Fulvio Stel

The climatology of hail in the plain of Friuli Venezia Giulia

Hail is quite common on the plain of Friuli Venezia Giulia (FVG), for this reason a network of polystyrene hailpads was established in 1988. This network remains active today monitoring hail frequency and aspect. We use the data of this network to define the hail climatology for Friuli Venezia Giulia, in particular, its spatial and temporal frequency, and to give a physical interpretation of the observed behavior. The main features that emerge from the analysis are that, even if hail is a local phenomenon, its climatology can be interpreted only by accounting for the mesoscale mechanisms triggered by the interplay between synoptic perturbations and the complex orography of Friuli Venezia Giulia. The analysis also reveals the importance of the lower level (say below 1500 m) water vapor amount for the occurrence of hail. The yearly frequency of hail days in Friuli was also compared with that of other regions and Countries, as well as with the time series of the North Atlantic Oscillation index. The results reveal some significant, even if weak, connections.

Heuristic considerations pertaining to hailstone size distributions in the plain of Friuli-Venezia Giulia

Abstract In this work, the hailstone size distributions at the ground in the plain of Friuli-Venezia Giulia are presented, as revealed through a network of polystyrene pads (hailpads), managed by volunteers, which has been active since 1988. The aim of this work is to highlight possible differences in the diurnal and seasonal behavior of hail at the ground, both from Friuli-Venezia Giulia and other countries, in order to improve the knowledge of this meteorological phenomenon. In the comparison between different countries, differences are found between the yearly size distributions of Friuli-Venezia Giulia and those of North-East Colorado, measured during the National Hail Research Experiment (NHRE). The size distributions obtained in South West France and in Friuli-Venezia Giulia are quite similar and they are slightly different from those of the Grossversuch experiment. In the comparison between different periods of the year, relevant differences are found between April and May and the other months. In particular, thunderstorms are less efficient in producing big hailstones during the former months. The most prolific month in producing hailstones is June, followed by September. This feature is interpreted as due to a form of synergy between the frequency of the synoptic forcing of storms and the amount of available energy at the ground. Analyzing the size distributions at different times of the day, the greatest differences are found in the intervals [00–06] and [06–12] in local time (respectively, [22–04] and [04–10] in UTC). These differences cannot be ascribed to the melting of the hailstones during their fall.

The June 4th 1999 severe weather episode in San Quirino, Italy: a tornado event?

Abstract On the morning of June 4th 1999, a severe weather event took place in San Quirino, a small village of Friuli-Venezia Giulia in the northeast of Italy. This village is located near the piedmont of the Alps, 40 km west from Udine and 60 km north from Venice. Around 0900 UTC (1100 local time), a thunderstorm with an intense hail fall affected the area of San Quirino. A few minutes later (around 0920 UTC, source: a farmer), a funnel cloud from a cumulonimbus touched the ground, producing damages to houses, trees and sheds. The damaged area was quite narrow (about 300 m) and short (less than 10 km). No injuries to people were reported. In spite of the smallness of the area interested by the phenomenon, this storm is studied here starting from the synoptic scale, moving to the mesoscale and finishing with the storm scale, trying to underline its characteristics. These analyses, especially those coming from the Doppler radar images, bring us to the conclusion that the San Quirino episode was produced by a supercell storm.

Atmospheric Convection: Research and Operational Forecasting Aspects

Markowski, P., An Overview of Atmospheric Convection.- Markowski, P., The Concept of Buoyancy and Its Application to Deep Moist Convection.- Markowski, P., Pressure Fluctuations Associated with Deep Moist Convection.- Markowski, P., Convective Storm Initiation and Organization.- Markowski, P., Supercell Thunderstorms.- Markowski, P., Tornadoes and Tornadogenesis.- Steinacker, R., Dynamical Aspects of Topography: the Role of Obstacles.- Steinacker, R., Thermodynamic Aspects of Topography: the Role of Elevation.- Steinacker, R., Topography: the Global Player in Meteorology.- Brooks, H., Environmental Conditions Associated with Convective Phenomena: Proximity Soundings.- Brooks, H., Development and Use of Climatologies of Convective Weather.- Brooks, H., Ingredients-Based Forecasting.- Brooks, H., Practical Aspects of Forecasting Severe Convection in the United States: Environmental Conditions and Initiation.- Brooks, H., Practical Aspects of Forecasting Severe Convection in the United States: Storm Evolution and Warning.- Giaiotti, D. B., Stel, F., General Considerations on the Operational Forecasts of Severe Convective Events: from Medium to Short Range.- Giaiotti, D. B., Stel, F., General Considerations on the Operational Forecasts of Severe Convective Events: from Short Range to Nowcasting.- Stel, F., Giaiotti, D. B., Weather Forecast Verication.

