February 16, 2007

The Rain in Spain

Filed under: Precipitation

What film dominated the Academy Awards in 1964 winning Best Picture, Best Director, Best Actor in a Leading Role, Best Art Direction, Best Cinematography, Best Costume Design, Best Music, Best Scoring of Music, and Best Sound? The answer (that should be obvious from our title for this piece) is My Fair Lady starring Audrey Hepburn and Rex Harrison. Somehow, just mention the four words “The Rain in Spain” and everyone starts humming or whistling a very famous melody from that musical.

An article in the recent Journal of Geophysical Research puts the spotlight directly on the rain in Spain as four scientists from the Universidad of Extremadura examined precipitation records from throughout the Iberian Peninsula from 1958 to 1997. We have been told many times before that global warming will impact precipitation and generally increase the rainfall for the planet as a whole. While many might argue that increased precipitation would be a good result of warming, the fear mongers claim that global warming will increase extreme rainfall events which in their eyes would be yet another disastrous outcome of the continued buildup of greenhouse gases.

The just-released Summary for Policymakers of the new Intergovernmental Panel on Climate Change (IPCC) report (AR4) states that “The frequency of heavy precipitation events has increased over most land areas, consistent with warming and observed increases of atmopsheric water content” and “is very likely” that “heavy precipitation events will continue to become more frequent” during the 21st century. The last we checked our atlas, Portugal and Spain are certainly land areas. When we saw the title of the article “Changes in frequency and intensity of daily precipitation over the Iberian Peninsula,” we took a hard look, and as you’ve probably have guessed already, the rain in Spain might be a Spain pain to greenhouse predictions.

Gallego et al. collected daily rainfall data from 35 observatories distributed throughout the Iberian Peninsula (IP) for the period 1958 to 1997. They broke the data into four categories including light (between 0.2 and 2.5 mm per day), moderate (2.5 to 7.5 mm), intense (greater than 7.5 mm), and very intense (greater that 15 mm). They examined annual and seasonal trends in the amounts of rain falling in each category and they examined the number of days with rain in each category. They used a Mann-Kendall (MK) statistical test to determine if significant trends existed in the data.

The colorful figure below is one of many presented in their article, and this figure, like the others, suggests that intense precipitation (lines d and e in the figure) is decreasing in all seasons (columns are spring, summer, autumn, winter from left to right) while light precipitation (line b) is increasing. Line a in the figure suggests that total precipitation has generally decreased in all seasons over much of the peninsula.


Figure 1. Mann-Kendall test for accumulated rainfall in the spring, summer, autumn, and winter seasons (from left to right) for (a) total, (b) light, (c) moderate, (d) intense, and (e) very intense rainfall categories. Triangles point up and down to denote the sign of the trend, and the size of the triangles denotes the statistical importance of the trend. Black triangles show trends that are significant at the 0.05 level of confidence (from Gallego et al., 2006)

Gallego et al. conclude that “one saw that there was an increasing trend in the number of days with light rainfall in every season over much of the IP, while number of days with moderate, intense, and very intense rainfall was diminishing, especially in spring and winter.” Further, they note that “The more generalized patterns of behavior found in the number of events were internally consistent. The increasing trends in the number of days of light rainfall in summer and autumn were sufficient to explain the aforementioned increase in the total number of rainy days. The spring and winter increase observed in the number of days of light rainfall was at the cost of the decrease in the rest of the rainfall categories (moderate, intense, and very intense) in these seasons. This implies that the real distribution of the daily peninsula precipitation is becoming ever more asymmetric.”

They also state “The proportion of light rainfall relative to the total represented an increase at the peninsula level in every season. There were decreasing trends in spring and summer, and in spring and winter, in the proportion of intense and very intense rainfall relative to the total, respectively. This indicates that the increase in the proportion of light rainfall is occurring at the cost of a decrease in the proportion of intense and very intense precipitation.”

OK – we get the point, light precipitation amounts and days with light precipitation are increasing, but the days with and/or amounts of moderate to very intense precipitation are decreasing. Not surprisingly, they also found “there was a decreasing trend in the mean precipitation value per rainy event in every season. This decrease is consistent with the aforementioned increase of the proportion of light rainfall relative to the total.”

Gallego et al. present a very interesting set of patterns that seem to fly in the face of predictions about what is happening and will happen to precipitation patterns in the future. Once again, numerical models of climate tell us to keep an eye out for increases in intense precipitation, but in the Iberian Peninsula, it appears just the opposite is occurring.

Imagine if Gallego et al. had discovered significant upward trends in intense precipitation – you would see “Rain in Spain” headlines everywhere. But because they found significant downward trends in extreme rainfall, “The Rain in Spain” headline may appear only in the World Climate Report.

Reference

Gallego, M. C., J. A. Garcia, J. M. Vaquero, and V. L. Mateos, 2006. Changes in frequency and intensity of daily precipitation over the Iberian Peninsula, Journal of Geophysical Research, 111, D24105, doi:10.1029/2006JD007280.




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