NASA/GSFC/Global Modeling and Assimilation Office
This seminar addresses several questions concerning the role of moisture and moist physics in the dispersion of short term ensemble forecasts. Singular vectors are used as the examination tool. These are defined for several norms, including ones measuring initial moisture or final precipitation rate and using weights based on analysis error statistics. Four 12-hour synoptic cases are examined. Results show that forecasts are generally as sensitive to initial moisture perturbations as they are to the dry fields. In most other respects, results vary greatly among the cases and norms. In some cases, the same structures that maximize mean-squared precipitation perturbations also maximize the final-time energy norm, but in other cases they can be quite distinct. Some initial moisture SVs produce the same perturbations at hour 12 as initially dry SVs, presumably due to a sensitivity to moist enthalpy. Examples of weak remote effects, presumably due to gravity wave propagation, are shown. For some SVs, the corresponding nonlinear behavior with meaningful magnitudes of initial perturbations correlates remarkably well with the tangent linear behavior, even when precipitation is concerned. For other SVs, however, the two behaviors are not very similar. The results therefore show both the power and limitations of moist SV analysis.