* Improve our understanding of the aerosols, clouds and chemistry-climate interactions.
* Use the data collected during INDOEX for validation of GCMs and chemistry-transport models.
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 the solar absorption at the surface and in the troposphere including the 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 measured to study how aerosols influence it. The specific issues that have to be unraveled are:
Do models systematically underestimate clear sky solar absorption?
If so, how does the magnitude of the discrepancy depend on water vapor concentration, aerosol characteristics and concentration of pollutants?
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 aerosols (sulfate, black carbon, organics) and natural aerosols (dust) in the clear sky and cloudy sky solar absorption?
How does this excess heating alter the meridional heating gradient in the ocean and the atmosphere?
|Figure 8. Surface ozone distribution during the cruise of the ship Mme Butterfly of the Wallenius Line. Height of the bars is proportional to the ozone mixing ratio.|
The first question we will address is:
To what extent are anthropogenic emissions altering the chemical composition of the "free" troposphere?
Our own (by scientists from the Max Planck Institute for Chemistry (MPIC), Mainz, Germany) worldwide measurements of surface O3 on board a commercial ship with automated instrumentation have 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 upper troposphere 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 and its role in changing the oxidation capacity of the atmosphere, but also about the role of the ITCZ in stratosphere-troposphere and interhemispheric exchange.
Does air entering the stratosphere in the ITCZ come predominantly
from the northern or southern hemisphere?
The three primary objectives are strongly linked experimentally as well as scientifically.
First, the chemical, radiative and microphysical measurements required for accomplishing the three objectives are very similar. For example, to estimate the role of aerosols in the excess absorption, we need to understand the optical properties and composition of aerosols (sulfates, dust or carbonaceous). Likewise, to understand the role of anthropogenic sulfates, we need to identify the origin of the air (marine or continental), which requires chemical measurements such as concentration of CO, O3, NOx and CH4. Lastly, to understand the role of aerosols in direct and indirect radiative forcing, we need solar radiation flux observations for clear and cloudy skies under pristine and polluted conditions.
Second, the regional emissions which lead to sulfates can also lead to increased tropospheric ozone. The greenhouse effect of increased tropospheric ozone (particularly if it is mixed vertically through the upper troposphere) may partially offset the sulfate cooling effect, thus linking Objective 1 with 3.
Third, the possible importance of anthropogenic emissions from the Indian sub-continent is a critical issue for all of the three objectives listed above.
|Figure 9. Schematic of air flow into the upper tropical troposphere. Horizontal lines are contours of constant potential temperature.|
Is there any evidence for the occurrence of routes B and C? Very
recently, papers by Chen (1995) and Dunkerton (1995) have
argued from large scale meteorological analysis that there is exchange
between the upper tropical troposphere and the lower midlatitude
stratosphere at potential temperatures above about
340 K, particularly in the summer hemisphere. Kent et al (1995) have argued for stratospheric input to the UTT from satellite aerosol observations. Figure 10 shows average vertical profiles of chlorofluorocarbon-11
(CFC-11) (Elkins, private communication) above Hawaii (21°N) and near the equator (0°-7°) at longitudes close to the dateline taken in October 1994 from the ER2. The CFC-11 mixing ratios are shown relative to the tropopause; there are two important features from the point of view of INDOEX. One is that the value of
CFC-11 at the tropical tropopause is less than the free tropospheric or surface values in either hemisphere, the other is the negative lapse rate of CFC-11 in the upper tropical troposphere. The implication of these observations is that there is stratospheric air in the upper tropical troposphere in detectable amounts. The negative lapse rate and variability are greater above the tropical tropopause, and can only be accounted for by transport of older stratospheric air from midlatitudes. Note that the lifetime of CFC-11 against photodissociation immediately above the tropical tropopause is years, so the CFC-11 loss there cannot be occurring in situ. It is not clear what the division is between horizontal transfer via routes BS and BN in Figure 9, or via vertical transfer C down through the tropical tropopause. Irrespective of the route, the fact is that stratospheric air, bringing with it stratospheric composition and aerosol loading, is present in the upper tropical troposphere. It is clear then that the balance of the chemical composition of the upper tropical troposphere is between convection from below and transport from the stratosphere. We pose the following questions to address the larger issues, noting that each has a myriad of finer associated questions.
|Figure 10. Vertical profiles of CFC-11 from the ASHOE experiment (1994). Altitude is measured relative to the tropopause.|
* What effect, if any, does the presence of detectable amounts of stratospheric air in the upper tropical troposphere have upon cirrus formation there?
* What is the chemical composition of the airflow in the lower troposphere leaving the Indian subcontinent in the NE monsoon during the early part of the year?
* How does this chemical composition change as a function of time as the flow approaches and enters the ITCZ, and how is it related to cloud formation?
* What are the vertical profiles of gaseous species (particularly ozone) and aerosol particles in the ITCZ?
* What is the chemical composition, both particulate and gaseous, of the outflow from the ITCZ in the upper tropical troposphere and lower stratosphere?
* Does air entering the stratosphere in the ITCZ come predominantly from the northern or southern hemisphere?
A complete examination of the various mechanisms under each objective must include studies with 3-dimensional atmospheric models. Carrying out these studies will require additional information, which is encompassed by the following supporting objectives:
* Measure the atmospheric boundary layer depth and the height of the monsoonal inversion, with particular emphasis on diurnal variability and variation from the Indian coast up to the ITCZ.
* Measure vertical profiles of water vapor distribution from 15 to 25 km.
* Measure the 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, 1996; McFarquhar and Heymsfield, 1996]. 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 11 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 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 11. Ice particles collected in subvisual cirrus located below the troposphere in the vicinity of Kwajalen, Marshall Islands.|