A soon-to-be published paper by Hamilton et al., "Observations of an Atmospheric Chemical Equator and its Implications for the Tropical Warm Pool Region", has been publicized in the scientific media recently. The paper describes aircraft observations of a sharp boundary of chemical species in the atmosphere, particularly carbon monoxide (CO), and also including ozone and volatile organic carbons (VOCs). These observations took place north of the northern Australia city of Darwin. The researchers termed this sharp boundary a "chemical equator".
In brief summary, what Hamilton et al. observed was that the sharp boundary where the atmospheric concentrations of CO rose markedly over a short distance (30 km) did not coincide with the well-known boundary zone in Earth's atmosphere called the Intertropical Convergence Zone (ITCZ). The ITCZ is a zone where the Northeast trade winds and the southeast trade winds converge, inducing warm moist air to rise, resulting in increased cloud cover and frequently heavy rainfall. The ITCZ can be easily perceived in satellite weather images from geostationary satellites as a band of clouds lying approximately along the Equator. The ITCZ migrates seasonally, depending on where the warmest sea surface temperatures are located.
Satellite image of the eastern Pacific Ocean. The ITCZ is marked by the bright band of clouds aligned horizontally over the equatorical Pacific. (Image courtesy of Cllimate and Radiation Branch, Goddard Space Flight Center)
Giovanni possesses several data products of relevance to the Hamilton et al. research results. Notably, we can examine the CO mixing ratio using AIRS data and view the chemical equator that the scientists measured with atmospheric measurements obtained by aircraft. We can also look at other data products using Giovanni's visualization tools to observe the meteorological factors described in the paper.
The first image from Giovanni is of the CO mixing ratio at a pressure of 802 hPA, which is fairly near the surface. The aircraft measurements were made between 400 meters and 2500 meters above the ocean surface. After creating the image with Giovanni, a line indicating the approximate location of the chemical boundary (between low and high concentrations) and the location of Darwin, Australia (where the flights orginated) was drawn on the image with MS Powerpoint. The period chosen corresponded to when the first aircraft measurements were obtained, on January 30, 2008.
What the researchers noted specifically was that the CO chemical equator did not coincide with the approximate location of the ITCZ. So now we'll look at some other data using Giovanni to examine the meteorological environment and the location of the atmospheric boundaries and processes described in the paper.
First we'll examine the MODIS cloud fraction data product. Cloud fraction indicates the fraction of the sky that is covered by clouds. Because the ITCZ is an area of very persistent cloudiness, the location of the ITCZ can be indicated by looking at where the clouds are most persistent. So for the same time period as the image above, the MODIS cloud fraction was examined, with the palette customized to only show areas with 90% cloud cover or higher. On the image, several meteorological features described in the paper have been sketched, which will be discussed below the image.
The thick black line locates what the researchers describe as "a large band of convection (running south west from 120 ºE, 10 ºN to 180 ºE, 30 ºS and over 100 km wide". This is termed the South Pacific Convergence Zone. The dotted black line is the position of the ITCZ, which according to the paper was determined by the "conventional pressure definition (a narrow band of low pressure due to convergence and uplift)".
The white line follows the boundary of the 90% cloud fraction area, which bulges south over Australia due to a monsoon low pressure system (trough). East of this region, the 90% cloud cover boundary corresponds well to the approximate location of the ITCZ described in the paper. The yellow line is the approximate location of the chemical boundary from the first image.
The researchers noted that the chemical equator was found in clear skies north of Darwin; this appears
to have been fortuitous, as indicated by the area of lower cloud cover immediately north of Darwin. The chemical equator formed "by confluence between the north-westerly monsoonal flow out of Indonesia and the south-westerly southern hemisphere air flowing cyclonically around the monsoon trough to the south." This set of meterorological circumstances meant that the chemical boundary was not near the regions of atmospheric convergence.
One other way to look at the ITCZ and the meteorological conditions is with Tropical Rainfall Measuring Mission (TRMM) precipitation data. In order to visualize areas with lighter rainfall during this period, the palette was adjusted for accumulations between 0 and 100 mm; higher accumulation will thus be seen as bright red and pink in the image. This image shows some interesting characteristics.
The same lines as shown in the figure before are superimposed on this image. This image shows that the chemical boundary was associated with areas of lighter rainfall, particularly over Sumatra and Java, so the TRMM data provides another way to visualize the zones and boundaries occurring in the atmosphere at this time. Note how clearly the monsoon low pressure system south of Darwin is shown in this image.
The last image shows the CO boundary image again, now with the ITCZ, SPCZ, chemical boundary and 90% cloud fraction lines superimposed.
So Giovanni visualizations have allowed us to examine the features and processes described in Hamilton et al., and perhaps to provide some additional insight using the data products available in Giovanni.
Hamilton, J. F., G. Allen, N. M. Watson, J. D. Lee, J. E. Saxton, A. C. Lewis, G. Vaughan, K. N. Bower, M. Flynn, J. Crosier, G. D. Carver, N. R. P. Harris, R. J. Parker, J. Remedios, and N. Richards (2008), Observations of an Atmospheric Chemical Equator and its Implications for the Tropical Warm Pool Region, J. Geophys. Res., doi:10.1029/2008JD009940, in press.