Assessing the representation of north-‐west Australian rainfall in GCMs
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Assessing the representation of northwest Australian rainfall in GCMs Duncan Ackerley Gareth Berry, Christian Jakob and Michael Reeder Monash University [email protected]
Transcript of Assessing the representation of north-‐west Australian rainfall in GCMs
Assessing the representation of north-‐west Australian rainfall
in GCMs
Duncan Ackerley
Gareth Berry, Christian Jakob and Michael Reeder
Monash University
Why do we care about north-‐west Australian rainfall?
• Figure: 1950 – 2008 linear trend in DJF rainfall (mm
yr-‐1).
• Only positive trend
shown.
• Strong increase. • Increased water supply –
resource?
• Data from Bureau of Meteorology.
• Several hypotheses as to the cause:
– Aerosol, Tropical Atlantic SST, Indian Ocean SST and ozone depletion.
• Different models have opposing signs for the rainfall change associated
with ozone depletion.
• AR4: 8-‐13 models out of 21 predicted increased rainfall for north-‐west
Australia:
– Little consensus?
– Precipitation change is small?
– Combination of the two?
• It therefore seemed sensible to go back and evaluate whether any model
could actually represent the physical mechanisms correctly before we
could make any statements about the cause of the trend.
How?
• Start simple: Assess our own model in Australia.
• Use ACCESS1.3 general circulation model.
• Simulate December-‐January-‐February (DJF) 1979/80 – 2007/08.
• Forced using AMIP boundary conditions.
• Ask the question – can ACCESS represent the physical processes that lead to rainfall in north-‐west
Australia?
• Use ACCESS as a test case to evaluate the important processes that lead to rainfall.
– Seasonal mean climatology (not shown).
– Diurnal cycle.
– Air flow and moisture sources.
• These analyses can then be applied to AMIP simulations of other models.
Diurnal cycle of the heat low circulation.
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(h) ACCESS 1.3 02 AWST
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• 6-‐hourly MSLP and 925 hPa wind
composites for all DJFs over AMIP
period.
• AWST – Australian Western Standard
Time (UTC + 8).
• Shading indicates regions of low wind
speed.
• Simple proxy for identifying the centre
of the heat low circulation.
• ERA-‐Interim and ACCESS1.3 agree well
during the day.
• Disagreement overnight.
• Heat low circulation displaced in
ACCESS1.3.
• E to NE flow tendency in ERA-‐Interim.
• E to SE flow tendency in ACCESS1.3.
6-‐hourly ageostrophic flow and convergence.
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(b) ERA-Interim: 14 AWST
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(d) ERA-Interim: 02 AWST
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(e) ACCESS1.3: 08 AWST
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• Ageostrophic flow vectors at 925 hPa .
• Convergence: shaded.
• Convergence occurs near the coast
during the day.
• Nocturnal convergence occurs inland.
• Similar patterns in ERA-‐Interim and
ACCESS1.3.
• However, inland ageostrophic flow
penetrates further south and west into
north-‐west Australia (ERA).
• HOW DOES THIS INFLUENCE
RAINFALL?
6-‐hourly rainfall as a percentage of the daily mean total
• TRMM 3B42 data: 1998/99 –
2007/08.
• ACCESS1.3: 1979/80 – 2007/08.
• Rainfall occurs too early in much
of eastern Australia.
• TRMM and ACCESS1.3 compare
well in the afternoon.
• Nocturnal rainfall maximum
shifted south and west in
ACCESS1.3.
• No rainfall at 02-‐08 AWST anyway,
so good agreement then.
Not a daily occurrence…
• The diurnal cycle dominates in the
composites.
• However, there are many days without
rainfall.
• Therefore the diurnal cycle must not be the
only factor governing rainfall.
Rainfall initiation
• Perhaps the difference comes from the circulation?
• ERA-‐Interim 925 hPa geopotential height and wind
composites.
• Asked the question: What does the geopotential
height and 925 hPa air flow look like on dry and wet
days at these places?
• Top: Wet days (when rainfall was initiated) at
pluviograph sites (dots).
• Middle: Dry days.
• Bottom: Differences.
• Enhanced westerly at coast.
• Enhanced north-‐easterly inland.
Image taken from Figure 6 in: Berry, G., M. J. Reeder and C. Jakob, 2011: Physical mechanisms regulating summertime rainfall over northwestern Australia. Journal of Climate, 24, 3705-‐3717. © American Meteorological Society. Used with permission.
ACCESS1.3 rainfall initiation
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• Corresponding composites for
ACCESS1.3.
• Weakened westerlies at
the coast – incorrect.
• North-‐easterlies inland
– correct.
Conclusions (for ACCESS1.3)
• The diurnal cycle of the circulation corresponds well between ACCESS1.3 and reanalyses; however, there
are some spatial differences.
• The diurnal cycle of rainfall in ACCESS corresponds well with observations at certain times, again there
are spatial differences between the observations and model.
• North-‐easterlies responsible for bringing moisture inland – well represented.
• Coastal rainfall initiation is poor in the model.
• All of these features discussed in a paper submitted to Journal of Climate – speak to me for details.
• These processes in particular must be analyzed in models before we can even start to look at forcings –
diurnal cycle and day-‐to-‐day initiation processes.
• Method described above can be applied to CMIP5 models where the relevant data exist.
• So what about other models?
CMIP5
• Some early analysis on the CMIP5 models.
• Similar to the figures above:
– 6-‐hourly TRMM accumulation (1998-‐2007)
– Corresponding figures for 21 models
– Data taken from AMIP simulations (1979-‐2008)
– Look at the afternoon and nocturnal cases.
• Interesting similarities and differences!
Final conclusions • Afternoon rainfall:
– No rainfall in some models – all rained out in morning (not shown) possibly due to early convection scheme
trigger.
– Approximately 8 models capture the pattern and relative amounts quite well.
• Nocturnal rainfall:
– Difficult test for the models, which two seem to pass.
– Some models (ACCESS for example) have a nocturnal peak but it is in the wrong place, which is still a good
result!
• Next step is to go back and identify why these models do this in a similar way to ACCESS (where data exist):
– Circulation
– Heat low
– Moisture sources
• Try to pick the best one(s) to understand what’s forcing the increase in rainfall.
• TAKE HOME MESSAGE: ASSESS ALL OF THE ABOVE BEFORE USING A MODEL TO EXPLAIN THE TREND!
