jennifer catto supervisors: len shaffrey – ncas climate and kevin hodges - essc the representation...
TRANSCRIPT
Jennifer Catto
Supervisors: Len Shaffrey – NCAS Climate
and Kevin Hodges - ESSC
The Representation of Extratropical Cyclones in HiGEM
Motivation – Importance of Extratropical Cyclones
Huge socioeconomic impacts Strong winds and heavy
precipitation Important for large-scale flow
in transporting heat and moisture,
Want to be able to forecast how they will change in the future.
3rd February 2009
Higher resolution atmosphere models should be able to represent structures of storms better – e.g. fronts, distribution of precipitation.
For regional climate studies including hazards and impacts need higher resolution.
Higher resolution ocean gives better representation of Gulf Stream and Kuroshio current – impacts on baroclinicity.
Motivation – High Resolution Climate Modelling
HiGEM HadGEM
Introduction to HiGEM
HiGEM Based on HadGEM1 – the Met
Office coupled climate model 1.25° lat x 0.83° lon in
atmosphere (N144) 38 vertical levels in the
atmosphere 1/3° x 1/3°in the ocean, 40
levels
HadGEM 1.875° lat x 1.25° lon in
atmosphere (N96) 38 vertical levels in atmosphere 1° x 1° in the ocean going to 1°
x 1/3° at the equator
ERA-40 45 years of global gridded
data Approximately 1.1° x 1.1°
resolution (110km)
ERA-Interim New reanalysis from ECMWF Approximately 80km
resolution Uses 4DVAR
Outline
Climatology Eulerian Statistics Objective Feature Tracking SST experiment
Structure Compositing
Future Climate Predictions Conclusions
Eulerian Statistics – 250hPa TEKE
ERA-40 HiGEM
2-6 day bandpass Transient
Eddy Kinetic Energy (m2s-2)
2 21
2TEKE u v where uand vare the 2-6 day bandpass filtered winds.
Eulerian Statistics – 250hPa TEKE
ERA-40 HadGEM
2-6 day bandpass Transient
Eddy Kinetic Energy (m2s-2)
Eulerian Statistics – MSLP variance
ERA-40 HiGEM
2-6 day bandpass
filtered standard
deviation of MSLP (hPa)
Eulerian Statistics – MSLP Variance
ERA-40 HadGEM
2-6 day bandpass
filtered standard
deviation of MSLP (hPa)
Objective Feature Tracking
Frequently used method to investigate extratropical cyclone activity
Using Kevin Hodges Feature tracking method described in Hoskins and Hodges (2002)
Fields filtered to T42 and background field - (wavenumber ≤ 5) removed
Once tracks are found they are referenced back to full resolution for further analysis
Vorticity preferred for tracking as it picks up more small scale features and is not an extrapolated field.
NH Tracking Statistics – Genesis Density
ERA-40 HiGEM
HiGEM captures main features of cyclogenesis over Gulf Stream and Kuroshio current although there is too much cyclogenesis over the Pacific.
Genesis density –
cyclones per month per 5°
spherical cap.
NH Tracking Statistics – Genesis density
ERA-40 HadGEM
HadGEM also captures main features of cyclogenesis with roughly the right amount of cyclogenesis over the Kuroshio but slightly less than ERA-40 in the mid-Pacific.
Genesis density –
cyclones per month per 5°
spherical cap.
NH Tracking Statistics – Track Density
ERA-40 HiGEM
Track density –
cyclones per month per 5°
spherical cap.
NH Tracking Statistics – Track Density
ERA-40 HadGEM
Track density –
cyclones per month per 5°
spherical cap.
NH PDFs of cyclone strength - vorticity
More extreme high vorticity events in HiGEM compared to ERA-40 and HadGEM.
HadGEM compares very well with lower resolution reanalysis – ERA-40. HiGEM compares very well with higher resolution reanalysis – Interim.
NH PDFs of cyclone strength – 925hPa wind speeds
Wind speed affected less by resolution than vorticity. Higher wind speeds seen in storms in HiGEM than in other datasets.
SST Experiment
HiGEM HiGAM - HiGEM
Comparing HiGEM and HiGAM (atmosphere only run using AMIP2 SSTs).
HiGAM doesn’t have big excess of cyclones in Pacific.
SST Experiment
Cold bias in North Pacific leading to increased SST gradient in Kuroshio current.
SST experiment
Constant mean DJF anomaly applied to AMIP2 SSTs. Ensemble of 20 winters (October to March) of atmosphere only
model run.
HiGAM SST experiment - Results
SST anom HiGAM
Genesis region shifts further off coast of Japan when SST anomaly is included.
Maximum genesis in this region has not changed.
HiGAM SST experiment - Results
SST anom HiGEM
Still large differences in genesis in this region. Need to consider advection of moisture, upper level seeding of
cyclones.
Summary
Transient eddy kinetic energy represented well by models.
MSLP variance looks similar between datasets but does not show up Mediterranean storm track.
Genesis density and track density well represented in HiGEM and HadGEM.
