Their electrical activity is also measured, as well as precipitation in every form (rain, snow, hail, etc), which is one of the characteristics of clouds that is hardest to measure. The map obtained using the climatology from another international programme for rainfall, the Global Precipitation Climatology Project, underlines the importance of tropical regions for the water budget. Available since 1979, these data have not for the moment shown any alteration to rainfall patterns in the Tropics: the signal being sought is extremely small compared to the uncertainty in the data, and it will be several years before we are able to identify the signs of climate change with the help of these observations.
Now that we have looked at the whole atmosphere, both clear and cloudy, let's go right up to the top of it and consider energy exchanges with space. Notions like reflected or emitted energy, or the greenhouse effect, are a little abstract, and not as evocative as clouds, rain or dust. Nonetheless, there exists a direct link between the components of the atmosphere, their chemical composition, and the energy balance of our system. Yet again, this is revealed by satellites. The atmosphere and its components control the way in which the energy exchanges that drive the climate engine occur.
The Earth's radiation budget seen by the ScaRaB instrument
The left-hand column represents the long wavelength, short wavelength and net radiation budget at the top of the atmosphere during a winter season. The right-hand column shows the same data for summer. Looking at the net budget (fi gures 3 and 6) it can be seen that the tropical regions show a surplus of energy (area in red), while the north of the Northern hemisphere shows a large defi cit (area in violet) in winter. The same is true for the south of the Southern hemisphere in summer. Apart from these major features, not much else can be made out on these maps. However, if we look at the two components of the energy budget at the top of the atmosphere, the solar component (Figure 2 and 4) and the infrared component (Figure 1 and 3), separately, a number of patterns appear. For example, off Peru and Chile, a region can be seen which refl ects more solar radiation than its vicinity and yet emits a large amount of thermal radiation out to space. This is because this region is home to a great deal of very bright low cloud, and is characterized by a very dry troposphere which therefore has a weak greenhouse effect. In winter over Indonesia, a bright area can be seen which doesn't emit much in the infrared. At this season there are many deep cumulonimbus as well as tropical storms, which are very bright high clouds, but with a strong greenhouse effect, strengthened in this region by very high concentrations of atmospheric water vapour. This pattern is also seen in southeast Asia, where at this time of year there are many deep convective clouds associated with the Asian monsoon. This effect is especially marked in the Bay of Bengal.
The left-hand column represents the long wavelength, short wavelength and net radiation budget at the top of the atmosphere during a winter season. The right-hand column shows the same data for summer. Looking at the net budget (fi gures 3 and 6) it can be seen that the tropical regions show a surplus of energy (area in red), while the north of the Northern hemisphere shows a large defi cit (area in violet) in winter. The same is true for the south of the Southern hemisphere in summer. Apart from these major features, not much else can be made out on these maps. However, if we look at the two components of the energy budget at the top of the atmosphere, the solar component (Figure 2 and 4) and the infrared component (Figure 1 and 3), separately, a number of patterns appear. For example, off Peru and Chile, a region can be seen which refl ects more solar radiation than its vicinity and yet emits a large amount of thermal radiation out to space. This is because this region is home to a great deal of very bright low cloud, and is characterized by a very dry troposphere which therefore has a weak greenhouse effect. In winter over Indonesia, a bright area can be seen which doesn't emit much in the infrared. At this season there are many deep cumulonimbus as well as tropical storms, which are very bright high clouds, but with a strong greenhouse effect, strengthened in this region by very high concentrations of atmospheric water vapour. This pattern is also seen in southeast Asia, where at this time of year there are many deep convective clouds associated with the Asian monsoon. This effect is especially marked in the Bay of Bengal.
Satellites provide us with an original viewpoint, and are very useful for understanding how this energy is redistributed in the climate engine, thanks to the combination of all the various missions and instruments. And lastly, analysis of these long series of data enables us to estimate the change in the climate's operating mechanisms over time. And the climate is changing. The chemical composition of the atmosphere is evolving. A strong reminder is the well-known curve showing the carbon dioxide concentration in the atmosphere at the Mauna Loa observatory in Hawaii increasing every year.
The first satellite estimations of greenhouse gas concentrations in the atmosphere are now becoming available. This is also the case for long series for precipitation, humidity, cloud properties, etc.
This progress has been made thanks to enthusiastic researchers who share the same goal: to use meteorological data from the past to study the climate, pushing back the methodological and algorithmic* limits. This requires long-term high-level research in the science of measurements. It also requires the development, in collaboration with cutting-edge industry, of new space systems henceforth dedicated to monitoring and understanding the climate. The constellations and long-standing families of meteorological satellites are already showing us the way forward.
This progress has been made thanks to enthusiastic researchers who share the same goal: to use meteorological data from the past to study the climate, pushing back the methodological and algorithmic* limits. This requires long-term high-level research in the science of measurements. It also requires the development, in collaboration with cutting-edge industry, of new space systems henceforth dedicated to monitoring and understanding the climate. The constellations and long-standing families of meteorological satellites are already showing us the way forward.
Rémy Roca
| < Climatology of clouds |
|---|
