Role of EarthCARE
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Investigating the effect of clouds and aerosols on global warming

The temperature drops differently on a cloudless winter night than on a cloudy winter night because clouds are closely related to temperature changes on the Earth's surface and in the atmosphere. Clouds warm the surface by absorbing heat radiated from the land and oceans and returning some of it but also cool the Earth by reflecting sunlight.
The degree of greenhouse and cooling effects depends on the thickness, height, how clouds overlap, the size and shape of the cloud particles, and the moisture content.

Observation by EarthCARE satellite

Observation by EarthCARE satellite

For example, it is known that clouds high in the sky warm the ground, while clouds low in the sky cool it.

Aerosols also reflect and absorb light from the sun, and the presence or absence of aerosols can significantly change the properties of clouds. For example, the time it takes for a cloud to form and disappear (its lifetime), optical properties such as reflection and absorption, and physical properties such as the size of the cloud particles differ between the ones formed in areas with high aerosol levels and those formed in areas with low levels.

However, scientific understanding of how these phenomena occur has been lacking because precise global observations of the three-dimensional structure of clouds and aerosols have been very limited. Because clouds can range from several kilometers to several hundred kilometers across, it has not been possible to observe their details globally using only ground observations.

EarthCARE will use satellite observations of clouds and aerosols on a global scale to determine how they affect global warming and how they are connected to the global climate system.

Clouds high above and near the ground

Clouds high above and near the ground

Contribute to improving the accuracy of climate change prediction models by observing cloud and aerosol details

To understand the impact of clouds and aerosols on the Earth's radiation budget (i.e., the energy balance due to solar and terrestrial radiation), it is important to accumulate detailed vertical information on clouds and aerosols over the entire Earth, for which there is little observational data.

Cloud Profiling Radar was jointly developed by JAXA and the National Institute of Information and Communications Technology (NICT) and uses high-frequency (94 GHz) radio waves to determine cloud particle size, moisture content, and vertical structure. For the first time in satellite observations, the radar will also measure the ascent and descent speeds of cloud particles globally to reveal convection currents within clouds.

Radar is a technology that can detect the presence or absence of an object, its condition, and its distance. In a cloud composed of many small particles, the radar signal penetrates the cloud and returns information about the cloud particles as a signal that depends on the distance. In other words, it is possible to measure the internal structure of clouds by obtaining cross-sectional information on how many cloud particles are present and at what distance. Satellite cloud profiling radar is pointed directly downward to obtain vertical internal information about the altitude and particle composition of the clouds below.

Since cloud particles are small, they can only be observed with high-frequency radio waves. This requires a transmitter that transmits high-power radio waves and high mirror-precision antenna. Moreover, the radio waves must be transmitted continuously over the three years of the mission, which has been technically difficult to develop.

However, NICT's outstanding research on high-power transmitter tubes has enabled us to reach the required operating life. JAXA has developed an antenna with a diameter of 2.5 meters, which will be the world's largest 94 GHz satellite radar. This will enable observations with a sensitivity about 10 times that of conventional satellite-borne cloud radar.

Appearance of cloud profiling radar developed by Japan

The cloud-profiling radar developed by Japan

Atmospheric lidar, on the other hand, developed in Europe, observes aerosols (fine particles suspended in the atmosphere) and very thin clouds with smaller diameters that cannot be observed by cloud profiling radar. Lidar, like radar, can obtain distance information. By combining lidar and cloud radar, we can simultaneously measure the extent and height of cloud particles and aerosols of different sizes and clarify their three-dimensional structures.

As already mentioned, EarthCARE's cloud profiling radar will be the first in the world to provide information on cloud ascent and descent rates on a global scale. Generally, an evolving cloud rises by gaining updrafts and buoyancy, and when sufficiently grown, it loses its buoyancy, and the heavier particles are dragged down by gravity. Although the vertical particle motion is strongly linked to the cloud growth process, detecting vertical cloud motion has been limited to ground observations.

The EarthCARE satellite will be able to observe clouds and aerosols simultaneously because clouds and aerosols are closely related. Cloud particles are formed when water vapor comes into contact with aerosols and becomes water or ice particles (condensation). Aerosols in clouds change the properties of the clouds in complex ways, such as when clouds composed of liquid water reduce the size of their particles. Conversely, some aerosols encourage the growth of ice particles in clouds with a mixture of water and ice cloud particles.

The EarthCARE project, which simultaneously equips a cloud profiling radar and an atmospheric lidar, is expected to provide new scientific knowledge not only on the vertical structure of clouds but also on the processes of cloud formation and dissipation and the interaction between clouds and aerosols. The vertical motion of clouds has not been adequately represented in numerical models due to a lack of global observational data and scientific understanding, a major source of error in climate change prediction. The observational data obtained from EarthCARE will dramatically improve the accuracy of numerical models for predicting climate change.