Modelling the local surface exchange over a grass-field site under stable conditions

Observations of near-surface variables are used with the Met Office Surface Exchange Scheme to calculate the exchange of heat and moisture between the surface and the atmosphere for a grass-field site at the Met Office, Cardington, UK. Different methods for modelling the stability near the surface are investigated with the surface temperatures and turbulent heat fluxes compared with data collected from the site. The results show that, whilst using Monin-Obukhov similarity theory can give good results in stable conditions, the form of the stability functions can significantly affect the accuracy in certain stability regimes. The use of log-linear stability functions can cause the turbulent fluxes of heat and moisture to cut off at stabilities which are too close to neutral compared with the observations. The results also show that unrealistically high surface temperatures are obtained in clear-sky nocturnal conditions with the standard surface energy balance. An alternative energy balance which includes a canopy of vegetation above the underlying soil gives better agreement with the observations in these conditions.