Analysis of extreme rainfall in Barcelona using a microscale rain gauge network

Extreme storms registered by the urban rain gauge network installed and supported by CLABSA (Clavegueram de Barcelona S. A.) in Barcelona in the period 1994–2001 have been investigated. Eleven rain events presenting intensities for durations between 5 min and 24 h with return periods equal to or larger than 5 years for any of the network gauges have been found. A cluster analysis has yielded four main classes of extreme rainfall events in this area, related to the meteorological scales involved: local (18%), mesoscale (37%) and synoptic storms (27%), as well as more complex rain events originated by multiscale mechanisms acting together (18%). An intensity index to classify extreme rainfall events in order to their complexity and severity, taking into account the contribution of the different scales implied in the rainfall processes, has been calculated. The frequency distribution of the intensity index values obtained for the urban network has resulted very similar to that calculated for rain data recorded by the Jard´ı gauge of the Observatory Fabra of Barcelona during 1927–1992 inclusive.Postprint (published version

A single scaling parameter as a first approximation to describe the rainfall pattern of a place: application on Catalonia

As well as in other natural processes, it has been frequently observed that the phenomenon arising from the rainfall generation process presents fractal self-similarity of statistical type, and thus, rainfall series generally show scaling properties. Based on this fact, there is a methodology, simple scaling, which is used quite broadly to find or reproduce the intensity–duration–frequency curves of a place. In the present work, the relationship of the simple scaling parameter with the characteristic rainfall pattern of the area of study has been investigated. The calculation of this scaling parameter has been performed from 147 daily rainfall selected series covering the temporal period between 1883 and 2016 over the Catalonian territory (Spain) and its nearby surroundings, and a discussion about the relationship between the scaling parameter spatial distribution and rainfall pattern, as well as about trends of this scaling parameter over the past decades possibly due to climate change, has been presented.Peer ReviewedPostprint (author's final draft

Multifractal analysis of the rainfall time distribution on the metropolitan area of Barcelona (Spain)

In most of the studies on scale properties in the rainfall process, multifractal behavior has been investigated without taking into account the different rain generation mechanisms involved. However, it is known that rain processes are related to certain scales, determined by climatological characteristics as well as regional and local meteorological features. One of the implications derived from these correspondences is the possibility that the multifractal parameters of the rainfall could depend on the dominant precipitation generation mechanism. Fractal analysis techniques have been applied in this work to rainfall data recorded in the metropolitan area of Barcelona in the period 1994–2001, as well as to a selection of synoptic rainfall events registered in the same city in the period 1927–1992. The multifractal parameters obtained have been significantly different in each case probably showing the influence of the rain generation mechanisms involved. This influence has been revealed also in the analysis of the effects of seasonality on the multifractal behavior of rainfall in Barcelona.Postprint (published version

Quality control process of the daily rainfall series available in Catalonia from 1855 to the present

The quality control of weather data is a necessity and a responsibility of meteorological services that store, distribute, and use these data. In the present work, a newly designed quality control procedure for daily rainfall data is presented after it has been adjusted and tested with more than 10^7 data from 1726 daily rainfall measurement sites in Catalonia. It is applicable to data from different origins (e.g., automatic weather stations or manual historical measurements). The procedure is focused on relative comparison of daily data with reference stations that are automatically selected after an initial estimation of their quality and a proximity study regarding location and correlation. The presented procedure has been verified taking advantage of an available network in the study area that has been routinely quality controlled by technicians of the Meteorological Service of Catalonia. The newly designed quality control procedure for daily precipitation yields good results, especially for extreme values: type I error under 10% is found for values up to 150 mm (error decreasing for lower values) and type II error is under 16% when reported values are twice a measure of 50 mm or more (error decreasing for more extreme values). After the application of the quality control procedure, a selection of series with the minimum desired quality is achieved.Peer ReviewedPostprint (author's final draft

Estimation of the probable maximum precipitation in Barcelona (Spain)

The main objective of this study is to estimate the probable maximum precipitation (PMP) in Barcelona for durations ranging from 5 min to 30 h. To this end, rain records from the Jard´ı gauge of the Fabra Observatory located in Barcelona (1927–1992) and the urban pluviometric network supported by Clavegueram de Barcelona, S.A. (CLABSA, 1994–2007) were analysed. Two different techniques were used and compared: a physical method based on the maximization of actual storms, and the Hershfield’ statistical method. The PMP values obtained using the two techniques are very similar. In both cases, the expected increasing behaviour of the PMP with duration was found, with the increase especially notable for the mesoscale durations 2–9 h, and not significant from 12 h on up. This result seems to be related to the scale of the meteorological situations producing high intense rainfall amounts over our territory.Postprint (published version

Influence of regional and seasonal rainfall patterns on the ratio between fixed and unrestricted measured intervals of rainfall amounts

