VERTICAL STRUCTURE AND DOMINANT HORIZONTAL SCALES OF BAROCLINIC WAVESIN THE NASA DAO AND NCEP REANALYSES

The reanalyses of the Data Assimilation Office (DAO) of the National A eronautics and Space Administration (NASA) are compared to those of th e National Centers for Environmental Prediction (NCEP) with regard to the vertical structure and important horizontal scales of the baroclin ic transients. Attention is focused on the eight Northern Hemisphere w inters of 1985/86-1992/93 and on (bandpass) transients of timescales 2 -8 days. The local seasonal mean vertical shear (normalized by the squ are root of the static stability) is very similar between the two sets of analyses. The upper-level vorticity gradient (dominated by the mer idional derivative) also shows little sensitivity to which reanalysis is used. The condition for barotropic instability (change of sign of t otal vorticity gradient) is satisfied. The vertical structure of bandp ass kinetic energy, meridional sensible heat Bur, and variance of temp erature gradient all show consistent differences between the NCEP and NASA reanalyses, with the NCEP signal significantly stronger at upper levels. The difference is modest for the kinetic energy (similar to 10 %) and is much stronger for the heat flux (similar to 100%) and the va riance of temperature gradient (similar to 70%). The NCEP reanalyses a lso have a stronger midlevel temperature gradient variance by about 20 %. The differences in this quantity reflect the treatment of the Natio nal Environmental Satellite, Data, and Information Service (NESDIS) op erational retrievals used by both reanalyses, and these satellite data affect the NASA reanalyses more strongly. There are significant diffe rences in the synoptic waves. The positive difference between the 300- hPa bandpass kinetic energy (NCEP minus NASA) as a function of the glo bal wave number used to truncate the fields reaches nearly half (two-t hirds) its total value by wavenumber 15 in the eastern Pacific (Atlant ic). For the 200-hPa sensible heat flux the difference is a maximum at wavenumber 10 over the whole midlatitude belt. Differences in midleve l temperature gradient variance between the first three winters (using NESDIS statistical retrievals) and the last five winters (using NESDI S physically based retrievals) include (i) NASA deficit compared to NC EP is slightly greater in the latter period and (ii) NASA variance is nearly 20% less in the latter period over the Pacific.