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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.