Determining the accuracy of gridded climate data and how this varies with observing-network density

Abstract. The Met Office National Climate Information Centre regularly produces assessments of mean monthly, seasonal and annual values of weather parameters and their anomalies over the UK. However the gridded values, and corresponding areal-average values, are subject to error. Experiments have been done in an attempt to quantify the mean errors in gridded monthly values and monthly areal averages of temperature and rainfall, and how these errors vary when we artificially thin out the observation network. But there are two additional reasons for this work: firstly we wish to determine how far back we can realistically extend the historical areal series, and secondly, we want to estimate the size of error bars on the historical values. For the UK as a whole, we estimate that error bars of around 0.1 °C would arise for monthly-mean temperatures.