Remote Sensing of the Atmosphere

Scientists utilize various remote sensory technologies, including passive imaging systems, radar, and lidar (light direction and ranging), to study gases and aerosols in the five layers of Earth’s atmosphere. Remote sensors analyze and forecast meteorological phenomena and conditions. The field of remote sensing has particular relevance in light of efforts to assess and reverse global warming caused by human-made greenhouse emissions. Remote sensory technologies are ground-based, airborne, and space-based.

The Atmosphere

One of the keys to life on Earth is the planet’s atmosphere, a multilayered field that spans thousands of miles from the ground surface. The atmosphere is the origin of weather patterns and the location in which surface temperatures are regulated. Although the atmosphere comprises three primary gases—oxygen, nitrogen, and argon—it also contains aerosols (mists), particles, and many other gases.

Five basic layers make up the atmosphere. Each layer has distinct thermal patterns, rates of motion, chemical compositions, and density. The first of these layers is the troposphere, which begins at the planet’s surface and extends skyward between 6.5 kilometers, or 4 miles (at the poles), and 20 km, or 12.5 mi (at the equator), to the tropopause (a transitional boundary between the troposphere and the above layer). These two regions make up the lower atmosphere. Earth’s weather patterns occur in this layer.

The second layer is the stratosphere, extending from the tropopause to about 50 km (31 mi) above the ground surface. In the stratosphere, heat (which is produced by the development of ozone) increases with height. This phenomenon is contrary to what occurs in the troposphere, wherein heat decreases with increased height. Although little water vapor is present in the stratosphere, about 19 percent of the gases of Earth’s atmosphere are found in the troposphere. Separating the stratosphere from the level above it is another transitional boundary, the stratopause.

The third level is the mesosphere, extending from the stratopause outward about 90 km (56 mi) above the surface. The gases in this layer are thinner, and temperatures (caused by ultraviolet radiation from the Sun) decrease with an increase in height. In the mesosphere, most space debris, such as meteors, burn up. The section separating the mesosphere from the layer above is called the mesopause. The mesopause, mesosphere, stratopause, and stratosphere are called the middle atmosphere.

The thermosphere is the fourth level, located about 600 km (375 mi) above the Earth. In this layer (which makes up the upper atmosphere), gases are thin and increasingly heated with height, a condition created by high-energy ultraviolet and X-ray solar radiation. Finally, the fifth level, the exosphere, is located about 1,000 km (620 mi) above the surface. It is the region in which satellites orbit the planet.

Types of Remote Sensor Systems

One type of remote sensory technology is ground-based technologies and systems, usually housed in observatories or mobile units (such as ships and weather balloons). Scientists use ground-based remote sensors to analyze precipitation patterns, turbulence, temperature, and other atmospheric conditions and trends.

Aircraft-borne remote sensors, another type of remote sensory technology, often prove highly effective. For example, scientists may fly high-altitude aircraft into clouds to take readings and photograph images unavailable to sensors on the ground. One such study occurred in 2000 when a high-altitude aircraft operated by the National Aeronautics and Space Administration (NASA) was flown into cloud fields over Oklahoma. Using a system known as a multi-angle imaging spectroradiometer, researchers generated simulations of weather patterns at high altitudes.

In later decades, the evolution of satellite technologies has added another dimension to scientists’ study of the atmosphere. Satellites now play an integral role in examining the many atmospheric layers, providing greater range and depth to atmospheric science. For example, the European scientific satellite MetOp includes in its cache of onboard equipment a thermal, infrared, and spectral-imaging system. This technology enables scientists to develop a comprehensive and detailed profile of the changes in atmospheric composition caused by pollution.

Scientists also use remote sensing equipment to forecast changes in atmospheric conditions. Weather forecasting continues to evolve, enabling meteorologists and other scientists to analyze complex weather patterns and, based on the data compiled, predict atmospheric phenomena. Doppler radar, lidar (light direction and ranging), thermal imaging, and other active and passive remote sensors are employed for this purpose, developing models that predict how weather systems (such as storms and even fog) form, how they will be constituted, and the tracks they will follow.

