Chapter 3

Scientific Goals and Objectives

The fundamental goals, behind the scientific objectives described below, are two fold:

* to improve our understanding of the aerosols, clouds and chemistry-climate interactions

* to use the data collected during INDOEX for validation of GCMs and chemistry-transport models.

The three primary objectives described below are strongly linked. First, the Indian Ocean is a unique region for attacking all three objectives since it contains a mixture of persistent clear skies, marine stratocumulus and deep ITCZ cloud systems. Second, the chemical, the radiative and the microphysical measurements required for accomplishing the three objectives are very similar. Lastly, the possible importance of anthropogenic emissions from the Indian sub-continent is a critical issue for all of the three objectives listed below:

Primary Objectives

Assess the significance of sulfates and other continental aerosols for global radiative forcing.

In particular, we are interested in addressing the following questions:

Is the radiative cooling effect of anthropogenic particles confined regionally to urban and surrounding land areas?


Can it spread to remote regions and influence the ocean heat budget and the planetary (clear and cloudy) albedo thousands of kilometers away from the source of the pollution?

What is the importance of inter-hemispheric differences on upper-tropospheric aerosols, cirrus clouds, and their radiative properties?

We emphasize the importance of understanding the sulfate effects in the presence of other continental aerosols such as dust and carbonaceous aerosols. Again, the Indian Ocean provides this opportunity.

Assess the magnitude of solar absorption in ITCZ cloud systems.

This objective, while it is important on its own merit, is inextricably linked with the first objective because the atmospheric cloud forcing has to be observed to study how aerosols influence it. The specific issues that have to be unraveled are:

Do ITCZ convective/cirrus systems enhance atmospheric solar absorption?

If so, what is the physics and chemistry behind this excess absorption?

What is the role of anthropogenic (sulfate, black carbon, organics) and natural aerosols (dust) in the cloud solar absorption?

How does this excess heating alter the meridional heating gradient in the ocean and the atmosphere?

Assess the role of the ITCZ in the transport of trace species and pollutants.

The first question we will address is:

To what extent are anthropogenic emissions altering the chemical composition of the "free" troposphere?

Figure 8. Surface ozone distribution during the cruise of the ship Madame Butterfly of the Wallenius Line (Coutesy of P.J. Crutzen)

Our own (by scientists from the Max Planck Institute for Chemistry in Mainz, Germany) worldwide measurements of surface O3 on board a commercial ship with automated instrumentation has clearly shown much higher ozone concentrations during the dry season over the ocean in the vicinity of India (Figure 8). At this stage the spatial extent of elevated anthropogenic ozone concentrations in the troposphere over the Indian Ocean is unknown. If the extent is sufficiently large, and if anthropogenic ozone is transported into the uppertroposphere in the ITCZ, there will be an effect on the radiation budget of the troposphere in this important region of the globe. Because of strongly expanding human activity the effect will be enhanced in the future.

The reality of the large population density on the Indian subcontinent in close proximity to the ITCZ forces us to think not only about the chemical perturbation of the tropical free troposphere, its role in changing the oxidation capacity of the atmosphere, but also about the role of the ITCZ in stratosphere-troposphere and interhemispheric exchange. In this context, we will examine the following question:

Does air entering the stratosphere in the ITCZ come predominantly from the northern or southern hemisphere?

This question is crucial to issues of age of stratospheric air and global transport of greenhouse gases such as CO2, CH4, SF6 and the CFCs, as well as the timescale for transport of these critical species.

Targets of Opportunity

The observations that will be collected to accomplish the 3 primary objectives above will also provide some new data to make a modest beginning on three important issues: A) meteorology of the NE monsoon and the transition period between the NE and SW monsoons; B) regulation of the sea surface temperature in the Indian Ocean warm pool; and, C) interhemispheric and geographical differences in cirrus properties.

Meteorology of the Asian winter monsoon period

Since much of the focus in the past has been on the SW monsoon, very little is known about the January-April period which includes the NE monsoon and possibly the transition between the NE to SW monsoon. The cloud, water vapor, chemistry and radiation data collected during INDOEX will shed insights into the following aspects of the NE monsoon and the transition period:

* Better understanding of the north-south variation of the monsoonal inversion. INDOEX needs this data to understand how the continental aerosols and gases are transported southwards; species that penetrate this inversion will most likely be deflected eastwards and will not make it to the Arabian Sea and the tropical Indian Ocean.

* The vertical variation of the water vapor and ozone across the ITCZ. These data are required for the chemistry and cloud forcing objectives of INDOEX.

* The frequency and spatial extent of the westward propagating cyclonic disturbances (which constitute the ITCZ). This is needed to obtain a better understanding of the coupling of the ITCZ, chemistry and clouds.

* Surface heat budget and sea surface temperature across the ITCZ. These data will be collected to unravel the aerosol effects on the sea-surface energy budget.

It should be noted that the INDOEX data collected during a limited time period over a limited spatial domain while not adequate for a climatology, will be useful for the validation of models used in monsoon studies.

Indian Ocean warm pool SST regulation

The CEPEX campaign in the western Pacific determined that evaporative cooling decreased, the water vapor greenhouse increased unstably, and the shortwave cloud forcing was greatly enhanced (¾ -100 Wm-2) with increasing SST [Zhang and Grossman, 1995; Weaver et al., 1995; Valero et al., in preparation; Collins et al., 1995]. These findings were based on spatial increases in SST from the eastern to the western Pacific. The large scale dynamical forcing in the western Pacific is strongly influenced by east-west SST gradients. On the other hand, the ITCZ circulation over the Indian Ocean is strongly modulated by land-sea heating contrasts.

The equatorial Indian Ocean offers an unique, if not the only opportunity to find out whether the CEPEX findings are valid for temporal changes in SST. During January to June, the SSTs warm rapidly from 301.5 to 303 K and then cool back to about 301.5 K. Some of the fundamental questions that can be answered include:

Is the warming accompanied by an unstable increase in the water vapor greenhouse effect?

How does the evaporation respond to the temporal SST changes?

Is the reduction in solar energy at the surface much larger than the increased reflection at the tropopause, or is it moderated by the sulfate and dust aerosols?

Additional measurements required for this study are turbulent evaporation fluxes from the sea surface, and drifter (or buoy) data for SST and mixed layer depth.

Microphysical and radiative properties of cirrus clouds

Microphysical measurements during CEPEX demonstrated that the mid-levels of cirrus anvils are optically more dense than the other levels [Heymsfield and McFarquhar, 1995; McFarquhar and Heymsfield, 1995]. Inability to sample the upper-most cloud layers and thin subvisual cirrus located at the base of the tropopause placed some uncertainty on these interpretations. Figure 9 shows the complexity of the crystals that form in subvisual cirrus and the need to collect representative data from the upper levels of anvils.

INDOEX offers an opportunity to characterize cirrus microphysics at all levels in an area of great importance to climate models. Inter-hemispheric differences can be used to evaluate the influence of anthropogenic forcing. Tropical cirrus cloud properties can be contrasted with measurements in mid-latitude from such experiments as First International Satellite Cloud and Climatology Project (ISCCP) Regional Experiment (FIRE) and International Cirrus Experiment (ICE). INDOEX will help in the collection of a global cirrus cloud property data base. Concurrent radiative measurements can help determine the relationship between cloud albedo and particle size distributions.

0 to 20 µm
Figure 9. Ice Particles collected in subvisual cirrus located just below the tropopause in the vicinity of Kwajalen, Marshall Islands. [Courtesy of A. Heymsfield]