Measurement of tropospheric radio meteorological data
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Tropospheric sounding is the measurement of tropospheric radio meteorological data.Tropospheric detection is divided intoRefractive index measurementAnd water vapor condensate measurement.
Chinese name
Tropospheric sounding
Definition
Measurement of tropospheric radio meteorological data
Category
Refractive index measurement and water vapor condensate measurement
Measurement of tropospheric radio meteorological data.Tropospheric detection is divided intoRefractive index measurementAnd water vapor condensate measurement.The former includes the measurement of temperature, humidity, pressure, refractive index, turbulence and stratification;The latter includes the measurement of clouds, fog, especially precipitation.Tropospheric sounding data isTropospheric radio wave propagationPhysical basis of the study.
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Measurement of tropospheric refractive index The refractive index is usually determined by the measurement data of temperature, humidity and pressure according to the following formula N=(77.6/T) (P+4810e/T)
Where N is the refractive index (N unit);T is the temperature (K);E is water vapor pressure (millibar);P is the atmospheric pressure (millibar).The refractive index can also be measured directly by using the refractive index meter.There are many kinds of refractometer. In the refractometer with cylindrical or coaxial cavity as the sensing element, the change of air refractive index N causes the change of cavity resonant frequency f (墹 f), and the relationship is 墹 N=- (墹 f/f) × 10
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.
The short-term average value and distribution of the ground refractive index, refractive index gradient 1 km above the ground and within 100 m around the country can generally be obtained from the ground temperature, humidity, pressure records and sounding data of conventional meteorological stations and stations.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. ①Direct measurement: place the measuring instrument on the meteorological tower, tethered balloon or aircraft to 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 in a very fine way, but it cannot measure all day long. ②Remote sensing: use radiometer, laser radar, acoustic radar or microwave radar to measure refractive index.The radiometer generally passes through theRadiation intensity measurementThe vertical distribution of atmospheric temperature is retrieved, and the height distribution of water vapor density is determined by measuring the solar radiation attenuation of water vapor absorption zone or the temperature of atmospheric bright spots;Lidar measures temperature using the Roman backscattering of nitrogen.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.The response of sound waves to temperature and water vapor changes is much more sensitive than that of radio waves. The intensity and location 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 acoustic waves is a function of frequency and humidity, the humidity profile can be measured using a multi frequency acoustic system;Microwave radar can also measure stratification and turbulent structure.
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 areas, atmospheric stratification, waveguide and small inhomogeneity have also been investigated in detail.
Precipitation measurement includes rainfall measurement and snowfall measurement.The measurement items include rainfall rate or snowfall rate and its spatio-temporal change, microstructure of rainfall or snowfall (particle shape, inclination, terminal velocity and titre distribution, etc.).Rainfall rate measurement is usually conducted with fast response rain gauge orTipping bucket rain gaugeconduct.The routine rainfall measurement data of the meteorological department can be used as data in a large range after the integral time correction. The long-term distribution of reference rainfall rates in various rain climate regions of the world and the preliminary model of temporal and spatial changes of rainfall rates have been proposed.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 is not more than 8mm, the axis of symmetry is close to the vertical line, and it inclines slightly under the vertical gradient of wind speed.
R. Geng and GD. Kenzer used an electronic device to measure the final velocity of raindrops, and achieved good results.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.
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 sensorGenerally known as raindrop distributor, it changes the impulse or water on the rigid membrane into 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 determine the size of raindrops by measuring the charge of raindrops and the size of the shadow formed by raindrops when they pass through the beam, respectively.The raindrop distribution models commonly used in communication research are Routh Parsons distribution and Marshall Palmer distributionNegative exponential distribution。
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.For snowflake particle size distribution, K.L.S. Gung and JS. Marshall put forward a negative exponential model which is completely similar to the Marshall Palmer raindrop distribution in form, except that the parameters are different.
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.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.The dual polarization differential reflectivity and attenuation of hail are different from that of rain, so the use of dual polarization and dual frequency radar can separate hail and rain.
