Archive for July, 2016

22-July-2016 storm-flood and radon washout

July 28, 2016

About 15 km away from Diekirch, the small village of Larochette and some neighbouring villages sufferered the 22 July 2016 from a disastrous downpour of more than 50 mm during less than one hour: the result were very disruptive floods which caused much destruction: bridges that crumbled, roads torn open and many houses with their low levels flooded. The phenomenon was very localized, and in Diekirch the very short-lived rain pulse was not more than about 12.9 mm (in 30 minutes).

As seen in a couple of former blog posts (see here , here and here) these rain pulses caused very visible radioactive peaks measured by our Geiger counter. We know that these peaks are essentially caused by a radon washout (the peaks are the fingerprint of the gamma-emitting radon daughters).

We all like nice and clean cause-effect relations, preferentially linear ones. When we look at what happened during the week from the 21 to the 26 July, we see that things are a bit more complicated:

22JUL2016The upper plot shows in blue the intensities of the precipitation peaks (in mm per 30 minutes, also given by the corresponding labels) and in red the cumulative precipitation. The lower plot gives the dose-rates in nSv/h, with the yellow boxes showing the approximative numbers for clarity. If we assume an usual background about 83 nSv/h, you should subtract 83 from these numbers to yield the peaks caused by the washout.

In the preceding blogs I suggested that after a first washout the atmosphere requires a minimum time to “recover”: a rain pulse, even much more intense, prior to that minimum (3 days were suggested) yields a lower radioactive response. What happened the 22 July 2016 shows both the same and an opposite behavior: the storm rain pulse of 12.9 mm follows less than 24 hours a normal heavy downpour of 4.2mm. This first precipitation event lashes out a radiation peak of 16 nSv/h; the storm event, with a 4 times higher intensity, produces practically the same radiation rise. But even more intriguing is the next rain event some 5 hours later: a meagre 1.8 mm rain produces a radiation peak of 13 nSv/h, not much lower than the preceding one.

So we have in this week two conflicting situations:

a. the first confirms the hypothesis of an atmospheric recovery-time: you can not washout what isn’t yet there; the very heavy storm event happens too early to produce a bigger radiation peak.

b. the small precipitation event following the 19: 00 UTC “monster”  causes an important radiation peak, in violation of both the effect-proportional-to-cause and the time-lag hypothesis. I have no explanation to this for the moment. A first enquiry would be looking for a precipitation measurement error. I checked this with our Davis VantagePro II backup station, mounted at a distance of about 4 m. The next table shows the details of the measurements:

22Jul2016_data._xls

The last column corresponds to the increase of the dose-rate, taking 83 nSv/h as the reference.

Clearly, there is a delay of about 1 hour between the precipitation and the radiation peaks. What remains puzzling, is the strong radiation pulse after a very modest rain peak following the “big one”.

Let us finish this short analysis by plotting the radiation increase versus the rain pulse which is its cause. I will add all the data of this week to the values measured in  2013 and 2014. First the table with these data:

radon_washout
and now the graph:

rad_versus_rain
We see that the big storm from 22-Jul 19:00 is an outlier, and a linear model might not be the best. The Pearson correlation between rain-pulse and radiation-peak is 0.23, statistically not significant at the alpha = 0.95 level. Omitting the outlier betters the correlation to 0.32, but it remains not significant. Now this discussion on significance is moot, as clearly the observations show that radon washout does exist. What remains deeply puzzling is not the situation in cases 11 & 12 (the last two lines of the preceding table): a first small rain pulse might cause a distinct radiation peak, and to get the same peak a second rain pulse following shortly must be much higher.

But the situation in cases 8&9 remains for me disturbing: why does the small rain pulse following the big one create a similar intense radiation peak?

Thank you for an answer, if you have a clue what might happen here….

_____________________________

01-Aug-2016: added as a supplementary information to Marcel’s comment.
The link to the full paper of Livesay “Rain induced background radiation….” is here.

Modulation of Ice Ages (part 2)

July 5, 2016

In the first part of this blog I recalled some fundamentals of Milankovic’s climate-relevant cycles: precession, obliquity and eccentricity. In this second part I try to resume as simple as possible the main points of the new paper by Ellis & Palmer.

1. Five major insights

The following 5 points are known and accepted by most scientists:

a. Each major deglaciation coincides with maximum NH (North Hemisphere) solar insolation.

b. Not all insolation maxima (“Great Summers”) trigger deglaciations.

c. Eccentricity governs the strength of the Great Summer.

d. During an ice age atmospheric CO2 levels plunge (colder oceans absorb more), ice sheets extension and albedo (the earth’s reflectivity) increase.

e. When CO2 levels are at a minimum and albedo is at maximum, a rapid warming will begin and start an interglacial period. Conversely when CO2 levels are at a maximum and albedo is at mininum (during an interglacial) a new cooling (= ice age) will begin.

2. The Ellis & Palmer paper

The new theory postulated by Ellis & Palmer can be summarized as follows: A minimum CO2 ( e.g 150 -200 ppm) starves plant life, creates a die-back of forests and savanna’s, which increases soil erosion and produces more dust storms. The dust deposits on the ice sheets diminish the albedo, increasing the absorption of solar energy. This increase of about 180 W/m2 at higher NH latitudes starts a global warming, i.e. an intra-glacial.

