# Derived Parameters¶

POWER provides an estimate of the total Photosynthetically Active Radiation (PAR) is available in all temporal levels from 1984 to within months of Near Real Time (NRT) for the daily. Total PAR from CERES SYN1Deg Ed. 4.1 is computed from both diffuse and direct horizontal irradiance PAR, and is available in both clear sky and all sky conditions. The methodology and validation of these parameters is described in Su et al., 2007.

The daily total PAR clear sky is not available for GEWEX SRB R4-IP; it is only available from January 1, 2001. The monthly total PAR is available for both clear sky and all sky from CERES SYN1deg Edition 4.1 from 2001.

Equation: Total Photosynthetically Active Radiation (PAR)

Total PAR is the sum of diffuse and direct PAR irradiances.

\begin{align} \text{PAR Total} = \text{Diffuse Incident PAR} + \text{Direct Incident PAR} \end{align}

The POWER derived Direct Normal Irradiance (DNI) is available in all temporal levels from 1984 (2001 for hourly) to months of Near Real Time (NRT). DNI is computed from both GEWEX SRB 4-IP and CERES SYN1Deg Ed. 4.1. DNI is the irradiance incident to a horizontal plane normal (perpendicular) to the direction of the sun's position. It is available in both all sky and clear sky conditions at the surface of the Earth and at the Top-Of-Atmosphere (TOA).

DNI is the direct irradiance incident divided by the cosine of solar zenith angle.

\begin{align}\ \text{DNI} &= \text{Direct Irradiance Incident} / \text{CSZA} \end{align}
\begin{align} Where&: \\ & DNI:\text{The direct normal irradiance.} \\ & CSZA:\text{The cosine of solar zenith angle.} \\ \end{align}

### Surface Albedo¶

The POWER derived Surface Albedo is available in all temporal levels from 1984 (2001 for hourly) to within months of Near Real Time (NRT). Surface Albedo is computed from both GEWEX SRB 4-IP and CERES SYN1Deg Ed. 4.1. The Surface Albedo is the ratio of the sunlight reflected by the surface of the Earth to the total solar energy incident on the surface. It is available in both all sky and clear sky conditions at the surface.

Equation: Surface Albedo

Surface Albedo is the upward irradiance incident at the surface of the Eathr divided by downward irradiance incident.

## Indexes¶

### Insolation Clearness Index¶

The POWER derived Insolation Clearness Index is available in all temporal levels from 1984 (2001 for hourly) to within months of Near Real Time (NRT). Insolation Clearness Index is computed from both GEWEX SRB 4-IP and CERES SYN1Deg Ed. 4.1. The Insolation Clearness Index is the fraction representing clearness of the atmosphere; the all sky insolation that is transmitted through the atmosphere to strike the surface of the Earth divided by the average of top of the atmosphere total solar irradiance incident. It is available in both all sky and clear sky conditions at the surface.

Equation: Insolation Clearness Index

To compute the Insolation Clearness Index the total solar irradiance incident is divided by the top of the atmosphere irradiance.

\begin{align} Where&: \\ & \text{KT}:\text{Insolation clearness index.} \\ & \text{TOA Irradiance}:\text{Top of the atmosphere irradiance.} \\ \end{align}

### Normalized Insolation Clearness Index¶

The POWER derived Normalized Insolation Clearness Index is available in all temporal levels from 1984 (2001 for hourly) to within months of Near Real Time (NRT). Normalized Insolation Clearness Index is computed from both GEWEX SRB 4-IP and CERES SYN1Deg Ed. 4.1. The Normalized Insolation Clearness Index is the average zenith angle-independent expression of the all sky insolation clearness index. It is available in both all sky and clear sky conditions at the surface.

Equation: Normalized Insolation Clearness Index

To compute the Insolation Clearness Index the total solar irradiance incident is divided by the top of the atmosphere irradiance.

\begin{align}\ \text{NKT} &= \text{KT} / (1.031 * (-1.4 / (0.9 + 9.4 * \text{CSZA}))^2 + 0.1) \end{align}
\begin{align} Where&: \\ & \text{NKT}:\text{Normalized insolation clearness index.} \\ & \text{KT}:\text{Insolation clearness index.} \\ & \text{CSZA}:\text{The cosine of solar zenith angle.} \\ \end{align}

### Clear Sky Days¶

The POWER derived Clear Sky Days is available in all temporal levels from 1984 (2001 for hourly) to within months of Near Real Time (NRT). Normalized Insolation Clearness Index is computed from both GEWEX SRB 4-IP and CERES SYN1Deg Ed. 4.1. The Clear Sky Days is the count of the days when daytime cloud amount is less than 10 percent.