Evaluation of boundary‐layer type in a weather forecast model utilizing long‐term Doppler lidar observations

Abstract

Many studies evaluating model boundary‐layer schemes focus on either near‐surface parameters or short‐term observational campaigns. This reflects the observational datasets that are widely available for use in model evaluation. In this article, we show how surface and long‐term Doppler lidar observations, combined in such a way as to match model representation of the boundary layer as closely as possible, can be used to evaluate the skill of boundary‐layer forecasts. We use a two‐year observational dataset from a rural site in the UK to evaluate a climatology of boundary‐layer type forecast by the UK Met Office Unified Model. In addition, we demonstrate the use of a binary skill score (Symmetric Extremal Dependence Index, SEDI) to investigate the dependence of forecast skill on season, horizontal resolution and forecast lead time. A clear diurnal and seasonal cycle can be seen in the climatology of both model and observations, with the main discrepancies being the model overpredicting cumulus‐capped and decoupled stratocumulus‐capped boundary layers and underpredicting well‐mixed boundary layers. Using the SEDI skill score, the model is most skilful at predicting the surface stability. The skill of the model in predicting cumulus‐capped and stratocumulus‐capped stable boundary‐layer forecasts is low, but greater than a 24 h persistence forecast. In contrast, the prediction of decoupled boundary layers and boundary layers with multiple cloud layers is lower than persistence. This process‐based evaluation approach has the potential to be applied to other boundary‐layer parametrization schemes with similar decision structures.