Structure function characteristics for 2 metre temperature and relative humidity in different horizontal resolutions

Abstract

The isotropic correlations of forecast errors in the HIRLAM system are investigated for differenthorizontal grid sizes in order to achieve an improved representation of the structure functionsfor high-resolution surface analysis. The investigation is performed for 2 metre temperature andrelative humidity and makes use of operational forecasts from DMI-HIRLAM at the DanishMeteorological Institute (DMI), which can support the background for a surface analysis inthree different horizontal resolutions. Two different well-known methods for determining isotropicforecast error correlations are applied. The first method compares forecasts to observations(the Observation Method), while the second makes use of two different forecasts validfor the same time (the NMC Method). The latter method is also used to investigate isotropyas well as the influence of land’sea contrast and orography. A comparison of the two mentionedmethods reveals a good correspondence between them, and the investigation of monthly changesshows some seasonal tendencies. The isotropy assumption is shown to be acceptable to a firstapproximation, despite a slight dependency on the predominant flow. The results further suggesta decrease in the background error correlation scales when going to higher horizontal resolutionin the forecast model.