A Comparison of Tropical Rainforest Phenology Retrieved From Geostationary (SEVIRI) and Polar-Orbiting (MODIS) Sensors Across the Congo Basin

Abstract

The seasonal and interannual dynamics of tropical rainforests play a critical role in the global carbon cycle and climate change. This paper retrieved and compared land surface phenology from observations acquired by the Spinning Enhanced Visible and Infrared Imager (SEVIRI) onboard geostationary satellites and the Moderate Resolution Imaging Spectroradiometer (MODIS) on polar-orbiting satellites over the Congo Basin. To achieve this, we first retrieved canopy greenness cycles (CGCs) and their transition timing from two-band enhanced vegetation index (EVI2) derived from SEVIRI and MODIS data between 2006 and 2013. We then assessed the influences of SEVIRI and MODIS data quality on the reconstruction of the EVI2 temporal trajectory, the detection of the CGC onset and end timing, and the total number of successful CGC retrievals. The significance of influences was determined using the one-tailed two-sample Kolmogorov-Smirnov test. The results indicate that diurnal SEVIRI observations greatly increased the probability of capturing cloud-free daily EVI2 in the rainforest-dominated region of the Congo Basin, where the proportion of good quality (PGQ) observations during a CGC was up to 80% higher than that from MODIS. As a result, the double annual CGCs of the Congo Basin rainforests were well identified from SEVIRI but sparsely detected from MODIS, whereas the single annual CGC in the savanna-dominated northern and southern Congo Basin was successfully retrieved from both SEVIRI and MODIS. Moreover, the decreases of PGQ in an EVI2 time series were found to significantly increase the uncertainties of retrieved phenological timings and increase the probabilities of CGC retrieval failures. Index Terms-Congo basin, Moderate Resolution Imaging Spectroradiometer (MODIS), Spinning Enhanced Visible andInfrared Imager (SEVIRI), tropical rainforest phenology.

Characterizing Land Cover Impacts on the Responses of Land Surface Phenology to the Rainy Season in the Congo Basin

“…According to a previous study on LSP retrieval uncertainty in Congo Basin, a three-day EVI2 time series from SEVIRI is able to provide sufficient cloud-free data for LSP retrieval purposes [16]. In order to reduce data volume while retaining a fine temporal resolution for CGC detection, we composited the daily angularly-corrected EVI2 into a three-day EVI2 time series by extracting the maximum EVI2 acquired under cloud-free conditions.…”

“…LSP in the Congo Basin rainforest has been previously generated in studies using data acquired by sensors such as the Moderate Resolution Imaging Spectroradiometer and Satellite Pour l'Observation de la Terre Vegetation, in which the observations were averaged over multiple years to reduce cloud contamination [13,14]. However, the inter-annual variations in Congo Basin rainforest LSP have not been retrieved until recently, when the observations from the METEOSAT Second Generation series of geostationary satellites become available [15][16][17]. On the other hand, since ground meteorological measurements in the Congo Basin are extremely sparse [2], satellite measurements represent the only viable way to retrieve rainfall seasonality in this region.…”

“…The characteristics of LSP across the Congo Basin vary across different ecosystems [13,14,16,21]. Results from previous studies have shown that the phenology of rainforest trees is highly asynchronous, and characterized by the simultaneous abscissions of old leaves and emergences of new leaves [22][23][24].…”

“…In contrast, other types of land cover in the Congo Basin present much stronger seasonal leaf variations with distinct growing and dormant phases. In order to present a more precise characterization of LSP, CGC was employed to represent the seasonal variation in canopy greenness extracted from the time series of the SEVIRI two-band enhanced vegetation index (EVI2) [16].…”

“…Previous studies have shown that there are two annual CGCs for the rainforests in ECB and a single Remote Sens. 2017, 9, 461 3 of 17 annual CGC for other types of land cover [13,16]. To avoid confusion, we hereby refer to the CGC or the rainy season initiated before and after July as the first and the second cycle, respectively.…”