The phenomenon that electromagnetic radiation energy is redistributed regularly in all directions
synonymAtmospheric scattering(The phenomenon that the incident energy is redistributed in all directions with a certain rule) Generally refers to scattered radiation
Scattering radiation is a phenomenon that electromagnetic radiation energy is redistributed in all directions with a certain rule due to the action of air molecules and aerosol particles, or due to the fluctuation and inhomogeneity of air density.
Chinese name
Scattered radiation
Foreign name
diffuse radiation
Cause
The role of air molecules and aerosol particles
Phenomenon
The electromagnetic radiation energy is redistributed regularly
Scattered radiation refers to sunlight passing through air molecules, cloud droplets andaerosolIt is formed by the scattering effect of (sky scattering) and diffuse reflection of the ground (ground scattering).
Scattered radiation is an important part of the solar radiation reaching the ground. In addition to being affected by the solar altitude angle, it is extremely sensitive to the changes of cloud, aerosol, water vapor content and surface conditions in the atmosphere.Klein (1977) pointed out that on a sunny day in winter in the Northern Hemisphere, the solar radiation reaching the sunny slope is three times that of the shady slope with the same slope, while on a cloudy day, the solar radiation reaching the shady slope and the sunny slope is almost equal. Scattered radiation also increases with the increase of cloud cover and aerosol particles.
In addition, the changes of scattered radiation are related to thephotosynthesisIt also has a close relationship, thus affecting the carbon source status of the ecosystem.Especially sinceindustrial revolutionSince then, the increasingly serious atmospheric pollution, the change of atmospheric composition, the increase of aerosol particles and other environmental problems have had a great impact on the scattered radiation, which has attracted the attention of scholars at home and abroad. There are many reports on the research of scattered radiation abroad, while the research of scattered radiation in China is mostly short-term or discontinuous observation.[1]
background knowledge
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Solar radiation is the main source of energy on the earth's surface, the main power to promote the activities and changes of water, atmosphere and biology on the earth, and the basis for the formation of weather and climate. When passing through the atmosphere, solar radiation is absorbed, reflected and scattered by air molecules, clouds and aerosol particles in the atmosphere,The solar radiation reaching the ground is obviously weakened. The solar radiation reaching the ground is composed of direct radiation and scattered radiation.The solar radiation reaching the ground in the form of parallel light is called direct radiation, and the solar radiation reaching the ground after atmospheric scattering is called scattered radiation.
features
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basic feature
(1) Diurnal variation of scattered radiation: in a day, the total radiation increases with the rise of the solar altitude angle. Therefore, the scattered radiation also increases with the rise of the solar altitude angle. The highest value of the daily variation of scattered radiation in sunny days usually occurs around noon when the solar altitude angle is highest.
(2) In addition to the influence of the clear sky index, the variation of the scattering radiation is also related to the solar altitude angle. Generally, when the solar altitude angle increases, the scattering radiation reaching the near ground will also increase accordingly. On the contrary, when the solar altitude angle is low, the scattering radiation will also be weak.
(3) Scattered radiation has an annual variation law.
Characteristics of scattered radiation over the tibetan plateau
(1) The daily maximum value of scattering radiation of alpine meadow ecosystem in the Qinghai Tibet Plateau appears in the afternoon, lagging behind that of low altitude areas;The annual maximum value of scattering radiation appears in April.
(2) The change of the clear sky index has a great influence on the scattered radiationAt about 45, the scattering radiation accounts for the highest proportion of the daily total solar radiation outside the atmosphere, that is, the largest contribution to the scattering radiation. When the clear sky index is too high or too low, the scattering radiation will weaken. The daily clear sky index is greater than 0At 3:00, the proportion of scattered radiation to the total daily total radiation decreases linearly with the increase of clear sky index.
(3) Scattered radiation increases with the increase of solar altitude angle, but its increase rate is different under different clear sky index, which is at3 ^-0. In the range of 7, the scattered radiation changes rapidly with the solar altitude angle.[1]
Influencing factors
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Scattered radiation is mainly affected by the solar altitude angle and atmospheric transparency. Generally, the scattered radiation increases with the increase of the solar altitude angle. The atmospheric transparency is mainly affected by clouds, water vapor and aerosols. The impact on the scattered radiation is more complex, not a simple increase and decrease relationship.
