Evaluation and Wind Speed Dependence of MODIS Aerosol Retrievals Over Open Ocean

The Maritime Aerosol Network (MAN) data set provides high-quality ground truth to validate the Moderate Resolution Imaging Spectroradiometer (MODIS) aerosol product over open ocean. Prior validation of the ocean aerosol product has been limited to coastal and island sites. Comparing MODIS Collection 5 ocean aerosol retrieval products with collocated MAN measurements from ships shows that MODIS is meeting the prelaunch uncertainty estimates for aerosol optical depth (AOD) with 64% and 67% of retrievals at 550 nm and 74% and 78% of retrievals at 870 nm, falling within expected uncertainty for Terra and Aqua, respectively. Angstrom exponent comparisons show a high correlation between MODIS retrievals and shipboard measurements (R = 0.85 for Terra and 0.83 for Aqua), although the MODIS aerosol algorithm tends to underestimate particle size for large particles and overestimate size for small particles, as seen in earlier collections. Prior analysis noted an offset between Terra and Aqua ocean AODs, without concluding which sensor was more accurate. The simple linear regression reported here is consistent with other anecdotal evidence that Aqua agreement with the Aerosol Robotic Network is marginally better. However, we cannot claim based on the current study that the better Aqua comparison is statistically significant. A systematic increase of error as a function of wind speed is noted in both Terra and Aqua retrievals. This wind speed dependence enters the retrieval when winds deviate from the 6-m/s value assumed in the rough ocean surface and white cap parameterizations. Wind speed dependence in the results can be mitigated by using auxiliary National Centers for Environmental Prediction wind speed information in the retrieval process.