Heavy smoke from forest fires in the Amazon was observed to reduce cloud droplet size and so delay the onset of precipitation, which affects the water cycle, the pollution burden of the atmosphere, and the dynamics of atmospheric circulation.
Abstract:
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. Suppression of low-level rainout and aerosol washout allows transport of water and smoke to upper levels,where the clouds appear “smoking” as they detrain much of the pollution. Elevating the onset of precipitation allows invigoration of the updrafts,causing intense thunderstorms,large hail,and greater likelihood for overshooting cloud tops into the stratosphere. There,detrained pollutants and water vapor would have profound radiative impacts on the climate system. The invigorated storms release the latent heat higher in the atmosphere. This should substantially affect the regional and global circulation systems. Together,these processes affect the water cycle,the pollution burden of the atmosphere,and the dynamics of atmospheric circulation. Several hundred thousand deforestation and agricultural fires burn in Amazonia during the dry season each year, covering vast areas with dense smoke (1, 2). The smoke’s radiative impact suppresses surface heating and evaporation and stabilizes the lower troposphere. In turn, this suppresses the formation of convective clouds and precipitation and thus slows down the hydrological cycle (3). 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. Convective clouds forming in smoky air show substantially reduced droplet size compared to that of similar clouds in clean air (4), with a mean satelliteretrieved effective droplet radius of 9 m in smoky clouds compared to 14 mi n clean clouds (5). This reduction of cloud droplet size by smoke is associated with an inhibition of the onset of precipitation radar echoes up to heights of 6.5 km, compared to 3 km in smoke-free clouds (6, 7). Here, we report in situ measurements for
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Q1. What have the authors contributed in "Smoking rain clouds over the amazon" ?
In this paper, 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.
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.