= localize_covar_serial_pdaf = The page document the user-supplied call-back routine `localize_covar_serial_pdaf`, which was introduced with PDAF V3.0. The routine `localize_covar_serial_pdaf` (`U_localize_covar_serial` inside PDAF) is a call-back routine that has to be provided by the user. The routine is used in the ENSRF/EAKF and is called during the analysis step. The purpose of the routine is to apply covariance localization to the vectors '''Hi P''' and '''Hi PH^T^''' for the assimilation of a single observation (determined by the index `iobs` related to the observation operator '''Hi'''). Here '''Hi PH^T^''' is for the vector relating to the observed covariance matrix for the full observation vector, which is required for parallelization. The interface is the following: {{{ SUBROUTINE localize_covar_serial_pdaf(iobs, dim_p, dim_obs, HP_p, HXY_p) }}} with arguments: {{{ INTEGER, INTENT(in) :: iobs !< Index of the assimilated single observation INTEGER, INTENT(in) :: dim_p !< Process-local state dimension INTEGER, INTENT(in) :: dim_obs_f !< Number of full observations REAL, INTENT(inout) :: HP_p(dim_p) !< Process-local part of matrix HP for observation iobs REAL, INTENT(inout) :: HXY_p(dim_obs_F) !< Process-local part of matrix HX(HX_all) for full observations }}} Hints: * To compute the localization one can use the routine `PDAF_local_weight` after computing the distance between two elements in the vector '''Hi P''' or '''Hi PH^T'''.