Kippzonen BSRN Scientific Solar Monitoring System Manual do Utilizador descarregar pdf (Página 29)

(6) near vehicle parking areas; and

(7) where heat is exhausted by vehicles or buildings.

Conversely, BSRN stations must be located where facilities exist, preferably on a full-time 24-hour

basis. Ideally, the site should be co-located with a synoptic station and within 50 km of an upper

air station.

3.1.2 Horizon

The ideal site for the m easurement of solar and terrestrial radiation for meteorological purposes is

one that has a completely flat horizon. The WMO Guide to Meteorological Instruments and

Methods of Observation (WMO No. 8) recommends that if possible no obstruction should be

present, particularly within the azimuth range of sunrise and sunset over the year (see Annex I for

a solar position algorithm). In cases where obstructions do occur, the instrument should be

located where these subtend an elevation angle of less than 5° to minimize their effects. The total

diffuse radiation received by a surface from elevation angles of less than 5° accounts for only

about 1% of the total global radiation. The determination of the change in radiation fluxes with

respect to changing climate, and the use of surface measurements to test and ground-truth

satellite retrieval algorithms do not require strict adherence to this guideline when distant

topography is considered. In the latter case, measurements in areas of complex topography are

required to determine the capabilities of the retrievals.

While the distant horizon may be influenced by topography, the local horizon should be as clear

as practically possible. A distance of 12 times the height of any object to the location of the sensor

will ensure that the elevation of the object is less than 5° above the horizon. The site should be

located such that all objects are to the poleward side of the installation and do not interfere with

the direct beam radiation at any time during the year. The instruments should be removed, as far

as is practical, from any highly reflective objects. Where a site is to be developed in a built-up

area, the sensors can be located on the roof of a building to overcome problems with the local

horizon.

While antennas and other slender objects should be avoided, their effect is minimal and can be

endured if they are less than 1° wide, and do not block the direct beam radiation during any tim e

of the year.

3.1.3 Latitude, Longitude, Elevation

A detailed description of the m easurement site and its surroundings is probably one of the most

significant pieces of metadata provided to other researchers. It is of utmost importance to describe

the site and its surroundings, not only in terms of latitude, longitude and elevation, but also with

respect to the topography and land use surrounding the measurement location. One must

consider this description in terms of the pixel size of present-day satellite measurements and the

potential for influences on the radiation regime due to multiple scattering.

The first and foremost information required are the geographic coordinates of the site: latitude,

longitude and elevation above mean sea level (msl). These normally can be obtained from high-

quality topographic maps obtained through the mapping agencies of national governments. The

BSRN archive records this inform ation in a floating point form at with three decim al places. This is

equivalent to an accuracy of approximately 3.5 seconds of arc, or about 108 metres in latitude

and 76 metres in longitude at 45°. To obtain such accuracies, a map with a scale of better than

1:100000 is required. The latitude and longitude should be recorded in decimal degrees, North

and East positive with both the South Pole and 180° W defined as zero. For example, a station

located in the Northern Hemisphere and east of Greenwich, such as Potsdam, Germany (52 N, 13

E) would be encoded 142.000, 193.000, while for a similar latitude, but in the Canadian prairies

(52 N, 105 W) the location would be encoded 142.000, 75.000. This is for consistency with the

Archive station-to-archive file format.

Elevation can also be read from topographic maps, normally to within 5 metres. More accurate

measurements require site surveys. The Archive records the elevation to within 1 metre.

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