Satellite estimation of photosynthetically active radiation in Southeast Asia: Impacts of smoke and cloud cover

Since large‐scale variations in photosynthetically active radiation (PAR) influence the terrestrial carbon sources and sinks through the plant photosynthesis variations, large‐scale evaluation of PAR is required. In the present study a simple PAR estimation model was developed for Southeast Asia, where large‐scale forest fires occurred during El Niño years. The model considered the smoke aerosol released by forest fires using satellite‐based smoke detection methods. A comparison study with ground‐based solar radiation data for Malaysia and Thailand indicated that the current model could estimate monthly PAR with 10% (root‐mean‐square) accuracy and would successfully trace the seasonal and year‐to‐year variations in PAR, including the forest fire periods. During the peak‐smoke month in Indonesia, September 1997, the reduction of PAR by smoke reached 63–75% in the center of the Kalimantan and Sumatra Islands. From the analyses of the smoke and cloud cover impacts on PAR in 1997–1999, annual PAR variations were found to be mostly regulated by smoke variations on the Malay Peninsula, Sumatra, and Kalimantan Islands and cloud cover variations on the Indo‐China Peninsula. Thus annual variations in PAR changed with location. These variations did not simply correlate with year‐to‐year variations in cloud cover associated with the El Niño and La Niña cycle, but exhibited more complicated spatial variations due to the existence of smoke in Southeast Asia.