Troposphere refers to the nearestearthA layer of surfaceatmosphereIs also the lowest layer of the atmosphere,densityIt is the largest, containing air quality accounting for almost 75% of the total air quality, and almost allsteamandaerosol。[1]
The earth's troposphere is the layer of atmosphere closest to the earth's surface, concentrating about 75% of the atmosphere's mass and more than 90% of thesteamQuality.The troposphere is affected by the groundforest、lake、Grassland、beach、mountain ridgeUnder the influence of different terrain, the temperature changes caused by sunlight, resulting in a large number of vertical and horizontal wind, that is, the airConvection phenomenon, so it is called troposphere.
Vertical structure of atmospheric temperature from the earth's surface to an altitude of about 110km[1]
The lower boundary is connected with the ground, and the upper boundary height isGeographic latitudeAnd seasons, its height depends onlatitudeHowever, the average altitude is 17-18 km in low latitudes, 10-12 km in mid latitudes, and 8-9 km in polar regions.[1]For example, the height of the troposphere in Beijing is about 11 km, that in Guangzhou is about 16 km, and that in Northeast China is 10 km.And summer is higher than winter.Even on the same day in the same region, the tropospheric height will change with the change of morning, middle and evening.[3]
The troposphere extends from the surface of the earth to the upper air untilTropopause, i.estratosphereTo the starting point of.In the high latitude area, because the friction of the surface will affect the air flow, an average thickness of 2 kmPlanetary boundary layer。The formation of this layer is different mainly depending on the terrain, and it will also be separated from other parts of the troposphere by the separation of the countercurrent layer.[4]
This layer is the lowest layer of the atmosphere and has the highest density. It contains almost 75% of the air quality of the whole atmosphere.[1]In the troposphere, air can flow up and down,thunderstorm、dense fog、wind shearAnd other weather phenomena occur in this layer.Because the troposphere isturbulenceIn the most frequent layer, most jet airliners will fly over the top of this layer (tropopause) to avoid the airflow that affects flight safety.[3]
In the universefixed starThere is also the troposphere, where the energy inside the sun is transmitted outwardradiationAnd convection.The temperature at the periphery of the radiation zone drops rapidly, and the radiation loss on the solar surface increases, making the temperature difference between the upper and lower levels also increase, which forms a strong troposphere dominated by turbulence.The gas properties in this layer vary greatly and are very unstable.[5]
Composition
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The troposphere is mainly composed of the following components (in no particular order):[6]
Nitrogen (Ntwo)
Oxygen (Otwo)
Carbon dioxide (COtwo)
Methane (CHfour)
Nitrous oxide (NtwoO)
Carbon monoxide (CO)
Ozone (Othree)
Sulfate radical (SOfour2-)
Nitrogen dioxide (NOtwo)
Hydroxide (OH-)
[6]
Under the condition of no air pollution, the composition of the troposphere is very uniform, which is mainly due to the strong mixing caused by the continuous circulation of air masses in the troposphere.However, due to the formation of cloudsrainfallAnd the evaporation of water from the earth's water body, so that the content of water in the troposphere changes greatly.[6]
main features
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Temperature inversion phenomenon
In the troposphere, the temperature decreases with the increase of altitude, and the tropospheric temperature decreases by about 0.65 ℃ every 100 meters.[3]The temperature decreases with altitude because the main heat source of the troposphere atmosphere is the groundLong wave radiationThe higher it is above the ground, the less heat it receives, and the lower the temperature is.However, under certain conditions, the temperature in the troposphere will also rise with the increase of height, which is called“Temperature inversion phenomenon”。
Inversion is separableRadiation inversion、Turbulent inversion、advection inversion 、Subsidence inversion、Frontal inversion。Inversion layerThe existence of, causing the local atmosphere to be hot and cold, hindering the development of air convection movement, a large number of smoke, pollutants, water vapor condensate, etc. gathered under it, making visibility worse and air pollution worse;Especially over cities and industrial and mining areas, due to the large number of condensation cores, it is easy to have dense fog weather, and some even cause serious air pollution events, such asPhotochemical smog。[7]
layered
In the troposphere, the troposphere can be further divided into the following three layers according to the movement state of the atmosphere in the troposphere, the vertical variation characteristics of temperature and the variation characteristics of weather phenomena:[8]
(1) Lower layer: the lower layer is also called disturbance layer or friction layer.The range is generally from the ground to a height of 2 kilometers.The scope of the lower layer also varies with the season, day and night, generally higher in summer than in winter, and higher in daytime than at night.In this layer, the air flow is greatly affected by the friction of the ground,Turbulent exchangeThe effect is particularly strong. Generally, with the increase of height, the wind speed increases and the wind direction deflects.This layer is affected by the thermal effect of the ground, and the temperature also has obvious diurnal changes.Due to the water vaporDust particleThe content is high, so low clouds, fogFloating dustAnd so on.[8]
