Observation system simulation experiments have been performed at the
National Centers for Environmental Prediction primarily for the purpose
of evaluating the forecast improvement potential of proposed new
observation instruments. Validation of the simulations have been
conducted primarily by comparing results from corresponding data denial
experiments in both simulated and real data assimilation contexts.
Additional validation is presented here using comparisons of some
statistics of analysis increments determined from a baseline simulation
using the entire suite of observations utilized during a reanalysis for
February 1993. By exploiting the availability of a data set
representing "truth" in the simulations, the background and analysis
errors produced for the baseline simulation are computed. Several
statistics of these errors are then determined, including time means
and variances as functions of location or spherical harmonic wave
number, vertical correlations, Kalman gains, and balance as measured by
projections onto normal modes. Use of an OSSE in this way is one of the
few means of estimating analysis error characteristics. Although this
work with the present NCEP OSSE should not be considered definitive, it
reveals some important ways in which OSSEs may be utilized beyond their
original purposes.