THEGREENGROK

Rains Come and Come Again the Next Day


by Bill Chameides | March 24th, 2010
posted by Erica Rowell (Editor)

Permalink | 12 comments

Do you find that raindrops keep falling on your head? You may be right.

More Refrains of ‘Here Comes the Rain Again’ Likely in Store

Scientists have been predicting for some time that global warming is about a lot more than increasing global temperatures. It’s really about climate disruption: changes in almost all aspects of our climate, including the thing we probably care the most about, rainfall.

We don’t like it when it never rains, because we need that water for drinking and growing things. And we don’t like it when it rains too much because it can lead to flooding and, well, too much rain is just a drag.

The consensus in the scientific community has been that global warming means more rain — more rain and intense rainstorms. (For all you folks who got buried by blizzards this past winter, this includes more snow, when temperatures are below freezing.)

There are probably a number of reasons for this, here’s the most basic one: Higher temperatures cause the amount of water vapor in the atmosphere to increase, and more water in the atmosphere should bring about more water falling out of the atmosphere.

The idea of more rain with global warming is not some wild idea cooked up by a bunch of ivory-tower climate scientists. Data from weather stations show that rain amounts and rain intensity have in fact increased over the past few decades. And by the way, the increase in overall precipitation has been accompanied by longer, more intense dry periods (aka droughts) as well. More wet when it’s wet; more dry when it’s dry; less of when it’s just right.

Paper: Longer Stretches of Wet Periods Found

Now comes a new paper in Geophysical Research Letters by Olga Zolina, of the University of Bonn’s Meteorological Institute, and colleagues, that adds a new twist to the global warming-increasing rainfall story. These investigators used data from European daily rain gauges to assess the length of rainfall events between 1950 and 2008. They defined a wet period (WP) as the number of consecutive days with rain in excess of one millimeter a day (or 0.04 inches/day).

Not surprisingly, most precipitation events (i.e., ~ 50 percent) in Europe last only one day. (More than likely, wherever you are, the same is true in your neck of the woods.) And about 90 percent of all wet periods last about 3 to 4 days or less. Only about one percent of precipitation events last 10 days or more.

All well and good, and sort of ho hum. But things get interesting when the authors looked at the temporal change in the distribution of wet periods from 1950 to 2008:

  • the frequency of short WPs (i.e., 1, 2, and 3 days) decreased significantly,
  • the frequency of long WPs increased significantly,
  • one-day WPs declined by about 0.5 percent per decade,
  • 12-day WPs increased in frequency by about 0.4 percent per decade, and
  • WPs of 5–11 days increased by about 0.2–0.3 percent per decade.

There’s one more thing worth noting: the trends also show that heavy precipitation events not only increased in frequency but they increasingly occurred during longer WPs.

So some might argue that more consecutive rainy days are a drag but it’s not that big of a deal — after all, look at the folks in Seattle — they’re wet but they’re happy. (Tip of the hat to Yusuf, the artist who used to be known as Cat Stevens.) And if you’re in the rain slicker business, what could be better?

Well, in fact the trend could be trouble and it’s spelled F-L-O-O-D. Rainfall events that last for days at a time can saturate soils, push rivers over their banks, and overwhelm urban sewer systems. And one more thing: the trend is consistent with humongous snowstorms. Hmmm, any evidence of that occurring?

filed under: climate change, faculty, global warming, rainfall
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12 Comments

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  1. MattN
    Mar 25, 2010

    The degree to which you guys have swallowed that bilge hook, line and sinker cannot be measured…just unreal… Europe was in a drought in 1950. That is undeniable. Google these three words: “European” “drought” “1950” and tell me what you get. I don’t care about spatial this and spatial that. Just handwaving. Europe was in a drought in 1950. Period. So it is obvious to anyone with 2 brain cells to rub together that a return to normal precipitation conditions will appear to be an upward trend in precipitation from that starting point forward. This isn’t that hard fellas…it’s an old trick. (Did I say “trick”???)

