Effects of Clouds on the Global Climatic System

Towering columns of cumulonimbus stretch vertically in the summer skies, while cirrocumulus spread broadly across the autumn skies. We have sensed the arrival of seasons through particular cloud formations since the ancient time. We have also known their deep relationship with weather variations, sensing such things like when the weather may break by observing a thin veil of cirrostratus clouds far up in the sky – in fact, the layer of cirrostratus clouds tends to grow thicker as a warm front approaches.

Clouds move and transform in response to fine changes in atmospheric composition, providing us important clues for understanding the airflow that is not directly discernible. Clouds do not only mirror atmospheric conditions, but are also determinant factors that influence the ground temperature. Typically, clouds generate so called the umbrella effect and greenhouse effect, which contribute to controlling the ground temperature. You may feel a little cooler on a cloudy day in midsummer because the clouds serve as an umbrella to reflect solar radiation back into space. On the other hand, severity of a winter morning may get lessened on a cloudy day because the clouds prevent some of the outgoing infrared radiation from the Earth's surface to escape into space. Through these two effects, clouds play a very important role in meteorology and climatology that concerns everything from tomorrow’s weather forecast to a prediction of climate in the next century.

However, the formations and distributions of clouds are very transient and manifold. Accurately estimating the effect of clouds on solar radiation and thermal emissions would be as difficult as catching the clouds in your own hands. It is very difficult to make an accurate estimate of impacts of clouds on future changes in temperature. Indeed, the 2001 report issued by the Intergovernmental Panel on Climate Change (IPCC) recognizes the cloud as the largest indeterminate factor for global warming prediction.

It is now considered one of the best solution in coping with these difficulties is to identify the physical mechanism that relates clouds and climate by conducting a thorough examination past and current observation data. And doing so is the solid first step for improving the accuracy of climate change prediction.

Cloud Patterns Observed from Remote Sensing Satellites

Let us take a look at some of the cloud research projects conducted through Earth observation satellites.

Including the well-known meteorological satellite Himawari, a geostationary satellite that provides weather forecasts for newspapers and television, satellite-based cloud observations often utilize visible and infrared radiometers. Recently, NASDA’s Earth Observation Research Center (EORC) successfully mapped the distribution of physical parameters of clouds, as shown in Fig. 1, using MODIS data from the U.S. satellite Terra. The image was observed around noon on April 13, 2001, as the satellite passed over East China Sea.

We will briefly explain the terminology used in Fig. 1. Optical thickness refers to the magnitude of effect that clouds have on solar radiation. Effective radius indicates the process of the cloud droplet growth, and, in turn, the efficiency that cloud droplets are transformed into raindrops. The cloud top temperature and liquid water path contribute for determination and classification of cloud types (thickness and altitude). This MODIS data analysis was conducted as a JST core research project (Asian Atmospheric Particulate Environment Change Studies: APEX, under the leadership of Prof. Teruyuki Nakajima, Center for Climate System Research, Univ. of Tokyo). Future plan calls for identifying the interaction between the climate, clouds and aerosol-particulates suspended in the atmosphere by integrating this satellite data with the results of airborne experiments and an analysis of ground observation data.

Both visible/infrared radiometers and microwave sensors have long been used to obtain cloud image data. Projects combining visible / infrared data with microwave data have been initiated to gain a greater understanding of cloud formation and cloud characteristics. One example of these projects is being continued using data obtained by the Tropical Rainfall Measuring Mission (TRMM) satellite, which was launched atop the H-II rocket at the end of 1997 with a Visible and Infrared Scanner (VIRS) and TRMM Microwave Imager (TMI). Fig. 2 shows the effective cloud particle radius estimated from a simultaneous analysis of VIRS data and TMI data.

The top image shows the effective radius calculated by dividing the liquid water path retrieved from microwaves with optical thickness. As in the lower left image in Fig. 1, the bottom image in Fig. 2 shows the effective radius estimated from VIRS data. As found in Fig. 2, the derived results differs greatly between the two methods. It is because microwaves have a higher transmittivity and can detect the particle sizes of raindrops below clouds (top image), while visible and infrared measurements are limited to the cloud particle radius near the cloud tops, regardless of the presence of raindrops below clouds. Distinctive differences in particle size can be seen when comparing tropical regions, where cumulus clouds often develop and produce showers, to subtropical regions off the west coast (the waters off California and Peru), where low-level clouds such as non-precipitating stratocumulus and fog usually spread (compare the top and bottom images in Fig. 2).

Expectations for Future Cloud Observations from Space

Thanks to the successful launch of the HII-A launch vehicle No.1, NASDA is accelerating its planned space projects. The Advanced Earth Observing Satellite (ADEOS)-II is scheduled to be launched in 2002. The Global Imager (GLI) and the Advanced Microwave Scanning Radiometer (AMSR) onboard ADEOS-II are expected to become a useful tool in cloud observation. The GLI will have a multi-channel (a total of 36) system for short wave and long wave observations, and the AMSR makes observations in microwaves at eight frequencies. With these two systems combined on a satellite, ADEOS-II further extends the potential of the simultaneous multi-spectral Earth observation. In other words, it will become possible to obtain physical parameters of clouds on a global basis, including the high latitude region, by enhancing the approach applied to the creation of Fig. 2.

The space-based Earth atmospheric observations have been mainly conducted through passive sensors, such as radiometers and spectrometers. However, beginning with Precipitation Radar (PR) onboard TRMM, we will see a number of active sensors (such as radar and lidar) observing precipitation and clouds in the first decade of the 21st century.

CloudSAT, a U.S. -Canada joint project will launch a cloud radar satellite around 2003. The objective of CloudSAT is to obtain the three-dimensional structure of clouds.

Also, NASDA and ESA plan to launch a cloud radar and a lidar onboard a satellite between 2007 and 2010 as part of EarthCARE project. This joint project will allow us to directly estimate the vertical distribution of cloud particle radii by simultaneously analyzing the data from these instruments, and dramatically enhance our understanding of dynamic microphysical processes of clouds (i.e., the growth process of cloud droplets and their transformation into raindrops).

Furthermore, NASDA and NASA is planning Global Precipitation Mission (GPM) together. The GPM will consist of a core satellite equipped with a dual-frequency precipitation radar and eight satellites equipped with microwave radiometers working concurrently. NASDA and NASA expect the mission to be launched as early as 2007. Once deployed, the GPM satellites will observe precipitation every three hours on a global basis.

These future projects will bring us the world where passive/active radars, through visible to microwave, working together to contribute the understanding of global cloud/participation observation in less than 10 years. An enormous amount of observation data will be analyzed to provide more complete picture of the climatic change mechanism. This enables the Earth scientists to propose better solutions for environmental problems such as global warming, flood disasters and desertification through their unique viewpoint.