Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Modeling rainfall and flooding

  1. Nov 15, 2009 #1

    Xnn

    User Avatar

    I will not comment about earthquakes and tsumani's.
    However, there has been an increase in extreme precipitation
    events due to global warming/greenhouse gases.

    Basically, greenhouse gases have increased the ability
    of the atmosphere to hold water vapor. It is not that
    there are more storms than in the past, instead they are more intense.

    http://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-chapter3.pdf


     
  2. jcsd
  3. Nov 15, 2009 #2

    mheslep

    User Avatar
    Gold Member

    Re: Is there any scientific explanation for increasingly violent natural disasters?

    That may be, but I don't believe the language you used (highlighted) is supported by the IPCC. They make a great many observations, and express concern:
    but do not state the causation as a fact.
     
  4. Nov 16, 2009 #3

    Xnn

    User Avatar

    Re: Is there any scientific explanation for increasingly violent natural disasters?

    Theory and modeling both predict that hurricane intensity should increase with
    increasing global temperatures and we already know that rising levels of CO2
    cause rising global temperatures. However, the degree to which the rise
    in extreme precipitation events and storms are due to the rise in greenhouse
    gases has not been quantified. There are after all, natural variations some
    of which span decades.

    Here is a letter in the Journal Nature defining an index for potential destructiveness
    of hurricanes that factors in both duration and intensity. There has been a marked
    increase since the 1970's.

    http://www.nature.com/nature/journal/v436/n7051/full/nature03906.html

     
    Last edited by a moderator: May 4, 2017
  5. Nov 16, 2009 #4

    Xnn

    User Avatar

    Re: Is there any scientific explanation for increasingly violent natural disasters?

    Here's another paper that has found an increase in floods during the 20th century
    due to global warming with the expectation that the trend will continue.

    http://www.nature.com/nature/journal/v415/n6871/full/415514a.html

     
    Last edited by a moderator: May 4, 2017
  6. Nov 16, 2009 #5

    Evo

    User Avatar

    Staff: Mentor

    Re: Is there any scientific explanation for increasingly violent natural disasters?

    What?

    Please point out the floods it has cited, the specific number of floods, location, and how they positively linked this to "global warming" by ruling out any possibility of natural occurance. Such as natural or man made changes to the terrian that could cause flooding. Why don't I find anything about an increase of actual floods?

    This appears to be nothing more than maybe, might, possibly, perhaps...

    Where are you reading this stuff you stated as fact?
     
    Last edited by a moderator: May 4, 2017
  7. Nov 16, 2009 #6

    mheslep

    User Avatar
    Gold Member

    Re: Is there any scientific explanation for increasingly violent natural disasters?

    Thanks for the pointer to Emanuel; I'd heard of it like everyone else but never reviewed it.
    I don't have access to the full text at the moment, perhaps Emanuel does quantify? But without some kind of quantification of the effect, of what use is the prediction? Suppose (and I have no idea) the effect is predicted to be a cyclone wind speed increase of 1 part in 10000 for a 10 deg K rise in SST?

    Here are some comments on Emanuel 2005 from W. Grey, submitted to Nature in response:
    http://arxiv.org/ftp/physics/papers/0601/0601050.pdf

    If you are interested, Emanuel informally answers some of his critics on his MIT web site here.
    Skip down to:
    5. Empirical Evidence for Increasing Tropical Cyclone Activity (and a response to its critics)
    http://wind.mit.edu/~emanuel/anthro2.htm
     
    Last edited by a moderator: May 4, 2017
  8. Nov 17, 2009 #7

    Xnn

    User Avatar

    Re: Is there any scientific explanation for increasingly violent natural disasters?

    Notice the last sentance of the abstract reads as follows:

    Seems clear enough and no maybe about it.

    Don't have the full paper with all the details (it's available, but not free,).
    Have emailed the author for more infomation.
     
  9. Nov 17, 2009 #8

    Xnn

    User Avatar

    Re: Is there any scientific explanation for increasingly violent natural disasters?

    Here's a link to a free version of the Nature article on increased risk of great floods:

    http://www.gfdl.noaa.gov/bibliography/related_files/pcm0201.pdf

    Notice it is a 2002 paper.

