Atmospheric Sciences

Research >> Atmospheric Sciences

Earth atmospheric research at ARIES - a High altitude (~2km) site in Central Himalayas

At present a multi-wavelength solar radiometer (MWR) is operational at ARIES to measure aerosol optical depth at different wavelengths under ISRO-GBP ACE Project. An augmentation of instrumental set up to carry out atmospheric research is essential.
The instruments include

(a) Aethalometer for aerosol black carbon measurements
(b) Radiation sensors for surface solar radiation measurements
(c) Condensation nuclei counter to study aerosol-cloud interactions

The 'Excess' Atmospheric Absorption

Recent studies suggest that the clear-sky atmosphere absorb more short-wave radiation than predicted by radiative transfer models. These investigations have shown that radiative transfer models consistently overestimate diffuse downward irradiance at the surface in a cloud-free atmosphere by 9 to 40 % while correctly calculating direct normal solar irradiance. They reported significant discrepancies at low altitude stations while showed good agreement at high altitude sites. These studies have led to a hypothesis that an unidentified absorber may be present in the atmosphere, which has optical properties similar to that of black carbon. Several investigators, on the other hand, have found agreement between model and observations.

Aerosol-Cloud Interactions: Indirect Effect

Aerosols play an important role in determining the global mean cloud cover. An increased concentration of aerosols result in an enhanced concentration of cloud droplets which in turn increases the albedo of clouds and this causes a decrease in the short wave solar radiation reaching the earth's surface. Cloud albedo has a significant role in determining the global energy balance. Effect at IR may be negligible because tropospheric clouds are already optically thick at IR. The increase in condensation nuclei (CN) also influence the cloud life time. An increase in CN increase the cloud droplet concentration and reduce the mean droplet size. This increases the cloud life time and inhibits precipitation. This also lead to an increase in fractional cloud coverage and influence both short wave and long wave radiation. Cloud albedo depends on the cloud droplet number. For a given water vapor content, the average cloud droplet size is larger for less number of aerosols and is small for more number of aerosols. This is because the water vapor availability per CN is more in the former case compared to latter case. Not all aerosols are capable of acting as CN. For acting as CN, it should be larger than a critical size (~1 µm) and should be hygroscopic (water soluble). As the number of aerosol increases, the super saturation reduces. The inverse correlation is due to the fact that as more drops form, the water supply available will be less and as a result S is reduced.

Role of Black Carbon in Radiative Forcing
Even though soot contribute only about 5 - 10 % to the aerosol optical depth, it has very significant role in the forcing because of its absorption properties. Soot contributes about 35% of the total reduction of solar radiation at the surface. That is over ocean where surface reflection is very low (~ 6%). If the same aerosol system were present over the land the effect of soot absorption would be significantly larger because of the radiation reflected from below and interacting with aerosol again. Estimates show that the negative (cooling) radiative forcing in clear skies can even become positive (heating) forcing in the presence of clouds. According to the present estimates, the expected change in climate due to greenhouse gases is not observed in the real case.
It was the general belief that sulfate aerosols contribute a major fraction of the aerosol system and the net effect of aerosol is cooling the planet. But recent investigations have observed significant amount of carbonaceous aerosols (black carbon for e.g.). Since black carbon (BC) is highly absorbing, the net effect is heating the planet. So question of whether aerosol cools or warm the planet depends on the relative contribution of various chemical species which constitute the aerosol. An aerosol with significant BC content can have net warming effect and complement to the green house warming. The effect of aerosol on climate is the best example of how human activities interfere with climate.

Regional consequences of global warming depend critically on the potentially large cooling or warming effect of aerosols. Aerosols scatter sunlight back to space and cause a regional cooling effect. These aerosols consisting of sulfates, soot, organic carbon and mineral dust are produced both naturally and by human activities. Results of numerous global warming models suggest that the aerosol cooling or warming is one of the largest, if not the largest, source of uncertainty in predicting future climate. Still, the complex influence of aerosol cooling on global warming is not clearly understood.

Reduction of Cloud Cover by BC Heating

Measurements and models show that enhanced aerosol concentrations augment cloud albedo not only by increasing the total droplet number but also by reducing the precipitation. This increases the cloud liquid water content and cloud coverage. Aerosol pollution is expected to exert a net cooling on the global climate. There is another mechanism through which aerosols can reduce the cloud cover and thus significantly offset aerosol induced radiative cooling on regional scale. This is because of the presence of soot, which intensified the solar heating.

Mesosphere Lower Thermosphere Photometer (MLTP)

Gravity wave and solar tides are most important coupling agents in the lower and upper atmospheric processes. They are generated at lower atmospheric altitudes and propagate upwards. Their interaction with background wind and other wave modes leads them to dissipate their heat and momentum at different altitudes. Most significant dissipation occurs at mesospheric altitudes and this in-turn affects the upper atmospheric processes through turbopause. Identification and exploration of these wave processes is one of the prime objectives of this airglow photometer. Using Airglow emissions coming from mesospheric altitudes we can study complex mesospheric wave dynamics. MLTP uses Airglow as a tool to find out atmospheric wave characteristics through observed intensities and derived temperatures.