|
Approach
|
|
Approach
A modified version of the Global Energy and Water Cycle Experiment (GEWEX)
/Surface Radiation Budget (
SRB) model (Whitlock et al., 1995; Pinker et al., 1995) was used to generate
PAR and related parameters. The approach is based on a
physical inference scheme to derive surface spectral radiatiative components
from satellite observations at the top of atmosphere. Model output parameters
are:
total surface PAR (downwelling and upwelling)
diffuse surface PAR (downwelling and upwelling)
Top of the atmosphere (TOA) upwelling PAR
Conversion Factors (CF), defined as:
as computed from independently derived PAR and total
shortwave radiative (SW) flux. Global climatologies of CFs are useful
because these factors can later be applied to independently derived
values of SW, produced with models which do not have capabilities to estimate
PAR directly.
The widely used quantum sensor for measuring PAR,
provides photosynthetic photon flux density (PPFD) in units of Einstein
per m**2 per second. It is an estimate of the number of photons in the
400-700 nm spectral region which are available to initiate photochemical
reaction within a cell. It is the PPFD which should be related to dry
matter production of plants or CO2 fixation in phytoplankton. Usually,
when PAR is available in units of W/m**2, a nominal conversion factor
at 485 nm is used to convert it into units of photon flux density. Preferably,
such conversion should be done by spectral integration. Since the inference
model we use is spectral, it is possible to get an improved estimate
of PPFD.
Maintained by Banglin Zhang zhang@atmos.umd.edu
and Chuan Li cli@atmos.umd.edu
|
|
