1. 13 th ALADIN Workshop, Prague, November 24 th -28 th 2003 1 Coupling frequencies in ALADIN/Hrn8 domain Stjepan Ivatek-Šahdan Meteorological and Hydrological Service of Croatia ivateks@cirus.dhz.hr

2. 13 th ALADIN Workshop, Prague, November 24 th -28 th 2003 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. 13 th ALADIN Workshop, Prague, November 24 th -28 th 2003 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 couplingDavies 3 h coupling 1.5 hrs - with 3 h coupling cyclone is in domain, it should enter later. 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. 4.5 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.

4. 13 th ALADIN Workshop, Prague, November 24 th -28 th 2003 4 Model version and domains ALADIN25T1_op2 was used for experiments LACE domain with coupling zone from HRn8 domain

5. 13 th ALADIN Workshop, Prague, November 24 th -28 th 2003 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 ? 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 4 th of February and October 5 th. Results for that two days will be presented. Experiments for 7 days in year 2003 are performed: February 4 th, August 26 th & 27 th, September 2 nd and October 5 th, 14 th & 15 th 2003.

6. 13 th ALADIN Workshop, Prague, November 24 th -28 th 2003 6 4 th of February 2003 - I - during the 4 th and 5 th of February, cyclone is moving over Adriatic sea and few cyclones in inland near the right border of the domain

7. 13 th ALADIN Workshop, Prague, November 24 th -28 th 2003 7 4 th of February 2003 - II Varazdin Gospic Ploce Split

8. 13 th ALADIN Workshop, Prague, November 24 th -28 th 2003 8 4 th of February 2003 - III

9. 13 th ALADIN Workshop, Prague, November 24 th -28 th 2003 9 5 th of October 2003 - I Again a cyclone over a Adriatic sea.

10. 13 th ALADIN Workshop, Prague, November 24 th -28 th 2003 10 5 th of October 2003 - II L_1hL_3h ELSCF 1004.31 1007.29 1006.85

11. 13 th ALADIN Workshop, Prague, November 24 th -28 th 2003 11 5 th of October 2003 - III Komiza Sibenik Lastovo Palagruza Zadar

12. 13 th ALADIN Workshop, Prague, November 24 th -28 th 2003 12 Conclusion It is better to use 1 hour coupling frequencies, perturbations from bigger domain are imported in smaller domain.