Why is hail raining in the Amazon?

by Ina Taraldsen and Yael Knoll

Recently there have been reports about hail in the tropical rain forests in Barzil.

This is not a regular phenomenon, so we decided to look into it.

The human impact on the tropical rain forest

"The eradication of the Amazon Rainforest is wreaking havoc on the climate of our Earth".  (Solcomhouse website)

Human habitation, agricultural development and massive industries productions are some of the most important reasons for rainforest destruction.

As the population grows more and more areas are turned into by living areas and food growing fields.

Mostly the rainforests are being cleared by massive fires.

Moreover, the industrial pollution (e.g. CO2, CO gasses and other organic compounds) have an effect on the rain forests.

According to the "Solcomhouse" website, the annual report of the Brazilian government on the destruction of the rain forests declared that the pace of destruction is steady, even with the increased policing of the threatened areas.

"The Brazilian Amazon, which alone is larger than Western Europe, lost 16,926 square kilometers (6,347 square miles) of forest last year, according to satellite imagery".

Aerosols

Aerosols are actually a collective name for small particles that are suspended in the atmosphere.

The aerosols can react chemically with other substances, interfere with the sun radiation and disturb the rain clouds.

Aerosols are important in cloud droplet formation – water vapor condenses around the particles, thus enabling rain clouds formation.

We will focus on human-made aerosols, made by fires and industrial activity.

The research

Between September and November 2002 an international research team, led by M. O. Andrea from the Max Planck Institute in Germany, set off to the Amazon area in Brazil to investigate the effects of the massive fires on the formation of clouds, nature of precipitation and global climate.

By satellite images, chemical and physical tests they could analyze the "weird" meteorological phenomena and suggest a model.

According to the research there are several kinds of "cloud and precipitation regimes":

1.     Blue Ocean:  relatively clean area above the ocean with low concentration of CCN, producing clouds with a small amount of big drops, that coalesces into rain drops quite rapidly.  Moreover, weak updrafts allow more time for precipitation of the rain drops.

2.     Green Ocean: "Over the unpolluted Amazon, especially in the rainy season" we can see that the CCN concentration is relatively low, duo to the effect of the aerosols washout and clean air.  Therefore, clouds in that area and time have a small amount of big droplets that condense rapidly into rain drops.

3.     Smoky Clouds: The fires in the Amazon release a large amount of aerosols that nucleate the cloud's water vapor resulting in high concentration of small droplets, slowly to precipitate. The slow precipitation results in the constant presence of aerosols in the air, thus maintaining the rainless condition. Moreover, the water vapor is being transferred to higher, supercooled areas, enabling intense ice precipitation, hail and thunder formation.

4.     Pyro-clouds: an extreme version of the smoke clouds that exist from the heat and smoke of the mass fires. There is a conflict in this case:  on one hand, the high concentrations of the CCN stop the precipitation. On the other hand, ash particles may serve as giant initiating particles for precipitation.

Representative values of CO, CN, CCN (1%), and cloud droplet

number concentrations in the different regimes sampled during LBA-SMOCC.

(Taken from the research publication at www.sciencemag.org)

During the research the team made flights at different altitudes over the above mentioned "areas", detecting the concentration and diameter of the present CCN.

The following observations were made:

1.     Tail of drops:  at the Blue Ocean a wide tail of large drops was seen at the base 

of the clouds, due to salt aerosols.                                                                                                                                                                                                

            At the Green Ocean there was a smaller tail of drops at the base of the clouds..

            The Pyro clouds showed the smallest tail, due to the ash particles of the fires.

2.     Connection between the altitude and precipitation:  at the altitudes between 1000 m and 1500 m there was precipitation at the Blue and Green Oceans, but not in the Smoky and Pyro Cloud areas.

           Only at the altitude of 6600 and 7600 m, respectively, precipitation occurred.

3.     When the water vapor condenses around aerosols and when they freeze, heat is released.

conclusions

1.     pyrogenic and biogenic aerosols contain soluble material that allows them to absorb water vapor, activating clouds at slight saturation and only at high altitudes.

2.     The CCN serve as "recovery" nucleating agents for polluted clouds at high altitudes.

3.     For polluted areas, only at high altitudes, where the temperature is sufficiently low, there can be precipitation.

4.     Aerosols do not let some of the sun's radiation reach the surface and they reduce evaporation from the oceans and reduce the necessary heating that energizes the convective clouds (Hansen et al., 1997; Ackerman et al., 2000).

5.     In a global scale, the released heat can change the circulation of air, creating movement of  aerosols to higher atmosphere levels and by that - across the globe. Consequently, the effects discussed above will spread over the globe.

Another module for the relationship between the CCN and the aerosols population

Köhler equation tries to quantify the phenomena.

Super saturation has to exceed a certain critical value ( S_c) and only then a droplet will start to grow to cloud-droplet size.

"S_c is a function of, among other things, the concentration of solute in the wetted aerosol particle and its surface tension".

The mass fires and industrial activity add soluble material to the atmosphere, so the number of CCN increased and by that the concentration of the droplets in the polluted clouds increases as well.

Surface tension is another factor in the equation: surfactant organic material could lower the surface tension of the aerosols consequently lowering the critical value for super saturation.  

More particles will be activated and grow to cloud droplets, the climate will be affected as described above and there will be less precipitation.

So, why is hail raining on the Amazon?

Concentrating on deeper clouds at the smoky and pyro areas shows us precipitation processes different then those of the blue and green areas.

At high altitudes the water vapor condenses around the aerosols and the surrounding temperature is low enough to freeze the drops, resulting in 2 cm drops.

As the drops freeze, intense heat is released to the surrounding, resulting in strong turbulent updrafts.

This makes the clouds more vigorous and by that allowing short bursts of hail, rain, thunderstorms and lightening.

The solution

 We believe that the only solution is to monitor the industrial activity, screening the gasses and particles coming out as the by-products.

A strong lobby against rainforest's destruction should be founded, concentrating on alternative solutions for the parties making a living from the destruction.

As for the damage that was already done, such as the "resident" aerosols in the higher altitudes, we hope that time will help them precipitate and eventually eliminated from the atmosphere.

Bibliography

1. Max Planck's Institute joint research at www.sciencemeg.org
 http://bio.univet.hu/SALVE/Student_essay/libr/hail_science.pdf

2. Atmospheric aerosols
 http://www.greennature.com/article278.html

3. Article in Hebrew concerning the phenomena
http://news.walla.co.il/?w=/5/513226

3. Introduction to tropical rain forests.
 http://library.thinkquest.org/26634/forest/intro.htm?tqskip1=1

4. Solcomhouse website – destruction of the rain forests.
http://www.solcomhouse.com/rainforest.html

5. The effect of aerosols on cloud properties and precipitation 
 http://earth.huji.ac.il/research.asp