Earthdata Forum

Hello, I have some questions regarding the Instantaneous Photosynthetically Available Radiation (IPAR) product (both L2 and L3 level) over the ocean.

Does Instantaneous Photosynthetically Available Radiation (IPAR) apply exclusively to clear-sky (cloud-free) conditions?
I downloaded PACE_OCI L2 products:
PACE_OCI.20240316T092652.L2.PAR.V2_0.NRT.nc
PACE_OCI.20240316T092652.L2.OC_BGC.V2_0.NRT.nc
PACE_OCI.20240316T092652.L2.CLDMASK.V2_0.NRT.nc
PACE_OCI.20240316T092652.L2.CLD.V2_0.NRT.nc
PACE_OCI.20240316T092652.L2.OC_IOP.V2_0.NRT.nc

I found that the variables 'ipar_***' and 'par_***' in the PACE_OCI.20240316T092652.L2.PAR.V2_0.NRT.nc are valid under both clearsky and cloudsky (checked the ''cloud_flag" in the PACE_OCI.20240316T092652.L2.CLDMASK.V2_0.NRT.nc).

I reviewed the algorithm for 'IPAR' on the Ocean Color website (https://oceancolor.gsfc.nasa.gov/resources/atbd/ipar/). The algorithm is based on the reference: "Carder, K. L., Chen, F. R., & Hawes, S. K. (2003). MODIS Ocean Science Team Algorithm Theoretical Basis Document 20: Instantaneous Photosynthetically Available Radiation and Absorbed Radiation by Phytoplankton." However, the document only addresses the calculation of 'IPAR' under clear-sky conditions.

I’m curious if someone from the Ocean Color team could provide clarification on the PACE_OCI PAR product?

Thank you.

Hello,

A short description of each:
IPAR: PAR at the ocean surface at the time of the satellite observation
PAR: Daily mean photosynthetically available radiation at the ocean surface

A longer description of PAR (from our product suites description page; https://oceancolor.gsfc.nasa.gov/resources/docs/product-suites/):
Phytoplankton pigments absorb sunlight and use it for photosynthesis, but not all light can be absorbed and used, such as ultraviolet light can damage the cell. Phytoplankton pigments (chlorophyll and accessory pigments) absorb light in the wavelength range of 400-700 nm. This part of the solar radiation can be used in photosynthesis and the quantum energy flux of this wavelength range is called photosynthetically available radiation. PAR changes seasonally and varies depending on the latitude and time of day.

The PAR ATBD lives here: https://oceancolor.gsfc.nasa.gov/resources/atbd/par/

Take a look and let us know if you would like more clarification.

Hello,

Thank you for your reply.

The PAR ATBD appears to only describe the calculation of PAR under clear-sky conditions, while the PACE OCI PAR product currently provides valid values for both clear-sky and cloudy-sky conditions.

I am curious about the algorithm used to derive PAR under cloudy skies. Does it incorporate any cloud properties?

What cloud optical model is used, and is it the same as the one utilized in the PACE OCI CLD suite? Modis collection 6 cloud model?

What is the source of the cloud information? Merra2?

Additionally, how is ocean albedo calculated under cloudy skies, and are there any specific assumptions involved?

Thank you very much.

Best regards