Clear sky index
The water droplets and ice crystals in clouds are the direct factors affecting the scattering radiation. There have been many reports on this aspect, but it is difficult to accurately quantify the cloud because the current cloud observation is mainly regular visual observation, and the clouds on the plateau are changing rapidly. The clear sky index can well reflect the change of clouds,It is an ideal parameter to quantitatively discuss the impact of sky conditions on scattered radiation. There is less dust in the atmosphere over the plateau, and the path of solar radiation through the earth's atmosphere is also short. Therefore, the clear sky index is high in sunny and cloudless weather, and the scattered radiation on the plateau is lower than that in low altitude areas at the same latitude. However, the temperature on the plateau changes sharply, especially between May and September during the plant growing season,The water vapor content in the air is high, and it is easy to form thermal convection in the afternoon, which provides conditions for the formation of clouds. In particular, the amount of clouds in the northeast of the Qinghai Tibet Plateau is the most in summer, followed by spring, and the least in autumn and winter.
Solar altitude angle and atmospheric mass
In addition to the influence of the clear sky index, the variation of the scattering radiation is also related to the solar altitude angle. Generally, when the solar altitude angle increases, the scattering radiation reaching the near ground will also increase accordingly. On the contrary, when the solar altitude angle is low, the scattering radiation is also weak. However, under different conditions of the clear sky index, the rate of the scattering radiation changing with the solar altitude angle is different.
Scattered radiation increases with the increase of solar altitude angle.Scattered radiation changes with solar altitude angle under different sky conditions. The rate of scattered radiation changes with solar altitude angle is very different for different clear sky indexes. With the increase of solar altitude angle, scattered radiation is more sensitive to the change of clear sky index. When the clear sky index is greater than 0At 7:00, the variation of the scattered radiation on the plateau with the solar altitude angle is slower than that in low altitude areas.The variation of scattering radiation on the plateau is mainly concentrated in the range of air quality less than 4, and decreases rapidly with the increase of air quality. When the air quality exceeds 4, the variation of scattering radiation is relatively small.[1]
Change trend
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Annual exposure distribution
The distribution of annual exposure of scattered radiation is that the low latitude area is larger than the high latitude area, the low altitude area is larger than the high altitude area, and the western dry area is larger than the eastern wet area.Its high value area is located in the east and south of China, more than 700kWh/mtwo, mainly because there are many clouds and aerosols, high value centers mainly appear in the hills of Guangdong and Guangxi, around Shanghai, Hubei and Sichuan basins, exceeding 900 kWh/mtwo。The air in the Qinghai Tibet Plateau is clean and the scattering of air molecules is weak. However, due to the high altitude of the region, the total radiation is large and the scattering radiation is relatively large.In the area north of the higher latitude of 45 ° N, the scattered radiation is generally less, and the minimum annual exposure is less than 400 kWh/mtwo。
Change trend in China
The trend of scattered radiation has rising and falling stations in all regions of the country.However, major cities such as Guangzhou, Shanghai, Tianjin, Shenyang, Jinan, Zhengzhou, and Harbin are on the rise.According to relevant research, it may be related to the increase of scattering radiation caused by the increase of aerosol in urbanization development (this may be related to the increase and decrease of atmospheric pollution and aerosol caused by the speed of urbanization development).The scattering radiation decreases significantly in some small and medium-sized cities in the northwest and Qinghai Tibet Plateau, but the scattering radiation decreases slightly in some large cities, such as Beijing, Wuhan, Chengdu, Lanzhou, Urumqi, Hangzhou, Nanjing, etc., which may be related to the recent improvement of the atmospheric environment in the region.[2]
Scattering radiation model
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Research and development stage
The scattering radiation is reasonably calculated through the scattering radiation model. The research on the solar scattering radiation model can be divided into three stages:
The first stage model is dominated by isotropic scattering radiation model, which is not suitable for cloudy and sunny weather conditions, and Liu and Jordan models are represented.
