Derived Parameters¶

Irradiance¶

Photosynthetically Active Radiation (PAR)¶

POWER provides an estimate of the total Photosynthetically Active Radiation (PAR) and it is available in all temporal levels from 1984 to within months of Near Real Time (NRT). Total PAR from CERES SYN1deg Ed. 4.2 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 CERES SYN1deg Ed. 4.2 product derives surface PAR fluxes within an energy-balanced radiative-transfer framework whose computed surface irradiances have been evaluated against ground-based reference measurements Rutan et al., 2015; CERES SYN1deg Ed4.2. The PAR retrieval approach attributed to Su et al., 2007 should be confirmed against surface-irradiance validation networks such as BSRN Driemel et al., 2018.

Only a limited number of BSRN sites have PAR measurements as ancillary data, and the satellite-based PAR data thus have been validated agaist only a limited number of BSRN sites, such as the SURFRAD sites. According to Su et al., 2007, the bias and RMS with a sample size of 1485 from Desert Rock, Nevada, under all-sky condition is 4.5 and 25.0 W/m2, respectively; under clear-sky conditions with a sample size of 583, the bias and RMS are 6.7 and 13.1, respectively.

The daily total PAR clear sky is not available for GEWEX SRB R4-IP; it is only available starting from January 1, 2001. The monthly total PAR is available for both clear sky and all sky from CERES SYN1deg Edition 4.2 starting from 2001. The GEWEX SRB Release 4 Integrated Product supplies the shortwave surface fluxes from which PAR is derived, and its surface irradiance estimates are documented and validated against the BSRN and PMEL ground networks GEWEX SRB R4-IP; Driemel et al., 2018. The PAR data, however, have not been validated.

Direct Normal Irradiance (DNI) and Diffuse Horizontal Irradiance (DHI)¶

The POWER derived Direct Normal Irradiance (DNI) is available in all temporal levels from 1984 (hourly starting from 2001) to months of Near Real Time (NRT). DNI is computed from both GEWEX SRB 4-IP and CERES SYN1deg Ed. 4.2. DNI is the irradiance incident to a 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 conventionally obtained by dividing the direct horizontal irradiance by the cosine of the solar zenith angle, a relationship standard in solar radiation modeling Iqbal, 1983; Gueymard, 2008. The GEWEX SRB R4-IP and CERES SYN1deg Ed. 4.2 source fluxes used here have been independently evaluated against surface reference measurements GEWEX SRB R4-IP; Rutan et al., 2015.

The validation of GEWEX SRB R4-IP can be found in Zhang et al., 2020, and the validation of CERES SYN1deg of different editions can be found in Rutan et al., 2015 and Zhang et al., 2024. The DNI derived from GEWEX SRB is monthly means derived empirically from monthly mean GHI and DHI, and the methodology can be found in Zhang et al. (2024). The DNI derived CERES SYN1deg is hourly mean derived from hourly GHI and DHI, and the methodology and validation of results can be found in Zhang et al. (2024). For instance, the Ed4.2 hourly DNI validation statistics bias/RMS/N against the BSRN for the years 2000 to 2024 are -1.27/186.94/3,510,867, and the units of bias and RMS are W/m2, where N is the number of data points.

The Diffuse Horizontal Irradiance (DHI) was corrected in a similar fashion. The Ed4.2 hourly DHI validation statistics of bias/RMS/N for the years 2000 to 2024 are 0.01/63.07/4,051,942, where the units of bias and RMS are W/m2 and N is the number of data points.

Surface Albedo¶

The POWER derived surface albedo is available in all temporal levels from 1984 (hourly starting from 2001) to within months of Near Real Time (NRT). Surface albedo is computed from both GEWEX SRB 4-IP and CERES SYN1deg Ed. 4.2. 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. Surface albedo is defined as the ratio of upwelling to downwelling shortwave irradiance, and the CERES and GEWEX SRB surface flux products underlying this ratio have been validated against ground observations Rutan et al., 2015; CERES SYN1deg Ed4.2. The broadband shortwave albedo derived from these fluxes is consistent with the energy-balanced approach documented for the SRB and BSRN reference frameworks GEWEX SRB R4-IP; Driemel et al., 2018.

For reference regarding the assessment of surface albedo, refer to Rutan et al., 2009.

Indexes¶

Insolation Clearness Index (KT)¶

The POWER derived insolation clearness index is available in all temporal levels from 1984 (hourly starting from 2001) to within months of Near Real Time (NRT). Insolation clearness index is computed from both GEWEX SRB 4-IP and CERES SYN1deg Ed. 4.2. 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 TOA total solar irradiance incident. It is available in both all-sky and clear-sky conditions at the surface. The clearness index KT is the standard dimensionless ratio of surface global irradiance to extraterrestrial (TOA) irradiance widely used to characterize atmospheric transmission Iqbal, 1983; Gueymard, 2008. The surface and TOA irradiances input to KT are taken from the validated GEWEX SRB R4-IP and CERES SYN1deg Ed. 4.2 flux products GEWEX SRB R4-IP; Rutan et al., 2015. The validation of GHI can be viewed as indirect validation of KT.

Clear Sky Days¶

The POWER derived clear-sky days is available in all temporal levels from 1984 (hourly starting from 2001) to within months of Near Real Time (NRT). Normalized insolation clearness index, a modified version of clearness index with the effect of solar zenith angle or airmass removed, is computed from both GEWEX SRB 4-IP and CERES SYN1deg Ed. 4.2. The clear-sky days is the count of the days when daytime cloud amount is less than 10%. Cloud-amount thresholds such as the 10% daytime limit used to identify clear-sky days follow conventions in surface radiation and cloud climatology assessments Rutan et al., 2015; CERES SYN1deg Ed4.2. While the threshold value of cloud fraction varies depending on scientific applications, POWER uses 10% as the threshold value to identify clear-sky days.