J.L. Marcos

Discriminant analysis applied to the forecasting of thunderstorms

SummaryThe combination of several thermodynamic variables based upon the data provided by a radiosounding can be useful for the forecasting of thunderstorms. As a matter of fact, there are many indices that allow the establishment of a storm risk prediction once they have been gauged. The problem comes when not all indices lead to the same prediction. In these cases, it is necessary to establish one single function based on the information provided by all the variables employed, which should be able to determine a two-fold prediction: risk or no risk. This article presents a statistic model for the short tem prediction of thunderstorms in the region of León (Spain). To reach this aim 15 meteorological variables were selected. These variables were easy to handle by non-expert staff, and they allowed the characterisation of the preconvective environment early in the morning on thunderstorm days. The variables have been properly combined and gauged with the help of a dense network of meteorological observers. The result has led to the construction of a reliable model. The discriminant quadratic model has been easily applied to determine in an objective and binary way the risk/no risk for the occurrence of thunderstorms.

CAPE values and hailstorms on northwestern Spain

A study has been carried out in Leon (in the northwest of the Iberian Peninsula) on atmospheric convection during summer periods, by analyzing the values of the Convective Available Potential Energy (CAPE) around 07:00 UTC. The project analyzed the data provided by a network of voluntary observers, a hailpad network, and a meteorological station on a sample of 224 days. The CAPE values found were not high: they never reached 2000 J/kg, not even on hail days, i.e. on days with high convective activity. These values are much lower than the ones measured in convective situations in tropical regions, but they are within the usual values found in Europe. The same happens with the wet bulb potential temperature measured in Leon. The frequency distribution of the CAPE values shows a clear prevalence of very low or zero values. The group of days with the lowest CAPE value is that which included days with no storm. The mean value increases on storm days, and it is even higher for the days with recorded hailfalls. All these differences are significantly marked. Nevertheless, the differences across the years are not significant enough to be able to speak of an influence of the climatic change on the CAPE. The correlation of the CAPE with some of the variables previously used for hail forecasts was analyzed, and the correlation found was higher for the lifted index. The possible use of the CAPE as a thunderstorm and hailstorm forecasting method was considered. The results were encouraging, especially for hail forecasting, although the CAPE should not be used as the only variable, but combined with other parameters. Moreover, the relationships between the CAPE and the wet bulb potential temperature and between the CAPE and the physical parameters of the hailstones were also analyzed. A relationship was observed with the parameters of the hailstone size distribution. Nonetheless, these results are provisional, and they should be confirmed by analyzing a more representative sample. With a more detailed analysis of these and other relationships, the present forecast model used by the Laboratory for Atmospheric Physics at the University of Leon is expected to be greatly improved by including the CAPE in this model.

Noteworthy C-band radar parameters of storms on hail days in northwestern Spain

Abstract The data provided by a meteorological C-band radar have been employed in order to analyze the behavior of 224 storms that occurred on hail days in the provinces of Leon and Zamora, in the northwest of Spain. A network of more than 700 voluntary observers supplied the necessary information to distinguish hailstorms from storms that did not produce hail precipitation. The observers also reported the size of the hailstones found. Among the variables studied are the maximum reflectivity factors, the vertical developments, and the motion speed of the storm for hailstorms as well as for storms with no hail. The altitude at which the precipitation processes take place inside the storm cell and its average lifetime are the variables that determine the subsequent evolution of hailstorms and no-hail storms in the same day. Furthermore, these variables have been analyzed taking into account the type of storm: unicellular, multicellular or supercellular. Finally, a correlation between the characteristics of the storms and the size of the hailstones registered simultaneously on the ground has been searched for using the RHI detected on the radar exactly at the time of the precipitation.

Concentration of ice nuclei in continental and maritime air masses in León (Spain)

Among the objectives of the Plan de Actuacion de Lucha Antigranizo, (PALA, Hail Suppression Activity Plan), is the on-going analysis of the mechanisms operating during the formation of storms. There is a relationship between the processes at work in the interior of cloud masses and the concentration of natural ice nuclei (IN) at ground level. Through the rigorous analysis of 954 measurements made with an isothermal cloud chamber, it has been possible to establish that a dependence exists between the background concentration of IN and the type of air mass. The work shows that the concentration of IN in the samples analyzed in Leon was, on average, higher for continental than for maritime air masses, with a substantial presence of continental air masses originating from the Sahara desert. Furthermore, from the analysis of 4-day back trajectories, it has been observed that slow-moving maritime air masses gradually become altered during their passage over the peninsula, which causes an increase in the number of active IN.

A logistic regression model applied to short term forecast of hail risk

Abstract In areas where high-quality agricultural products are grown that are also highly susceptible to hailfall damage, short-term forecasting is a prerequisite, in order to evaluate the risk of hail falling. In cases such as this, where a relationship exists between a response variable and one or more explanatory variables, and the outcome variable is also binary (risk/no risk) the multiple Logistic Regression Model is a good tool to use. A collection of 15 independent meteorological variables was obtained from a sample of 229 days. The determination of risk/no risk situations was based on the meteorological information supplied by a dense network of over 500 observation points, distributed across an area of more than 6,500 km2.

Construction and assessment of a logistic regression model applied to short-term forecasting of thunderstorms in León (Spain)

Abstract Forecasting thunderstorms is one of the most difficult tasks in weather prediction due to the scarce knowledge on how to characterise the mechanisms taking part in the formation of thunderstorms. New tools based upon the objective recognition of appropriate conceptual models have been developed in the last years. A good example of this are the statistical models, based on variables that characterise the preconvective atmospheric conditions. This paper presents the results obtained by applying a short-term forecast model to thunderstorms during the summer periods in Leon (Spain). This model makes use of the logistic function as a binary forecasting technique determining storm/no storm. The logistic function used was a combination of 15 previously selected meteorological variables. The model has been constructed with the data collected on 152 occasions, and it has been verified on 77 other occasions. The skill scores obtained show that the use of statistical models combining the data, provided by variables characterizing the preconvective conditions and the triggering mechanisms, represent an important step in the difficult task of short-term thunderstorm forecasting.