Measured Radiation Quantities and Terminology
Following is a brief description of the quantities measured at the earth's surface within the ESRL GMD Radiation (G-Rad) network.
- "Irradiance" is the amount of electromagnetic energy incident on a surface per unit time per unit area. In the past this quantity has often been referred to as "flux".
- "Solar" refers to electromagnetic radiation in the spectral range of approximately 0.30 µm to 3.0 µm. Solar radiation is also often referred to as shortwave with the shortest wavelengths of solar known as UV, the middle wavelengths are in the visible part of the spectrum, and the wavelengths longer than visible are know as the near or solar infrared (IR).
- "Thermal" refers to the 4.0 µm to 100. µm range in the spectrum. The thermal radiation is often called longwave or sometimes infrared, although one must distinguish the thermal IR from the solar IR.
"Total" frequently appears as a modifier to a measured radiation
quantity. It can mean either an integral over a wide spectral range, or can
refer to sum of different direction components such as diffuse & direct
solar as defined in the following.
"Spectral" is used to indicate higher spectral resolution than the
terms "solar" and "thermal". "Broadband"
generally refers to either the complete solar or complete thermal spectral
generally refers to a subdivision of the broadband that still covers at least
several 10s of percent of the broadband range. "Narrowband"
typically refers to a spectral band of several to a few 10s of nanometers.
Various terms such as "high or moderate spectral resolution" refers
to spectral resolution less than a couple nanometers.
- Direct solar irradiance - This is the solar radiation that passes directly though the atmosphere from the sun without being scattered or absorbed by the atmosphere. Typically it is measured on a surface that is kept normal to the direction of the center of the sun's disk. That surface is held normal to the sun's position by a solar tracker. Pyrheliometers are most commonly used to measure total direct solar irradiance. The most accurate measurements of this quantity are obtained from absolute cavity radiometers which serve as calibration standards and are being increasing used operationally.
- Diffuse sky solar irradiance - Diffuse solar irradiance is solar irradiance that reaches the ground that has been scattered by an atmospheric constituent such as air molecules, dust, or clouds. Diffuse irradiance is measured with a pyranometer that is continually shaded from the direct solar irradiance. The continuous shading is accomplished with a shading solar tracker.
- Downward total solar irradiance - This quantity is given many names by the scientific community. It is the total amount of solar irradiance on an upward-facing horizontal surface and is the sum of the vertical component of the direct solar irradiance and the diffuse sky irradiance. This is the fundamental quantity from which the world's weather/climate system obtains it energy. Downward total solar irradiance is measured either with a pyranometer, or typically more accurately by summing the direct and diffuse horizontal irradiance on a horizontal surface.
- Upward solar irradiance - The quantity is the solar irradiance incident a downward-facing surface. The source of the quantity is the downward solar irradiance that is reflected off the earth's surface. The upward solar irradiance is typically diffuse and is measured with an inverted pyranometer held a representative distance above the ground.
- Downward longwave irradiance - This quantity is thermal irradiance emitted in all directions by the atmosphere; gases, aerosols, and clouds as received by an horizontal upward facing surface. Downward longwave irradiance is measured with a pyrgeometer.
- Upward longwave irradiance - This is the thermal irradiance emitted for the earth's surface passingthrough a horizontal surface at a representative distance above the ground. Upward longwave is measured with an inverted pyrgeometer.
- Direct solar irradiance - The direct solar irradiances in several widebands; 0.3 µm to 0.530 µm, 0.3 µm to 0.63 µm, and 0.3 µm to 0.695 µm, are measured at certain GMD observatories. A filter wheel pyrheliometer using Schott glass filters is used for discrete manual observations under clear skies.
- Downward solar irradiance - The total downward solar irradiance in the 0.3 µm to 0.695 µm band is measured at several G-Rad sites with a RG8 Schott glass filter mounted instead of a clear dome on a pyranometer.
Direct, diffuse, and total relative irradiance - Observations of the
direct, diffuse, and total spectral solar irradiance on a horizontal surface
are measure at some of the sites. These data are used to determine spectral
aerosol optical depth and can be used to learn more about the scattering and
absorbing features of the atmosphere. The measurements are made in relative
units because of limitations in maintaining an absolute calibration on the
radiometer. The radiometer used is known as a Multi-Filter Rotating Shadowband
Direct solar irradiance - The direct solar irradiance in certain narrowband
channels is measured by sunphotometers.
These observations can be used to derive the total transmittance of the atmosphere
within the respective spectral band which can in turn be used to deduce aerosol
- The data signals produced by most radiometers used by G-Rad produce a low level DC voltage signal. The signal is typically measured with a 13 (or greater) bit resolution A/D converter utilizing several millisecond integration times. Individual data channels are sampled at 1-hz and 1-minute average data are recorded and stored at the field site. Data are transferred several times daily to G-Rad in Boulder via regular phone lines, except from the South Pole where the data are transferred weekly via FTP.
- Meteorological measurements are important for the understanding and interpretation of various radiation measurements. Basic meteorological surface and upper air soundings are made at or near each of the G-Rad sites.