In order to understand Earth's climate system, it is necessary to
observe the living earth itself. One of the useful tools for such
observation is satellite remote sensing. On a daily basis, we are
using satellite products to validate climate model outputs such as
temperature, cloud amount, and precipitation. It is expected that the
satellite remote sensing technique will become more important in the
next century, when we have many more earth- observing satellites in
space to generate a large volume of radiance data. One of the CCSR's
research activities is to study satellite remote sensing in order to
develop products useful for climate studies. We have obtained the
following important results from the studies.
Various Climate External Forcing (Figure 1)
Evaluation of radiative forcing in the last 140 years, as shown in
Fig. 1, indicates that our climate system has been disturbed by
various anthropogenic influences such as an increase in greenhouse
gases. However, aerosol effects on the climate system have not been
well understood and should be studied in detail.
[Figure 1]: Radiative forcing at the top of the atmosphere since
1850, as reported by the Intergovernmental Panel on Climate
Change. There is great uncertainty concerning the magnitude of
radiative forcing due to anthropogenic aerosols.
Modeling the Radiative Process (Figure 2)
Satellite sensors observe the earth through electromagnetic waves at
various wavelengths. We have to understand the mechanism of the
interaction between radiation and matter composing the atmosphere and
Earth's surface in order to obtain related geophysical
parameters. Such modeling efforts of radiative transfer in the
earth-atmosphere system are also useful for climate modeling.
[Figure 2]: Atmospheric transmission spectrum and satellite sensor channels. There are many applications using various wavelengths.
A Study of Aerosol Effects on Climate (Figure 3)
CCSR has succeeded in pioneering a work generating the global
distribution of the aerosol optical thickness and Ångström exponent,
as shown in Figure 3. Since dominance in small or large aerosol
particles is indicated respectively by large or small Ångström
exponent values, we can observe the characteristic distribution of
small anthropological aerosols and large mineral dust particles by
such analyses.
[Figure 3]: The global distribution of the aerosol optical
thickness at a wavelength of 500nm and Ångström exponent. It was
found that aerosols of various sizes cover the earth. Monthly mean
values of July 1990.
A Study of Cloud and Aerosol Interaction (Figure 4)
Aerosols modify the cloud field. Figure 4 shows the effective cloud
droplet radius retrieved by satellite remote sensing. The effective
droplet radius is reduced around the continents where there are many
aerosol particles. Such a study is important to delineate the climate
effects of cloud-aerosol interaction, which has not been well
evaluated, as indicated in Figure 1. Current knowledge tells us that
anthropological aerosols cause significant cooling by an direct
increase of solar reflection and an indirect effect due to increase
of cloud reflectivity.
[Figure 4]: The global distribution of the effective radius of
lower cloud droplets in microns. There is a large contrast in the
magnitude of effective radius between continental and remote
maritime air masses. Monthly mean values of July 1990.
A Study of Climate Effects of Clouds (Figure 5)
It is important to understand the climate effects of clouds,
especially thin cirrus clouds, that may undergo change due to global
warming. Cirrus clouds can have positive radiative forcing (i.e.,
heating the system by cloud increase) and negative forcing (i.e.,
cooling) depending on the cloud properties and the temperature
profile. This situation makes forecasting climate change difficult.
[Figure 5]: Cloud type and cloud radiative forcing of the largest
cloud amount in each region. Arrows are vectors in a x-y diagram,
with the shortwave radiative forcing as x-axis and longwave
radiative forcing as y-axis. White arrows show cooling; red ones
show heating. An example of 1986.
A Study of Cirrus Cloud Properties (Figure 6)
There have been no good statistics on cirrus cloud top temperature for
studying the climate effects of cirrus clouds. The International
Satellite Cloud Climatology Project (ISCCP) is now producing cloud
climatology maps to overcome such a problem. Figure 6 shows the cloud
top temperature of cirrus clouds estimated by an algorithm newly
developed at CCSR. The figure shows that the cloud top temperature of
tropical cirrus clouds is as low as 210K, which is thought to be
caused by vigorous deep convection in this region. The fact that the
corresponding cloud temperature by ISCCP is 240K suggests a necessity
for further evaluation of cirrus cloud temperature.
[Figure 6]: A new evaluation of the cloud top temperature of
cirrus clouds.
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