Using data sets on the daily data of maximum atmospheric pressure at ground level collected at 14 weather stations located in the central zone of the Iberian Peninsula [Spanish central plateau (SCP)], the series of daily maximum pressure anomalies at each station, together with the difference between the daily value and mean daily value for each day of the year for the period between 1961 and 2003, have been established. The regional series of such anomalies were constructed for the whole study zone and for two differentiated parts of the same. As thresholds for the extreme values of the anomaly series, the values corresponding to the P 05 and P 95 percentiles were used. The series of annual frequencies of days with anomaly values below and above the threshold values were constructed for each of the weather stations, together with the average series for the whole study zone and each of its two parts. The corresponding regional average series of seasonal frequencies were also constructed.From an analysis of the trend of the series of the annual frequency of extreme anomaly values in daily maximum pressure it may be deduced that the lowest values show a decreasing annual trend, while the highest ones show an increasing frequency. This indicates that between 1961 and 2003 the number of days per year on the SCP with the highest extreme atmospheric pressure values at ground level increased along the study period. In contrast the number of days per year with lower extreme values decreased. Additionally, analysis of the seasonal frequency series indicated that it was the winter that dictated such behaviour.
RESUMENSe establece el comportamiento temporal de las frecuencias anuales de olas de calor y frío observadas entre 1961 y 2010 en la Meseta Central española y en las dos zonas que se pueden diferenciar en ella. A partir de los datos diarios de anomalías de temperatura se determinan las series de anomalías diarias de temperaturas máxima y mínima para las áreas de trabajo. Se obtienen los valores umbrales de dichas series de anomalías, determinados por los percentiles P 10 y P 90 . Se establece la existencia de ola de calor cuando se observan dos o más días consecutivos en los que las anomalías de temperatura máxima y de temperatura mínima superan, simultáneamente, los valores de los umbrales establecidos por el P 90 . Se identifican las olas de calor que han afectado a la Meseta Central española, y a las dos subzonas durante el periodo de estudio y se establecen sus frecuencias mensuales y anuales. Asimismo, considerando que existe una ola de frío cuando hay dos o más días consecutivos en que los valores de temperaturas máximas y mínimas diarias son inferiores, simultáneamente, a los umbrales establecidos por el P 10 , se identifican las olas de frío y se establecen sus frecuencias mensuales y anuales en el periodo de estudio. Los resultados indican que los meses con mayor número de olas de calor entre 1961 y 2010 son mayo (25 olas) y junio (23 olas). El análisis de tendencia de las series de frecuencias anuales obtenidas indica que existe una tendencia creciente de olas de calor a un nivel de confianza mayor del 99%. El modelo lineal establece que se ha producido un aumento en la frecuencia de olas de calor en la Meseta Central española del orden de 0.6 olas cada 10 años. En cuanto a las olas de frío, se detectan olas de frío todos los meses del año en número que oscila entre ocho y 16 olas. Los meses con menor número son abril (nueve), julio (ocho) y agosto (nueve), y los meses con mayor número son marzo, mayo, junio y octubre con 16 olas. Los años con mayor número de olas de frío son 1969, 1971 y 1977 con siete casos; en el resto de los años la frecuencia anual está comprendida entre uno y seis. El análisis de tendencia de la serie de frecuencias anuales indica que existe una tendencia decreciente de las olas de frío a un nivel de confianza del 99%. Si se considera un modelo lineal, en la Meseta Central española se ha producido, entre 1961 y 2010, una disminución de olas de frío del orden de 0.54 olas cada 10 años. ABSTRACTThe temporal behavior of the annual frequency of heat and cold waves observed between 1961 and 2010 is established for the Spanish Central Plateau and for the two sub-areas in it. The series of daily maximum and minimum temperature anomalies for the working areas were calculated from the daily data concerning temperature anomalies. The thresholds of these series of anomalies, determined by the P 10 and P 90 percentiles values, were obtained. Heat waves occur when there are two or more consecutive days on which the maximum and minimum temperature anomalies are simultaneously greater than the...
From anomalies of daily data of maximum and minimum temperatures, that have been obtained as difference between the real data of every day and the average data of the corresponding day of the year in each of the 14 observatories of the Spanish Central Plateau (8 in North subplateau and 6 in the South subplateau) between 1961-2010, the extremely cold and warm days are determined, throughout all the year, in all the study area and the two different sub areas. We consider a day as an extremely cold day (ECD) if achieves simultaneously the following conditions: the anomaly of minimum temperature of the day is lower than the value of the P05 percentile of the series of daily anomalies of minimum temperature, and the value of the anomaly of daily maximum temperature is lower, as well, than the P 05 percentile of the corresponding series of anomalies. The values of the thresholds determined by these percentiles are obtained considering the complete anomalies daily series of temperature for all the study regions. In order to establish the extremely warm days the methodology is the same but the conditions are: the anomaly of minimum temperature of the day is greater than the value of the P 95 percentile of the series of daily anomalies of minimum temperature, and the value of the anomaly of daily maximum temperature is greater as well than the P 95 percentile of the corresponding anomalies series. Once the extremely warm and cold days are determined, throughout the year in the period of time considered, for each zone, the corresponding series of annual frequencies are obtained. The analysis of tendency of these series indicates that in all the cases the tendency of the frequency of the extremely cold days is decreasing. Considering a linear model for the temporary behavior of the frequency, the reduce is of the order of 1 day every 10 years. In the case of the extremely warm days its annual frequency presents an increasing tendency, which indicates that the number of extremely warm days per year has been increased during the studied time interval. In agreement with the linear model of behavior, the * Corresponding author. A. L. Labajo et al. 200increase is, approximately, of the order of 1 day per decade.
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