Jet Stream Winds: Comparisons of Analyses with Independent Aircraft Data over Southwest Asia

Abstract

We have obtained cruise-level wind data from commercial aircraft, and compared this data with operational jet stream analyses over southwest Asia, an area of limited conventional data. We present results from an ensemble of 11 cases during January 1989 and individual cases during December 1988?March 1989. Our key results are: (a) European Centre for Medium-Range Weather Forecasts (ECMWF), National Meteorological Center (NMC), and United Kingdom Meteorological Office (UKMO) analyses of the subtropical jet in southwest Asia are 11%, 17%, and 17% weaker, respectively, than aircraft observations; (b) analyzed poleward shears range up to 1 f (7 ?10?5 s?1) compared with up to 3f(21 ? 10?5 s?1 in the aircraft observations where f is the local Coriolis parameter; (c) the ECMWF errors are larger at the base of the jet; (d) the mean ECMWF core location is latitudinally correct but has an rms latitude variance of 1.5°; (e) isolated erroneous radiosondes produce unmeterorological structure in the analyzed subtropical jet stream; and (f) the increased utilization of automated aircraft reports is likely to produce a spurious secular increase in the apparent strength of the jets. The magnitude and spatial extent of the errors seen are near limits of current general circulation model resolution (100 km) but should be resolvable. Our results imply that studies of general circulation model systematic jet stream wind errors in weather and climate forecasts must be interpreted with caution in this region.