Abstract
Excessive wind speeds are present above the subtropical jets in all current general circulation model forecasts. In this paper we study the longitude and time dependence of these errors in 14 five-day winter forecasts using the NASA Goddard Laboratory for Atmospheres (GLA) fourth-order general circulation model. We show that the forecast errors may be divided into four catagories, two of which appear above the jet, and two of which appear at the jet stream level. They are 1) error growth above the jet near the Himalayas; 2) error growth above the jet initiated elsewhere followed by extensive advection; 3) tropical moisture hums burst appearing in equatorial regions and migrating northeastward to merge with and distort the jet; and 4) undulatory growth of waves in the meridional component of the jet stream velocity. Two forecasts for Southern Hemisphere winters show similar problems.
For the error growth near the Himalayas, we have performed additional tests. Initializations based on analyses prepared by NMC, GLA, and ECM/WF and special forecasts made using the ECMWF model showed qualitatively results Both a simple orographically induced gravity-wave parameterization and a model with increased stratospheric resolution yielded a reduced forecast error north of 30°N but yielded continued problems south of the Himalayas. We suggest parameterization of shear-induced gravity waves as a possible solution.