The tighter SST gradient in the Kuroshio current impacts the position but not strength of the cyclogenesis.
Large impact of resolution on the maximum vorticity and 925hPa wind speeds of the storms HiGEM very similar to ERA-Interim HadGEM very similar to ERA-40
Storm Structure
Representing storm structures in climate models is very important for producing believable future predictions
Do the storms represent the key features of extratropical cyclones? Structures important for regional climate impacts – strong
winds and heavy precipitation have large socioeconomic impacts.
Need a statistical way to compare the structure of storms from HiGEM and ERA-40
Previous composite studies
Field & Wood 2007 Composite over all features at
all times (for 400 features over 2 years).
Doesn’t distinguish between different times in lifecycle.
Wang & Rogers 2001 Compositing explosive cyclones
in different regions of the Atlantic.
Composites taken at different times.
Don’t take into account direction of propagation of storms.
Composite Analysis - Methodology
Step 1: Identify tracks using Kevin’s tracking program
Composite Analysis - Methodology
Step 2: Choose at what stage in the lifecycle to composite the tracks e.g.
Maximum tendency
Maximum intensity
Maximum precipitation
Minimum pressure and identify where
this occurs
Composite Analysis - Methodology
Step 3: Extract the 20o radius around this point and note the direction of propagation of the storms.
Composite Analysis - Methodology
Step 4: Rotate these areas to the same direction of propagation and average them.
+ + =
Composite Analysis – Positions of tracks
Position of 50 of the tracks used for the composites and the point at which the maximum intensity occurs.
Conceptual Models
Composite Analyses – Surface Features
ERA-40HiGEM
Colours – 850hPa
equivalent potential
temperature (K) (anomaly from area average)
Composites Analysis – Surface Features
ERA-40
Colours – 925hPa wind speed (ms-1)
HiGEM
Results – 925hPa horizontal winds
Colours – system
relative wind speed (ms-1)
ERA-40
B
A
Results – 400hPa horizontal winds
Colours – system
relative wind speed (m/s)
ERA-40
B
A
Results – vertical slice along WCB
WCB
ERA-40
CCBAA
B
B
ERA-40
Results – vertical slice along WCB
AA
B
B
Results – 925hPa horizontal winds
Colours – system
relative wind speed (ms-1)
ERA-40HiGEM
B
A
B
A
Results – 400hPa horizontal winds
Colours – system
relative wind speed (m/s)
ERA-40HiGEM
B
A
B
A
Results – vertical slice along WCB
WCBWCB
CCB CCBHiGEM ERA-40
• Isentropes slightly shallower in HiGEM
CCB CCB
Results – vertical slice along WCB
HiGEMERA-40
• Very similar wind and temperature fields
• Differences in structure of RH• Possibly more convection triggered
along WCB in HiGEM than ERA-40
Results – 500 hPa vertical winds
ERA-40HiGEM
Colours – vertical velocity
(hPa/hour)B
A
B
A
Results – 500hPa Relative Humidity
ERA-40HiGEM
Relative Humidity at 500hPa (%)
Results – Vertical Slice along Dry Intrusion
• Slope of isentropes very similar between HiGEM and ERA-40.
• Weaker along isentropic wind speeds in HiGEM consistent with weaker descent behind the cyclone.
ERA-40HiGEM
B BA A
Summary
Compositing method capable of producing composite
cyclones that show the key features of conceptual models – CCB, WCB, dry intrusion.
These features compare well between HiGEM and ERA-40.
There are some differences in the vertical structure of moisture indicating that diabatic processes are handled differently in HiGEM and ERA-40.
Future Predictions
Bengtsson et al 2006 Northwards shift of
storm track Increase of storms
over UK A lot fewer tracks
over Mediterranean
Track density difference between 2071-2100 and 1971-2000 for A1B scenario – ECHAM5
Future Predictions
2 X CO2 - Control 4 X CO2 - Control
Track density –
cyclones per month per 5°
spherical cap.
Future Predictions
Distributions of wind speed for NH
Conclusions
The storm tracks in HiGEM and HadGEM compare well with ERA-40 although there are some differences that have yet to be understood.
The storms in HiGEM and the Interim reanalysis have higher maxima in vorticity than HadGEM and ERA-40.
The compositing methodology provides a novel way of looking statistically at the structure of cyclones.
Using conceptual models to guide the analysis, HiGEM does a very good job of representing the structures of extratropical storms when compared to ERA-40.
There are some differences in the vertical structure of moisture in the composites, indicating that diabatic processes are handled differently in HiGEM and ERA-40.
Initial results from the 2X and 4X CO2 experiments show some consistency with other studies.
Large reductions in track density over Mediterranean, Northwards shift.
Further Questions
SST Experiment What impact do the changed SSTs have on other
aspects of the atmospheric flow? Compositing
Will the results found here be robust over other climate models?
Will the results be the same for HadGEM? Would an isentropic analysis yield the same results?
Future Predictions How do the storm structures change in the 2x and 4x
CO2 experiments? Are there stronger winds in the most extreme storms?