Orographic influence on the distribution of accumulated rainfall with different wind directions

For the region of the Upper Rhine valley with its surrounding mountains the influence of the orography on the horizontal distribution of surface rain amounts is analyzed by use of data of a C-band Doppler radar. A 30-month data sample is evaluated by accumulating rain intensities for different time intervals (months, years). The rain intensities taken into account refer to a constant height of 1.0 km AGL, i.e., to a terrain-following level. It is found that the influence of the orography on the rainfall distribution depends strongly on the direction from which clouds and rain systems move into the area under consideration. Accordingly, the data of all rain events are investigated in three different ways: (i) for the whole period (ii) month by month regardless of wind direction and (iii) also for the whole period but considering eight different sectors of wind directions, with each sector 45° wide. The wind direction dependent rain accumulations presented show the following features: (1) well-known phenomena as maxima of rainfall in the updraft region of hills or minima in the lee-side of hills, (2) phenomena triggered by the mesoscale flow systems such as preferred developing areas or tracks of thunderstorms as well as rain maxima in the lee-side of small hills induced by a counter current to the geostrophic wind.