Doppler radar investigation into the representation of evaporating ice in operational models

Several previous studies have shown that ice particles evaporate in a shallow layer beneath the ice cloud. It is important that operational models represent this layer accurately as it has been shown that an incorrect latent cooling profile can lead to incorrect thermal structure and dynamics within the model. Using assumptions made in the operational models and the model output we have compared the evaporation of ice in several operational models to observations from the 94 GHz cloud radar at Chilbolton, and find that the evaporation depth scale can be as much as three times the observed depth. We investigate several possible causes of this problem and give strong evidence that the larger evaporation depth is due to incorrect representation of a sharp drop in humidity beneath the ice cloud. We then introduce a simple high-resolution model that uses parameterisations from the Met Office model, but can accurately represent changes in temperature and humidity with height in the evaporating layer. Results show that the Met Office model could accurately represent the evaporation depth scale if it could maintain the sharp humidity gradient within the evaporating layer.