The second stage model is mainly the anisotropic scattering radiation model obtained by various modifications on the basis of isotropic scattering radiation model. Generally, the first stage isotropic model is modified on the basis of the first stage isotropic model to adapt to the weather conditions of cloudy and sunny days. Typical models include Temps, Coulson model, Klucher model Cn7, Hay model, Skartveit and Olseth modelReindl model.Although the form of the model at this stage is relatively simple and has certain practical value, its physical meaning is not clear and its accuracy is relatively low because it is only modified on the basis of the isotropic model.
The third stage model adopts the method of integral solution after building scattering radiation micro element, in which the Gueymard model and Muneer model are developed on the basis of Steven and Unsworth models, while Steven and Unsworth models are obtained by integral solution of Moon and Spence: models. Its core index is that the brightness (radiation) distribution index has a greater impact on the accuracy of the calculation results.The Perez model is built according to the definition of radiation intensity and solid angle. It decomposes the solar scattering radiation into three regions, namely, circumsolar scattering radiation, skyline scattering radiation, and zenith scattering radiation. It is assumed that each region is isotropic. On this basis, the three regions are integrated and solved separately. However, the zenith scattering radiation region in this model is actually not isotropic,Many measured results and studies show that there is an obvious weakening of scattering radiation in the area with an angle of 90 ° to the sun (the corresponding scattering radiation in this area is referred to as orthogonal scattering radiation).
Accuracy comparison of several models
Based on the analysis of the existing scattering radiation model and the measured data of solar radiation, a new anisotropic scattering radiation model is established.
(1) The research on weather types shows that Klucher model, Hay model, Skartveit model and Olseth model are in good agreement with the measured values in cloudy or cloudy days;In sunny days, the calculated values of Perez model and NADR model are closer to the measured values. The analysis of typical weather shows that the calculated values of the other models are basically consistent with the measured values, and are closer to the calculated values of the isotropic Liu and Jordan models, except that the calculated values of the Temp, and Coulson models are slightly larger than the measured values in cloudy days.
(2) The research on orientation shows that Perez model and NADR model are more accurate for east and north directions;For the south and west directions, Liu and Jordan models and NADR models are more accurate. The analysis of typical orientations in sunny days shows that the calculated values of Liu and Jordan models, Temp, Coulson models, Klucher models and Uueymard models are generally greater than the measured values;The calculated values of Hay model, Skartveit and Olseth model, Reindl model and Muneer model tend to be 0 or equal to 0 without direct reflection, and there is obvious deviation from the measured values;The error between the calculated value of Perez model and the measured value of NADR model is the smallest.
(3) Research on seasons shows that Perez model and NADR model are more accurate in summer and autumn;In autumn and winter, the Iiu, Jordan and NADR models are more accurate.
(4) For the study of dip angle, Klucher model can be more consistent with the measured values for different dip angles;For 30.The dip angle, Hay model, Skartveit and Olseth model are in good agreement with the measured values;For 45.The dip angle, Temp, and Coulson models are in good agreement with the measured values;For 60 ° and 90 ° inclinations, the NADR model is also in good agreement with the measured values. In addition, the accuracy of scattering radiation of various scattering radiation models is better than that of large inclinations when calculating inclinations with small inclinations. The analysis of scattering radiation of different inclinations in the south direction on sunny days shows that the error of Gueymard model is large when the inclinations are large, the calculated values of other models are not significantly different from the measured values.
(5) The comprehensive consideration of weather, orientation, season, dip angle and other factors shows that Temp, Coulson modelThe accuracy of Uueymard model is relatively low, while the accuracy of NADR model is the highest. The main influencing factors of solar scattering radiation are clear sky index and solar altitude angle, which are suitable for areas with clear sky index distribution and geographical latitude similar to Shanghai, such as southern Jiangsu (Suzhou, Wuxi, etc.), Anhui, Henan, Hubei, Zhejiang, northern Jiangxi, and northern Hunan.[3]
Measurement method of scattered radiation
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In order to study the influence of scattered radiation, it is necessary to accurately observe or estimate the diffuse radiation fraction (DFR) of solar radiation.At present, there are mainly two methods to calculate DFR: direct measurement and model estimation.
