AMIP II Diagnostic Subproject No. 3:

Statistics of Transient Circulation Systems
Project coordinators:
James S. Boyle1
Kevin Hodges2
Ian Simmonds3
D. Jones4
Program for Climate Model Diagnosis and Intercomparison, LLNL, CA, USA1
University of Reading, U.K.2
University of Melbourne, Australia3
BMRC, Australia4

    The study of the cyclone/anticyclone frequencies has a long history. Study of these features has always been considered a fundamental element of mid-latitude meteorology. Proper simulation of the location and intensity of these features play a critical role in the definition of regional climates in GCMs. It is of critical interest to relate any systematic differences found in these climatologies to the other errors in the models. Especially important is to gain insight as to the interannual variations of the frequency distributions and how these might relate to regional and global climate anomalies. There are documented patterns of teleconnections but the agents that translate these patterns into climate anomalies of temperature, precipitation and wind in mid-latitudes are largely the transient cyclones and anticyclones.  Slingo et al. (1994) used tracks of vorticity centers to determine the effects of varying convective parameterizations in the UGAMP GCM. It would appear that this is a useful analog to the traditional cyclone tracking procedures in midlatitudes.The NCEP and ERA re-analyses provide sufficient data to perform a comprehensive validation of the statistics computed from the model integrations. It may be necessary to reduce the resolution of the observed data sets in order to carry out direct comparisons with the models having coarser resolution.
    The cyclone and anticyclone frequency climatologies and their interannual variability will be computed from the 6h sample mean sea level pressure from each model. These will be compared to historical climatologies and computations using the NCEP/NCAR and ERA reanalyses products. The statistics compiled will be the tracks, genesis,lysis, mean intensity, mean velocity and mean growth/decay distributions. The focus of this phase of the study will be on the extratropics both southern and northern hemispheres. We will endeavor to establish relationships between the MSLP cyclones and anticylones and the longer time scale phenomena such as blocking, NAO and SOI indices etc. The geographical distribution of the cyclone/anticyclone events will be examined as a function of season and of central pressure or intensity.The ability to track features in the  vorticity field enables studies to be carried out in the Tropics as well. Statistics analogous to the traditional cyclone/anticyclone studies will be compiled for the 850 hPa relative vorticity. Tropical cyclones and easterly waves are systems which will be compared to the observations.Vorticity centers will also be tracked in the extratropics to allow a precise diagnosis of the initial stages of cyclogenesis and also to facilitate the examination of the dependence of the transient statistics on the nature to the atmospheric signature used to define the transients. The potential vorticity is a variable that incorporates a great deal of information succinctly, Hoskins et al. (1985),  which is an important consideration when comparing the number of models that are in AMIP. The tracking of PV features at the upper theta levels should provide a valuable supplement to the interpretation of the MSLP and 850 hPa statistics. In addition the tracking code can be applied to other levels and variables, such as the 250 and 500 hPa geopotential and the 850 hPa temperature.

    The 2.5 to 6 day  band pass filtered 500 hPa geopotential will be used to identify the 'storm tracks' of the models and reanalyses. The relationship of the cyclone/anticyclone frequencies and intensities to the upper level features will be examined.

Methodology and Validation
    We will use the code and procedures developed at the University of Melbourne for locating and tracking cyclones, Jones and Simmonds (1993). David Jones has kindly provided the rather extensive code that he used in his doctoral work. We will also use the tracking code of Hodges (1996). This technique has been used extensively in model validation work, Hodges (1996) and Slingo et al. (1994). The filtering of the 500 hPa geopotential will follow the traditional path of Blackmon et al. (1977).

    NCEP/NCAR reanalyses, ERA reanalyses

Data Requirements
    Mean sea level pressure at 6h intervals .
    u,v wind components at 6h intervals
    Potential vorticity at 6h intervals.
    For the data above, 12h intervals are marginally acceptable.
    500 hPa geopotential at 6h intervals ( daily acceptable )
    Blackmon, M. L., Wallace, J. M., Lau, N.-C., and Mullen, S. L., 1977: An observational study of the Northern Hemisphere wintertime circulation. J. Atmos. Sci., 34, 1040-1053.

    Hodges, K. I., 1996: Spherical nonparametric estimators applied to the UGAMP model integration for AMIP. Mon. Wea. Rev., 124, 2914-2932.

    Hoskins, B. J.,M. E. McIntyre and A. W. Robertson, 1985:  On the use and significance of isentropic potential vorticity maps. Q. J. R.
    Meteorol. Soc., 111, 877-946.

    Jones, D. A. and I. Simmonds, 1993: A climatology of Southern Hemisphere extratropical cyclones. Climate Dynamics, 9, 131-145.

    Slingo et. al., 1994: Mean climate and transience in the Tropics of the UGAMP GCM: sensitivity to convective parameterization. Q. J. R. Meteorol. Soc., 120, 881-922.

For further information, contact Jim Boyle ( or the AMIP Project Office (

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