Optimal Vorticity Forcing of Blocking Derived from A Variational Approach

Investigations (Colucci and Alberta 1996) have found that cyclone over somewhere upstream prior to blocking may be a precondition of a blocking event. Statistical method is used to study the sensitive area of the vorticity forcing. The statistical results only give some possible sensitive area and hardly determine the magnitude of the vorticity forcing.

In the study, a variational approach is used to seek an optimal vorticity forcing of blocking event. Firstly, we define a distance measurement between forecasts and observations (cost function) over the area where a block occupies.Then, a global barotropic spectral model and its adjoint are employed to evaluate the cost function and its gradient with respect to the vorticity forcing. Finally, the cost function and the gradient are incorporated into a minimization algorithm for solving a optimal vorticity forcing.

The sensitivity of blocking to the vorticity forcing, including position and strength, derived from the adjoint model, is analysed and compared with Colucci's results. By minimizing the cost function, the optimal vorticity forcing is retrieved.