Impact on satellite retrievals of temporal changes in Karenia brevis harmful algal blooms in the West Florida Shelf

Abstract

We examine the impact of temporal changes on satellite retrievals of Karenia brevis Harmful Algal blooms (KB HABS) in the West Florida Shelf (WFS). These impacts are compared for retrievals from both VIIRS and MODIS-A using a number of retrieval techniques. The comparisons include our recently developed neural network (NN) technique. The neural network, previously developed by us, was trained on 10,000 data point part of a synthetic data of 20,000 inherent optical properties (IOPs) based on a wide range of IOP parameters for a large variety of natural conditions based on the NOMAD data. The NN then uses as inputs the remote sensing reflectance (Rrs) measurements at 486, 551 and 671 and 488, 555 and 667 nm which are available from VIIRS and MODIS-A respectively, to retrieve phytoplankton absorption at 443 nm in satellite images. This information, when combined with backscatter information has been shown by us to be effective for obtaining retrievals of KB HB HABs in the WFS. Other retrieval algorithms included in the present comparison are the blue/green OCI/OC3, the Generalized Inherent Optical Property (GIOP) model and the Quasi-Analytical Algorithm, (QAA version 5). The accuracies of VIIRS retrievals using all five techniques were then compared against all the in-situ measurements available over the 2012-2016 period for which concurrent or near concurrent match ups could be obtained with VIIRS retrievals. Analysis of retrieval statistics showed that the NN technique achieved the best accuracies. The analysis highlighted the impact of temporal variabilities on retrieval accuracies. The results showed the importance of having a shorter overlap time window between in-situ measurement and satellite retrieval. Retrievals where the maximum permissible overlap time window was shortened to 15 minutes, exhibited very significantly improved retrieval accuracies over those that were obtained with a 100 minute overlap time window, Retrievals that relaxed the overlap time window between in-situ measurement and satellite retrievals to simply the same day were hopelessly inaccurate by comparison. These results are believed to reflect the impact of temporal variabilities on retrieval accuracies. They underline the time limitations associated with satellite retrievals of inherently variable conditions and a changing scene. The temporal effects associated with KB HABs retrievals in the WFS were also examined by using images from consecutive VIIRS - MODIS-A – VIIRS overpasses overlapping the same parts of the WFS containing KB blooms. The consecutive images, all within an approximately 100 minute period, appear to confirm the changing bloom features. The temporal behavior of the KB blooms in the WFS over short time periods (less than100 minuntes) was further confirmed by a recent set of in-situ field measurements off the coast of Sarasota.