Abstract:
Three methods for estimating gridded fields of divergence from irregularly spaced wind observations are evaluated by sampling analytic fields of cyclones and anticyclones of varying wavelengths using a surface network. For the finite differencing method, divergence is computed from the objectively analyzed horizontal wind field. For the triangle method, which requires a triangular tessellation of the station network and assumes that the wind varies linearly within each triangle, divergence estimates are obtained directly from the wind observations and are assumed valid at triangle centroids. These irregularly spaced centroid divergence estimates then are analyzed to a grid. For the pentagon method, which requires a pentagonal tessellation of the station network and assumes that the wind varies quadratically within each pentagon, divergence estimates are obtained directly from the wind observations and are valid at the station lying within the interior of each pentagon. These irregularly spaced divergence estimates then are analyzed to a grid.
Results support previous studies showing that the triangle method provides analyses with lower root-mean-square errors that those of the finite differencing method. We find that for all wavelengths considered, the triangle method provides better analyses than the pentagon method, as well, despite the more restrictive assumption by the triangle method regarding the wind field. However, for most wavelengths considered, we find that the divergence estimates at the interiors of pentagons are superior to those at triangle centroids.