Smoking Rain Clouds over the Amazon
Summary (2 min read)
Introduction
- Heavy smoke from forest fires in the Amazon was observed to reduce cloud droplet size and so delay the onset of precipitation from 1.5 kilometers above cloud base in pristine clouds to more than 5 kilometers in polluted clouds and more than 7 kilometers in pyro-clouds.
- This should substantially affect the regional and global circulation systems.
- The microphysical effects of the aerosols on clouds and precipitation are no less important but have until now only been inferred from modeling and satellite observations.
- Therefore, clouds over the Amazon during the rainy season are predominantly microphysically maritime, hence the term “green ocean” (9). (iii) Smoky clouds: Vegetation burning produces high concentrations of aerosols, a large fraction of which are capable of nucleating cloud droplets.
- The lack of early precipitation allows updrafts to accelerate and transport cloud water in deep convection to the high and supercooled regions, where it can release additional latent heat of freezing, which it would not have delivered in the maritime case of early rainout.
The SMOCC Campaign
- The need to validate this conceptual model motivated the LBA-SMOCC (Large-Scale Biosphere-Atmosphere Experiment in Amazonia– Smoke, Aerosols, Clouds, Rainfall, and Climate) campaign from September to November 2002.
- 6Departamento de Ciencias Atmosfericas, Instituto de Astronomia, Geofisica e Ciências Atmosfericas, Universidade de Sao Paulo, Rua do Matao, 1226 CEP 05508-900, Sao Paulo, Brazil.
- To investigate the role of smoke aerosols with minimal influence of changes in meteorological conditions, the authors conducted a set of flights on 4 and 5 October 2002, which went from a moderately polluted region in Rondonia to an area with very clean air over the western Amazon (Fig. 1).
Results
- The fires emit smoke particles, which the authors quantified as condensation nuclei (CN) concentrations.
- These values are slightly higher than the lowest campaign-average values observed previously over the Amazon [380 to 390 cm 3 (8, 13)] and about twice as high as typical values in the marine boundary layer (MBL) over the Atlantic off Brazil.
- Cloud-processed or aged (days) particles in the regional haze and the aerosols in the clean (green ocean) BL are larger, with modal diameters around 130 to 170 nm.
- According to the CDSD (Fig. 4, A and B), the large-drop tail at cloud base appears to have played a role in creating the raindrops over the ocean but not over the almost similarly pristine land and definitely not over the smoky land (Fig. 4C), at least up to the aircraft operational height limitation of 4.5 km.
- They show a polluted BL below cloud base (at 1000 m) and a somewhat less polluted CDL.
Discussion
- In spite of the different source mechanisms and compositions of the aerosol particles in smoky and clean regions and their vastly different concentrations, they are strikingly similar in their ability to nucleate cloud droplets.
- The similarity in size distribution suggests that it is similar to the smoky BL aerosol shown in Fig. 27 FEBRUARY 2004 VOL 303 SCIENCE www.sciencemag.org1340the authors.
- The suppressed precipitation below H 6 km can also explain the observation that smoky Cb in the tropics enrich the lower stratosphere with water vapor (36) by allowing a greater amount of cloud condensates in the form of smaller particles to detrain from cloud tops.
- In the green ocean regime, low CCN concentrations favor efficient precipitation scavenging, which in turn reduces CCN concentrations, until a balance between natural CCN production rates and precipitation removal is achieved.
REPORTS
- Measurement of the Effect of Amazon Smoke on Inhibition of Cloud Formation Ilan Koren,1,2* Yoram J. Kaufman,1 Lorraine A. Remer,1 Jose V. Martins1,3.
- Aerosols can counteract regional greenhouse warming by reflecting solar radiation to space or by enhancing cloud reflectance (2) or lifetime (3, 4).
- Here, using data from the MODIS-Aqua space instrument, the authors report measurements of the effect of smoke on cloud formation over the Amazon basin during the dry season (August–September) of 2002—namely, the reduction of the fraction of scattered cumulus clouds with the increase in smoke column concentration.
- 2National Research Council, Washington, DC 20001, USA.
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Frequently Asked Questions (9)
Q2. Why were the measurements limited to altitudes below the zero isotherm?
The measurements were limited to altitudes below the zero isotherm ( 4.8 km above mean sea level) because of aircraft constraints.
Q3. What is the effect of the suppression of precipitation on the area?
Given that the suppression of initiation of precipitation is compensated by increased vigor of the storms, the net effect on the area amount of precipitation remains unknown.
Q4. What is the effect of the smoke radiative effect on the climate?
This response to the smoke radiative effect reverses the regional smoke instantaneous forcing of climate from –28 watts per square meter in cloud-free conditions to 8 watts per square meter once the reduction of cloud cover is accounted for.
Q5. What is the effect of CCN concentrations on the environment?
In contrast, high CCN concentrations suppress wet removal, at least in the lower and middle troposphere, and thus stabilize the pollution burden.
Q6. What is the CCN efficiency of the aerosol?
CCN efficiency spectra (the ratio CCN/ CN as a function of supersaturation, SS; Fig. 3) taken in the freshly polluted boundary layer show that about 40 to 60% of CN are able to nucleate cloud droplets at 1% SS, whereas the larger particles in aged smoke and in the clean BL have a distinctly higher efficiency (60 to 80%).
Q7. What is the significance of the large-drop tail in the smoky clouds?
This shows that the ash particles play a less important role as giant CCN in the smoky clouds than do sea salt particles that are entrained into polluted clouds (16).
Q8. What is the reason for the undercounting of aerosols?
On the other side, quality control data of the SPP-100 show a severe undercounting because of coincidence of droplets in the measurement volume in the high aerosolsituations.
Q9. How big is the DL for pyro-clouds?
At the extreme end, pyro-clouds have the smallest DL for the same H and reach only 16 m at H 3000 m, well below the size required for the onset of warm rain.