  2. MattN
    Mar 24, 2010

    http://forecast.mssl.ucl.ac.uk/docs/Lloyd-Hughes+Saunders.pdf Page 1584, figure 6, proportion of Europe experienceing moderate drought conditions. 1950 was a high point for drought in Europe. Therefore a return to “normal” rainfall patterns appear to be a trend to excessive wetness from that point forward. Pick 1900 as a starting point and there is no trend towards wetter conditions. In fact, there looks to be a .068% trend towards being *drier* in Europe. Why do you guys continue to swallow this crap??? Does it taste good or something?

    • Jim
      Mar 25, 2010

      This is a quote from the very paper you referenced [Lloyd-Hughes, Saunders] in Section 4: “Trends in drought/wetness, though small for Europe as a whole, are significant over several European regions. Upward trends in wetness are largest over northeast Europe during winter and spring. These results agree with the IPCC report findings on the regional impacts of climate change (Watson et al., 1997) which show a precipitation increase over the region from the Alps to northern Scandinavia since 1900. The drying tendencies found in central eastern Europe and western Russia since 1900 are also in accord with the IPCC findings.”

    • JoshU
      Mar 25, 2010

      You forget to mention that the paper you reference found the index of drought/wetness to be spatially variable across Europe, with a significant shift towards wetter conditions in northeast Europe and drier conditions in central Europe and western Russia over the 20th century. This is in accord with the IPCC predictions and is acknowledged as such by the author in the discussion. Furthermore, the author explicitly states in the results that “the mean number of extreme and moderate wet events have tended to increase from 1901-50 to 1951-99”. This is in accordance with the findings of the Zolina paper discussed by Dr.Chameides above. Although not your original intent, thank you for providing us with an additional peer reviwed study which lends support to the findings of the discussed paper as well as the predictions of the IPCC.

    • Bill Chameides
      Mar 26, 2010

      MattN – Yum. Determining trends of precipitation back to the early 1900s is problematic because the spatial coverage of rain gauges was less than today. Nevertheless, the data DO indicate that there has been significant trends in precipitation over the century that are consistent with what would be expected by global warming. See for example: http://pubs.giss.nasa.gov/docs/1997/1997_Dai_etal_2.pdf and http://www.nature.com/nature/journal/v448/n7152/full/nature06025.html and http://www.agu.org/pubs/crossref/2006/2006JD007103.shtml

  3. M_N
    Mar 24, 2010

    Anyone else wonder why they picked the start date of 1950? Anyone wonder what the trend would look like if they picked 1900 as the start date?(I’m sure the data is availible.) I don’t wonder. I’m positive it was chosen specifically to maximize the appearance of any short term trend in favor of “the sky is falling” crowd. Just more of the same crap science…..

    • Jim
      Mar 24, 2010

      The U.S. is a small part of the world…

      • MattN
        Mar 25, 2010

        see European data below….

      • MattN
        Mar 25, 2010

        Europe is only ~14% larger than the US. So it is only a slighy less small part of the world. You point was….?

    • Sandman
      Mar 24, 2010

      Yes, they probably picked 1950 because all the European nations were war-torn from 1939 to 1945 and were busily rebuilding from 1945 to 1950 and thereafter. Do you think many European rain gauges and their records survived the bombings? I’m glad you’re so “sure the data is available”; I’m not. More importantly, what has your argument to do with the actual data presented? 68 years is enough time to analyze a trend such as this.

      • MattN
        Mar 25, 2010

        As I told Jim, see European data below. Data IS availible. And the trend from 1950 to 2000 looks dramatically different than the trend from 1900 to 2000. With the highest % of Europe seeing drought conditions around 1950 than any other time in the 20th century, it is now obvious why 1950 was chosen as a starting point. The only cherries that have been picked are by the author(s) of this paper…

  4. MattN
    Mar 24, 2010

    Long term, there is no upward (or downward) trend: in US wet weather: http://www.climate-movie.com/wordpress/wp-content/uploads/2010/03/Slide70.jpg Long term, there is no upward (or downward) trend in US drought: http://www.climate-movie.com/wordpress/wp-content/uploads/2010/03/Slide69.jpg When you get caught in an unusually strong rainstorm (or snow storm), what you are experiencing is called W-E-A-T-H-E-R.

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