Boundary layer mixing is highly turbulent and complex and is therefore parameterized in weather forecast and climate models. In particular, it is often necessary to classify the boundary layer in these models in order to determine which mixing scheme to apply, or whether to apply a cloud-top entrainment parameterization. But are the parameterizations implemented in these models representative of the real world?
For the first time, Doppler lidar and sonic anemometer data have been used to objectively classify the observed boundary layer into nine different types based on the Met Office Lock scheme. Examples of these types are decoupled stratocumulus, cumuluscapped and stable with no turbulent cloud. This method has been applied to 3 years of data from the Chilbolton Observatory, UK and a climatology of boundary layer type has been created. This climatology shows a clear seasonal and diurnal cycle. More stable types are diagnosed in the cold seasons and more cumulus types are diagnosed in the warm seasons.
This observed time series is used to evaluate the 4km resolution version of The Met Office Unified Model. The model predicts more decoupled stratocumulus boundary layers, in both the stable and unstable regime, in all seasons than the observations. Stratocumulus cloud is present in the morning in the model but it is more common in the afternoon in the observations. The skill of the model in predicting boundary-layer class at the correct time has also been assessed using the SEDI skill score.
Figure 1: Observations taken on 8 August 2009: (a) attenuated lidar backscatter coefficient with the most probable boundary layer type shown in Roman numerals, (b) 2-hourly mean skewness of vertical velocity, (c) 2-hourly mean standard deviation of vertical velocity, (d) hourly-mean surface sensible heat flux. In (a), (b) and (c) the solid black indicates the diagnosed boundary-layer and the dotted line indicates the diagnosed cloud base.
- Indicates paper has been withdrawn from meeting
- Indicates an Award Winner