DIAGNOSIS AND CORRECTION OF SYSTEMATIC HUMIDITY ERROR IN A GLOBAL NUMERICAL WEATHER PREDICTION MODEL

Accuracy of humidity forecasts has been considered relatively unimport ant to much of the operational numerical weather prediction (NWP) comm unity. However, the U.S. Air Force is interested in accurate water vap or and cloud forecasts as end products. It is expected that the NWP co mmunity as a whole will become more involved in improving their humidi ty forecasts as they recognize the important role of accurate water va por distributions in data assimilation, forecasts of temperature and p recipitation, and climate change research. As a modeling community, we need to begin now to identify and rectify the systematic humidity for ecast errors that are present in NWP models. This will allow us to tak e full advantage of the new types of remotely sensed water vapor and c loud measurements that are on the horizon. The research reported in th is paper attempts to address this issue in a simple, straightforward m anner, using the Phillips Laboratory Global Spectral Model (PL GSM). I t was found that significant systematic specific humidity errors exist in the much-used FGGE [First GARP (Global Atmospheric Research Progra m) Global Experiment] IIIb (initialized) analyses. However, when a cor rection on the analyses was imposed and the PL GSM forecasts rerun, fo recast errors similar to the forecast errors generated from the uncorr ected analyses were observed. The errors were diagnosed through an eva luation of the tendency terms in the model's specific humidity prognos tic equation. The results showed that systematic low-level tropical dr ying and upper-level global moistening could be attributed to the conv ective terms and the horizontal and vertical advection terms, respecti vely. Alternative formulations of the model were identified in an atte mpt to reduce or eliminate these errors. In general, it was found that the alternative formulations that do not modify the convection parame terization of the model reduced the upper-level moistening, while thos e that do modify the convection scheme reduced low-level tropical dryi ng but introduced low-level and midlevel moistening in the summer hemi sphere extratropics. The authors conclude that the nonconvective modif ications could be instituted in the model as is. However, more work is needed on improving the way that convective parameterizations distrib ute water vapor in the vertical.