Satellite Thermal Sounders Atmospheric gases absorb radiation at specific wavelengths. They emit at those same wavelengths.
The intensity of the radiation emitted is proportional to the temperature of the gas. Stephan Boltzman Equation. As the radiation moves upward toward satellites, that radiation emitted by specific molecules can be absorbed by the same type molecules higher in the atmosphere. These molecules will then emit radiation, again at particular wavelengths. From the amount of energy received by a satellite at a particular wavelength, the temperature of the emitting gas can be determined.
Filters on the photomultiplier tubes that detect radiation are set to allow only specific wavelengths through. These wavelengths (channels) are chosen by the wavelengths of particular gases that are fairly evenly distributed throughout the atmosphere - primarily the troposphere; e.g., CO 2. Some wavelengths it strongly absorbs and emits, others it absorbs/emits less strongly. And, some wavelengths it hardly absorbs at all.
By choosing the proper channels, temperatures can be determined for levels high in the troposphere, (opaque), near the middle of the troposphere, (semi- transparent), or near the earth’s surface, (nearly transparent).
Clouds are opaque at all three wavelengths. This image shows temperature variations in upper troposphere. Warm tems over N. California - Ridge. Cool over central US - Trough. Blurry view of ground. Sierra Nevada shows as line of cooler temps. Warm temps off Baja California - middle and lower troposphere is warm. Shows surface temps. If no clouds are present. Southwest is warmest. Cool water temps off west coast. Low clouds show up over central US.
By choosing channels sensitive to water vapor, a profile of water vapor can be obtained. Clouds (liquid water) is opaque and so water vapor profiles can be made in these areas. (Shown as gray or white on image). Total column water vapor (precipitable water). Shows depth of water if all water vapor in column condensed and precipitated.
(2) Images and gradients? Most meteorological satellite images are presented at about 4 km resolution, (pixel resolution) although capabilities exist for much smaller resolution. The latest GOES-13 can give images to 1km resolution, and, of course, spy satellite at even much smaller scale. Since with infrared satellite images, a temperature for each pixel can be determined, then a gradient can be determined, (the derivative in the horizontal). One must be careful near the edges of the image because of the change of scale that occurs. It is often difficult to determine distance there.
(3) Water Vapor Images There are certain frequencies that water vapor molecules absorb and emit radiation. The intensity of emitted radiation is dependent on the temperature of the molecule. Meteorological satellites detects radiation from a variety of channels. Various instruments detect radiation from those channels in which water vapor emits radiation. (AVHRR, AMSU A/B, HIRS, etc.)
Water Vapor Images Channels are usually chosen to detect radiation from water vapor that is in the levels about 500 mb up to 300 mb. If a lot of water vapor in this region, radiation reaching the satellite is primarily from the upper portion (Temperatures are cold - Intensity is low). Shown as white on image. If the region is relatively dry, radiation is coming from lower in the layer (Temperatures are warm - Intensity is high). Shown as black on image. Cannot detect low level water vapor.
Upper troposphere off California is dry - descending air. This was region of low clouds on “transparent” image. Very white areas are high clouds - cirrus and cb’s. Circulation northeast of Hawaii. Cold front entering northwest Oregon.
(4) Infrared Images Receives radiation (11-12 m) which is in the Atmospheric Window (8-12 m). Atmosphere is transparent. Satellite also detects emitted radiation for earth’s surface. Can determine surface temperatures - if no clouds are present. Can determine cloud top temperatures: Colder - higher. Warmer - lower. Often colorized by temperature.
High clouds - cold - are white in this image. Low clouds - warm - are gray.
(5) Visible Images Satellite detects radiation emitted from clouds and the earth’s surface. Can more easily distinguish high clouds from low clouds. Low clouds and snow covered surfaces may be hard to detect. Only available when Sun is shining.
(6) Wind Profilers and Radar General weather radars emit radiation at 10-cm which is a good wavelength for scattering of raindrops and ice crystals, hail. Other weather radars may use 3-cm or 5-cm wavelengths. The signal is emitted. Some reflects off the object of interest and returns to the radar antenna. The time it takes for the signal to go out and return determines how far away the object is. The direction the antenna is point when the signal is emitted and returns along with distance determines the location of the object.
For raindrops, ice crystals, etc., what is being detected is a group, not each individual particle.
Doppler Radar The difference between Doppler radar and non-Doppler radar is that with Doppler, the phase change that occurs due to the object moving can be determined. That difference is dependent on the speed with which the object (or at least a component of the speed) is moving toward or away from the radar antenna.
If the object is a volume of raindrops and it moves horizontally because the wind blows it horizontally, then the wind speed and direction can be determined by how the volume of raindrops move. Because the weather radar usually does not point horizontal but rather at some elevation angle above horizontal, the objects reflecting the signal are higher above ground the farther the objects are from the radar.
Since Doppler radar will give the wind speed along the radar beam and whether the wind is moving toward the radar or way, the horizontal wind can be determined. The vertical wind component is usually much less then the horizontal wind components if the radar is not directed upward at a high angle, the vertical wind component can usually be neglected.
Notice the black region roughly to the east and west of the center. And black indicates very weak winds, nearly calm. Does this mean the wind is not blowing in this region?
Wind Profilers Wind profilers are relatively low-power, highly sensitive, clear-air radars, operating with wavelengths from 33-cm to 6 meters. The radars detect minute fluctuations in atmospheric density caused by the turbulent mixing of volumes of air with slightly different temperature and moisture content. The resulting fluctuations of the index of refraction are used as a tracer of the mean wind in the clear air.
The antenna is really a group of antennas in a grid. The radar sends a signal vertically (all antennas emitting at the same time). Then it emits a signal with antennas emitting at slightly later times so the wave front of the signal moves upward at an angle 15 o away from vertical and toward the north. Then another signal is emitted 15 o away from vertical toward the east.
The signal emitted vertically will give the component of the wind moving directly up or down. The signal emitted 15 o from vertical toward the north will contain the components of the wind blowing both vertically and toward the north. The signal emitted 15 o from vertical toward the east will contain the components of the wind blowing both vertically and toward the east.
The components: u, v, w of the wind can then be determined from these equations: Knowing the u- and v-components, the horizontal wind can be determined.
Wind information from wind profilers are customarily plotted either with time increasing to the right, or with time increasing toward the left. With time running backward, weather features that pass a station must have started out farther east than others.
This VAD (velocity azimuth display) shows time increasing toward the right.
Profiler data from Bonneville, Utah. Time increases toward the left.
(8) Other Time-Height Sections Time-Height analyses may be made for any parameter that is measured or forecasted. The next two examples show analyses of temperature and potential temperature.
Time-Height of Temperature Warm afternoon temperatures confined to roughly the lowest 100 - 150 mb. (isotherms begin to flatten horizontally) Note wind shift about 18/06Z
Time-Height of Potential Temperature Note: A well- mixed atmosphere (lot of vertical motion) exhibits a dry- adiabatic lapse rate and constant potential temperature with height. A stratified atmosphere exhibits an increase of potential temperature with height.
(9) Vertical Sections Often used to depict the characteristics off some particular feature; such as, a jet stream, front, etc., by orienting the section along a line perpendicular to the feature. Data plotted on the section is taken from stations within a designated distance of the line. Then the parameters of interest are analyzed.