Amit Teller - The interaction of aerosols and clouds and the effect on precipitation.
The increased demand for fresh water and the high vulnerability of different regions of the world to climate change and especially to drought events leads to extreme water shortages and may endanger existing fresh water sources. Cloud physics research is aimed at studying how dynamic and microphysicsal processes lead to the formation of precipitation in clouds. Aerosols have a crucial role in these processes.
The complex interactions between aerosol particles and clouds depend on the particles’ characteristics (concentrations, size distributions, and chemical compositions) and on the type of clouds that are involved (continental or maritime, convective or stratiform, cold or warm).
Aerosols, on which cloud droplets form, determine the initial concentrations and sizes of the droplets; they influence the production of precipitation and affect clouds’ radiative properties. In addition, the clouds modify the aerosol composition and size by various chemical reactions that take place in the drops. The mutual interactions of aerosol particles and clouds have received much attention by the scientific community in the last few decades. The interest grew partly due to the still unclear effects that clouds have on climate.
Amit Teller’s research focuses on the interactions between clouds, aerosols and precipitation with special emphasis on the contribution of different types of aerosols from anthropogenic and natural sources on the development of precipitation. The research has important implications for scientific understanding of weather modification, as weather modification is based on seeding aerosols in clouds in order to increase the formation rates and sizes of droplets and ice particles. The objective of these operations is to enhance precipitation.
The research in this field is composed of in-situ airborne measurements to characterize the aerosol and the cloud particles’ characteristics and the use of the data in detailed cloud models that simulate the dynamic and the microphysical processes. Amit Teller is involved in cloud measurements and characterization projects as well as in cloud model studies aimed at interpreting the measurements.
As an example for the effect of aerosols on precipitation, we show results from simulations of precipitation formation from orographic clouds (Fig. 1). Orographic clouds are clouds that develop in response to the forced lifting of air by the earth's topography (mountains for example).
The study was carried out with Weather Research and Forecast (WRF) simulations coupled with a new bin microphysics scheme. The advantage of this tool is in its ability to provide an accurate description of number and mass distributions of different cloud species (liquid, and ice) following cloud microphysical processes (condensation, collision and coalescence, aggregation and many more cloud processes).
In this example we compare the total precipitation in 4 simulation runs where we used Winter (350 cm-3) and Summer (790 cm-3) initial aerosol concentrations and two soundings in which the ground temperatures were 0°C and 7°C. The results shows that the clean environment produces more precipitation than polluted. Most of the precipitation in the 0°C cases comes as snow, while in the 7°C precipitation mainly comes as raindrops. A rigorous analysis of the results reveals that the increased aerosol concentration in the polluted cases delays the onset of precipitation and forms a larger concentration of droplets with smaller sizes. In addition, more droplets reach the so-called mixed phase region of the cloud in the polluted cases. These droplets can freeze to form more ice crystals and snow.
The results shown here reveal that air pollution modifies precipitation. However, it has been shown that the estimation of this effect should also take into account other atmospheric factors such as differences in topography, temperature profiles and humidity.
(a)
(b)
Fig. 1 – Total ground precipitation produced after 6h simulation of orographic cloud over 800m bell shape mountain using the WRF bin-microphysics scheme. The shape of the mountain is shown in the bottom subplots, (a) – Atmospheric profile with ground temperature 7°C, (b) - Atmospheric profile with ground temperature 0°C
ASP Spotlight October 2008
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