Climate Sense

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bserving

, P

redicting

and

rOjecting

limate

OnditiOns

Oscillations and westward propagating Rossby waves in the lower

atmosphere significantly modulate AOD and its spectral depend-

ence over the tropics. These intra-seasonal oscillations contribute as

much as 45 per cent to long-term seasonal mean AOD.

Changes

in circulation parameters (wind convergence and vorticity) also

produce strong perturbations in the spatial distribution of AOD, on

timescales of 7-10 days over a fairly large area.

Field campaigns

Field campaigns address region-specific problems, through intense

multi-instrumented and multi-platform observations, but over a limited

spatial and/or temporal scale. In this context several field experiments

were applied over India during the last decade. The Indoex – (India)

programme, satellite validation cruises and Arabian Sea Monsoon

Experiment

– focused on oceanic regions around India under varying

continental impacts. In addition, two land campaigns (LC-1, LC-2)

organized by I_GBP examined the mainland features.

While LC-1 was a mobile campaign focusing on the spatial varia-

tion of aerosol parameters over peninsular India, LC-2 examined the

spatio-temporal variations over the Indo-Gangetic Plain using a chain

of observatories. A synthesis of LC-1 data revealed potential hotspots of

enhanced aerosol concentration (near surface) and AOD difference over

the peninsula implying a heterogeneous vertical distribution.

This was

confirmed fromairbornemeasurements of concentration and extinction.

ICARB

The pilot field campaigns eventually developed into fully integrated

campaigns. These involved concurrent measurements over the entire

domain for long periods, employing multiple instruments for accu-

rate and independent measurements of several aerosol properties over

the land, ocean and atmosphere. The first of the Integrated Campaign

for Aerosols, gases and Radiation Budget (ICARB) series, executed in

March-May 2006, was the biggest and most exhaustive field experiment

ever conducted over this region,

and made considerable inroads into

regional characterization of aerosols and trace gases. ICARB brought out

new observations, raising several serious issues relating to the regional

climate implications of aerosols, including possible modifications to the

Indianmonsoons. Surprisingly, low single scattering albedo

– implying high aerosol absorption –were observed in the

central oceanic regions, far from the coastal regions that are

believed to be strong sources of anthropogenic aerosols.

For the first time in India ICARB brought in a synergy

of multi-platformmeasurements, satellite data, and multi-

layer radiative transfer models. This synthesis led to the

discovery of the prevalence of extensive elevated layers of

aerosol extinction over India between one and five kilo-

metres above the surface – within which aerosol extinction

was several times higher than that at the surface.

The

consequent absorption resulted in strong warming at

higher levels of the atmosphere. It also provided the first

observational evidence of a meridional gradient in this

warming, which increased from ~0.4 K day-1, at ~2 kilo-

metres over the northern Indian Ocean, strengthening

gradually to reach more than 1.5 K day-1 at ~3.5 kilo-

metres over Central India. Viewed in the light of recent

simulations suggesting that aerosol warming over the

Tibetan Plateau acts like an elevated heat pump drawing in

warm and moist air over the Indian sub-continent leading

to advancement of the Asian monsoon, the Icarb findings

of the meridional gradients assume regional importance.

Anthropogenic trace gases

Anthropogenic trace gases (such asCO, NO

, SO

andhydro-

carbons) have significant climate forcing, so there has been

focused study on these under I_GBP. Establishing ‘environ-

mental laboratories’ to quantify the regional distribution of

these species has now become a national concern. Rising

levels of these gases are of major concern for global warming

and climate change. Moreover, increased levels of these gases

alter the chemistry of the atmosphere, affecting the biosphere

and human health as well as crop yields. Ozone in the tropo-

sphere is photochemically produced from these trace gases.

Extensive investigations of ozone and its precursors

have been made using network observatories, rockets and

AOD measurements in mega cities

Source: PRL, (DOS)

Monthly mean 550nm aerosol optical depths obtained by averaging the data from 2001 to 2005 over Chennai, Mumbai, Kolkata and New Delhi. Vertical bars

indicate ±1

from the mean