Coupling frequencies in ALADIN/Hrn8 domain

1. Coupling frequencies in ALADIN/Hrn8 domain Stjepan Ivatek-Šahdan Meteorological and Hydrological Service of Croatia ivateks@cirus.dhz.hr

2. Introduction - Domain zones in ALADIN The horizontal domain of ALADIN can be divided in three zones. Central zone (C) - region of meteorological interest, Intermediate zone (I) - the Coupling zone, Extension zone (E) - bi-periodic fields. Central zone noted C, represents the region of meteorological interest, where the forecast is fully adapted to small-scale conditions. Any limited area model needs information about the state of the atmosphere outside of its integration domain. In order to solve this difficulty, one defines an Intermediate zone (Coupling) I where a large-scale solution computed with the ALADIN on LACE domain (12.2) is mixed with the solution resulting from the ALADIN integration (8). The use of spectral methods imposes for the fields similar boundary condition to those characterising the basis of function used for expansions i.e. the field must be periodic in the x and in the y directions (bi-periodic). An artificial zone, Extension zone E is defined like an outer belt only for this previously mentioned mathematical reason. Its size is chosen to avoid too sharp slopes at the boundaries of the domain.

3. Experiment with 1D model Problem: fast cyclone enters LAM domain between coupling times and is not recognised by grid point (Davies) coupling. The cyclone propagates with unnaturally high velocity, so CFL is broken in LAM. Davies 1 h coupling Davies 3 h coupling 1.5 hrs - with 3 h coupling cyclone is in domain, it should enter later. 4.5 hrs - 3 h coupling cyclone is in not deep enough and wind speed is too low. 10.5 hrs - 1 h coupling, problem near the Extension zone. With 1 hour coupling frequencies cyclone is much better represented. That was a reason to start with testing of 1 hour coupling in 3D ALADIN.

4. Model version and domains • ALADIN25T1_op2 was used for experiments • LACE domain with coupling zone from HRn8 domain

5. Experiments Question: Is it better to use 1 hour coupling frequencies instead of 3 hours frequencies what is now case in ALADIN/Vienna and ALADIN/HRn8 ? Experiments for 7 days in year 2003 are performed: February 4th, August 26th & 27th, September 2nd and October 5th, 14th & 15th 2003. Results: Changes in MSL Pressure for different coupling frequencies is usually less than 0.5 hPa. Days with difference bigger than 0.5 hPa are 4th of February and October 5th. Results for that two days will be presented.

6. 4th of February 2003 - I - during the 4th and 5th of February, cyclone is moving over Adriatic sea and few cyclones in inland near the right border of the domain

7. 4th of February 2003 - II Varazdin Gospic Split Ploce

8. 4th of February 2003 - III

9. 5th of October 2003 - I Again a cyclone over a Adriatic sea.

10. 5th of October 2003 - II L_1h L_3h 1004.31 1006.85 ELSCF 1007.29

11. 5th of October 2003 - III Zadar Sibenik Lastovo Komiza Palagruza

12. Conclusion It is better to use 1 hour coupling frequencies, perturbations from bigger domain are imported in smaller domain.