Assimilation of radiation by the earth's surface. Albedo. Anthropogenic increase in the Earth's albedo as an effective measure to combat global warming Albedo values ​​for different surfaces and territories

The total radiation reaching the earth's surface is not completely absorbed by it, but is partially reflected from the earth. Therefore, when calculating the arrival of solar energy for a place, it is necessary to take into account the reflectivity of the earth's surface. Reflection of radiation also occurs from the surface of clouds. The ratio of the entire flux of short-wave radiation Rk reflected by a given surface in all directions to the radiation flux Q incident on this surface is called albedo(A) given surface. This value

shows how much of the radiant energy incident on the surface is reflected from it. Albedo is often expressed as a percentage. Then

(1.3)

In table. No. 1.5 albedo values ​​are given various kinds earth's surface. From the data in Table. 1.5 shows that freshly fallen snow has the highest reflectivity. In some cases, a snow albedo of up to 87% was observed, and in the conditions of the Arctic and Antarctic, even up to 95%. Packed, melted and even more polluted snow reflects much less. Albedo of various soils and vegetation, as follows from Table. 4, differ relatively slightly. Numerous studies have shown that the albedo often changes during the day.

Wherein highest values albedo is recorded in the morning and evening. This is explained by the fact that the reflectivity of rough surfaces depends on the angle of incidence of sunlight. With a vertical fall, the sun's rays penetrate deeper into the vegetation cover and are absorbed there. At a low height of the sun, the rays penetrate less into the vegetation and are reflected to a greater extent from its surface. The albedo of water surfaces is, on average, less than the albedo of the land surface. This is explained by the fact that the sun's rays (the short-wave green-blue part of the solar spectrum) penetrate to a large extent into the upper layers of water that are transparent to them, where they are scattered and absorbed. In this regard, the degree of its turbidity affects the reflectivity of water.

Table No. 1.5

For polluted and turbid water, the albedo increases noticeably. For scattered radiation, the albedo of water is on average about 8-10%. For direct solar radiation, the albedo of the water surface depends on the height of the sun: with a decrease in the height of the sun, the albedo value increases. So, with a sheer incidence of rays, only about 2-5% is reflected. When the sun is low above the horizon, 30-70% is reflected. The reflectivity of the clouds is very high. The average cloud albedo is about 80%. Knowing the value of the surface albedo and the value of the total radiation, it is possible to determine the amount of radiation absorbed by a given surface. If A is the albedo, then the value a \u003d (1-A) is the absorption coefficient of a given surface, showing what part of the radiation incident on this surface is absorbed by it.

For example, if a total radiation flux Q = 1.2 cal / cm 2 min falls on the surface of green grass (A \u003d 26%), then the percentage of absorbed radiation will be

Q \u003d 1 - A \u003d 1 - 0.26 \u003d 0.74, or a \u003d 74%,

and the amount of absorbed radiation

B absorb \u003d Q (1 - A) \u003d 1.2 0.74 \u003d 0.89 cal / cm2 min.

The albedo of the surface of water is highly dependent on the angle of incidence of the sun's rays, since pure water reflects light according to Fresnel's law.

At the position of the Sun at the zenith, the albedo of the surface of a calm sea is 0.02. With an increase in the zenith angle of the Sun Z P albedo increases and reaches 0.35 at Z P\u003d 85. The excitement of the sea leads to a change Z P , and significantly reduces the range of albedo values, since it increases at large Z n due to an increase in the probability of rays hitting an inclined wave surface. Excitement affects the reflectivity not only due to the inclination of the wave surface relative to the sun's rays, but also due to the formation of air bubbles in the water. These bubbles scatter light to a large extent, increasing the diffuse radiation coming out of the sea. Therefore, during high sea waves, when foam and lambs appear, the albedo increases under the influence of both factors. Scattered radiation enters the water surface at different angles. cloudless sky. It also depends on the distribution of clouds in the sky. Therefore, the sea surface albedo for diffuse radiation is not constant. But the boundaries of its fluctuations are narrower 1 from 0.05 to 0.11. Consequently, the albedo of the water surface for total radiation varies depending on the height of the Sun, the ratio between direct and scattered radiation, sea surface waves. It should be borne in mind that the northern parts oceans are heavily covered with sea ice. In this case, the albedo of ice must also be taken into account. As you know, significant areas of the earth's surface, especially in middle and high latitudes, are covered with clouds that reflect solar radiation very much. Therefore, knowledge of the cloud albedo is of great interest. Special measurements of cloud albedo were carried out with the help of airplanes and balloons. They showed that the albedo of clouds depends on their shape and thickness. The albedo of altocumulus and stratocumulus clouds has the highest values. clouds Cu - Sc - about 50%.

