Geoscience Reference
In-Depth Information
As described in Sec. 2.2, both kinds of mini radar, the “wind-mill” (vertical plane scanning
along valley) and the “super-gauge” (which implements the traditional horizontal scan)
currently deliver an image of precipitation every minute; furthermore, the temporal
resolution can easily be reduced to 30 or even 15 s.
We here describe in more details how the time-averaged (1-minute-sampled) radar
reflectivity measurements are acquired. For each of the 9 consecutive shots, 2 contiguous
pulses have been acquired: the 2 contiguous pulses are separated by the pulse width,
which is 400 ns; the 9 consecutive shots are separated by the pulse repetition interval,
which is 1.25 ms. Every minute, the antenna performs 22 revolutions; however only data
from the first 16 revolutions (out of 22) were averaged on a linear power scale (algebraic
average: dBm values are antilog transformed, then averaged, then again transformed on a
decibel logarithmic scale). This means a total of 288 (18 times 16) samples; among them, at
least 216 are independent, if we assume a decorrelation time of ~10 ms (Fig. 1.14,
Sauvageot
[1992]): sample #1 and # 9 of the 9 consecutive rays are in fact separated by
9×1.25 = 11.25 ms.
3. A few qualitative examples
3.1 An hostile environment: Detecting precipitation even inside a narrow valley
At the beginning of November 2011 (from 3 to 9 November around noon) six days of
continuous, wide-spread precipitation hit the north-western part of Italy (see Fig. 2). In the
south-western Alps and in the surrounding flatlands and hills, in fact, autumn is the season
in which the longest and heaviest rainfalls occur. This fact has long been known: a
description of these “late-summer” Mediterranean storms can already be found in the works
by the old-Roman author, Plinius. It can been explained in simple terms as follows: in
autumn the Mediterranean Sea surface temperature is still high, while cold air is already
forming over the central-northern part of Europe. This has two effects: first of all, the
thermal contrast facilitates the deepening of pressure low over the north-western part of the
Mediterranean Sea; secondly, the warm air that arrives from the south, flowing over the
Mediterranean, provides a ready source of moisture.
The enforced rising of this warm-humid convectively unstable air, thanks to the Alpine
barrier, causes extensive and heavy rainfall. One has the impression of being subject to a
long storm, but, in reality, it is the continuous formation of stormy cells over the same
place.
The first study site here presented is located in north-western Italy in the “Aosta Valley”,
which is the smallest region in Italy. It is set between the Graian and Pennine Alps, which
are very steep. Among the more than “four-thousand” massifs, the most famous are: Mont
Blanc, Monte Rosa, the Matterhorn and Gran Paradiso. The Dora Baltea river together with
its tributaries have formed the tree-leaf-shape veining of the Valley. A Digital Elevation map
of the investigated area is shown in Fig. 2.
Being surrounded by such high relieves (> 4000 m MSL), the deep Aosta Valley (< 500 m
MSL) cannot be effectively monitored by any of the surrounding weather radars (Dole,
close to Geneva; Bric, close to Torino; Monte Lema, close to Maggiore Lake). Among these
