Comparison of large-scale dynamical variability in the extratropical stratosphere among the JRA-55 family data sets: impacts of assimilation of observational data in JRA-55 reanalysis data

摘要

This study compares large-scale dynamical variability inthe extratropical stratosphere, such as major stratospheric sudden warmings(MSSWs), among the Japanese 55-year Reanalysis (JRA-55) family data sets.The JRA-55 family consists of three products: a standard product (STDD) ofthe JRA-55 reanalysis data and two sub-products of JRA-55C (CONV) andJRA-55AMIP (AMIP). CONV assimilates only conventional surface and upper-airobservations without assimilation of satellite observations, whereasAMIP runs the same numerical weather prediction model without assimilationof observational data. A comparison of the occurrence of MSSWs in NorthernHemisphere (NH) winter shows that, compared to STDD, CONV delays several MSSWs by 1 to4 days and also misses a few MSSWs. CONV also misses the Southern Hemisphere(SH) MSSW in September 2002. AMIP shows significantly fewer MSSWs in Northern Hemisphere winter andespecially lacks MSSWs of the high aspect ratio of the polar vortex in which thevortex is highly stretched or split. A further examination of dailygeopotential height differences between STDD and CONV reveals occasionalpeaks in both hemispheres that are separated from MSSWs. The delayed andmissed MSSW cases have smaller height differences in magnitude than suchpeaks. The height differences for those MSSWs include large contributionsfrom the zonal component, which reflects underestimations in the weakening ofthe zonal mean polar night jet in CONV. We also explore strong planetarywave forcings and associated polar vortex weakenings for STDD and AMIP. Wefind a lower frequency of strong wave forcings and weaker vortex responsesto such wave forcings in AMIP, consistent with the lower MSSW frequency.