Vertical Structure and Dominant Horizontal Scales of Baroclinic Waves in the NASA DAO and NCEP Reanalyses

Abstract

The reanalyses of the Data Assimilation Office (DAO) of the National Aeronautics and Space Administration (NASA) are compared to those of the National Centers for Environmental Prediction (NCEP) with regard to the vertical structure and important horizontal scales of the baroclinic transients. Attention is focused on the eight Northern Hemisphere winters of 1985/86?1992/93 and on (bandpass) transients of timescales 2?8 days. The local seasonal mean vertical shear (normalized by the square root of the static stability) is very similar between the two sets of analyses. The upper-level vorticity gradient (dominated by the meridional derivative) also shows little sensitivity to which reanalysis is used. The condition for barotropic instability (change of sign of total vorticity gradient) is satisfied. The vertical structure of bandpass kinetic energy, meridional sensible heat flux, and variance of temperature 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 (?10%) and is much stronger for the heat flux (?100%) and the variance of temperature gradient (?70%). The NCEP reanalyses also have a stronger midlevel temperature gradient variance by about 20%. The differences in this quantity reflect the treatment of the National Environmental Satellite, Data, and Information Service (NESDIS) operational retrievals used by both reanalyses, and these satellite data affect the NASA reanalyses more strongly. There are significant differences in the synoptic waves. The positive difference between the 300-hPa bandpass kinetic energy (NCEP minus NASA) as a function of the global wave number used to truncate the fields reaches nearly half (two-thirds) its total value by wavenumber 15 in the eastern Pacific (Atlantic). For the 200-hPa sensible heat flux the difference is a maximum at wavenumber 10 over the whole midlatitude belt. Differences in midlevel temperature gradient variance between the first three winters (using NESDIS statistical retrievals) and the last five winters (using NESDIS physically based retrievals) include (i) NASA deficit compared to NCEP is slightly greater in the latter period and (ii) NASA variance is nearly 20% less in the latter period over the Pacific.