Identifying and Explaining subtle AMRE-E vapor and wind speed differences

As part of the NASA MEaSUREs DISCOVER Project, we are merging ocean observations of SST, wind speed, water vapor, cloud and rain rates from a variety of passive microwave sensors including SSM/I, TMI and AMSR-E. The merged products remove the need for handling multiple satellite data products. An important step in the merging is an analysis of each ocean parameter to identify any biases between instruments. Though similar processing algorithms were applied to the data and extensive inter-calibration was performed, we still find some small differences between SSM/I and AMSR-E ocean products. In particular, the wind speed and water vapor values are both lower than SSM/I values by roughly 3%. Specifics of these ocean parameter differences will be presented.

We are not surprised that these subtle differences exist despite careful data handling as the AMSR-E instrument has a number of unique characteristics in its design. For example, the channel frequencies are different than both SSM/I and TMI, the larger AMSR-E antenna results in higher footprint resolutions, and the local equatorial crossing time of the ascending node is in the early afternoon (as opposed to the SSM/I which all have morning/evening node times). The AMSR-E instrument has also presented several challenges, including: errors in geolocation, a flaw in the hot load design, radio frequency interference, moon reflection in the cold mirror, and more sunlight reflecting into the feedhorns due to the later observation times. We will discuss the effect each of these has on the water vapor and wind speed retrievals and how these effects are mitigated by the algorithm. We will explain the source of the remaining subtle differences and propose a solution to remove these differences in the next reprocessing.