Radar and Lidar

One of the best-known ground-based remote sensors is radar, which directs radio waves at a target and interprets the returning waves. Scientists use radar to study weather systems, including wind speeds, precipitation, and other elements. Radar has evolved considerably since the 1980s to include systems that can produce highly detailed images of atmospheric conditions and events.

One such radar system is pulse Doppler radar, which focuses electromagnetic radiation at a target at various frequencies. Arrays of Doppler radars can provide a comprehensive profile of a given target, generating three-dimensional models and enabling researchers to more accurately predict phenomena.

Another increasingly popular application in the study of the atmosphere is a variation of radar that uses lasers rather than radio waves and electromagnetic radiation. Lidar, as it is known, focuses laser beams primarily on lower-level atmospheric targets (such as weather systems) and gathers data on precipitation, particulates, clouds, and other features. Lidar can in many cases provide data on specific targets with greater detail than can traditional radar.

Remote Sensing, Absorption, and Scattering

Many different types of remote sensors rely on spectral images of targets as illuminated by the Sun. Depending largely on the size and composition of the target in question (and on the wavelength at which the light is radiated), light is either absorbed or scattered by the target.

Oxygen and nitrogen molecules in the atmosphere, for example, scatter solar radiation along a short wavelength of light only. This selective scattering ability is known as Rayleigh scattering. The colors that are emitted through this scattering process are blue and violet, which is why the sky appears blue on clear days. However, through Mie scattering, some molecules are large enough to scatter virtually any light wavelength equally. Cloud droplets, for example, are large enough to demonstrate Mie scattering properties: With every wavelength of solar radiation scattered, clouds appear white (a combination of all colors on the spectrum).

Scattering has evolved into an important research method for scientists studying the atmosphere. As mentioned, many different types of molecules, particulates, gases, and clouds are found in this broad region, and each type of molecule is composed so that it absorbs some light and scatters others. Lidar has proved to be an effective tool in this arena.

Using ground-based lidar, scientists emit a series of beams into a cloud or target area in the atmosphere. The backscattering (Rayleigh or Mie) that occurs enables researchers to profile the composition of the cloud. Some airborne remote sensors employ lidar, pointing the beam downward into a specific area. Satellites, which can profile a much larger area by virtue of their location, also sometimes include lidar.

An example of the successful study of scattering concerning specific phenomena is a 2011 survey conducted near Bozeman, Montana. Scientists attempting to assess the composition and origin of an aerosol layer in the overhanging atmosphere used lidar (in addition to other remote sensor systems designed to detect scattered light signatures) to generate backscatter. Based on the data retrieved from various angles, scientists determined that the aerosol cloud was made up of smoke from forest fires burning in California more than 1,600 km (1,000 mi) to the southwest.

Active and Passive Remote Sensors

The atmosphere is a highly complex environment, containing many different types of particles, aerosols, and gases. Scientists using remote sensors to study multifarious atmospheric conditions must, therefore, utilize different types of technology.

One of these technologies is passive in nature. Passive sensory systems focus on targets that reflect natural energy or that emit their own energy. Often, passive sensors are effective during the day, when sunlight is reflected off target molecules and other objects. However, remote sensors that detect infrared signatures (such as thermal energy emitted naturally from a source) are also used during the day or night.

Active sensors, however, emit radiation at a target to analyze that target. Such radiation includes microwaves, light waves, or radio waves. Active sensors are useful for several reasons. First, when analyzing a target that does not emit an energy signature, active sensors are equally effective in daylight as during periods without illuminating sunlight. Second, these sensors can be adjusted to detect energy emissions other than solar radiation (energy released at different wavelengths). Third, by adjusting the wavelengths of the emissions focused on a target, scientists can study the target’s different properties from multiple angles.

