A methodology of exploratory data analysis investigating the phenomenon
of orographic precipitation enhancement is proposed. The precipitation
observations obtained from three Swiss Doppler weather radars are
analysed for the major precipitation event of August 2005 in the
Alps. Image processing techniques are used to detect significant
precipitation cells/pixels from radar images while filtering out
spurious effects due to ground clutter. The contribution of topography
to precipitation patterns is described by an extensive set of topographical
descriptors computed from the digital elevation model at multiple
spatial scales. Additionally, the motion vector field is derived
from subsequent radar images and integrated into a set of topographic
features to highlight the slopes exposed to main flows. Following
the exploratory data analysis with a recent algorithm of spectral
clustering, it is shown that orographic precipitation cells are generated
under specific flow and topographic conditions. Repeatability of
precipitation patterns in particular spatial locations is found to
be linked to specific local terrain shapes, e.g. at the top of hills
and on the upwind side of the mountains. This methodology and our
empirical findings for the Alpine region provide a basis for building
computational data-driven models of orographic enhancement and triggering
of precipitation. Copyright (C) 2011 Royal Meteorological Society
.