Dynamics of orographic precipitation

Orographic influence on climatological distribution of precipitation Orographic influence on the formation of clouds and its associated precipitation amount and distribution is dramatic. The influence of orography was well recognized very early in human history and documented in numerous meteorological and climatological literatures. When a moist airflow impinges on a mountain, the dynamical and cloud microphysical characteristics of the airflow are modified by orographic lifting and blocking which may modify and/or trigger cloud and precipitation systems in the vicinity of the mountain. Figure 11.1 shows the mean annual precipitation for the period 1971–1990 over Western Europe. Areas of heavy precipitation are concentrated on the Alpine mountains. Note that precipitation over the Alps is produced by weather systems coming from different directions, in particular, from the northern and southern sides. Over a meso- α / β or large-scale mountain range, precipitation is triggered or enhanced on the windward slope of a prevailing wind due to orographic lifting on the upwind slope. A larger advection time is required for an air parcel to pass over the windward slope compared to the formation time of orographic clouds and precipitation. On the lee side of the mountain, there is little or no rain due to the depletion of moisture over the upwind slope and the adiabatic warming associated with the descending air, known as rain shadow . In short, the overall influence of mountains on climatological precipitation is orographic precipitation enhancement and suppression in the windward and lee sides of mountains, respectively .