Atmospheric sounding

Atmospheric sounding or atmospheric profiling is a measurement of vertical distribution of physical properties of the atmospheric column such as pressure, temperature, wind speed and wind direction (thus deriving wind shear), liquid water content, ozone concentration, pollution, and other properties. Such measurements are performed in a variety of ways including remote sensing and in situ observations. Atmospheric sounding or atmospheric profiling is a measurement of vertical distribution of physical properties of the atmospheric column such as pressure, temperature, wind speed and wind direction (thus deriving wind shear), liquid water content, ozone concentration, pollution, and other properties. Such measurements are performed in a variety of ways including remote sensing and in situ observations. The most common in situ sounding is a radiosonde, which usually is a weather balloon, but can also be a rocketsonde. Remote sensing soundings generally use passive infrared and microwave radiometers: Sensors that measure atmospheric constituents directly, such as thermometers, barometers, and humidity sensors, can be sent aloft on balloons, rockets or dropsondes. They can also be carried on the outer hulls of ships and aircraft or even mounted on towers. In this case, all that is needed to capture the measurements are storage devices and/or transponders. The more challenging case involves sensors, primarily satellite-mounted, such as radiometers, optical sensors, radar, lidar and ceilometer as well as sodar since these cannot measure the quantity of interest, such as temperature, pressure, humidity etc., directly. By understanding emission and absorption processes, we can figure out what the instrument is looking at between the layers of atmosphere.While this type of instrument can also be operated from ground stations or vehicles—optical methods can also be used inside in situ instruments—satellite instruments are particularly important because of their extensive, regular coverage. The AMSU instruments on three NOAA and two EUMETSAT satellites, for instance, can sample the entire globe at better than one degree resolution in less than a day. We can distinguish between two broad classes of sensor: active, such as radar, that have their own source, and passive that only detect what is already there. There can be a variety of sources for a passive instrument, including scattered radiation, light emitted directly from the sun, moon or stars—both more appropriate in the visual or ultra-violet range—as well light emitted from warm objects, which is more appropriate in the microwave and infrared. A limb sounder looks at the edge of the atmosphere where it is visible above the Earth. It does this in one of two ways: either it tracks the sun, moon, a star, or another transmitting satellite through the limb as the source gets occultated behind the Earth, or it looks towards empty space, collecting radiation that is scattered from one of these sources. In contrast, a nadir-looking atmospheric sounder looks down through the atmosphere at the surface. The SCIAMACHY instrument operates in all three of these modes.