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Extra-tropical cyclones and Storm Tracks

This study compares HiGEM, HadGEM, and ERA-40 models in representing storm structures. Findings reveal differences in track density and genesis density, indicating variations in storm intensity and scale. Through case studies, the differences in track statistics, genesis, maximum intensity, and growth rates between the models are explored. The research suggests that HiGEM produces more storms, particularly extreme high-intensity ones, of smaller scale compared to HadGEM. Future work includes further investigation into mechanisms generating these differences, such as spatial analysis, latent heating effects, and Greenland tip-jets.

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Extra-tropical cyclones and Storm Tracks

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  1. Extra-tropical cyclones and Storm Tracks Jennifer Catto Supervisors: Len Shaffrey and Kevin Hodges

  2. Introduction • Higher resolution atmosphere models should be able to represent structures of storms better – e.g. fronts. • Will compare HiGEM, HadGEM and ERA-40 • would like to look at ECMWF interim reanalysis in the future. • Using Kevin Hodges’ tracking program and some case studies to do this.

  3. Example of storm tracks – pressure tracks generated within 75W to 65W and 30N to 40N. 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 Feature tracking

  4. ERA-40 HiGEM1.2 HadGEM1.2 Vorticity (850hPa) Tracking Statistics I - Track Density • Vorticity preferred for tracking as it picks up more small scale features and is not an extrapolated field. • Both models capture main features of storm tracks including the Mediterranean track. • Both too strong in some places especially HiGEM in the Pacific (Cyclones per month per 5° spherical cap)

  5. HiGEM1.2 – ERA-40 HadGEM1.2 – ERA-40 HadGEM1.2 – HiGEM1.2 Vorticity (850hPa) Tracking Statistics I - Track Density Errors • Quite different error patterns between models in Atlantic • Track density too strong near Greenland • Big differences in Mid-East Pacific - Tracking of trailing cold fronts? (Cyclones per month per 5° spherical cap)

  6. ERA-40 HiGEM1.2 HadGEM1.2 Vorticity (850hPa) Tracking Statistics II - Genesis Density • Both models capture main features of genesis including Mediterranean cyclogenesis. • Overestimated genesis in mid-Pacific in HiGEM • Underestimated genesis in mid-Atlantic in both models. (Cyclones per month per 5° spherical cap)

  7. Summary so far… • Tracking statistics show that both models do a reasonable job representing the storm tracks. • Large differences between the models – HiGEM generally produces more storms. • Have looked at atmosphere only runs, SSTs, baroclinicity. • Want to investigate differences further by: • looking at individual cyclones in the models – case studies • investigating the mechanisms producing these differences – PDFs of cyclone variables

  8. Case Studies - MSLP HiGEM1.2 HadGEM1.2 Mean Sea Level Pressure (hPa)

  9. Case Studies - vorticity HiGEM1.2 HadGEM1.2 850hPa Vorticity (x10-5s-1)

  10. Maximum intensity – vorticity on vorticity tracks • HiGEM storms a lot more intense than HadGEM. • More high intensity storms in the Atlantic than Pacific

  11. Maximum intensity – pressure on vorticity tracks • Smaller difference between the two models – what we expect as vorticity is more sensitive to scale. • Indicates that storms may be of smaller scale in HiGEM.

  12. Growth Rates Pressure tendency Vorticity tendency • Some cyclones of larger deepening rates in HiGEM than HadGEM • Difference more obvious in vorticity tendency – many more rapidly intensifying cyclones

  13. Conclusions so far… • HiGEM produces more storms generally than HadGEM – shown by track density and genesis density. • There are more extreme high intensity storms in HiGEM than HadGEM. • Storms in HiGEM are smaller scale and more coherent as can be seen in the case studies. • It is possible that there is more secondary cyclogenesis in HiGEM than HadGEM.

  14. Future Work • Investigate mechanisms further: • Spatial analysis • Latent heating - vertical velocity and precipitation distributions • Deformation strain • Greenland – tip-jets. • Atmosphere only model runs. • ECMWF interim reanalysis • Teleconnections. • Role of coupling to the ocean. • Climate change run (1% CO2 increase per year) of HiGEM1.2

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