An adaptation technique based on the synoptic atmospheric circulation
to forecast local precipitation, namely the analogue method, has
been implemented for the western Swiss Alps. During the calibration
procedure, relevance maps were established for the geopotential height
data. These maps highlight the locations were the synoptic circulation
was found of interest for the precipitation forecasting at two rain
gauge stations (Binn and Les Marécottes) that are located both in
the alpine Rhône catchment, at a distance of about 100 km from each
other. These two stations are sensitive to different atmospheric
circulations.
We have observed that the most relevant data for the analogue method
can be found where specific atmospheric circulation patterns appear
concomitantly with heavy precipitation events. Those skilled regions
are coherent with the atmospheric flows illustrated, for example,
by means of the back trajectories of air masses. Indeed, the circulation
recurrently diverges from the climatology during days with strong
precipitation on the southern part of the alpine Rhône catchment.
We have found that for over 152 days with precipitation amount above
50 mm at the Binn station, only 3 did not show a trajectory of a
southerly flow, meaning that such a circulation was present for 98%
of the events.
Time evolution of the relevance maps confirms that the atmospheric
circulation variables have significantly better forecasting skills
close to the precipitation period, and that it seems pointless for
the analogue method to consider circulation information days before
a precipitation event as a primary predictor. Even though the occurrence
of some critical circulation patterns leading to heavy precipitation
events can be detected by precursors at remote locations and 1 week
ahead (Grazzini, 2007; Martius et al., 2008), time extrapolation
by the analogue method seems to be rather poor. This would suggest,
in accordance with previous studies (Obled et al., 2002; Bontron
and Obled, 2005), that time extrapolation should be done by the Global
Circulation Model, which can process atmospheric variables that can
be used by the adaptation method.