1. Gillian Baxter University of Reading Departments of Mathematics and Meteorology Supervisors: N. K. Nichols, S. L. Dance, A. S. Lawless, S. P. Ballard Sponsored by NERC and CASE studentship with the Met Office Multiscale Data Assimilation for High Resolution Nested Models

2. Contents  A brief introduction to Data Assimilation  High resolution models  The “long wave” problem  A few numerical results  Conclusions

3. In order to run a weather forecasting model we need initial conditions. We want initial conditions which most accurately describe the observed reality. Data Assimilation allows us to combine observational data with a previous model forecast (background). The initial conditions are known as the “analysis”. What is data assimilation?

4. time 4D-Var constrained by the numerical model

5. Accurate forecasting of convective storms is important because these storms can lead to dangerous flooding events, such as the Boscastle flood in 2004. High resolution models Pictures from BBC website

6. Boscastle Flood: Comparison of 00 UTC 12, 4 and 1 km forecasts 12-18 from 00 UTC 12km 12-18 from 00 UTC 4km 12-18 from 00 UTC 1km Actual peak accumulations reached about 200 mm (gauge), 130 mm (2 km radar) Plots from the Met Office, slide provided by S. L. Dance

7. If we want a model with high resolution it can only have a limited domain size due to restrictions in computer power. This can create some problems; The “long wave problem”. A limited area model (LAM) needs boundary conditions. Met Office HRTM domains

8. The “long wave” problem

9. The 1D heat equation Discretisation where with homogeneous boundary conditions Some numerical results Model set up

10. buffer zone Truth LAM Parent model Boundary conditions provided by the parent model the LAM has 4 times the spatial resolution and 16 times the temporal resolution of the parent model The truth has twice the spatial resolution and 4 times the temporal resolution of the LAM domain is [0,1] the LAM starts at the first internal grid point of the parent model and covers exactly half of its domain

11. The 1D heat equation Discretisation Davies Relaxation scheme where b is the width of the buffer where Model set up with homogeneous boundary conditions

12. Aim: To consider and compare the treatment in the parent model and the LAM of wavelengths which are 1.Shorter than the resolution of the parent model

13. Parent model outputs --- truth --- parent background __ parent analysis + observations where

14. LAM model outputs --- truth --- parent analysis __ LAM analysis + observations where LAM domain

15. __ LAM __ parent model __ truth The Discrete Fourier Transform of the function is Wavenumber k

16. Aim: To consider and compare the treatment in the parent model and the LAM of wavelengths which are 1.Shorter than the resolution of the parent model 2.Longer than the domain of the LAM

17. __ LAM __ parent model __ truth 102345678 Wavenumber k

18. Summary The LAM can represent wavelengths that are missed by the parent model. However, the LAM may actually be worse at reproducing the longer wavelengths. When the wavelength is longer than the domain of the LAM it cannot be correctly reproduced.

19. Future Work The “long wave” information is important and the challenge is how to assimilate observations of these “long waves” in order to feed information from the large scales into the analysis for the limited area model. One option may be to split the scales. Take longer wavenumbers from the parent model and shorter wavenumbers from the LAM.

20. Plot taken from “Development of 1-4km Resolution Data Assimilation for Nowcasting at the Met Office”. S. Ballard, Z. Li, M. Dixon, S. Swarbrick, O. Stiller and H. Lean. WMO Nowcasting Meeting Abstract 3.02

21. Parent model outputs --- truth --- parent background __ parent analysis + observations where

22. LAM model outputs --- truth --- parent analysis __ LAM analysis + observations where