Orographic Effects on Frontal Cloud

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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.

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Common Remarks

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.

Problems are connected to:

• Only a few cases really show clear features of front delay (62 frontal passages over the Alps within 1 year)
• There were less than 10 cases of front delay within a year
• About 6 cases of decoupling of clouds at different heights within a year
• But: many fronts show orographic effects (Lee, barrage cloud, Foehn, orographic convection, Wave developments at the lee side, frontogenesis, frontolysis), or deformation of frontal cloud band (streaming around the orographic obstacle, but not over it)
• There is another aggravating circumstance: many fronts are already in the dissipation phase when reaching the Alps

Similar problems appear over the Cantabrics and Pyrenees:

• Just a few clear features of front delay or decoupling of clouds at different heights are noticed (about 50 frontal passages over Cantabrics and Pyrenees within approximately one year)
• Many fronts move from west to east but with their majority more to the north, where only a "tail" of frontal cloud band reaches the mountain chain

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