Forecast‐observation pattern relationships in NCEP medium range forecasts of non‐winter northern hemisphere 500‐mb height fields: Research note

Abstract

Empirical orthogonal function (EOF) and singular value decomposition (SVD) analyses are applied to the Dynamical Extended Range Forecasting 90 (DERF90) dataset to find the most predictable areas and patterns in the non‐winter Northern Hemisphere 500‐mb height fields. Global 500‐mb height forecast and analysis fields covering 4 months of 1990 are used. The EOF method is applied to the forecast and analysis fields separately. A close correspondence between the forecast and analysis modes is found for 1‐day lead time. The correlation relations between these modes rapidly decrease with increasing forecast lead time. At 10‐day lead, the projections of the forecasts and corresponding analyses onto the strongest Northern Hemisphere EOF mode of the analyses correlate at 0.65, which is much higher than the general forecast‐analysis correlation of about 0.20. This particularly predictable mode consists of a Pacific/North America‐like pattern. Joint forecast vs. analysis correlation matrices were used to apply the SVD method to forecast and analysis fields. By design, the correlations between the forecast and analysis SVD modes are stronger than between the forecast and analysis EOF modes. Maps of temporal correlations between gridpoint values of analysis fields and the first and second EOF as well as SVD expansion coefficients of the forecast fields (i.e., heterogeneous EOF and SVD correlation maps) are used to find the regions of the best prediction skill. It is shown that the Pacific/North American, western and southeastern Pacific, the southern Atlantic and Indian ocean areas have relatively better prediction skill.