A comparison of North Atlantic storms in HiGEM, HadGEM and ERA-40 Jennifer 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 Europe Main source of precipitation Huge socioeconomic impacts Important in determining largescale atmospheric flow Unclear 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) centres Maxima of synoptic timescale filtered variances of pressure, vorticity, geopotential height Transient eddy kinetic energy maximum

Comparing HiGEM and ERA-40 PDFs  Area on east coast of USA chosen due to max of cyclogenesis in this region.

Comparing HiGEM and ERA-40 PDFs Mean sea level pressure (hPa)  In this region the model captures the distribution of vorticity and MSLP very well  Extreme low pressures and extreme positive vorticities match well HiGEMERA-40 HiGEM Mean sea level pressure (hPa)Relative vorticity (x10 5 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 HiGEMHiGEM- 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 HiGEMHiGEM- ERA40 HadGEM- ERA40  Unusual error pattern east of Newfoundland where in MSLP tracking the error is negative.

Tracking Diagnostics – Vorticity Genesis Density HiGEMHiGEM- ERA40 HadGEM- ERA40

Bandpass Filtered 250hPa TEKE  In Atlantic HiGEM matches well  HadGEM too zonal in Atlantic HiGEMHiGEM- ERA40 HadGEM- ERA40

Possible mechanisms for differences SSTs  Maximum gradient of surface temperature further North in HiGEM and HadGEM  Changes patterns of wind shear and moisture fluxes HiGEMERA-40HadGEM

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.