AMIP II Diagnostic Subproject No. 3:
Methodology and Validation
- 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.
- 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
- 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.
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 (firstname.lastname@example.org) or the AMIP Project Office (email@example.com).
Last update: 6 January 1998. This page is maintained by firstname.lastname@example.org