This work documents the effective use of X-band radar observations for monitoring severe storms in an operational framework. Two severe hail-bearing Mediterranean storms that occurred in 2013 in southern Italy, flooding two important Sicilian cities, are described in terms of their polarimetric radar signatures and retrieved rainfall fields. The X-band dual-polarization radar operating inside the Catania airport (Sicily, Italy), managed by the Italian Department of Civil Protection, is considered here. A suitable processing is applied to X-band radar measurements. The crucial procedural step relies on the differential phase processing, being preparatory for attenuation correction and rainfall estimation. It is based on an iterative approach that uses a very short-length (1 km) moving window, allowing proper capture of the observed high radial gradients of the differential phase. The parameterization of the attenuation correction algorithm, which uses the reconstructed differential phase shift, is derived from electromagnetic simulations based on 3 years of drop size distribution (DSD) observations collected in Rome (Italy). A fuzzy logic hydrometeor classification algorithm was also adopted to support the analysis of the storm characteristics. The precipitation field amounts were reconstructed using a combined polarimetric rainfall algorithm based on reflectivity and specific differential phase. The first storm was observed on 21 February when a winter convective system that originated in the Tyrrhenian Sea, marginally hit the central-eastern coastline of Sicily, causing a flash flood in Catania. Due to an optimal location (the system is located a few kilometers from the city center), it was possible to retrieve the storm characteristics fairly well, including the amount of rainfall field at the ground. Extemporaneous signal extinction, caused by close-range hail core causing significant differential phase shift in a very short-range path, is documented. The second storm, on 21 August 2013, was a summer mesoscale convective system that originated from a Mediterranean low pressure system lasting a few hours that eventually flooded the city of Syracuse. The undergoing physical process, including the storm dynamics, is inferred by analyzing the vertical sections of the polarimetric radar measurements. The high registered amount of precipitation was fairly well reconstructed, although with a trend toward underestimation at increasing distances. Several episodes of signal extinction were clearly manifested during the mature stage of the observed supercells.
Visibility maps for the considered radar system as retrieved
using a 250
Dual-polarization technology has greatly improved the quality of radar
precipitation measurements and reduced the gap between the qualitative and
quantitative use of radar observations. Several operational S- and C-band
radar networks have been (or will soon be) upgraded to adopt
dual-polarization technology. In the past decade, many studies have been
undertaken to explore the benefit of polarimetry for quantitative
precipitation estimation (QPE) using X-band radars
Examples of observed and filtered
Brightness temperature at
Mean vertical profiles of reflectivity, differential reflectivity, correlation coefficient, and Doppler velocity observed on 21 February 2013 at 16:50 UTC.
Vertical maximum intensity maps as retrieved on 21 February 2013 at 15:00, 15:30, 16:00 and 17:00 UTC.
The Selex-Gematronik 50 DX polarimetric system with a 3 dB beam width of
Vertical cut of
Range plots of the polarimetric radar parameters
Map of cumulated precipitation as retrieved by the combined polarimetric algorithm between 15:00 and 18:00 UTC on 21 February 2013.
The main steps of the applied processing chain, i.e., low signal ratio (SNR) compensation of
It is worth mentioning that the retrieved differential phase is not
affected by the system offset, which is removed through the derivative
computation. This feature is particularly useful for attenuation
correction purposes based on differential phase shift measurements. Figure The length of the adopted moving window (1
Steps (iii)–(iv) are repeated iteratively to reduce the expected
Brightness temperature at
Vertical maximum intensity maps as retrieved on 21 August 2013 at 04:00, 04:30, 04:50, 05:00, 05:30 and 06:00 UTC.
Mean vertical profiles of reflectivity, differential reflectivity, correlation coefficient and Doppler velocity observed on 21 August 2013 at 08:00 UTC.
The weight
The first precipitation event discussed here was observed on 21
February 2013. It was a winter convective system that hit Sicily, marginally
causing a flash flood in Catania, located on the central-eastern coastline.
Fortunately, most of the precipitation fell over the sea. According to press
reports, hail was also observed. Looking at the brightness temperature images
at The vertical
maximum intensity is a polar 2-D map of
Error scores computed on hourly cumulated rainfall for the event observed on 21 February 2013. RMSE refers to the root-mean-square error; FSE refers to the fractional standard error; “adim” represents adimensional ratios.
Vertical cut of
Range plots of the polarimetric radar parameters
Vertical cut of
Range plots of the polarimetric radar measurements
Map of cumulated precipitation as retrieved by the combined polarimetric algorithm between 04:00 and 10:00 UTC on 21 August 2013.
The precipitation nucleus moved southeast, leaving the city of Catania around
17:00 UTC. Focusing on the shielded azimuthal sector, which manifested at
15:00 UTC, Fig.
A Mediterranean low pressure system was the origin of the severe mesoscale
convective system that hit Sicily on 21 August 2013. The resulting intense
precipitation flooded several towns on the eastern coast, including Syracuse.
The system originated around 04:00 UTC and lasted about 6 h. The maximum
precipitation amount, registered in just 2 h by the rain gauge located
in Syracuse, exceeded 180
Error scores computed on hourly cumulated rainfall for the event observed on 21 August 2013. RMSE refers to the root-mean-square error; FSE refers to the fractional standard error; “adim” represents adimensional ratios.
Figure
This manuscript has documented the effective monitoring of intense
precipitation events in the Mediterranean area using an
operational X-band dual-polarization weather radar operated in Catania
(Sicily, Italy) by the Italian Department of Civil Protection. Two severe
hail-bearing storms that occurred in 2013 in south Italy have been
described in terms of their polarimetric radar signatures and estimated
rainfall fields. On 21 February 2013, a winter convective system
originating in the Tyrrhenian Sea caused a flash flood in the city of
Catania. Due to an optimal radar location, it was possible to
effectively reconstruct the storm characteristics in spite of the known limitation of X-band systems
due to attenuation. A few cases of signal extinction, caused by
close-range hail core generating significant differential phase shift
in a very short-range path, were documented. However, intense
precipitation rarely occurred above the radar, keeping the
influence of radome attenuation generally negligible regarding rainfall estimation, which benefited from the extensive use of
L. Baldini and N. Roberto acknowledge the support of the agreement between CNR-ISAC and DPC for 2014. Edited by: F. S. Marzano