An important consideration for analyses or reanalyses is the presence of observations, or the lack thereof. When multiple observations of the same parameter are assimilated, they "compete" to contribute to the analysis. Each observation carries a certain amount of uncertainty, as does the model forecast. The analyzed observations are often reported in observations space, in other words, at the observation location in precise space and time coordinates. These files are rarely used outside the centers that have developed the reanalyses.
In an effort to facilitate more user interaction with the observations and the analysis, NASA GMAO has produced a gridded data set called the MERRA Gridded Innovations and Observations (GIO) data. The observation-space data has been binned to the native reanalyses grid, so that with an understanding of the reanalysis data format, one could also evaluate the observing system. For example, this data was used to identify a systematic change in a central African radiosonde water vapor that adversely affects MERRA (see Bosilovich et al 2011). These data were also used for a general observing system assessment by Rienecker et al. (2011).
As an example, the following figure shows the RAOB meridional wind forecast bias (OmF) and the analysis bias (OmA) for JJA 1979-2011, area averaged over the central Great Plains states. At first glance, it looks like the radiosondes have an increasing trend in the forecast bias. Consider that the wind profiler network becomes available after 1991 and wind observations from aircraft are more greatly available after 2000. So, the analysis of radiosonde wind observations changes noticeably as more data becomes available.
The following three maps show the OmF for meridional wind across the United States for the radiosondes, profilers and aircraft observations. Data are plotted filling the MERRA grid box and provide an idea on the spatial distributions of the observations. The radiosondes and profiler network show that MERRA 6 hour forecasts are underestimating the central plains jet stream.
Data are available alongside the MERRA data files. Six hourly and monthly frequencies have been produced, all conventional and most radiances are also included. In addition, the data can be accessed through and OpenDAP GrADS Data Server (GDS): http://opendap.nccs.nasa.gov/dods/MerraObs