(2) Middle layer: the bottom boundary of the middle layer is at the top of the friction layer, and the height of the upper layer is about 6 kilometers.Due to the weakening of friction, turbulence movement in the middle layer is reduced, and advection movement is enhanced. The influence of the ground is much smaller than that of the friction layer. The movement law of the atmosphere is relatively simple and clear. The air flow condition can basically represent the trend of the air movement in the entire troposphere.Major in the atmosphereWeather phenomenonClouds and precipitation are mostly produced in this layer.[8]
(3) Upper layer: The upper layer extends from a height of 6 kilometers to the top of the troposphere.This layer is less affected by the ground, the temperature is below 0 ℃ all the year round, the moisture content is less, and all kinds of clouds are composed of ice crystals and supercooled water droplets.stayMid latitudeandtropicIn this layer, strong wind zones with wind speed equal to or greater than 30 m/s often appear, namely the so-calledrapids。[8]
In addition, between the troposphere and the stratosphere, there is a transition layer with a thickness of several hundred meters to 1~2 kilometers, calledTropopause。The main feature of this layer is that the temperature decreases with height, and there is a sudden change.The temperature decreases slowly with the increase of height, or it is almost isothermal.The position of tropopause is determined according to the initial height of this change.The average temperature of tropopause is about - 83 ℃ in low latitudes and - 53 ℃ in high latitudes.The tropopause has a great blocking effect on the vertical air flow, and the rising water vapor and dust particles gather under it,visibilityOften poor.[9]
Temperature change
The main direct source of heat in the troposphere isGround radiation, so the temperature decreases with the increase of altitude,[7]for exampleQinghai Tibet PlateauThe tropospheric temperature in the region is significantly higher than that in other regions at the same height, because the Qinghai Tibet Plateau has raised the position of ground radiation.[10]
The general law of troposphere changes with height: the temperature drops 0.65 ℃ for every 100 meters of height rise.[3]
Pressure temperature structure
pressure
AtmosphericpressureIt will descend as the height rises.This is because theSurfaceThe air above will be pressed by all the air above it. On the contrary, at a high place, the air will be pressed by less air, so the pressure will also decrease.
The change of air pressure with height can be described by the pressure height formula:
。Where: g is the acceleration of gravity;ρIs density;H is the height;P is the pressure;R is the gas constant;T is the temperature.[11]
air temperature
In the troposphere, the temperature will drop by 6.49 ℃ on average for every 1 km height rise.This temperature decline is due toAdiabatic coolingThe occurrence of.When the air rises, the air pressure will drop and the air will expand with it.In order to expand the air, a certain amount of work needs to be done around, so the temperature will drop.（First law of thermodynamics）[3]
staypolar region(High latitude area), the temperature will only drop to - 45 ℃ due to the relatively thin troposphere.In mid latitude, the temperature will drop from about+17 ℃ at sea level to about - 52 ℃ at tropopause.equatorRegional (low latitude) temperature can drop to - 75 ℃.[7]
Tropopause
The tropopause is the transitional layer between the troposphere and the stratosphere.The position about 11km from the ground is called tropopause.The ground height varies with latitude, season and weather.Generally speaking, it is 17 kilometers high near the equator, and about 9 kilometers high near the poles, while the average height is about 11 kilometers above the ground.Most long-distance passenger planes fly at this border.[3]
To calculate the temperature change in the troposphere due to height, it is necessary to understandstratosphere, because the stratosphere defines the position of the troposphere.In the troposphere, the temperature decreases with height, whereas in the stratosphere, the temperature increases with height.When the temperatureDecline rateWhen the phenomenon of changing from positive (troposphere) to negative (stratosphere) occurs, it is the region of tropopause.[12]
atmospheric circulation
The atmospheric circulation refers to the state of large-scale atmospheric movement.The atmospheric circulation is to complete the earth atmosphere systemangular momentum、quantity of heatandwater contentThe transport and balance of these physical quantities, as well as the important mechanism for the conversion of various energies, are also important results of the transport, balance and conversion of these physical quantities.[13]
In terms of geographical scope, there is atmospheric circulation in a region (such as Eurasia), a hemisphere (such as the Northern Hemisphere) or the whole world;In terms of vertical level, there are atmospheric circulation in the troposphere, stratosphere, mesosphere or the whole atmosphere;In terms of time scale, there are atmospheric circulation from one to several days, January, one season, half a year, one year to multi-year average.[13]
Phenomenological distinction
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The atmosphere in the lower troposphere will have friction with the surface, but the air in the upper troposphere is not affected by this friction.So the weather phenomenon in the troposphere is different.Based on the difference of this phenomenon, the troposphere will be divided into three layers:[14]
(1) Ground plane: the sea level is 0~100m, and the friction with the ground is relatively large, so the atmospheric movement and turbulence are very irregular and active.[14]
(2) Ekman layer: 100m~1km, affected byCoriolis force, air pressure inclination force andfrictionThese three forces combine to move.[14]
（3）Free atmosphere: 1km~tropopause, not affected by the friction of the ground, and the atmosphere is in a free state of motion.[14]