    Wonder if all the recent flooding has made the correlation stronger.
     
  10. Nov 17, 2009 #9

    mheslep

    User Avatar
    Gold Member

    Re: Is there any scientific explanation for increasingly violent natural disasters?

    What do you mean by "all the recent flooding"? Are there data or news reports showing global flooding since 2002 are above normal?
     
  11. Nov 17, 2009 #10

    Evo

    User Avatar

    Staff: Mentor

    Re: Is there any scientific explanation for increasingly violent natural disasters?

    That's not saying that there "has been an increase in floods", it says "risk". As in "it hasn't actually happened", but there is a chance. That's not the same thing. :smile: This is just a prediction, not fact.
     
  12. Nov 17, 2009 #11

    mheslep

    User Avatar
    Gold Member

    Re: Is there any scientific explanation for increasingly violent natural disasters?

    I've been quickly through the Milly 2002 paper since Xnn's post. Part of the paper concerns model predictions for the future, which is off topic for this thread, and part of the paper concerns observations of existing food data. I'm having some trouble with the latter.

    So far I have this:
    Milly et al and others in their references admit that that there's no detectable signal in small floods, so they look at major 100-year flood events, that is events that have a probability of 1% on a given year. They globally look at 29 large watershed sites spanning the 135 yr period 1865 to 1999. That would give them a maximum of 3915 site-years, but the data does not cover the full time period for all sites and they end up with 2066 site-years. From that data, if flooding were normal, we would expect to observer 20.6 100-year floods. They look at all this data, and find there were exactly 21 100-year events globally. I don't follow how that finding produces the abstract statement "We find that the frequency of great floods increased substantially during the twentieth century". Anybody? I'm still re-reading the 2nd half of Milly.
     
    Last edited: Nov 18, 2009
  13. Nov 17, 2009 #12

    sylas

    User Avatar
    Science Advisor

    Re: Is there any scientific explanation for increasingly violent natural disasters?

    It is not the existence of 21 events that is the basis of the quoted sentence, but the fact that those 21 events occur disproportionally in the second half of the record.

    Extract:
    Under the assumption that flood events were independent outcomes of a stationary process, we used binomial probability theory to determine a probability of 1.3% of having 16 or more of 21 events during the second part of the record. For observations from an extratropical subset of the basins (see below), the corresponding probability is 3.5%, for 7 out of 8 flood events in the second half of the record. Supplementary analyses for shorter return periods (2–50 yr) did not reveal significant trends, but 200-yr flood frequency increased significantly.

    The hypothesis in the paper is explicitly described in the final paragraph as "tentative", but the statistical property they measure seems clear enough. The open issues identified in the paper are described in the final paragraph as follows:
    Our detection of an increase in great-flood frequency and its attribution to radiatively induced climate change are tentative. The frequency of floods having return periods shorter than 100 yr did not increase significantly. Potentially significant effects of measurement non-stationarity are not easily assessed. The forced signal and unforced variability in the model contain errors of unknown magnitude. Absent from the model are forcings such as solar variability, volcanic activity, land-cover change9, and water-resource development10, and potential biospheric feedbacks such as CO2-induced stomatal closure11 and water-stress-induced root extension12. Especially evident from our study are the needs for improvements in simulation of tropical hydroclimate and continued commitment to stream-gauging programmes worldwide.

    (added in edit) This paper is not only about making predictions. It is also about a measured increased in "great" floods, and that is the portion most relevant to the thread. The key finding is: We find that the frequency of great floods increased substantially during the twentieth century. This is not an increase since 2001 of course, but it does have a clear connection to a topic of increasing disasters in present times. The development of discussion into a larger view of "present times" seems a pretty normal kind of thread development; but I would not object to locking the thread if it is to be limited so tightly as to preclude even this kind of associated discussion. I agree that the OP was not well founded.
     
    Last edited: Nov 17, 2009
  14. Nov 17, 2009 #13

    D H

    User Avatar
    Staff Emeritus
    Science Advisor

    Re: Is there any scientific explanation for increasingly violent natural disasters?