Many active and passive remote sensors focus on microwave emissions. Microwaves exist along wavelengths of between 1 centimeter (0.4 inch) and 1 meter (3 feet). Because of these relatively long wavelengths, microwaves do not scatter like sunlight. Additionally, many microwaves (either emitted by an active sensor system or captured by a passive one) that are radiated at longer wavelengths penetrate thick clouds, haze, and even steady rainfall. In other words, microwave-oriented passive and active sensors can be utilized in most types of weather. Because of this flexibility, meteorologists and other atmospheric scientists utilize such sensors to study phenomena and create models for weather prediction.

Computer Modeling

Computer modeling is an important complement to remote sensing. Such models are highly useful for compiling large amounts of data from studies that encompass broad target areas or complex systems. This application is particularly useful when scientists conduct large-scale observations, such as global studies on air pollution and national weather forecasting.

One example of the successful application of computer modeling to remote sensing is found in ongoing research on transpiration (the process by which water is evaporated and carried into the air). Since the early 1990s, scientists have used ground-based, airborne, and satellite-based remote sensory systems to detect temperature, humidity, surface radiation, and other atmospheric elements. These global studies connect the conditions of many different target areas to create a composite of transpiration systems.

Such studies require the consolidation of enormous amounts of remote sensor data, and based on the arrangement of such data, the creation of a number of computer models to better understand trends and to more accurately predict future conditions. One such project, the International Satellite Cloud Climatology Project, utilizes three different computer models that compile remote sensor data from around the world and generate models on such areas as surface energy and future transpiration rates. Although the original dataset for the project concluded in 2009, the project was extended under the guidance of the National Ocean and Atmopsheric Association's National Centers for Environmental Information.

Future Implications

Remote sensors continue to evolve as a technology. They create greater image clarity and provide more comprehensive data on atmospheric targets and trends.

Satellite-based systems are enhancing the use of remote sensors in studying specific areas. These systems also provide data on atmospheric trends and systems on a much broader (even global) scale. Coupled with the ability of scientists to view and share remote sensor data through the Internet (often in real time), this technology is likely to continue its contributions to meteorology and earth science.

This evolution is important, particularly in light of demands to better predict severe weather and to accurately monitor greenhouse gas emissions. An apparent increase in severe weather (such as multiple severe outbreaks of tornadoes across the United States in several consecutive years) has prompted scientists to explore the factors contributing to severe storms. Remote sensors that can detect wind velocities, transpiration, and other key elements are vital.

Furthermore, heightened public attention to the effects of human-made greenhouse gas emissions (such as industrial pollution and automobile exhaust) continues to create the need to examine the types and volumes of particulates in the atmosphere. In this arena, state-of-the-art remote sensors are also making such studies possible.

Innovations continued to be made in the field of atmospheric remote sensing. Two-dimensional infrared limb-emission sounding utilizes high-altitude aircraft to analyze atmospheric conditions and collect data on composition and temperature. Commercial aircraft are often equipped with remote sensors so that they can provide data on atmospheric conditions as they perform regular duties. The sensors on satellites have become increasingly sensitive and offer more precise data readings. Finally, machine learning and Artificial Intelligence are increasingly used to analyze data. 

Principal Terms

active sensor: a type of remote sensor that emits radiation at a target to study its composition and condition

lidar: a type of remote sensor that operates similarly to radar but uses lasers instead of radio waves

lower atmosphere: region of the atmosphere comprising the troposphere and the tropopause, reaching an altitude as high as 19 kilometers, or 12 miles

middle atmosphere: region of the atmosphere comprising the mesosphere, mesopause, stratosphere, and stratopause

passive sensor: a type of remote sensor that detects naturally emitted energy, such as reflected sunlight, from target sources

pulse Doppler: type of radar system that emits waves of electromagnetic energy at an atmospheric target; provides a detailed profile of motion, precipitation, and other conditions and objects

thermosphere: region of the atmosphere marked by thin gases and ultraviolet radiation; also known as the upper atmosphere

Bibliography

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