Communication mode
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Consider the impact of clouds and precipitation
The study of modern tropospheric propagation mainly focuses on theRadio wave propagationProblems, propagation problems in broadcasting and mobile communication, multi-path effects, etc.Millimeter wave has outstanding advantages in practice (see radio wave propagation above 10 GHz), so the research on tropospheric propagation is expanding towards millimeter wave.[15]Tropospheric scattering propagation isTroposcatter communicationTechnical basis of.The tropospheric scattering transmission system designed based on the tropospheric scattering propagation mechanism can realize over the horizon transmission;At the same time, it has moderate transmission capacity, transmission performance and reliability, and particularly strong nuclear explosion resistance.Tropospheric scattering transmission system always occupies an irreplaceable position in a variety of transmission systems, especially in a variety of wireless transmission systems, because of its special properties.[15]
Do not consider the influence of cloud and precipitation
It can be divided into:Atmospheric refraction, waveguide propagationTropospheric scattering、Multipath propagation、Atmospheric absorptionAnd absorption and scattering of water vapor condensate and other atmospheric particles.According to the scope of communication, there are:Line of sight propagation、Over the horizon propagationAnd ground to air communication.Ground air propagation can also be classified as line of sight propagation.The basic mode of line of sight propagation is direct propagation, but it is affected by the troposphere and the ground.Tropospheric scattering is a common way of over the horizon tropospheric propagation, and sometimes it may be waveguide propagation.According to the frequency band, there are:ultra-short wave propagation , microwave propagationMillimeter waveAndSubmillimeter waveDissemination andlight waveCommunication, etc.Ultra short wave and longer microwave can be used for line of sight propagation, and can also be used for over the horizon propagation.Radio waves and light waves in the frequency band above 10 GHz are generally limited to line of sight propagation.[16]
The development of tropospheric propagation research is closely related to communication.the Second World WarLater, the need for long-distance, high-quality multi-channel communication led to the discovery of tropospheric scattering propagation mechanism.This discovery not only led toTroposcatter communicationAnd led tothe ionosphereScattering communication andMeteor trail communicationOccurrence of[16]。satellite communicationThe emergence and the need for further development of, has promoted the research of ground to air propagation, especially in the frequency band above 10 GHz.Since tropospheric propagation is closely related to the characteristics of troposphereTropospheric soundingTechnology also promotes each other.Many techniques have been used to detect the tropospheric refractive index and the macrostructure and microstructure of clouds, fog and precipitation, which has promoted the study of tropospheric propagation;The propagation research results on the connection between the troposphere structure and the generated signal characteristics also provide a detection basis for the generation and improvement of relevant radio detection means.All precision radars use the tropospheric transmission mode, especially the line of sight transmission mode.Especially in microwave and higher frequency band, tropospheric effect between radar and target is a prominent propagation problem.Microwave andMillimeter waveRemote sensing also directly or indirectly uses atmospheric absorption and cloud attenuation effects.[16]
When studying the propagation of light in the atmosphere, the refractive index of the atmosphere is an important parameter.Under some predictable or unpredictable dynamic conditions, the refractive index of air is very complex because many variables often change.Mainly affected byastronomyThe impact of observation has been studied since 1700.For example, due to the change of atmospheric refractive index, different refractions will be generated, and in addition, different observation wavelengths will lead to the imaging of objects at different positions on the focal plane of the telescope.[18]
When the temperature is T=15 ℃, the atmospheric pressure is P=760mmHg and the air contains
COtwoWhen, yes[18]
, where n is the refractive index;λIs the wavelength in vacuum.[18]
Therefore, the change of refractive index can be determined by measuring the change of resonant frequency of the cavity.Some refractive index meters use air capacitors as sensing elements.[18]
The short-term average value and distribution of the ground refractive index, the refractive index gradient 1 km above the ground and within 100 m around the country can generally be obtained by using theGround temperature, humidity, pressure records and sounding data.However, the fine refractive index structure and its variation can only be obtained by special measurement.There are two kinds of measurement methods: direct measurement and remote sensing.[19]
① Direct measurement: place the measuring instrument on theMeteorological tower, tethered balloon or aircraft, directly measure the refractive index of the point where the instrument is located.The meteorological tower can obtain continuous and simultaneous records of refractive index or temperature, humidity and pressure, but is limited by height and location;The tethered balloon can measure the refractive index structure more precisely within 500 meters, but it is only suitable for good weather;The airborne refractive index meter has a wide range of measuring heights, and can measure stratification and turbulence with considerable precision, but it cannot measure all weather.[19]
