A comparison of North Atlantic storms in HiGEM, HadGEM and

ERA-40Jennifer Catto – University of Reading

Supervisors: Len Shaffrey Warwick Norton

Acknowledgement: Kevin Hodges

Outline

Introduction to HiGEM and companion HadGEM run

Extra-tropical cyclones and Storm Tracks in the North Atlantic

Comparing HiGEM, HadGEM and ERA-40 Probability Distribution Functions Tracking Diagnostics Bandpass filtered variances Sea Surface Temperatures

Future Work

Introduction to HiGEM

Collaboration between NCAS-Climate, Met Office, UEA, NOCS, BAS,UJCC.

Using Met Office model HadGEM1 as a base to create new 90km resolution coupled atmosphere-ocean model.

Mainly the same dynamics and physics as HadGEM1 but with a few improvements.

Introduction to HiGEM

HiGEM Atmosphere: 90km resolution in

horizontal (1.25ºx0.83º)

38 vertical levels

HiGEM Ocean: 30km resolution

(1/3ºx1/3º) 40 vertical levels

HiGEM Land Surface: 9 surface types, fixed

vegetation River Routing

HiGEM Ice: 5 ice categories

Comparison datasets

HadGEM version 1.2 – includes the improvements added to HiGEM but at lower resolution (1.875ºx1.25º in the atmosphere) Allows a more direct comparison between the models

as only resolution is different. ERA-40 (ECMWF Reanalysis) – comparable in

resolution to HiGEM Can be thought of as “real-life” but must remember

there are still uncertainties HiGEM and HadGEM runs both for 50 years but only

using last 30 years for analysis

Why Higher Resolution?

Example from HadCM3 coupled climate model. Much better representation of precipitation

distribution around low pressure systems

Why Higher Resolution?

Example from OCCAM ocean model Much more detailed ocean eddies and better

representation of coasts

Extra-tropical Cyclones and Storm Tracks

Extra-tropical storms Important for the everyday weather of the

UK and EuropeMain source of precipitationHuge socioeconomic impacts Important in determining largescale

atmospheric flowUnclear how they will change with global

warming

Extra-tropical Cyclones and Storm Tracks

Different ways of defining “storm-tracks”Feature tracking of low pressure (or positive

vorticity) centresMaxima of synoptic timescale filtered

variances of pressure, vorticity, geopotential height

Transient eddy kinetic energy maximum

2 21

2TEKE u v

Comparing HiGEM and ERA-40PDFs

Area on east coast of USA chosen due to max of cyclogenesis in this region.

Comparing HiGEM and ERA-40PDFs

Mean sea level pressure (hPa)

In this region the model captures the distribution of vorticity and MSLP very wellExtreme low pressures and extreme positive vorticities match well

HiGEMERA-40 ERA-40HiGEM

Mean sea level pressure (hPa) Relative vorticity (x105 s-1)

Feature Tracking

Using feature tracking method described in Hoskins & Hodges (2002).

Fields filtered to remove planetary scales with wavenumber less than or equal to 5.

Fields truncated to T42 resolution to exclude noise and (for vorticity) frontal features.

Max or min in fields identified and tracked . Tracks with lifetimes less than 2 days are

removed.

Tracking Diagnostics – MSLP track density

HiGEM shows northward shift of tracks

HadGEM has a quite different pattern over east of USA

HiGEM HiGEM-ERA40

HadGEM-ERA40

Tracking Diagnostics – Vorticity Track Density

More features here than in MSLP tracking

HiGEM does quite well in the Atlantic (better than HadGEM)

Again a different pattern in HadGEM

HiGEM HiGEM-ERA40

HadGEM-ERA40

Unusual error pattern east of Newfoundland where in MSLP tracking the error is negative.

Tracking Diagnostics – Vorticity Genesis Density

HiGEM HiGEM-ERA40

HadGEM-ERA40

Bandpass Filtered 250hPa TEKE

In Atlantic HiGEM matches well

HadGEM too zonal in Atlantic

HiGEM HiGEM-ERA40

HadGEM-ERA40

Possible mechanisms for differencesSSTs

Maximum gradient of surface temperature further North in HiGEM and HadGEM

Changes patterns of wind shear and moisture fluxes

HiGEMERA-40 HadGEM

Conclusions

HiGEM captures extremes of MSLP and vorticity well.

Overall the storm tracks are well represented. The northward shift of the tracks in HiGEM

could be due to warmer SSTs There are some large differences between

HiGEM and HadGEM which need to be investigated.

At upper levels TEKE is weaker in HiGEM than ERA-40

Future Work

Investigate reason for northward shift and stronger track density looking at SSTs, moisture fluxes, wind shear.

Try to find mechanisms for differences between models

Investigate large error to east of Greenland in track density

Do similar analysis for Pacific storms.