Validation of OCM-2 sensor performance in retrieving chlorophyll and TSM along the southwest Bay of Bengal coast

Abstract

The Chlorophyll and Total Suspended Matter (TSM) data retrieved from Ocean Colour Monitor (OCM-2) onboard Oceansat-2 were tested for the accuracy using in-situ measurements made along the southwest Bay of Bengal coast during cruises and monthly samplings synchronized with satellite overpass from January 2010 to May 2011. The observed range of in-situ chlorophyll a and TSM concentrations were 0.10-4.60 μgl-1 and 12.70-34.56 mgl-1 respectively, while OCM-2 derived chlorophyll a and TSM concentration ranged from 0.324 to 1.552 μgl-1 and 3.537 to 32.11 mgl-1, respectively. The in-situ dataset was grouped into low (0.1-0.5 μgl-1), moderate (0.51-1.0 μgl-1) and high (>1μgl-1) chlorophyll concentration and low (12.7-17.81 mgl-1), moderate (18.1-29.0 mgl-1) and high (>30 mgl-1) TSM concentration for evaluating the performance of algorithms against different ranges of field measurements. The OCM-2 chlorophyll retrieval algorithm (OC4V4) showed a systematic and large overestimation of low chlorophyll values with r2 = 0.607, root mean square error (RMSE) = 0.33 μgl-1 and mean normalized bias (MNB) = 1.57 and consistent underestimation of high chlorophyll values with r2 = 0.497, RMSE = 1.486 μgl-1 and MNB = 0.52 especially at nearshore waters due to the interference of suspended matter and coloured dissolved organic matter. However, moderate range of chlorophyll values showed better performance of OC4V4 algorithm in chlorophyll retrieval with r2 = 0.676, RMSE = 0.254 μgl-1 and MNB = 0.09 when compared to low and high chlorophyll values. The TSM algorithm (modified algorithm of Tassan 1994) showed large underestimation in TSM retrievals and this was proved by the statistical results which shown maximum r2 = 0.551 for low TSM values with less RMSE = 0.909 mgl-1 and MNB = 0.616 error compared to moderate and high TSM values. OCM-2 retrieved TSM values were not well correlated with in-situ TSM concentration and constantly underestimates four times lesser than the in-situ measurements especially near the coast when TSM concentration was measured high. Though there was significant correlation exists between OCM-2 retrieved chlorophyll and TSM with in-situ measurements, the empirical algorithms employed did not give logical retrieval of both chlorophyll and TSM for the southwest Bay of Bengal (BoB). Thus, the present study revealed that the OCM-2 chlorophyll and TSM retrieval algorithms need to be tested further with extensive in-situ dataset around BoB to improve the regional algorithms for accurate measurements of chlorophyll and TSM in this region. © Indian Academy of Sciences.