    From the paper (emphasis mine):
    Our detection of an increase in great-flood frequency and its attribution to radiatively induced climate change are tentative. The frequency of floods having return periods shorter than 100 yr did not increase significantly. Potentially significant effects of measurement non-stationarity are not easily assessed. The forced signal and unforced variability in the model contain errors of unknown magnitude. Absent from the model are forcings such as solar variability, volcanic activity, land-cover change, and water-resource development, and potential biospheric feedbacks such as CO2-induced stomatal closure and water-stress-induced root extension.​

    How did this paper get published in Nature?
     
  15. Nov 17, 2009 #14

    Evo

    User Avatar

    Staff: Mentor

    Re: Is there any scientific explanation for increasingly violent natural disasters?

    I was really surprised at this also.
     
  16. Nov 18, 2009 #15

    mheslep

    User Avatar
    Gold Member

    Re: Is there any scientific explanation for increasingly violent natural disasters?

    Yes thanks, on re-read I see that point now.

    Edit: Yes, though I question whether that changes the answer to the query: What is the statistical significance of the additional floods in the latter part of the date? That is, given a coin that I test for randomness, if I toss the coin 20 times and came up with 10 heads, and also found that I observed only 4 heads in the first 10 tosses and 6 heads came in the last 10 tosses, am I justified in making in conclusions about the randomness of the coin somehow changing during the second ten tosses?

    Edit: I see the paper comment's on 'stationarity'; I'm rusty on stationary processes but I believe that pertains to my question above.

    Its the statement published in the abstract, I'm not sure it should be called the 'key' finding, given the conclusion.
     
    Last edited: Nov 18, 2009
  17. Nov 18, 2009 #16

    mheslep

    User Avatar
    Gold Member

    Re: Is there any scientific explanation for increasingly violent natural disasters?

    I don't see any problem with publishing a narrow observation of one aspect a complex system, given that the authors freely admit that there may be a number of various drivers for observations as they do in the conclusion you posted above.

    HOWEVER, in my view the stand-alone and qualitative statement in the abstract, "We find that the frequency of great floods increased substantially during the twentieth century", should have had more context or should have been dropped. For instance, as I read the paper, it would be equally appropriate make the following statement in the abstract: "We find that the frequency of 100 year floods was as expected over the observed 139 year time period (X% confidence), with an increasing frequency trend during the twentieth century". Mentioning the 100 year flood level and the time period immediately draw the readers attention to the tentativeness of the data set.
     
    Last edited: Nov 18, 2009
  18. Nov 18, 2009 #17

    sylas

    User Avatar
    Science Advisor

    Re: Is there any scientific explanation for increasingly violent natural disasters?

    I presume we are still okay discussing this? I don't mind if a new thread is made; although it seems to me that this paper is a pretty close development from the OP; if the thread originator would like to stick to post 2001 developments then by all means we can split the thread.

    A "stationary" process is stochastic process without a trend; or more formally the probabilities remain the same over time (this is even stronger). What we have here is basically testing the null hypothesis; under the assumption of a stationary process they see how likely the observed distribution might be. If is it very unlikely, then that can be taken as evidence that the null hypothesis is falsified, and that the process of producing floods is not stationary, but has a real underlying trend.

    There's a fair bit more involved, and some statistics to deal with independence assumptions that I haven't tried to follow in detail, plus also the comparison of the observations with the behaviour of climate models, which would suggest that the trend seen in observations should be expected to persist; and that is where you get a hypothesis for the scientific explanation for the observed increase.

    My reaction to this paper is that there's nothing especially dubious or unusual about publication of a study like this, that tests ideas without claiming to have a clear proof, and which indicates in the conclusion the areas of uncertainty, the limits of the approach and the useful directions for future work. It's appropriately tentative, while still have useful results.

    Fair enough. One of the key points, then.

    Cheers -- sylas
     
    Last edited: Nov 18, 2009
  19. Nov 18, 2009 #18

    sylas

    User Avatar
    Science Advisor

    Ah. We have a new thread! Thanks. There seems to be a possible point of confusion: some readers may have thought that the paper is about predictions.

