Concentration of daily precipitation in the contiguous United States
Introduction
The fine temporal structure of precipitation, for example, at a daily resolution, is vitally important, not only for studying climatic factors such as intensity and torrential rainfalls and dry periods and drought, but also in relation to the soil and ecosystems (erosion, desertification, etc.), the management of water resources, floods and flash flooding, the design of drainage systems, the availability and safety of transport systems, urban planning, and more. In particular, extreme rainfall events at a daily scale are characterised by durations limited to a small number of days; however, they account for a high percentage of monthly, seasonal and annual precipitation (Brooks and Carruthers, 1953, Jolliffe and Hope, 1998, Easterling et al., 2000, Martin-Vide, 2004, Alexander et al., 2006, Burgueño et al., 2010, Monjo, 2016). Daily rainfall can be considered a discrete process, because of its elevated number of null values, and can be depicted by means of a negative exponential distribution (Brooks and Carruthers, 1953). In order to study the temporal structure of daily precipitation, Martin-Vide (2004) developed an index called the Concentration Index (CI) which describes and evaluates the percentage contribution of days with precipitation to the total amount of precipitation. This index, or a similar one from which it derives, the Gini Index, has been applied in several areas: in Spain (Martin-Vide, 2004, Burgueño et al., 2005), Iran (Alijani et al., 2008), China (Li et al., 2011), Peru (Zubieta, 2009), Europe (Burgueño et al., 2010, Cortesi et al., 2012), Peninsular Malaysia (Suhaila and Jemain, 2012), Southern Italy (Coscarelli and Caloiero, 2012), Chile (Sarricolea and Martin-Vide, 2014), Algeria (Benhamrouche et al., 2015) and finally, at a global scale (Monjo and Martin-Vide, 2016). The study of daily precipitation is particularly relevant in the context of global warming (IPCC, 2014). Wuebbles et al. (2014) indicate that there is a statistically significant increasing trend in the frequency and intensity of extreme precipitation events in the USA. An increase in wet and light precipitation days has been detected in southern South America, Western Europe, Australia and the US (Rajah et al., 2014). There is also significant evidence of a relationship between trends in temperature and the hydrological cycle in regions of the United States; this relationship manifests as an inverse correlation between the trend of daily temperatures and average daily rainfall (Portmann et al., 2009). Added to this is evidence of long-term changes in the increases in precipitation associated with heavy daily events over the north-eastern United States and decreases over the south-western United States. These observed trends are primarily due to the internal decadal ocean variability, which is dynamically driven by an atmospheric circulation reminiscent of teleconnections linked to cold tropical eastern Pacific SSTs, and only secondarily to a smaller contribution from anthropogenic climate change (Hoerling et al., 2016). Climatic projections up until the end of the 21st century suggest that rainfall variability will increase as a consequence of the strengthening of the global hydrological cycle and increased precipitation intensity (Lionello and Giorgi, 2007, Gloor et al., 2013, Wu et al., 2013, Chou et al., 2013, Singh et al., 2013, Wuebbles et al., 2014, Ning et al., 2015). In California, the projections of rainfall variability are linked to extra-tropical cyclonic activity (Chang et al., 2015). In this article, the spatio-temporal behaviour of daily precipitation is analysed through the application of the Concentration Index (CI) (Martin-Vide, 2004) in the contiguous United States, which has a variety of physiographic units, atmospheric, oceanic and continental influences, climates and ecosystems. This is the first time that the Concentration Index has been applied to the entire contiguous US, and in a high-resolution spatial scale. The spatial patterns of the CI are related to other precipitation indices such as the coefficient of variation of annual precipitation (Section 3.1.1), seasonal rainfall regime (Section 3.1.2), percentage of summer precipitation (Section 3.1.2) and probability of a day with precipitation (Section 3.1.3).
Section snippets
Objectives
The principal objective of this article is to determine the daily concentration of precipitation in the contiguous United States, i.e. evaluate the importance of the rainiest days to the annual total. The principal objective is divided into three specific objectives: 1) characterise precipitation in the study area by means of different variables in addition to the amount of precipitation, such as inter-annual variability, seasonal regime and the likelihood of a days with precipitation (>0.1 mm);
Characterisation of precipitation
Apart from the average annual quantities of precipitation mentioned above in the climatic characterisation of the study area, for the purposes of analysing the daily concentration of precipitation, it is appropriate to consider other pluviometric variables, such as inter-annual variability, seasonal regime or the probability of a days with precipitation, which will help explain and contextualise the results.
Discussion
The CI is an index at a daily resolution which provides information on the frequency distribution of daily precipitation quantities, or any other numeric variable, focusing on the more or less regular distribution of the various classes of daily quantities of precipitation. Thus, it is related to the rainfall weight in the annual total of a specific fraction of the days with the highest amounts of rainfall. Therefore, it provides new information, albeit related, with respect to the statistical
Conclusions
In general terms, the spatial distribution of the CI in the contiguous United States is geographically consistent, reflecting the principal physiographic and climatic units of the country. The probability or daily frequency of precipitation is significantly correlated with the CI. The central region of the country, which has summer rainfall maxima, exhibits relatively high CI values.
The CI values obtained range widely between 0.52 and 0.72, similar to those of Europe. 54.7% of the surface of
Acknowledgments
This study was funded by the WEMOTOR (CSO2014-55799-C2-1-R, 2015–2017) project of the Spanish Ministry of Economy and Competitiveness, and the Climatology Group (2014 SGR 300).
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