Direct measurement method
The sensors for direct measurement of DFR mostly use special shading devices to block all direct light, and then measure scattered light.Scattered radiation measuring instruments include simple scattered radiation meter and BF3 and SPN1 sensors with high sensitivity.Since the instrument shading device not only blocks the direct radiation of the sun, but also blocks the sky scattering in the shading direction, the observed scattering radiation is smaller than the actual.Therefore, the accurate scattered radiation can only be obtained by multiplying a correction coefficient of shading ring greater than 1 (China Meteorological Administration, 2007).Using BF3 sensor observation, it was found that the proportion of scattered photosynthetically available radiation (PAR) in the total PAR of the canopy at Blodgett Ameriflux Forest Station in the afternoon was higher than that in the morning, and the average scattered PAR in the afternoon was 22% higher than that in the morning.
Model estimation method
In areas where there is no direct measurement of scattering radiation, scattering radiation segmentation model is generally used for estimation.The input variables of the model are generally meteorological parameters such as cloud cover, longitude and latitude, sunshine hours, solar altitude angle, air temperature, relative humidity and clear sky index.Among them, the scattering radiation segmentation model with the clear sky index (also known as the clear sky factor) kt and the solar altitude angle (θ) as the input variables has high accuracy, and the variables are easy to obtain, which is conducive to continuous observation, so it is widely used in atmospheric science research.Kt is a comprehensive parameter describing the influence of atmosphere on solar shortwave radiation, which not only reflects theSolar radiation intensityIt can also reflect the atmospheric conditions when solar radiation passes through the atmosphere and the impact of solar radiation.Kt can be used to distinguish clear sky and cloudy weather, and indirectly reflect the changes of global solar radiation and DFR.The total PAR increases linearly with kt, while the proportion of scattering PAR increases with the decrease of kt, and reaches the maximum at 0.4 – 0.5, and then decreases with the decrease of kt, showing a nonlinear correlation.
When the cloud layer is not continuously distributed, the DFR value estimated by kt is small due to the "cloud gap effect".Since Liu and Jordan (1960), many empirical relationships between DFR and kt have been established.Iqbal (1980), based on the work of Bugler (1977), proposed two model factors for estimating the scattering radiation: kt and θ, and Skartveit and Olseth (1987) used these two factors to establish a scattering radiation segmentation model.Reindl et al.
The technology of direct observation of scattered radiation needs to be improved to eliminate the influence of shading devices. However, this method is still considered to be the most accurate method to calibrate the estimated results of the scattered radiation model.Different scattering radiation segmentation models can be used to estimate scattering radiation in different regions, different environmental conditions and different space-time scales.At the canopy scale, not only the influence of various environmental factors, but also the influence of terrain should be considered.For regional to global scale monitoring, the best method is to use satellite remote sensing technology to observe AOD and cloud changes for inversion, in addition to using observation data from multiple observation stations and scattered radiation segmentation model estimation.However, most of the current remote sensing models are based on AOD, without taking into account other factors such as cloud changes.[4]
Scattered radiation and direct radiation
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The direct radiation of the sun is the light from the sun through a straight path. When it is blocked by an object, it can form a shadow with clear boundaries behind the object.Scattered radiation is the solar radiation reflected by atmospheric molecules, water vapor, dust and other particles, which changes the direction.It seems to come to the surface of the earth from all directions of the whole sky, but most of it comes from the sky near the sun.Like cloudy and foggy days, the scattered light of the sun cannot be obscured by objects to form shadows with clear boundaries, nor can it be focused or reflected by convex lenses or mirrors.
The total radiation intensity of solar radiation is the sum of direct radiation intensity and scattered radiation intensity.The intensity of direct radiation is closely related to the position of the sun and the azimuth and altitude angle of the receiving surface.Scattered radiation is related to atmospheric conditions, such as the content of dust, smoke, water vapor, air molecules and other suspended solids, and the path of sunlight through the atmosphere.Generally, the component of scattered radiation is small in sunny and cloudless conditions;In cloudy days and with much smoke, the component of scattered radiation is large.The intensity of scattered radiation is usually expressed as the ratio of the total radiation intensity. Different places and meteorological conditions have great differences. The intensity of scattered radiation generally accounts for more than 10% to more than 30% of the total radiation intensity.
