
  • diffuse irradiance on a tilted plane*

*this problem needs to be kept tractable and within the limitations of what can be provided by climate models - many sophisticated tilted irradiance relationships exist in the literature but much beyond the isotropic diffuse model might be overkill given that most climate models use plane-parallel and two-stream radiation codes.

[byersiiasa]

Thanks @chrisroadmap - will share with Alessio. Nothing urgent form our side, but we may in time move away from current R implementation into a Python option - for conversion of horizontal to vertical radiation, so I think what you are doing is highly relevant!