F. de Pablo

Meteorological and geo‐orographical relationships with lightning activity in Castilla‐Leon (Spain)

Some meteorological and geo-orographical factors related to cloud-to-ground flashes in Castilla-Leon (Spain) are studied. The relationship between flash density and the number of days with thunderstorms (thunderdays) is analysed, and a clear dependence and geographical variability is found. It is also shown that flashes have an average delay of four hours with respect to the time of the beginning of convection. The effect on flashes of dry static instability and moisture content is analysed by classifying days into four types according to the average values of the 850–700 hPa temperature difference and the 850 hPa dew-point temperature. A similar classification using the 700 hPa wind direction is used to analyse the effect of the mean-level flow on flashes. The results suggest that the daily number of flashes is mainly affected by the moisture content. Finally, a relationship between the cloud-to-ground flash density and geo-orographical factors (altitude, latitude and longitude) is deduced using multiple linear analysis. Copyright

Study of lightning event duration and flash rate in the Iberian Peninsula using cloud-to-ground lightning data

Abstract The average monthly, seasonal and annual lightning event duration and lightning flash rates over the Iberian Peninsula were estimated using cloud-to-ground lightning flashes for the 1992–1994 period. The total lightning event duration was over 37,000 h year −1 . Maximum values were found during the summer and were seen to be associated with mountainous areas, while minimum values were found during the winter and in the western and southern parts of the Iberian Peninsula. The spatial pattern of lightning event duration exhibited significant seasonal variations. The annual average of the mean and peak lightning flash rate was found to be 8.7 and 78.3 flashes (fl) h −1 , respectively. The temporal and spatial variations in flash rate were similar to those of lightning event duration, but with significant differences. In general, lightning flash rate exhibited fewer temporal and spatial variations than lightning event duration.

Spatial patterns of the daily non‐convective rainfall in Castilla y León (Spain)

This work presents a regionalization of Castilla y Leon, an area located in the northwest portion of Spain, using 5-day rainfall. Daily data were available for 119 sites for the 6-year period 1990–1995, between the months of September and May. The region presents a special orography which affects the final results. Following similar studies (Van Regenmortel, 1995, International Journal of Meteorology15, 313–323), Principal Component Analysis (PCA) (VARIMAX rotation) has been used together with the Cluster Analysis (Ward and average-linkage) as methods of regionalization. The causes of rainfall in Castilla y Leon, synoptic systems that affect the region, are obtained by using the PCA. As a result of a considerable reduction in the number of data, the properties of the different homogeneous rainfall areas are easier to define. This reduction permits a simulation of rainfall with a small number of variables having only a slight loss of information (80.5% of the variance is explained). Copyright

A temperature calculation model: The case of minimum daily temperature

Abstract In the present work a model based on the linear relationship between variations in the relative humidity of the air and those of temperature is applied to calculate minimum daily temperatures as a function of the maximum humidity value and the value measured in a thermometer screen at 1800 UTC of the previous day. The accuracy of the model is analysed by comparing calculated temperatures with observed minimum temperatures.

Temporal and spatial characterization of cloud-to-ground lightning activity over Iberian Peninsula

The quantitative assessment of the impact on society of weather and climate extremes is challenging and recent years have seen a tremendous increase in economic losses from weather hazards. Is necessary to know the changes in the frequency and/or intensity of extreme events to evaluate the trend in losses. So the lightning activity plays a fundamental role as a cause of weather-related deaths, property damages and power system breakdowns. The temporal and spatial distribution of cloud-to-ground lightning activity in the Iberian Peninsula have been studied, using data of years 1992-1994,0ver 2.2 million cloud-to-ground lightning flashes were recorded. The highest lightning activity was found in the summer (79% of all lightning events in the months May to September and the diurnal variation shows a single peak with the maximum at 1700 local time. Just over 11% of all lightning flashes are observed between 0700 and 1100 local time; which represents the contribution of frontal thunderstorms to lightning activity. The lightning polarity changes throughout the year: during the warmer months positive flashes contribute only 5.9 °/0 whereas this percentage increases to 13.1‘Yo in the colder months. The geographical distribution of cloud-to-ground flashes shows a clear relationship with orography, with maximum lightning activity over Pirineos. The average maximum flash density is found to be 3,3 flashes km-zyr-l,