A New Clear-Sky Method for Assessing Photosynthetically Active Radiation at the Surface Level

Abstract

A clear-sky method to estimate the photosynthetically active radiation (PAR) at the surface level in cloudless atmospheres is presented and validated. It uses a fast and accurate approximation adopted in several radiative transfer models, known as the k-distribution method and the correlated-k approximation, which gives a set of fluxes accumulated over 32 established wavelength intervals. A resampling technique, followed by a summation, are applied over the wavelength range [0.4, 0.7] µm in order to retrieve the PAR fluxes. The method uses as inputs the total column contents of ozone and water vapor, and optical properties of aerosols provided by the Copernicus Atmosphere Monitoring Service. To validate the method, its outcomes were compared to instantaneous global photosynthetic photon flux density (PPFD) measurements acquired at seven experimental sites of the Surface Radiation Budget Network (SURFRAD) located in various climates in the USA. The bias lies in the interval [−12, 61] µmol m −2 s −1 ([−1, 5] % in values relative to the means of the measurements at each station). The root mean square error ranges between 37 µmol m −2 s −1 (3%) and 82 µmol m −2 s −1 (6%). The squared correlation coefficient fluctuates from 0.97 to 0.99. This comparison demonstrates the high level of accuracy of the presented method, which offers an accurate estimate of PAR fluxes in cloudless atmospheres at high spatial and temporal resolutions useful for several bio geophysical models.

Climatic traits on daily clearness and cloudiness indices

“…Although radiation itself is a proxy of cloudiness (Nyamsi et al, 2019;Oliphant et al, 2006), we use rainfall to separate the data sample into rainy and dry days keeping in mind the connection of our stochastic description of attenuation with the family of stochastic models of Rodríguez-Iturbe and and others. This way, bimodal distributions of c and k vanish in most of the cases when data are divided into rainy and dry days, reinforcing the idea that the modes are strongly related to clear and overcast sky conditions.…”

Assessment of five different methods for the estimation of surface photosynthetically active radiation from satellite imagery at three sites – application to the monitoring of indoor soft fruit crops in southern UK

“…We selected the methods of Udo and Aro (1999), and Jacovides et al (2004). We also referred to Yu et al (2015) which provides an excellent summary of previous studies to add a third method originally optimised for England: Szeicz (1974). The methods will respectively be named Udo et Aro, Jacovides and Szeicz in the rest of the document.…”

Using Copernicus Atmosphere Monitoring Service (CAMS) Products to Assess Illuminances at Ground Level under Cloudless Conditions

“…The spectral resampling technique has been described in Wandji Nyamsi et al [16,18,19] for the UV and PAR ranges respectively and is outlined here for a better understanding. The proposed method is a pure modeling concept with radiative transfer simulations made with libRadtran.…”

“…This paper describes the method to derive the illuminance from the clearness indices in the 13 KBs, which are themselves calculated from runs of libRadtran with inputs from CAMS. The method is not entirely new, since it has already been experimented in the case of UV fluxes using the most improved version of absorption parametrization included in libRadtran and named katoandwandji ( [16,17]) and photosynthetically active radiation (PAR) [18,19], but it has never been tested in the case of illuminance nor in combination with a spectral response. This study is part of an extensive project whose overarching goal is to freely and publicly offer datasets of illuminance at any time and any location in an operational way similar to the McClear model [13,14] with inputs on atmosphere variables originating from CAMS.…”