So ice ages are forced by orbital cycles and changes in NH insolation, but regulated by ice-albedo and dust-albedo feedbacks. The precession cycle is the main forcing agent through the induced albedo changes. The primary forcing and feedback for intra-glacial modulation is albedo.

As a consequence CO2 (by its greenhouse gas properties) can not be the primary feedback because high CO2 levels during or at the end of an intra-glacial result in cooling, and low CO2 levels during a glaciation maximum precede the warming.

Antarctic_T_and_dust
The grey bands in this figure correspond to maximal dust deposits  (>0.35 ppm): the Antarctic temperatures (from the Epica 3 bore-hole) start rising after most of these dust peaks.

3. Main conclusions of the paper.

Regarding IPCC’s AR5 published in 2013 the  authors write: “The IPCC has identified dust as a net weak cooling mechanism, when it is probably a very strong warming agent.”

And they conclude with these words: “The world’s dust-ice Achilles heel needs to be primed and ready to fire before an intra-glacial can be fully successful…in which case, intra-glacial warming is eccentricity and polar ice regrowth regulated, Great Summer forced and dust-ice albedo amplified. And the greenhouse attributes of CO2 play little or no part in this feedback system.

You should definitively read the full paper!

Modulation of Ice Ages (part 1)

July 4, 2016

Ralph Ellis and Michael Palmer have an extremely interesting paper in the Elsevier publication “Geoscience Frontiers” titled: Modulation of ice ages via precession and and dust-albedo feedbacks (link to open access version, May 2016). This long paper (19 pages!) is very readable, but nevertheless needs more than one reading to fully understand the important details. So I will try in this blog the resume the most important findings of that outstanding paper.

  1. The Milankovic cycles

The climate of the Earth is a system-response to the insolation of the sun. As this insolation (or irradiance) is not constant, it is not a big surprise that Earth’s climate is not constant, and never was. There are short variations, like seasons, El Nino’s, 11/22 years and 60 years changes from solar and oceanic oscillations etc. The much longer periodic changes like the ice ages are known since Milutin Milankovic’s seminal papers to be caused by variations of (at least) 3 astronomic parameters related to the Earth revolving in our solar system, varaitions that cause important changes in the insolation oft planet Earth,

The most important parameter is the precession of the Earth’s axis: this gyroscopic effect (first studied by the great mathematician Euler) means that the axis (which is inclined w.r. to the ecliptic plane, i.e. the plane of the orbit of the earth circling around the sun) makes a slow rotation around the perpendicular to the ecliptic plane. The axis oscillates between two extreme positions, where it points to Polaris (the Northern Star) or to Vega. When the axis is titled towards Polaris (which is close to the actual situation), the North Hemispheric (NH) winters correspond to a position where the globe is closest to the sun, and the NH summers where is it farthest. This precessional cycle (including a complication caused by the rotation of the elliptical orbit (=apsidal precession) has a cycle length of about 22200 years, a period often called a Seasonal Great Year (SGY). A Great Season takes 1/4 of this period, about 5700 years; one speaks of a Great Summer, a Great Winter and so on. This precession of the axis has by far the biggest influence on solar insolation (details will follow).

A second important astronomical parameter is the obliquity or axial tilt. The angle between the axis and the perpendicular to the ecliptic plane varies between 21.5° and 24.5°; the actual value is 23.5°. This angle essentially impacts the severity of the seasons. Actually the NH winters are moderate, as the solar rays are more close to the perpendicular of the globes surface, and the summers are moderate too, as the solar rays are more inclined, which diminishes their heating potential. The length of one obliquity cycle is 41000 years. Precession and obliquity cause a complicated wobbling movement of the Earth’s axis.

Finally the last important factor is the eccentricity of the Earth’s orbit. The orbit is an ellipse, close but not quite equal to a circle. The eccentricity describes the deviation from a perfect cycle, and in the case of our planet this parameter is not constant but varies slowly under the influence of the other planets with time. The cycle length is approx. 100000 years. The changes in eccentricity are small, between 0.034 and 0.058 (actual value is 0.0167, which means that the orbit actual is near circular). The main influence of the changing eccentricity is a (small) time shift of the seasons during the year.

For climate related questions, the most important parameter is the change in solar irradiance (or insolation) at high latitudes of the NH. Usually one looks at the changes observed (or calculated) at northern latitude 65° (NH 65). Here are the extreme changes caused by the variations of the three astronomical psrameters shown above:

Precession: 110 W/m2
Obliquity:      25 W/m2
Eccentricity: 0.4 W/m2

These changes can be lumped together in the so called Milankovic Cycle (figure from the Ellis/Palmer paper, the time axis is KY (kilo-years) before present):

Milankovitch_Cycle_with_zero_line

The upper plot shows the changes in solar irradiance, the lower the temperature deviations from a mean value of the Antarctic. I added a zero line to make clearer where the intra-glacial periods happen (the peaks above the zero line) and where the ice ages are (the periods in-between: note that the ice ages are the “normal” state of the Earth climate, and that the intra-glacials are, geologically speaking, exceptions to that state). The orange/red bands represent the (Seasonal) Great Summers. Clearly, not all Great Summers cause an intra-glacial warming, as there are about 4 to 5 Great Summers from one intra-glacial to the next. The Ellis/Palmer paper tries to explain this with a novel theory; I will discuss this in the part 2 of this blog.