1. Basics
Radiation occupies an important place among the quantities of interest in meteorology, since, on the one hand, solar radiation is the primary energy source for atmospheric processes, and, on the other hand, the various radiation fluxes account for a significant portion of energy transport in the atmosphere. These radiation fluxes, known in physical terms as radiation flux densities, i.e., the radiant energy passing through a horizontal unit area per unit time, are measured in W/m 2 In addition, UV radiation has gained interest in recent years.
2. Radiation flux densities used in meteorology
Radiation is wavelength-dependent (Planck's law), which is used, for example, in remote sensing through satellite radiation measurements. However, since radiation is always treated in meteorological terms in terms of its energetic aspects, the radiation integrated over the wavelength is of interest (Stefan-Boltzmann law). In terms of spectral distribution, the meteorologically significant radiation flux densities can be divided into two ranges: solar and terrestrial radiation.
2.1 Solar radiation
The shortwave radiation in the range of approximately 0.3…3 µm comes primarily from the sun and is therefore called solar radiation. At the upper edge of the atmosphere, Average 1367 Wm 2 (solar constant ) , part of which is absorbed in the atmosphere or scattered upwards or downwards. The solar radiation escaping downwards from the atmosphere therefore consists of direct and diffuse radiation, which together are referred to as global radiation.
1. The Direct sunlight S = l sin(h)
Here, l is the solar radiation falling on a surface perpendicular to the radiation direction and h is the sun's altitude (angle!). The position of the sun is often described by its zenith angle ?. Then S = l cos(?) and the resulting Change in solar radiation on the horizontal surface is called the "cosine law". When clouds are thick or there is a cloud in front of the sun, the sun is not visible, i.e. S = 0. In cases where no cloud obscures the sun, S is greater than D, except when the sun is low in the sky, where the direct sunlight is weakened by its long path through the atmosphere (shifting the sun's color to red).
2. The Diffuse solar radiation D (sky radiation)
This is the solar radiation scattered by the molecules, aerosol and cloud particles in the atmosphere.
3. Global radiation G
is the common name for all solar radiation emerging from the atmosphere below: G = S + D , regardless of the meteorological situation. The global radiation hitting the ground is partially reflected, the rest is absorbed.
4. The Reflected global radiation R
is the radiation reflected from the Earth's surface, also a radiant flux density in W/m2. The reflectivity of natural surfaces is, to a first approximation, isotropic and independent of the position of the sun. Thus, the reflectivity of a surface is assumed to be constant—except for water because of its reflective component. This means that R proportional to G ; it applies R= a G . Here, a is the reflectance of the surface, which for the spectral averaging over the solar spectral range considered here is called albedo a receives. If known a can R out of G It is therefore not routinely necessary to measure R. Albedo values for natural surfaces range from 6% to 15% for the ocean, depending on the position of the sun and cloud cover, around 10% for forests, around 20% for grass and asphalt, and around 50% for snow-covered surfaces. Higher values for snow only apply to fresh snow in large areas where no vegetation or similar structures break through the snow cover.
5. Solar radiation balance Qs
The energy absorbed by the Earth's surface is the amount of radiation absorbed by that surface, the balance between what arrives and what is re-radiated by reflection. This is the energy per area and time, again in W/m 2 , that is available for conversion into other forms of energy. Since the radiation falls on the Earth's surface, i.e., on an impermeable medium, so that it can only be reflected or absorbed, the following applies: a + e s =1 (the solar emission coefficient is equal to the solar absorption coefficient) and thus
Qs = G - R = e s . - G = (1 - a) • G = (1 - a) • (S + D)
2.2 Terrestrial radiation
Long-wave radiation in the range of approximately 4…60 µm originates primarily from terrestrial matter, i.e., the ground and atmospheric components, which radiate according to their temperature according to the Stefan-Boltzmann law. It is therefore referred to as terrestrial radiation. A distinction is made between:
1. Atmospheric counter-radiation A ,
The downward-directed terrestrial radiation of the atmosphere, i.e., the thermal radiation of atmospheric gases (mainly water vapor and carbon dioxide) and clouds. This radiation comes from the entire hemisphere and thus has no preferred direction, unlike direct solar radiation.
2 . The radiation from the Earth's surface.
E = e t • s • T ? 4 . Where et is the mean emissivity in the terrestrial range, which according to Kirchhoff's law is equal to the terrestrial absorptivity. a = 5.66956 10 -8 W• m 2 &#= 8226;K -4 is the constant of the Stefan-Boltzmann radiation law and T B the (absolute) temperature of the Earth's surface.