② Remote sensing: usingradiometer、Lidar, acoustic radar or microwave radar remote sensing measurement of refractive index.The radiometer generally passes through theRadiation intensity measurementThe vertical distribution of atmospheric temperature is retrieved bysolar radiationAttenuation or measurement of atmospheric bright spot temperature to determine the height distribution of water vapor density;Lidar utilizationnitrogenRomanBackscatteringMeasure the temperature.This backscattering intensity is related to the temperature of the scattering point.If the lidar operates at two wavelengths, one of which has water vapor absorption attenuation, the water vapor content can be calculated by comparing the echo attenuation of the two wavelengths.Response ratio of sound wave to temperature and water vapor changesair wavesIt is much more sensitive. The intensity and position of inversion layer can be detected by using a single station acoustic system.The radio acoustic system uses radio waves to measure the propagation speed of sound waves in the air, so as to obtain the height distribution of temperature.Since the absorption of water vapor to sound waves is a function of frequency and humidity, the use of multi frequency sound wave system canMeasuring humiditySection;Microwave radar can also measure stratification and turbulent structure.[19]
The global distribution of the monthly average values of the ground refractive index and the refractive index gradient within 1km above the ground has been measured;There are also many empirical models for the statistical distribution of refractive index gradients within 100 meters above the ground;In some areasAtmospheric stratification, waveguide and small inhomogeneity.[19]
Precipitation measurement
Precipitation measurement includes rainfall measurement and snowfall measurement.The measurement items include rainfall rate or snowfall rate and its temporal and spatial changes, and the microstructure of rainfall or snowfall（particleShape, inclinationFinal velocityAnd titre distribution, etc.).Multipurpose rainfall rate measurementtime resolution Very high fast response rain gauge orTipping bucket rain gaugeconduct.[20]The routine rainfall measurement data of the meteorological department can be used as data in a large range after the integral time correction, and various types of rain in the world have been proposedClimatic zoneThe long-term distribution of reference rainfall rate and the preliminary model of temporal and spatial variation of rainfall rate.The shape and inclination of raindrops can be measured by photography.The raindrop is generally flat spherical, and the larger the raindrop, the flatter the shape.In the study of radio wave propagation, the Prupache Pitt raindrop shape model is mostly used.Generally, the size of raindrops does not exceed 8mm, and the axis of symmetry is close to the vertical linewind speedSlightly inclined under the action of vertical gradient.[20]
The charged water drop with the selected titer passes through two induction loops in succession during the fall, and two potential pulses are successively generated at the vacuum tube grid connected with the induction loop.The final velocity of water droplets can be determined according to the distance of the induction loop and the time difference between the two pulses.The final velocity of raindrops increases with the increase of raindrops. The initial velocity increases rapidly, and it slows down when the titer exceeds 2mm.[20]
There are many methods for measuring rainfall rate distribution, including powder method, filter paper method, impact sensing method, electrostatic sensing method and optical detection method.The powder method and the filter paper method determine the size of raindrops according to the size of the powder ball formed by raindrops in the face plate and the spot formed on the filter paper with dye respectively.Shock sensorIt is generally called raindrop distribution meter, which applies theimpulseOr the water will become an electric pulse.Since the mass, terminal velocity and impact time of raindrops are all functions of raindrop drop density, the raindrop density distribution can be converted according to the electric pulse amplitude distribution.Electrostatic sensors and optical detectors measure the charge of raindrops and the passage of raindropslight beamTo determine the size of raindrops.[20]
Snow falls in the form of snowflakes, with a diameter of several millimeters to more than ten millimeters.The photographic measurement shows that the ratio of the maximum horizontal particle size to the height of snowflakes varies widely, with an average of close to 1.The angle variation is generally below 10 °, and the final velocity increases with the increase of the particle size and mass of snow, generally several meters per second.[20]
Multi parameter radar, including dual frequency radar, dual polarization radar andDoppler radar , has become a very important tool in precipitation measurement.Doppler radar can measure the frequency shift spectrum corresponding to various raindrop velocities.[20]The raindrop velocity is a function of the raindrop titer, so the frequency shift spectrum can be converted into the raindrop distribution.The dual polarization radar can measure the reflectivity of at least two orthogonal polarities, which can be used to determine two parameters in the negative exponential particle size distribution model.If the correlation and relative phase shift of the two polarization receiving signals are measured at the same time, the orientation of precipitation particles can also be determined at the same time.hailThe dual polarization differential reflectivity and attenuation of is different from that of rain, so the use of dual polarization and dual frequency radar can separate hail and rain.[20]