    It is actually nearly all about an empirical observation from the twentieth century, of an increase in the rate of large floods. There is a fair bit of statistical work to try and sort out whether the increase is significant and suggestive of a change in conditions over the last century. There are also some model experiments to test a hypothesis about the cause of the observed trend.

    The focus of the models in the latter half of the paper is to test a hypothesis for the cause of observed increases in great flood risk.

    Significance of the increased great flood risk

    The largest part of the paper is an evaluation of the significance of the observed increase in frequency of great floods. This is done in two ways:
    1. A simple statistical method assuming independence of events and a stationary process obtained a 1.3% probability of obtaining the observations by chance. A brief extract referring to this argument appears in [post=2446947]msg #12[/post], as the first extract in blue.
    2. Then a much more subtle method was used to deal with the independence assumption, which we should not actually expect to hold in general, even with a stationary process. The method used was to use output from a "control" experiment with coupled climate models and with constant radiative forcing. That is, the models here are not a prediction; but a basis for obtained a comparison with observations and estimating how unlikely they are under the stationary process null hypothesis.

    The result with the model experiment indicate that the observed increase in great flood frequency in real life occurs about 3.5% of times in the models of a stationary climate. As the paper says:
    Thus, the model-based significance analysis, which implicitly uses the space±time correlation structure of floods in the model, essentially confirms and reinforces the simpler binomial analysis.

    Cause of the increase in great flood frequency

    On page 516 there follows a smaller part of the paper, which considers a possible hypothesis for explaining the observed increase in great floods. This section of the paper starts with the sentence:
    The apparent increase in flood risk might be associated with radiatively forced climate change...
    followed by three paragraphs detailing some further experiments with models and non-constant radiative forcing.

    The conclusion of this part of the paper appears on page 517:
    Thus, the recent history of the observed trend index is generally consistent with the range of results from the scenario experiments.

    The experiments on scenarios here are therefore being used to test a hypothesis for the cause of the empirical observations described in the first part of the paper. This paper is not really a modeling paper; models are used simply as a tool in the paper to try and reveal details of the empirical observation.

    Summary

    This paper is focused on an empirical observation of floods in the past, and considers both the significance of the observation (could it be simply a random bit of bad luck?) and possible causes (could the cause of the observed increase be related to non-constant radiative forcing over the last century?).

    The "increasing risk" described in the paper title is an observation of an increased risk over the twentieth century. The paper considers the significance and a possible cause, all in the context of observed changes in climate over the twentieth century.

    The brief mention in the abstract that the trend is likely to continue is a fairly usual sort of comment in a paper like this, making explicit the relevance or importance of the work.

    Cheers -- sylas
     
  20. Nov 18, 2009 #19

    Evo

    User Avatar

    Staff: Mentor

    Does the paper take into consideration agriculture, animal farming, housing and business developments, roads, changes in drainage, redirection of streams and rivers, articial and natural, dams - artificial and natural, silting, etc... and that these things need to be studied all the way the down from the source of the water? This is a HUGE undertaking, but obviously very important in understanding and predciting where and why an area might flood. This is really complicted stuff.

    Am I wrong that this paper falied to take these critical flood causing conditions into consideration?
     
  21. Nov 18, 2009 #20

    mheslep

    User Avatar
    Gold Member

    As shown in the last section Sylas referenced in https://www.physicsforums.com/showpost.php?p=2446947&postcount=12", the authors specifically point out they consider none of those factors Evo. But nor do they make any claim what so ever about what's causing the floods. They simply did the work to collect global flood data (100 yr) and run trend studies on it. They report that.

    Unfortunately Xnn's post (https://www.physicsforums.com/showpost.php?p=2445234&postcount=4" in the thread that introduced the 100 yr flood paper makes the statement "another paper that has found an increase in floods during the 20th century due to global warming", which is completely unsupportable from that paper.
     
    Last edited by a moderator: Apr 24, 2017
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Modeling rainfall and flooding
  1. Ancient floods (Replies: 9)

  2. High rainfall event (Replies: 3)

  3. Noah's flood (Replies: 1)

Loading...