A BALANCED AGEOSTROPHIC INITIALIZATION WITH A FIXED EXTERNAL WIND BOUNDARY-VALUE FOR LIMITED-AREA MODELS

The wind in a limited region can be separated into the external and in ternal winds, which depend upon the vorticity and divergence only outs ide and inside the region, respectively. This is the basic property of the wind separation. A given case that the vorticity and divergence a re given within the region and the wind is prescribed at the boundary is used to represent a common feature that is often produced in the pr ediction of a limited-area model and its initialization. If the estima ted external wind for a given case satisfies the consistency condition at the boundary, the given case is a consistent case; otherwise, it i s an inconsistent case. For an inconsistent case, it is impossible to find a continuous wind that. satisfies the wind given at the boundary and yields the vorticity and divergence in agreement with those given within the region. A direct method with two Poisson equations of recon structing the wind for a given case can only be used if the given case is a consistent case. The implicit normal-mode initialization (Temper ton, 1988) is further illustrated from the wind reconstruction and win d separation. The goal of the initialization is to modify the analyzed ageostrophic wind to become the balanced ageostrophic wind. Only the imbalanced ageostrophic wind causes the fast gravity-inertia waves, an d needs to be filtered in the initialization. From one computed exampl e, the balanced ageostrophic wind is about 68 % of the ageostrophic wi nd, thus it is a good approximation to the ageostrophic wind. It is al so found that the rotational component of the ageostrophic wind (3.38 ms(-1)) is larger than that of the divergent component (1.64 ms(-1)). The balanced wind is the sum of the geostrophic wind and balanced ageo strophic wind: and it is a good approximation to the observed wind. It is proved that the fixed wind components at the boundary cannot be us ed in the initialization because the initialized case in this situatio n is an inconsistent case. Based on the basic property of the wind sep aration, it is very natural to use the fixed external wind as the late ral boundary condition in the initialization. The proposed initializat ion and boundary condition have been tested and the results show that the meteorological noise can be effectively removed at the start of th e forecast.