Rainfall projections relating to extratropical cyclone occurrence in eastern Australia
Andrew J. Dowdy, Graham A. Mills, Bertrand Timbal and Yang Wang
The Centre for Australian Weather and Climate Research - A partnership between the Bureau of Meteorology and CSIRO, Melbourne, Australia
A.Dowdy@bom.gov.au
AGU – Fall Meeting, December 2012, San Francisco
Introduction
Relationship between the diagnostic and heavy rainfall events
Extratropical cyclones are the main cause of extreme rainfall in subtropical
eastern seaboard of Australia, but they are poorly represented by current
GCMs (e.g. intense impacts often occur at small spatial scales). Most major
extratropical cyclones (known locally as East Coast Lows) are associated with
a high amplitude upper-tropospheric trough (Fig. 1), suggesting the potential
for developing a large-scale diagnostic suitable for application to GCMs.
The diagnostic was found to provide a good indication of heavy rainfall
events, with the proportion of events identified increasing for increasing
amounts of rainfall (Fig. 3).
“Sygna” storm, May 1974
“Pasha Bulker” storm, June 2007
Fig. 1: Upper-tropospheric conditions for two major storms: 300 hPa geopotential
height (red) and wind speed (shaded grey)
Diagnostic development
A systematic examination of East Coast Lows (using an observed dataset from
1989 to 2006) was undertaken in relation to a number of potential large-scale
diagnostic indicators of their occurrence:
• The diagnostic was designed to be large enough in scale to potentially be
applicable to GCM data: daily temporal resolution, covering an area of about
15 degrees in longitude and 10 degrees in latitude.
• The best diagnostic found was based on 500 hPa geostrophic vorticity (Fig.
2), with diagnostic events defined as days on which the diagnostic was above
its 90th percentile.
Fig. 3: Daily eastern seaboard rainfall in 2500 mm bins (upper panel) for all days
(black) and for diagnostic event days (orange), and the proportion of days
corresponding to diagnostic events (lower panel).
Applicability to GCMs
Due to data requirements (i.e. daily geopotential at 500 hPa), the
diagnostic was applied to 3 of the 25 GCMs available in the CMIP3
dataset (using the A2 emissions scenario). Interannual, seasonal and
spatial variability were examined for each GCM, as well as the relationship
between dataset resolution and diagnostic threshold (90th percentile).
Results for the 20th century from the HADCM3.0 model showed good
agreement with those from reanalyses (for ERA40, NCEP/NCAR and
ERA-Interim reanalyses), as was the case to a somewhat lesser degree
for the BCM2.0 model.
Projections and conclusions
The diagnostic was applied to future climate projections (Fig. 4), indicating
fewer heavy rainfall events associated with extratropical cyclones could be
expected in this region due to increasing greenhouse gas emissions.
It is also noted that although the number of heavy rainfall events could be
expected to reduce in the future, there are a range of factors that could
potentially influence the intensity of these events (e.g. changes in
subtropical static stability and atmospheric moisture content).
Fig. 2: Critical Success Index (CSI) for potential diagnostic indicators of extratropical
cyclone occurrence, calculated individually for each ERA-Interim grid point (1.5 degree
resolution), for geostrophic vorticity (left column), isentropic potential vorticity (middle
column) and the forcing term of the quasi-geostrophic height tendency equation (right
column), at 500 hPa (upper row), 400 (middle row) and 300 hPa (lower row). Latitude,
longitude and coastline are shown.
Fig. 4: Annual number of diagnostic events, for the HADCM3.0 (orange) and
BCM2.0 (blue) GCMs.
References for further details:
Dowdy, A. J., G. A. Mills and B. Timbal, 2012: Large-scale diagnostics of extratropical cyclogenesis in eastern Australia. Int. J. of Climatology, doi: 10.1002/joc.3599.
Dowdy, A. J., G. A. Mills, B. Timbal and Y. Wang, 2012: Changes in the risk of extratropical cyclones in eastern Australia. J. of Climate, doi: 10.1175/JCLI-D-12-00192.1.