Changes in frequency and intensity of daily precipitation over the Iberian Peninsula

Abstract

[1] There is a general consensus within the climate community that any change in the frequency or severity of extreme climate events would profoundly impact nature and society. Such changes can be studied in terms of rainfall indices derived from daily data. The great variability of the rainfall over the Iberian Peninsula and the irregularity of its water regime make any study of the rainfall in this geographical region very interesting. In this work, we contribute some information about the characteristics and distribution of the daily precipitation series over the Iberian Peninsula, in particular the study of trends in series of several selected indices all related to the frequency or intensity of the daily rainfall. Data used are taken from a set of 35 series of daily rainfall in the aforementioned region during the period 1958-1997. The selected observatories constitute a representative sample of the orography and geographical diversity present in the peninsula. From the daily rainfall data, series of frequency and intensity rainfall indices were constructed on the basis of a prior definition of four rainfall categories reflecting certain percentiles of the precipitation distribution in the peninsula: total rainfall (!0.2 mm), light (!0.2 mm and <2.5 mm), moderate (!2.5 mm and <7.5 mm), intense (!7.5 mm), and very intense (!15 mm). The indices studied were number of rainy days in each rainfall category, medians and maxima of length of dry spells (both related to the frequency of the precipitation), accumulated precipitation in each rainfall category, proportion of rainfall in each category relative to the total accumulated rainfall, and mean precipitation per wet day in each category (related to the amounts). All the indices were evaluated seasonally. Significant trends in many of the selected series were found. The results for all the indices are coherent and point to an increase of the light rainfall events at the cost of a decrease of more intense events.

Snow trends in Northern Spain: analysis and simulation with statistical downscaling methods

“…In this case, the annual frequency shows no significant trend during the same period 1975-2002 1960 1965 1970 1975 1980 1985 1990 1995 and Trigo (2007) and Gallego et al (2006), although the data and the period selected for these studies are quite different. Therefore, the reduction in the annual frequency of snow days doesn't seem to be related to an overall decrease of days with precipitation.…”

Links between circulation weather types and teleconnection patterns and their influence on precipitation patterns in Galicia (NW Spain)

“…From the daily rainfall data, we constructed five categories of rainfall magnitude: no rain, light rain, moderate, intense and very intense, following the method adopted in previous works (Osborn et al, 2000;Osborn and Hulme, 2002;Gallego et al, 2006). This classification was done (1) taking into account the whole year and (2) separately for each season.…”

Change of extreme rainfall indexes at Ebro River Basin

“…If the rainfall distribution has changed, it is necessary to determine whether there are specific sub-periods of time with a similar distribution. This type of treatment was not carried out in certain works that studied the evolution and probability of rainfall exceeding dangerous thresholds (Gallego et al, 2006;Rodrigo, 2010;Müller et al, 2009). These authors found a lack of stationarity in rainfall during the 1951-2002 period, especially in terms of extreme values.…”

“…In general, all estimates of this probability are made by assuming stationarity in rainfall series. This assumption is not entirely realistic, especially today, when a large number of scientists claim that climate is changing dramatically (Keeling, 1973;Palmer and Räisänen, 2002;Milly et al, 2002;Cox, 2005). If the rainfall distribution has changed, it is necessary to determine whether there are specific sub-periods of time with a similar distribution.…”