The most complete data on cloud albedo obtained in Ukraine. The dependence of the albedo and the transmission function p on the thickness of the clouds, which is the result of the systematization of the measurement data, is given in Table. 1.6. As can be seen, an increase in cloud thickness leads to an increase in albedo and a decrease in the transmission function.

Average albedo for clouds St with an average thickness of 430 m is 73%, for clouds Swith at an average thickness of 350 m - 66%, and the transmission functions for these clouds are 21 and 26%, respectively.

The albedo of clouds depends on the albedo of the earth's surface. r 3 over which the cloud is located. From a physical point of view, it is clear that the more r 3 , the greater the flux of reflected radiation passing upward through the upper boundary of the cloud. Since albedo is the ratio of this flow to the incoming one, an increase in the albedo of the earth's surface leads to an increase in the albedo of clouds. The study of the properties of clouds to reflect solar radiation was carried out using artificial Earth satellites by measuring the brightness of clouds. The average cloud albedo values ​​obtained from these data are given in table 1.7.

Table 1.7 - Average albedo values ​​of clouds of different forms

According to these data, cloud albedo ranges from 29 to 86%. Noteworthy is the fact that cirrus clouds have a small albedo compared to other cloud forms (with the exception of cumulus). Only cirrostratus clouds, which are thicker, largely reflect solar radiation (r= 74%).

Total radiation

All solar radiation reaching the earth's surface is called total solar radiation.

Q = S sin h c + D (34)

where S is the irradiance of direct radiation, h c is the height of the Sun, D is the irradiance of scattered radiation.

With a cloudless sky, the total solar radiation has a daily variation with a maximum around noon and an annual variation with a maximum in summer. Partial cloudiness, which does not cover the solar disk, increases the total radiation compared to a cloudless sky, while full cloudiness, on the contrary, reduces it. On average, cloud cover reduces radiation. Therefore, in summer, the arrival of total radiation in the pre-noon hours is greater than in the afternoon, and in the first half of the year more than in the second. The midday values ​​of the total radiation in the summer months near Moscow with a cloudless sky average 0.78, with the open Sun and clouds 0.80, with continuous clouds - 0.26 kW / m 2.

The distribution of total radiation values ​​over the globe deviates from the zonal one, which is explained by the influence of atmospheric transparency and cloudiness. The maximum annual values ​​of total radiation are 84*10 2 - 92*10 2 MJ/m 2 and are observed in the deserts of North Africa. Over areas of equatorial forests with high cloudiness, the values ​​of total radiation are reduced to 42*10 2 - 50*10 2 MJ/m 2 . To higher latitudes of both hemispheres, the values ​​of total radiation decrease, amounting to 25*10 2 - 33*10 2 MJ/m 2 under the 60th parallel. But then they grow again - little over the Arctic and significantly - over Antarctica, where in the central parts of the mainland they are 50 * 10 2 - 54 * 10 2 MJ / m 2. Over the oceans, in general, the values ​​of total radiation are lower than over the corresponding land latitudes.

In December, the highest values ​​of total radiation are observed in the deserts of the Southern Hemisphere (8*10 2 - 9*10 2 MJ/m 2). Above the equator, the total radiation values ​​decrease to 3*10 2 - 5*10 2 MJ/m 2 . In the Northern Hemisphere, radiation rapidly decreases towards the polar regions and is zero beyond the Arctic Circle. In the Southern Hemisphere, the total radiation decreases south to 50-60 0 S. (4 * 10 2 MJ / m 2), and then increases to 13 * 10 2 MJ / m 2 in the center of Antarctica.

In July, the highest values ​​of total radiation (over 9 * 10 2 MJ / m 2) are observed over northeast Africa and the Arabian Peninsula. Over the equatorial region, the values ​​of the total radiation are low and equal to those in December. To the north of the tropic, the total radiation decreases slowly to 60 0 N, and then increases to 8*10 2 MJ/m 2 in the Arctic. In the southern hemisphere, the total radiation from the equator rapidly decreases to the south, reaching zero values ​​near the polar circle.

Upon reaching the surface, the total radiation is partially absorbed in the upper thin layer of soil or water and converted into heat, and partially reflected. The conditions for the reflection of solar radiation from the earth's surface are characterized by the value albedo, equal to the ratio of the reflected radiation to the incoming flux (to the total radiation).

