Objective analysis of monthly climatological fields of temperature, sunshine, rainfall percentage and rainfall amount

An objective method of drawing climatological maps of sunshine, temperature, rainfall percentage and rainfall amount, which is used operationally at the Irish Meteorological Service in Dublin, is described. The interpolation is based on McLain's method of distance weighted least squares quadratic approximation. The observations are interpolated directly to a regular (rectangular) mesh, without the use of a first-guess, and the resultant field is contoured and shaded using a standard contouring package. This basic interpolation gave some spurious results near the coast and in data void areas when it was applied in its original form. These problems were overcome by the introduction of dummy values in such regions and a method was developed for calculating such values automatically based on nearby observations and various climatological normals. The method has been in operational use since February 1985 and the results are of quality comparable to the manual method which this computer method has replaced.