SATELLITE-DERIVED CLOUD PROPERTIES OVER CHINA
K. Kawamoto*, T. Hayasaka
Research Institute for Humanity and Nature, Kamigyo, Kyoto, 602-0878 Japan
kawamoto@chikyu.ac.jp
WG VIL/6
KEY WORDS: Atmosphere, Climate, Meteorology, Pollution, Satellite
ABSTRACT:
Low-level water cloud (cloud top temperature more than 273K) properties such as the optical depth, particle radius, and vertically-
integrated droplet number concentration are derived from satellite remote sensing technique with Advanced High Resolution
Radiometer (AVHRR) over the East Asia, mainly China area. Their annual mean characteristics thus obtained are consistent with
what the Twomey effect indicates, showing larger optical depth and droplet number concentration, and smaller particle radius over
land generally compared to those over ocean. Long-term analysis of cloud particle radius from 1985 to 1994 reveals gradual
decrease with time for clouds over both land and ocean. This trend, however, would be affected by some crucial reasons. We need to
be careful when we use long-term satellite data. Also, we estimate cloud spherical albedo and shortwave radiative flux by a fast
parameterization method. Development of fast computation of cloud-relevant quantities will contribute to better understanding of
climate problems.
1. INTRODUCTION
In considering the earth's climate formation and maintenance,
clouds play very important roles in terms of radiative and
hydrological processes. For example, they reflect solar radiation
and absorb and re-emit infrared radiation, and they produce
precipitation. Among climate issues, the aerosol indirect effect,
in other words, cloud modification due to interacting with
aerosol particles is one of the most uncertain (IPCC, 2001).
There are mainly two aspects in the aerosol indirect effects. One
is the change of cloud radiative properties, increasing the
optical depth and decreasing the particle size due to an increase
in droplet number. The other is the change of hydrological cycle,
prolonging cloud lifetime due to reducing precipitation
efficiency. They are called the aerosol indirect effect of the first
kind and the second kind, respectively. It is important to
monitor cloud properties such as the optical depth and particle
size for the aerosol indirect effect study, and satellite remote
sensing is a very effective method in terms of wide area
analysis.
Low-level water clouds covers about one thirds of the globe,
and have dominant effects on the earth radiation budget
(Harrison et al. 1990). Low clouds are expected to interact with
aerosols more than other types of clouds such middle and
higher clouds, since aerosols are also suspended in the lower
part of the atmosphere. Also water droplets are assumed to be
sphere, so we can apply the exact scattering in radiative transfer
calculation. On the other hand, ice clouds like cirrus and
tropical deep convective clouds have non-spherical and
complicated shape, and no exact scattering theories have been
proposed so far.
China had experienced substantial economic development
with the advent of so-called *open door policy' from the late
1970s. Due to this change, the energy consumption rapidly
increased, and emitted aerosols are also expected to increase.
China area is known to be a complex aerosol system of mixing
various chemical species from natural and anthropogenic
* Corresponding author. Kazuaki Kawamoto
origins. For example, there are dust particles from desert as
natural origin and sulfate and nitrate aerosols as anthropogenic
origin. Carbonaceous aerosols are generated both from
industrial activity and biomass burning. Kawamoto et al. (2001)
established an algorithm for deriving the water cloud properties,
and they argued annual mean features, seasonal changes and
altitudinal contrast of water cloud parameters on a global scale.
In this work, we concentrate on over China area (E.90- E135,
N15-N55) for the target region and discuss water cloud
properties about the annual mean and long-term (from 1985 to
1994) time series.
In chapter 2, algorithm and data used in this study are
described. In chapter 3, characteristics of cloud parameters over
China are illustrated. Finally, we summarize the results in
chapter 4.
2. Data and method
The algorithm of Kawamoto et al. (2001) was adopted for
retrieving the cloud optical depth at visible wavelength T,
effective particle radius r, and cloud top temperature 7. of
water clouds whose top temperatures were warmer than 273K.
The definition of r, is as follows.
3
BA (1)
[rncrdr
where n(r) is the number size distribution at a particle radius r.
Log-normal size distribution was assumed for n(r). It uses two
cloud-reflected solar radiances and one cloud-emitted thermal
radiance. The retrieval principle is that cloud-reflected solar
radiance at non-absorbing visible wavelength is a function of t,
while that at water-absorbing near-infrared wavelength is a
function of r, (Nakajima and King 1990). The current algorithm
was developed improving Nakajima and Nakajima (1995)'s
drawbacks in the limitation of regional application. It
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