Preliminary studies on the occurrence of local severe weather events in Friuli Venezia Giulia

In this work, the results obtained with the analysis of 3 years of data concerning the occurrence of local severe weather events (LSWE) in the plain of Friuli Venezia Giulia (FVG) are presented. These data are essentially collected by volunteers who personally observed the development of these phenomena or at least the damages produced by them. These data show that local severe weather events are much more frequent than previously expected just because of the occasional reports; moreover, they appear to be mainly associated to the approaching of a cold front over the Friuli region. The spatial distribution of these phenomena is not uniform, even if the study area is relatively small, but it is likely dependent from the interactions between synoptic perturbations and orography.

Dent Overlap in Hailpads: Error Estimation and Measurement Correction

The measurement of the physical characteristics of hailstones reaching the ground is usually carried out by means of hailpads, on which the impact of hailstones leaves dents. Hailstone dents provide information about parameters, such as the number N of hailstones, their size M, and their kinetic energy E. In the case of intense hailfalls, however, the dents often overlap and the final measurement may not be totally reliable. This paper presentsacomputerizedsimulationwiththeaimofassessingmeasurementerrorscausedbydentoverlap.The simulated dents represent several random hailfalls with both exponential size distributions and monodispersed size distributions. The simulated hailpads were measured following the procedure employed in the case of hailpads exposed to authentic hailfalls, and it was thus possible to assess the error due to dent overlap. The results show that dent overlap makes it impossible to measure all the dents, which means that in a real hailfall the number of hailstones registered will often be lower than the number of hailstones that actually hit the ground (up to 25% may go undetected). Consequently, the energy and mass of the hailstones are also underestimated (they may be up to 50% higher than the values registered on a hailpad). The maximum size registered, however, does not depend on the degree of overlapping and neither does the slope parameter l of the exponential distribution, except when l takes higher values. Finally, the authors suggest a heuristic correction of the data obtained by real hailpads based on the results of the simulations. An example is provided that applies these corrections to the 228 hailfalls registered by the Italian hailpad network over a period of 10 yr. The results show that, on average, the correction applied because of overlapping increases the number of hailstones in 3.2%, the mass in 1.9%, and the energy in 5.4%. However, there are cases in which these corrections reached much higher values of up to 6.9% in N and M, and up to 25.2% in E. It is therefore advisable to correct dent overlap before carrying out a regional climatic study of hail, since this study would certainly be affected by the errors accumulated by all the hailpads.

Weather Forecast Verification

These notes are voted to stress the importance as well as the intrinsic difficulty of the weather forecasts verification, giving some hints to solve specific problems and some tools to face various situations. In general weather forecasts cannot be fully wrong but they cannot be neither fully right, this because they are trying to represent a future state of an extremely complex system, which is defined by too many aspects to be fully well described. There is a quite general confidence on the fact that it is at least possible try to quantify the amount of good and bad information that forecasters are trying to give on that future state. Nevertheless it is not possible to define in a unique way this quantification process, then different verifications procedures might give different results even if correctly realized. The standardization of definitions and of procedures is generally still poor and sometimes contraddictory. This fact makes, if possible, even more difficult to deal with the weather forecast verification. Facing the verification of rare weather events, as can be the case for the phenomena related to deep moist convection, extra difficulty arise by the fact that the powerful tool represented by statistics becomes less effective and the intepretation of results becomes in those cases even more tricky. In any case the verification of weather forecasts is an extremely important and structural aspect of the forcasting activity, that cannot be considered complete without it. Moreover the verification of weather forecasts can be an important opportunity to have a different look to the atmospheric aspect toward which we are pointing our attention and, for whom it might interest, to have a different look at the forecasters mind.

General considerations on the operational forecasts of severe convective events: from medium to short range

In this lecture we will deal with the general aspects of an operational forecast of convective severe weather in the medium and short range, that is from 72h to 24h ahead the occurrence of the severe weather event. The attention will be focused on the information available to the forecaster, their reliability and their use. The role of the numerical model outputs generally available in the daily operational forecast activity are described and their limits are stressed. It is shown how the subjective contribution of the forecaster integrates the model outputs information. The main elements that characterize a severe weather occurrence are schematically described and their identification is explored by means of the useful information available at the medium and short range.

General considerations on the operational forecasts of severe convective events: from short range to nowcasting

In this lecture we deal with the general aspects of the convective severe weather forecast in the short range, that is from 24h ahead the occurrence of the event down to the threatening phenomena onset. When the forecast is very close to the event it relates to, lets say a few hours before, or it aims to track the event evolution in real time, the word nowcasting is used instead of forecast. The main elements relevant to issue a local severe weather forecast in the short range are here described, furthermore the information available for that purpose is analyzed with special attention to the limits and the constrains imposed by the operational forecasting activity. Some examples of environments prone to the severe weather onset and evolution are presented and discussed.