A \u003d Q neg / Q (35)

Theoretically, albedo values ​​can vary from 0 (perfectly black surface) to 1 (perfectly white surface). The available observational data show that the albedo values ​​of the underlying surfaces vary over a wide range, and their changes cover almost the entire possible range of reflectivity values ​​of various surfaces. In experimental studies, albedo values ​​were found for almost all common natural underlying surfaces. These studies show, first of all, that the conditions for the absorption of solar radiation on land and in water bodies are markedly different. The highest albedo values ​​are observed for clean and dry snow (90-95%). But since the snow cover is rarely completely clean, the average snow albedo in most cases is 70-80%. For wet and polluted snow, these values ​​are even lower - 40-50%. In the absence of snow, the highest albedo on the land surface is characteristic of some desert regions, where the surface is covered with a layer of crystalline salts (the bottom of dried lakes). Under these conditions, the albedo has a value of 50%. Slightly less than the albedo value in sandy deserts. The albedo of wet soil is less than the albedo of dry soil. For wet chernozems, the albedo values ​​are extremely small - 5%. The albedo of natural surfaces with a continuous vegetation cover varies within relatively small limits - from 10 to 20-25%. At the same time, the albedo of the forest (especially coniferous) in most cases is less than the albedo of meadow vegetation.

The conditions for absorption of radiation in water bodies differ from the conditions for absorption on the land surface. Pure water it is relatively transparent to short-wave radiation, as a result of which the sun's rays penetrating into the upper layers are scattered many times and only after that they are largely absorbed. Therefore, the process of absorption of solar radiation depends on the height of the Sun. If it stands high, a significant part of the incoming radiation penetrates into the upper layers of the water and is mainly absorbed. Therefore, the albedo of the water surface is a few percent when the Sun is high, and when the Sun is low, the albedo increases to several tens of percent.

The albedo of the "Earth-atmosphere" system has a more complex nature. Solar radiation entering the atmosphere is partly reflected as a result of backscattering of the atmosphere. In the presence of clouds, a significant part of the radiation is reflected from their surface. The albedo of clouds depends on the thickness of their layer and averages 40-50%. In the complete or partial absence of clouds, the albedo of the "Earth-atmosphere" system depends significantly on the albedo of the earth's surface itself. The nature of the geographical distribution of the planetary albedo according to satellite observations shows significant differences between the albedo of high and middle latitudes of the Northern and Southern hemispheres. In the tropics, the highest albedo values ​​are observed over deserts, in the zones of convective cloudiness over Central America and over the waters of the oceans. In the Southern Hemisphere, in contrast to the Northern Hemisphere, a zonal albedo variation is observed due to a simpler distribution of land and sea. The highest albedo values ​​are found in polar latitudes.

The predominant part of the radiation reflected by the earth's surface and the upper boundary of the clouds goes into the world space. A third of the scattered radiation also goes away. The ratio of the reflected and scattered radiation leaving into space to the total amount of solar radiation entering the atmosphere is called Earth's planetary albedo or Earth's albedo. Its value is estimated at 30%. The main part of the planetary albedo is radiation reflected by clouds.

Page 17 of 81

Total radiation, reflected solar radiation, absorbed radiation, PAR, Earth's albedo

All solar radiation coming to the earth's surface - direct and scattered - is called total radiation. Thus, the total radiation

Q = S? sin h + D,

where S– energy illumination by direct radiation,

D– energy illumination by scattered radiation,

h- the height of the sun.

With a cloudless sky, the total radiation has a daily variation with a maximum around noon and an annual variation with a maximum in summer. Partial cloudiness that does not cover the solar disk increases the total radiation compared to a cloudless sky; full cloudiness, on the contrary, reduces it. On average, cloudiness reduces the total radiation. Therefore, in summer, the arrival of total radiation in the pre-noon hours is on average greater than in the afternoon.
For the same reason, it is larger in the first half of the year than in the second.

S.P. Khromov and A.M. Petrosyants give midday values ​​of total radiation in the summer months near Moscow with a cloudless sky: an average of 0.78 kW / m 2, with the Sun and clouds - 0.80, with continuous clouds - 0.26 kW / m 2.

Falling on the earth's surface, the total radiation is mostly absorbed in the upper thin layer of soil or in a thicker layer of water and turns into heat, and is partially reflected. The amount of reflection of solar radiation by the earth's surface depends on the nature of this surface. The ratio of the amount of reflected radiation to the total amount of radiation incident on a given surface is called the surface albedo. This ratio is expressed as a percentage.

