A Balanced Ageostrophic Initialization with a Fixed External Wind Boundary Value for Limited-Area Models

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  • 領域モデルのための外成風境界値を固定したバランス非地衡風イニシャリゼーション
  • Balanced Ageostrophic Initialization wi

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Abstract

The wind in a limited region can be separated into the external and internal winds, which depend upon the vorticity and divergence only outside and inside the region, respectively. This is the basic property of the wind separation. A given case that the vorticity and divergence are 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 prediction of a limited-area model and its initialization. If the estimated external wind for a given case satisfies the consistency condition at the boundary, the given case is a consistent case; otherwise, it is 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 reconstructing the wind for a given case can only be used if the given case is a consistent case. The implicit normal-mode initialization (Temperton, 1988) is further illustrated from the wind reconstruction and wind 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, and needs to be filtered in the initialization. From one computed example, the balanced ageostrophic wind is about 68 % of the ageostrophic wind, thus it is a good approximation to the ageostrophic wind. It is also 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 ageostrophic wind, and it is a good approximation to the observed wind. It is proved that the fixed wind components at the boundary cannot be used in the initialization because the initialized case in this situation is an inconsistent case. Based on the basic property of the wind separation, it is very natural to use the fixed external wind as the lateral boundary condition in the initialization. The proposed initialization and boundary condition have been tested and the results show that the meteorological noise can be effectively removed at the start of the forecast.

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