Historically, most precipitation data have been measured by collecting rainfall, usually at intervals of 24 h, with a fixed starting time. Nonetheless, it is known that the use of fixed time intervals to measure rainfall quantities could lead to an underestimation of the true maximum precipitation amounts for the considered duration, so a single multiplicative correction factor is commonly applied, generally without taking into account the rainfall pattern of the place, nor regional or seasonal considerations. In the present work, hourly measurements from 120 stations of Catalonia (northeast of the Iberian Peninsula) have been used to analyse how the ratio between rainfall amounts measured by fixed and unrestricted intervals, i.e. the correction factor, depends on the considered duration and on the specific starting time of the fixed interval (local 00:00, 08:00, 12:00 or 16:00), as well as the influence ofgeographical location and seasonality and actual rainfall duration. For fixed sampling intervals starting at 16:00, the mean correction factor has been found to be higher (1.137) than at the usual 08:00 starting time (1.129). Some geographical patterns of the correction factor over Catalonia arose which, moreover, depend on the season, with a mean value of 1.161 in spring and a value of 1.093 in summer. Also, the value of the correction has been found to increase with the actual duration of the maximum rainfall events used in the analysis. Some of these extreme events had actual mesoscale durations between 6 and 9 h, linked to highly convective mesoscale organisations acting mainly in summer and the beginning of autumn. Other maxima episodes, with more advective rainfall lasting more than 12 h registered in the northern area of the territory, presented the highest values of the correction factor, especially in spring.Peer ReviewedPostprint (author's final draft

Rainfall spatial organization and areal reduction factors in the metropolitan area of Barcelona (Spain)

The rainfall spatial organization in the metropolitan area of Barcelona (Spain) has been studied from records of an urban rain gauge network in the period 1994–2009. Using statistical and regional analysis techniques, correlation between data recorded by the different rain gauges has been calculated, and the effective number of independent stations (neq) equivalent to the used network has been determined. It has been found out that for durations longer than 20 min, the areal rainfall return period observed for a storm registered by the network approximately decreases by a factor of 1/neq in relation to the current point rainfall intensity–duration–frequency relationships for the metropolitan area of Barcelona. Using objective analysis techniques, continuous precipitation fields have been generated on a regular grid with a spatial resolution of 300×300 m for the storms registered by the rain gauges from 1994 to 2009, for durations from 10 min to 24 h. The precipitation fields obtained have been useful to estimate the characteristic areal reduction factors in the metropolitan area of Barcelona. A direct relationship has been found between the areal reduction factor for all the area corresponding to the urban rainfall network of Barcelona and the effective number of neq for every duration consideredPostprint (published version

Introduction to the special issue on ‘‘hydro-meteorological time series analysis and their relation to climate change’’

Peer ReviewedPostprint (author's final draft

Rainfall intensity patterns derived from the urban network of Barcelona (NE Spain)

An analysis of the normalised rainfall intensity curves in Barcelona (NE Spain) has been undertaken from selected rain rate episodes recorded by an urban network of tipping buckets applying a 5-min integration time along the years 1994–2009. These curves, based on cumulative amount and time distributions, are modelled by a power law, this fact suggesting fractal behaviour. Four parameters characterise these curves. One of them is the exponent of the power law. Another one quantifies the intermittency of the rainfall along the episode. The other two are the coordinates of cumulative amount and time distribution from which the power law fits well the normalised curve. The total rainfall amount of the episode, its length and the coefficient of variation of the 5-min amounts are also considered as complementary parameters. Taking advantage of these seven parameters, patterns of rainfall intensity are determined for every episode. These patterns, together with the statistical distribution of 5-min amounts, maximum intensity and rainfall intermittence, should increase the knowledge on the urban rainfall regime with the aim of improving drainage design. In addition to present results, flood prevention should be complemented with extreme value analyses and quantification of return periods.Postprint (author's final draft

Return period curves for extreme 5-min rainfall amounts at the Barcelona urban network

Heavy rainfall episodes are relatively common in the conurbation of Barcelona and neighbouring cities (NE Spain), usually due to storms generated by convective phenomena in summer and eastern and south-eastern advections in autumn. Prevention of local flood episodes and right design of urban drainage have to take into account the rainfall intensity spread instead of a simple evaluation of daily rainfall amounts. The database comes from 5-min rain amounts recorded by tipping buckets in the Barcelona urban network along the years 1994–2009. From these data, extreme 5-min rain amounts are selected applying the peaks-over-threshold method for thresholds derived from both 95% percentile and the mean excess plot. The return period curves are derived from their statistical distribution for every gauge, describing with detail expected extreme 5-min rain amounts across the urban network. These curves are compared with those derived from annual extreme time series. In this way, areas in Barcelona submitted to different levels of flood risk from the point of view of rainfall intensity are detected. Additionally, global time trends on extreme 5-min rain amounts are quantified for the whole network and found as not statistically significant.Peer ReviewedPostprint (author's final draft