Apogee SP-610-SS Посібник користувача - Сторінка 4
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INTRODUCTION
Net radiation at Earth's surface is the source of available energy that drives key processes, including surface and
atmospheric heating, evaporation, sublimation, and transpiration. Shortwave radiation (approximately 280 to 4000
nm) is emitted by the sun, and a fraction incident at Earth's surface is reflected. Longwave radiation
(approximately 4000 to 100 000 nm) is emitted by molecules in the atmosphere and land surfaces. Net radiation is
the difference between incoming (downwelling) and outgoing (upwelling) shortwave and longwave radiation. Net
radiation at Earth's surface is spatially and temporally variable due to changes in position of the sun with respect
to Earth's surface, changes in atmospheric conditions, and differences in land surface conditions. Shortwave
radiation accounts for a larger proportion of net radiation during the day when the sun is shining. Longwave
radiation contributes to net radiation during the day and at night.
Typical values of the four components of net radiation (R
) for a clear summer day near solar noon over
net
-2
vegetation and a clear winter day near solar noon over snow are shown in the figure below (all units are W m
).
Net shortwave radiation is the difference between incoming shortwave (from sun, SW
) and outgoing shortwave
i
(reflected by surface, SW
). Net longwave radiation is the difference between incoming longwave (emitted by
o
molecules in the atmosphere, LW
) and outgoing longwave (emitted by elements at the surface, LW
). Net
i
o
radiation is the sum of net shortwave and net longwave radiation. Net radiation changes with solar zenith angle,
atmospheric conditions (e.g., degree of cloudiness), and surface conditions (e.g., bare soil, plant cover, snow).
Net radiometers are instruments designed to measure net radiation. Typical applications of net radiometers
include measurement of net radiation on surface flux towers and weather stations. Net radiation is a key variable
in the surface energy balance and influences turbulent fluxes, including evapotranspiration.
Apogee Instruments model SN-522 net radiometers are four-component instruments, with individual upward- and
downward-looking pyranometers and pyrgeometers. Each radiometer consists of a thermopile detector and filter
mounted in an anodized aluminum housing. Each radiometer is heated to minimize the effects of dew, frost, snow,
and ice on the filter and sensor head. Analog signals from each radiometer are measured with an onboard
voltmeter and converted to a digital value. That value can then be accessed via a Modbus RTU interface over an
RS-232 or RS-485 serial connection. This eliminates the need for multiple analog datalogger channels to make the
four-component measurement of net radiation. SN-522 net radiometers are small and lightweight to facilitate
rapid and simple mounting.