Validation of MODIS and CYCLOPES LAI products using global field measurement data

Abstract

The objective of this paper is to quantitatively validate the global MODIS and CYCLOPES leaf area index (LAI) products using a global LAI field measurement database created on the basis of a literature review and major validation campaigns. The MODIS LAI product suite, containing the Terra Collection 4 (C4), Terra Collection 5 (C5) and Terra+Aqua combined C5, was analyzed, with considerable attention paid to the quality control (QC) information. The CYCLOPES V3.1 LAI product was similarly analyzed with regard to the status map (SM) layer. In general, the MODIS LAI has improved consistently over all releases. MODIS C5 data retrieved with the main algorithm (QC<64) and CYCLOPES data showed a similar range of uncertainties (1.0–1.2). Uncertainties for the best MODIS C5 (QC=0) and CYCLOPES (SM=0) estimates were around 0.9–1.1. The overall mean differences between the best MODIS C5 and CYCLOPES were within ±0.10. The highest correspondence was obtained for woody biomes from the best MCD15 C5 data (RMSE=0.80). Results indicate that the uncertainties in current LAI products (around ±1.0) are still unable to meet the accuracy requirement of GCOS (±0.5). Although there are limitations, we recommend MODIS C5 retrieved with the main algorithm (QC<64) and CYCLOPES for the user community. This study demonstrates the necessity of exploring uncertainties related to the true and effective LAIs separately, and reveals the importance of referring to the quality assessment information. More field measurements are required for further studies, which should focus on under-sampled biome types and areas.