So, from the total flux of total radiation ( S sin h + D) part of it is reflected from the earth's surface ( S sin h + D)And where BUT is the surface albedo. The rest of the total radiation
(S sin h + D) (1 – BUT) is absorbed by the earth's surface and goes to heat the upper layers of soil and water. This part is called absorbed radiation.

The albedo of the soil surface varies within 10–30%; in wet chernozem, it decreases to 5%, and in dry light sand it can rise to 40%. As soil moisture increases, the albedo decreases. The albedo of vegetation cover - forests, meadows, fields - is 10–25%. The albedo of the surface of freshly fallen snow is 80–90%, while that of long-standing snow is about 50% and lower. The albedo of a smooth water surface for direct radiation varies from a few percent (if the Sun is high) to 70% (if low); it also depends on excitement. For scattered radiation, the albedo of water surfaces is 5–10%. On average, the albedo of the surface of the World Ocean is 5–20%. The albedo of the upper surface of the clouds varies from a few percent to 70–80%, depending on the type and thickness of the cloud cover, on average 50–60% (S.P. Khromov, M.A. Petrosyants, 2004).

The above figures refer to the reflection of solar radiation, not only visible, but also in its entire spectrum. Photometric means measure the albedo only for visible radiation, which, of course, may differ somewhat from the albedo for the entire radiation flux.

The predominant part of the radiation reflected by the earth's surface and the upper surface of the clouds goes beyond the atmosphere into the world space. A part (about one-third) of the scattered radiation also goes into the world space.

The ratio of reflected and scattered solar radiation leaving space to the total amount of solar radiation entering the atmosphere is called the planetary albedo of the Earth, or simply Earth's albedo.

In general, the planetary albedo of the Earth is estimated at 31%. The main part of the planetary albedo of the Earth is the reflection of solar radiation by clouds.

Part of the direct and reflected radiation is involved in the process of plant photosynthesis, so it is called photosynthetically active radiation(FAR). FAR - the part of short-wave radiation (from 380 to 710 nm), which is the most active in relation to photosynthesis and the production process of plants, is represented by both direct and diffuse radiation.

Plants are able to consume direct solar radiation and reflected from celestial and terrestrial objects in the wavelength range from 380 to 710 nm. The flux of photosynthetically active radiation is approximately half of the solar flux, i.e. half of the total radiation, and practically regardless of weather conditions and location. Although, if for the conditions of Europe the value of 0.5 is typical, then for the conditions of Israel it is somewhat higher (about 0.52). However, it cannot be said that plants use PAR in the same way throughout their lives and under different conditions. The efficiency of PAR use is different, therefore, the indicators "PAR use coefficient" were proposed, which reflects the efficiency of PAR use and the "Efficiency of phytocenoses". The efficiency of phytocenoses characterizes the photosynthetic activity of the vegetation cover. This parameter has found the widest application among foresters for assessing forest phytocenoses.

It should be emphasized that plants themselves are able to form PAR in the vegetation cover. This is achieved due to the location of the leaves towards the sun's rays, the rotation of the leaves, the distribution of leaves of different sizes and angles at different levels of phytocenoses, i.e. through the so-called canopy architecture. In the vegetation cover, the sun's rays are repeatedly refracted, reflected from the leaf surface, thereby forming their own internal radiation regime.

The radiation scattered within the vegetation cover has the same photosynthetic value as the direct and diffuse radiation entering the surface of the vegetation cover.

The long-term albedo trend is directed towards cooling. In recent years, satellite measurements show a slight trend.

Changing the Earth's albedo is potentially a powerful impact on climate. As albedo, or reflectivity, increases, more sunlight is reflected back into space. This has a cooling effect on global temperatures. On the contrary, a decrease in albedo heats up the planet. A change in albedo of only 1% gives a radiative effect of 3.4 W/m2, comparable to the effect of CO2 doubling. How has albedo affected global temperatures in recent decades?

Albedo trends up to 2000

The Earth's albedo is determined by several factors. Snow and ice reflect light well, so when they melt, the albedo goes down. Forests have a lower albedo than open spaces, so deforestation increases albedo (let's say that deforestation will not stop global warming). Aerosols have a direct and indirect effect on albedo. The direct influence is the reflection of sunlight into space. An indirect effect is the action of aerosol particles as centers of moisture condensation, which affects the formation and lifetime of clouds. Clouds, in turn, affect global temperatures in several ways. They cool the climate by reflecting sunlight, but can also have a heating effect by retaining outgoing infrared radiation.

