Historical Satellite Assimilation

When evaluating one of our first analyses in the 1980s, we found some results very different from what was apparent in the recent (2001-2006) experiments. The experiment was run to test the impact of SSMI on the reanalysis time series (See this report). THe increments were drying the lower tropospehre continuously, and precipitation was small and getting smaller. Several aspect of the system were studied to pin down the problem. One hypothesis was that older coefficients (for the radiative transfer model) for the historical satellite data were causing some of the problems. NOAA NESDIS was in the process of creating new coefficients, and were able to make the new values available for testing, and now for the whole reanalysis period.

The figure below shows the time series of analysis increments of temperature and moisture for the old coefficients compared to the new coefficients. The original coefficients lead to large drying increments in the lower troposphere, as well as dramatic jumps at the when the satellites change. NOAA10 data starts at the end of Oct 86, and a couple channels drop out for several days in January 1987. The new coefficients work much better with the radiance assimilation, and the increments are much more uniform.