Orographic Effects on Frontal Clouds
Orography has a strong influence on the propagation of frontal cloud bands; some parts can be retarded which might result in a delay or a decoupling of clouds at different heights. Also, fronts can be rapidly intensified or rapidly shifted.
Whilst passing a mountainous area a front can be modified by orography. Front delay is one possibility, decoupling of clouds at different heights is another. Two phenomena cannot be completely distinguished and are sometimes even difficult to detect because they occur in the same front. Therefore, these two modifications will be treated as one CM in the Manual. Front Delay occurs in cases where there is a long frontal cloud band which is orientated normal or almost normal to a major mountain chain. In the Alpine region this is the common north-south and northeast-southwest orientation of fronts. Moreover, it is observed that the deformation occurs more often if the front approaches the Alps from the west than if it comes from the northwest. If air is being advected towards an obstacle there are two possibilities: either going around or going above. Which possibility dominates depends on wind speed and stratification: low wind speeds inhibit ascent, as does stable stratification. Clouds are displaced slower in the mountainous region then the surrounding regions. As this phenomenon only lasts several hours, it is more important for nowcasting than for forecasting.