Summary:

## Solar Zenith Angle

The CLM (Community Land Model) uses the same formulation for the solar zenith angle as the Community Atmosphere Model. The cosine of the solar zenith angle (μ) is calculated using the latitude (φ), solar declination angle (δ), and solar hour angle (h).

The solar hour angle (h) is determined by the calendar day (d) and longitude (θ). The solar declination angle (δ) is calculated based on the Earth's obliquity (ε) and true longitude (λ).

The Earth's obliquity (ε) is calculated using a series expansion with a constant term (ε*) and various amplitude, mean rate, and phase terms. The true longitude of the Earth (λ) is calculated using the mean longitude (λm), Earth's eccentricity (e), and the longitude of the perihelion relative to the moving vernal equinox (ω̃).

The longitude of the perihelion (Π) is determined by the cosine and sine series expansions for the Earth's eccentricity (e). The general precession in longitude (ψ) is also calculated using various constants and series expansion terms.

The provided table lists the key orbital parameters used in these calculations, including the Earth's obliquity (ε*), the general precession constant (ψ̃), and the constant term (ζ).