Parameterisation of Orographic Cloud (2002)
AuthorsDean, Samuel Martinshow all
Orographic cloud is investigated in a global context using both observations and a global climate model. Climatological cloud amounts from the International Satellite Cloud Climatology Project (ISCCP) are used in conjunction with wind reanalyses to study orographic cirrus amounts over the globe. Significant increases in cirrus are seen over many land areas, with respect to any surrounding oceans. To aid in interpretation of this result special attention is given to the New Zealand region as a case study for orographic cloud formation. Cirrus is found be more prevalent over New Zealand when compared to the adjacent ocean to the west. ISCCP cloud amounts are also compared with a ten year simulation of the UK Meteorological Office's Unified Model. The model is found to be considerably lacking in both cirrus and total high cloud over major mountain ranges. The model is also found to lack trailing cirrus clouds in the lee of orography despite the inclusion of a prognostic ice variable capable of being advected by the model winds. To improve the simulations of orographic cirrus and high cloud in the Unified Model a linear hydrostatic gravity wave scheme that predicts both the amplitude and phase of subgrid orographic gravity waves is introduced. The temperature perturbation caused by these waves in the troposphere is used to modify the amount of both liquid and ice cloud. One important feature of the parameterisation is that the launch amplitude of the gravity waves is predicted by a directional variance function which accounts for anisotropy in the subgrid orography. The parameterisation is explored in the context of an off-line testbed before implementation in the Unified Model. In a ten year simulation the parameterisation is found to increase the high cloud amounts over a number of the world's major mountain ranges. However, this extra cloud is optically thick and unable to remove the deficiency in optically thin cirrus amounts. Suggestions, as part of future work, for improvements to the model and orographic cloud parameterisation are also made.