A Verification of Numerical Model Forecasts for Sounding-Derived Indices above Udine, Northeast Italy

Abstract

In this work, 40 different indices derived from real soundings and the corresponding ECMWF model forecasts for the same location (near Udine, northeast Italy) are compared. This comparison is repeated for more than 500 days, from June 2004 to December 2005. The comparison is performed in terms of linear correlation, bias, root-mean-square error, and relative absolute error. The results show that the quality of agreement varies considerably among the different indices and lead times. It is found that only 15 indices have strong correlations (R2 always larger than 0.5) that hold up until the fourth forecast day (+96 h). The indices having the best agreement are usually representative of the thermal fields while those having the worst agreement are usually representative of the wind fields, particularly in the low levels. It is also found that the correlations for all 40 of the indices change for the 4 times of the day (one sounding every 6 h), with the 1200 UTC comparison being usually the best one. Also, at 1800 and 0000 UTC the potential instability is strongly underestimated by the model. To test if these errors depend on the vertical resolutions, a similar comparison has been done between the observed sounding reduced in the World Meteorological Organization TEMP format (as assimilated by the NWP model) and the originals at high vertical resolution. It is found that the indices derived from the TEMP format vary from those with the highest resolution, in particular the indices that depend on the details of the low-level profile, such as the level of free convection and the initial height of the lifted parcel. Nevertheless, only the shear has a very low correlation (R2 = 0.15) with the corresponding index derived from the high-resolution soundings, because it is very sensitive to the number of vertical levels used.