All these factors should be taken into account when summing up the various radiative forcings that determine the climate. Land-use change is calculated from historical reconstructions of changes in cropland and pasture composition. Observations from satellites and from the ground make it possible to determine trends in the level of aerosols and cloud albedo. It can be seen that cloud albedo is the strongest factor of the various types of albedo. The long-term trend is towards cooling, the impact is -0.7 W/m2 from 1850 to 2000.

Fig.1 Average annual total radiative forcing(Chapter 2 of the IPCC AR4).

Albedo trends since 2000.

One way to measure the Earth's albedo is by the Moon's ashen light. This is sunlight, first reflected by the Earth and then reflected back to Earth by the Moon at night. The Moon's ash light has been measured by the Big Bear Solar Observatory since November 1998 (a number of measurements were also made in 1994 and 1995). Fig. 2 shows albedo changes from satellite data reconstruction (black line) and from lunar ash light measurements (blue line) (Palle 2004).

Fig.2 Changes in albedo reconstructed from ISCCP satellite data (black line) and changes in the moon's ash light (black line). The right vertical scale shows the negative radiative forcing (ie cooling) (Palle 2004).

The data in Figure 2 is problematic. Black line, ISCCP satellite data reconstruction" is a purely statistical parameter and has little physical meaning because it does not take into account the non-linear relationships between cloud and surface properties and planetary albedo, nor does it include aerosol albedo changes, such as those associated with Mount Pinatubo or anthropogenic sulfate emissions(Real Climate).

Even more problematic is the albedo peak around 2003, visible in the moon's blue ashen light line. It strongly contradicts the satellite data showing a slight trend at this time. For comparison, we can recall the Pinatubo eruption in 1991, which filled the atmosphere with aerosols. These aerosols reflected sunlight, creating a negative radiative forcing of 2.5 W/m2. This has drastically lowered the global temperature. The ash light data then showed an exposure of almost -6 W/m2, which should have meant an even greater drop in temperature. No similar events occurred in 2003. (Wielicki 2007).

In 2008, the reason for the discrepancy was discovered. The Big Bear Observatory installed a new telescope to measure lunar ashlight in 2004. With the new improved data, they recalibrated their old data and revised their albedo estimates (Palle 2008). Rice. 3 shows the old (black line) and updated (blue line) albedo values. The anomalous peak of 2003 has disappeared. However, the trend of increasing albedo from 1999 to 2003 has been preserved.

Rice. 3 Change in the Earth's albedo according to measurements of the moon's ashy light. The black line is the albedo changes from a 2004 publication (Palle 2004). Blue line - updated albedo changes after improved data analysis procedure, also includes data over a longer period of time (Palle 2008).

How accurately is the albedo determined from the moon's ashen light? The method is not global in scope. It affects about a third of the Earth in each observation, some areas always remain "invisible" from the observation site. In addition, measurements are infrequent and are made in a narrow wavelength range of 0.4-0.7 µm (Bender 2006).

In contrast, satellite data such as CERES is a global measurement of Earth's shortwave radiation, including all effects of surface and atmospheric properties. Compared to ash light measurements, they cover a wider range (0.3-5.0 µm). An analysis of the CERES data shows no long-term albedo trend from March 2000 to June 2005. Comparison with three independent datasets (MODIS, MISR and SeaWiFS) shows a "remarkable fit" for all 4 results (Loeb 2007a).

Rice. 4 Monthly changes in mean CERES SW TOA flux and MODIS cloud fraction ().

Albedo has been affecting global temperatures - mostly in the direction of cooling in a long-term trend. In terms of recent trends, the ashlight data shows an increase in albedo from 1999 to 2003 with little change after 2003. Satellites show little change since 2000. The radiative forcing from albedo changes has been minimal in recent years.

The predominant part of the direct radiation reflected by the earth's surface and the upper surface of the clouds goes beyond the atmosphere into the world space. About one third of the scattered radiation also goes into the world space. The ratio of all reflected and scattered solar radiation to the total amount of solar radiation entering the atmosphere is called Earth's planetary albedo. The planetary albedo of the Earth is estimated at 35 - 40%. The main part of it is the reflection of solar radiation by clouds.

Table 2.6

Magnitude dependency To n from the latitude of the place and time of year

Table 2.7

Magnitude dependency To in + from the latitude of the place and time of year

(according to A.P. Braslavsky and Z.A. Vikulina)