3. The reflected counter radiation.
If the emission level is not e t =1 is, ie if the ground cannot be understood as a "black body" in its emission behavior, a part of the counter radiation is reflected, resulting in the long-wave reflected radiation, r =(1 - e t ) A . This, together with the actual radiation of the ground, forms the thermal radiation coming from the earth's surface, which is thus additively composed of the two terms E and r which can only be separated using complex measuring methods. The Earth's surface is often seen as black in the long-wave range, since all natural surfaces, except for a few types of rock, e t ~ 1 Since r is simultaneously neglected, the small error is further reduced and even compensated within the limits of the measurement accuracy.
4. The terrestrial radiation balance
is the difference between the radiation arriving at the ground and the radiation leaving the ground: Q t = A - E - r With the assumed simplification e >t<e t = 1 surrendered:
Q t = A - E = A - sT B 4
2.3 Radiation balance
The radiation balance Q as the sum of Qs and Q t indicates the total radiation energy available at the Earth's surface for conversion into other forms of energy:
Q = Qs + Q t = S + D - R + A - E - r = (1 - a) -G + e t • (A - sT B 4 ) = (1 - a) -G + A - sT B 4 )
Q contains over a, e t and T B Properties of the soil surface: One variable free from these influences is the Effective radiation Q eff = S + D + A - s T L 4 , that is, the radiation balance of a horizontal black surface (a = 0, e = 1) with air temperature T L . Another size is the (nighttime) effective radiation E eff = sT L 4 - A used, which in the absence of solar radiation ( S + D = 0 ) as a negative value of Q eff This differs from the long-wave radiation balance of the ground, which is considered to be completely black Q l = A - sT B 4 except by the sign also by σT B 4 -σT L 4 = a s (? B - ? L , where a s the radiation transfer coefficient (5.6 Wm -2 K ~1 , ? B the soil and ? L the air temperature.
The presented radiation flux densities naturally vary with the time of day and year, but also with weather conditions (cloud cover). Typical annual mean values for the Earth are (all values in W/ m² ):
|
Global radiation |
S + D |
104 |
154 |
|
Reflected radiation |
R |
20 |
17 |
|
abbreviated radiation balance |
Qs |
84 |
137 |
|
Counter radiation |
A |
320 |
335 |
|
Charisma |
E |
357 |
390 |
|
long-term radiation balance |
Q t |
-37 |
-55 |
|
Radiation balance |
Q |
47 |
82 |
These annual averages show how small the radiation flux density actually available for conversion into other forms of energy on the ground is. Q especially when compared with the extraterrestrial solar radiation of 342 W m -2 incident on the Earth's surface area on average compares.
3.0 Spectral weighting
Visible and UV radiation are important for humans. Visible radiation is determined by the sensitivity with which the eye reacts to radiation. radiometric unit W m -2 radiant flux density, which describes the energetic aspect of radiation and is used in meteorology) corresponding photometric unit is the lux (ab = abbreviated lx - illuminance, which describes the impression of light or brightness). The wavelength-dependent brightness sensitivity of the human eye s has its highest value, normalized to 1, at 0.555 µm. Here, the following applies: 1 Ix =3D 1.47 10= -3 W m- 2 . At other wavelengths, the spectral luminosity s and the conversion factor f are smaller. Values for monochromatic radiation are given in the following table:
|
? |
0.4 |
0.45 |
0.5 |
0.55 |
0.60 |
0.65 |
0.70 |
0.75 |
µm |
|
s |
0.04 |
3.8 |
32.3 |
99.5 |
63.1 |
10.7 |
0.41 |
0.01 |
% |
|
f |
0.3 |
26.0 |
221 |
682 |
432 |
75.0 |
2.8 |
0.10 |
lx(W m 2) |
For non-monochromatic radiation, f can only be specified if its spectral composition is known. For global radiation with the sun high in the sky ("daylight"), f is approximately 100 lx/(Wm -2 ), the "daylight factor."
It should be emphasized once again that the visible spectral range, d= e r approximately 0.4…0.8 µm, is not identical to the solar spectral range. At longer wavelengths, the visible range is followed by the infrared range (> 0.8 µm), in which almost 50% of solar radiation lies.
The ultraviolet or UV radiation range (< 0.4 µm), i.e. the range with wavelengths shorter than visible light, contains only around 5% of solar radiation, but is important because of the photobiological and photochemical processes that this radiation can trigger. Similar to visible light, this radiation is usually weighted according to its spectral effect. This is particularly necessary in the UV range because, due to the strong absorption of ozone, the spectral irradiance within 30 nm, from 320 nm to 290 nm, decreases by around 5 orders of magnitude. On the other hand, the biological effect increases inversely, so that the effect of UV radiation can only be correctly described by taking such weightings into account. A typical weighting is that for sunburn, erythema. This is taken as representative of many biological UV effects and is also included in the UV index introduced by the WMO and WHO. This quantity, UVI, is defined as the erythema-weighted UV radiation in W/m 2 multiplied by 40 1/W/m 2 . Thus, the UVI is a dimensionless quantity that describes UV radiation flux densities and in Germany values between 0 and 8, on the Zugspitze = with sun and snow up to