X.
Radar Observations
Given the quite large amount of verbal evidence and damage reports, the radar
data of the June 4th 1999 were carefully considered in order to give a detailed
description of the severe weather event. A possible objective verification of the tornado
hypothesis was investigated especially through the analysis of the Doppler velocity
fields.
band
5420-5650 MHz
peak power
500 kW
antenna size
5m
beamwidth
0.9 °
parameters
ZH, ZDR, V, V
Table 1. GPM-500C radar characteristics.
The GPM-500C radar of Fossalon di Grado [1] is both a research and
operational radar, whose main characteristics are listed in Table 1. It is located near the
Northern Adriatic coast line (45°44’ N, 13°29’ E) and the event was observed at about
70 km range from the radar. With a beamwidth of 0.9 deg, the azimuthal resolution is
1100 m, while the resolution in range is 250 m.
Figure 1. VMI map at 09.20 UTC. The storm cell North of Pordenone is where the
tornado was observed.
The acquisition mode running during this event was a volumetric scan,
consisting of five elevations (0.5, 1.0, 1.5, 1.9, 2.9). As San Quirino is located 120 m
above sea level, the lowest available radar data refers to an altitude of about 800 m
above the ground.
Fig. 1 shows the VMI (Vertical Maximum Intensity) of June 4th at 09.20 UTC.
Several intense cells where present on the Western Friuli Alps and foothills. The cell
related with damage reports and tornado evidence is located about 10 km North of
Pordenone.
The high spatial resolution provided by the Fossalon radar even at far ranges
allows identifying inner features of single storm cells. In Fig. 2 and 3 are presented the
reflectivity and the Doppler velocity fields over an area of about 40x35 km within the
azimuthal sector between 290 and 325 deg.
90
85
80
[Km
]
75
70
65
60
55
290
300
310
320
[deg]
Figure 2. PPI of reflectivity at 0.5 deg over the area highlighted in Fig. 1. The arrow
points to the center of the cyclonic circulation (see text below).
90
85
80
[Km
]
75
70
65
60
55
290
300
310
320
Figure 3. Same as Fig. 2, but for the Doppler velocity.
[deg]
Considering the radial velocity in Fig. 3 it is possible to infer the movement of
the cell from South-West. The mountains in the right part of the picture (the clutter is
identified and represented in black) seem to induce an increase in the velocity, likely
due to the air mass compression over the terrain slope.
Nevertheless, the most interesting feature in the Doppler image is the strong
azimuthal shear (the Doppler velocity gradient along an arc at constant range) located
on the left boundary of the cell. The abrupt transition from negative to positive values of
Doppler velocity (from –15 ms-1 to +12 ms-1) is only partially related to the mean cell
motion. It rather gives a clear indication of a local cyclonic circulation. This
interpretation is also supported by the reflectivity minimum (characterised by a fall of
about 15 dBZ with respect to the surrounding values) correspondingly to the center of
the vortex (Fig. 2).
The radius of the observed cyclonic circulation (2-3 km), the height of the radar
beam (about 800 m) and the Doppler velocity values (12-15 ms-1) suggest the existence
of a mesocyclone (Fig. 3).
Figure 4. Meocyclone and tornadic vortex parameters used for the Binger tornado (22
May 1981) simulation [2].
It seems likely that the velocity pattern of the supposed (reported by verbal
evidences) tornado is not resolved because its size is too small compared to the radar’s
resolution volume. In fact the tornado radius deduced from the width of the damage
path is about 150 m, while the radar cell azimuthal extension is 1100 m at 70 km range.
Fig. 4 shows an example of radius and velocity values observed in a mesocyclone with a
tornado embedded in it, in Oklahoma (U.S.A.).
References:
[1] Dietrich, E.: GPM-500C/F - A Polarimetric Doppler Radar for Research, Hail
Detection, General Monitoring. ERSA Friuli Venezia Giulia, Gorizia. (1994).
[2] Doviak, R.J., D.S. Zrnic: Doppler Radar and Weather Observations. Academic
Press, inc. (1993).