The impact of ATOVS radiance in MERRA reanalysis

From a climate perspective, a retrospective-analysis (reanalysis) provides a systematic description of the climate system, which is anchored to reality by the assimilated observation. On the other hand, an important disadvantage of reanalysis is that changes in the observing system cause inhomogeneities in the time series. The inhomogeneity in reanalyses is a compelling problem for research focused on climate change and trends. Although Modern Era Retrospective-analysis for Research and Applications (MERRA) reanalysis implements several utilities to improve data homogeneity, it is evident that MERRA is not immune from the impacts of observing system changes. The introduction of ATOVS radiance from NOAA-15 satellite at the end of 1998 appears to cause one of the biggest impacts in MERRA. In this study, In order to characterize the impact of NOAA-15 ATOVS radiance, we performed a two year reanalysis segment (Oct 1998 – Nov 2000) parallel to MERRA stream but withholding NOAA-15 data. This reanalysis segment is a natural continuation of MERRA at the time before NOAA-15 ATOVS data is added, except that the new NOAA-15 data that comes along will not be assimilated. By comparing the original MERRA data with this NOAA-15_withholding reanalysis segment, a systematic picture of the NOAA-15 impact is shown. The introduction of NOAA-15 data causes a 0.15 mm/day increase of global mean precipitation, which is not evenly distributed. More increase in the tropical convective precipitation and more increase in the middle latitude large scale precipitation, especially over the south hemisphere. At the same time, there are systematic changes in temperature, moisture, cloud, energy fluxes, and general circulation, etc. By in detail analysis of these differences, we will give quantitative estimate of the NOAA-15 ATOVS impact in different fields, and try to understand how and why these changes happen, with the aid of analyzing the assimilation increments.