Assessments of the reliability of NCEP circulation data and relationships with surface climate by direct comparisons with station based data

An assessment is made of the climate simulations from the NCEP (National Centers for Environmental Prediction) Reanalyses over Europe. This assessment was initiated as part of the European Commission funded study on Atmospheric Circulation Classification and Regional Downscaling (ACCORD) and was designed to test the suitability of the Reanalyses for this type of application. Here NCEP Reanalyses (pressure, temperature and precipitation) from 1958 through 1997 are compared to station data of precipitation and temperature and composites of mean sea level pressure (MSLP) data, The comparison is made over a European window using monthly data with a focus on 3 land areas: Central and Eastern England and Italy, where daily timescale data are employed. MSLP data are generally well simulated; however, an input problem in the NCEP data prior to 1967 results in unrealistically low surface pressure. NCEP surface pressure over Greenland is also shown to be unrealistically high during the winter months. Spatially NCEP MSLP is shown to correlate quite well with UK Meteorological Office (UKMO) MSLP over the ocean and much of northeast Europe, while they correlate less well over high orographical regions. It is shown that, while daily temperature is well simulated, daily precipitation is less so, particularly during the summer months when convective precipitation is dominant. Total precipitation over the 2 UK areas is lower than observed, by as much as 22 % over Central England. The number of rain day events is underestimated over the 3 regions, although the anomaly of rain per rain day is shown to vary between the regions, being overestimated in NCEP in Eastern England and Italy. Mean daily temperature is shown to be much better simulated compared with precipitation, with a slight warm bias in all 3 grid boxes.