Formulation of a balanced Exner pressure in the Lateral ... - Met Office

Figure 6: PMSL difference [Pa] between the LAM and the global at the end of timestep 1 (top) and timestep9 (bottom). This was from the simulation where hydrostatic balance was imposed in the LBCs, but not inthe reconfigured start dump.2.3 Reproducing the problemThe initial aim of the experiment was to reproduce, in this idealised environment, the negative PMSLbias found in the operational forecasts when imposing hydrostatic balance in the LBCs. While theglobal model remained in a steady-state (as expected from theory), the LAM showed waves generatedalong the boundaries which spread across the domain. Figure 6 shows the difference in PMSL betweenthe idealised limited area model and the global model. It can be seen that a negative PMSL bias isestablished around the boundaries which then spreads into the interior of the domain, in a very similarfashion to that in the operational forecasts (see Figure 1). The phase speed of these waves indicatesthat these are shallow water gravity waves. This experiment successfully reproduced the problem beinginvestigated, allowing the causes to be diagnosed free from the complexities of the full operationalsetup.As the source of the PMSL bias was evidently the boundaries of the domain, further attention waspaid to these regions. Figure 7 shows a cross-section along the equator of the vertical velocity at the7