Jennifer Catto Supervisors: Len Shaffrey – NCAS Climate and Kevin Hodges - ESSC The Representation...

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 –


spherical cap.

NH Tracking Statistics – Track Density

ERA-40 HiGEM

Track density –


spherical cap.

NH Tracking Statistics – Track Density

ERA-40 HadGEM

Track density –


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


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


Step 3: Extract the 20o radius around this point and note the direction of propagation of the storms.


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


Colours – system

relative wind speed (m/s)

ERA-40

B

A

Results – vertical slice along WCB

WCB

ERA-40

CCBAA

B

B

ERA-40


AA

B

B


Colours – system

relative wind speed (ms-1)

ERA-40HiGEM

B

A

B

A


Colours – system

relative wind speed (m/s)

ERA-40HiGEM

B

A

B

A


WCBWCB

CCB CCBHiGEM ERA-40

• Isentropes slightly shallower in HiGEM

CCB CCB


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 –


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?

Thanks!

[email protected]

www.met.rdg.ac.uk/~swr06jlc

Jennifer Catto Supervisors: Len Shaffrey – NCAS Climate and Kevin Hodges - ESSC The Representation...

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