Underwater photography cos Guangzhou

1. Ultra-long wave: the largest wave in time and space in the earth's atmosphere. Usually, the wave number circling the earth along latitude is 1 ~ 3, and its characteristic wavelength is in the same order of magnitude as that of radius of the earth (104km). The activity of ultra-long waves can be clearly seen on the isobar diagram above 200 HPA (see stratosphere and mesosphere circulation); Taking the troposphere as an example, the existence of atmospheric fluctuations with different wavelengths can only be determined by spectral analysis on the 500 HPA isobaric map.

According to the space-time scale of ultra-long waves, its Ross is closer to the quasi-geostrophic equilibrium condition than Ro≈ 10-2. In the dynamic model of ultra-long waves, the vorticity equation (see atmospheric dynamic equation) can be simplified as fD+βv=0, where F is Coriolis parameter, D is divergence, β is Rossby parameter, and V is the horizontal velocity of the atmosphere. The formula shows that for ultra-long wave-scale motion, divergence effect and β term are quasi-balanced, so they have quasi-steady properties.

In the actual atmosphere, ultra-long waves are controlled by large terrain, cold and heat sources on land and at sea, and long-wave energy feedback. It is quasi-steady in a certain geographical area, which is in sharp contrast with the moving atmospheric long wave. Ultra-long waves can be regarded as one of the important backgrounds of hemispheric circulation situation, and the long-term weather evolution process is closely related to ultra-long waves.

Ultra-long waves are radio waves with the wavelength of 1- 10 megameter (frequency is 30-300 Hz). Ultra-long wave has small propagation loss, strong diffraction ability and long communication distance; And can bypass space stars and cross the ionosphere; Generally, it has the ability to penetrate 100 meter depth to seawater; When propagating underground, the absorption loss is also small, and it is not affected by external factors such as nuclear explosion and solar activity. Suitable for underwater submarines, long-distance ships and underground communication. However, ultra-long wave communication equipment is generally bulky, complex, expensive and high in operating cost, which is only suitable for one-way communication between shore station and submarine. Because the ultra-long wave channel is very narrow and the communication rate is very low, it takes about 15 minutes to send a group of three-letter signals. 1986, the United States built an ultra-long wave communication system for the first time, which was used to communicate with strategic nuclear submarines cruising in the oceans of the world from the mainland.

Application: ultra-long wave therapeutic instrument, ultra-long wave communication at sea.

2. Long wave: 1, electron long wave

Refers to the radio waves from 300 kHz to 30 kHz with the wavelength of 1000 ~ 10000. When the long wave propagates, it has the advantages of stable propagation and little influence by nuclear explosion and atmospheric disturbance. Its propagation mode is mainly in the form of ionospheric waves around the earth's surface, which can reach thousands to tens of thousands of kilometers. In addition, it can also spread through ground waves in a short distance (within 200 to 300 kilometers). The electric field intensity in this band increases at night than during the day, and the shorter the wavelength, the more it increases. The influence of electric field intensity on seasonal variation is small; The propagation conditions are less affected by ionospheric disturbance, and the stability is good, and there will be no sudden change of receiving intensity and sudden interruption of communication. Suitable for radio direction finding, radio navigation, etc. Because the long wave requires huge antenna equipment, the radio station in China does not use the long wave (LW) band, which is mainly used for submarine communication and ocean-going ship communication. Therefore, domestic radios generally have no long-wave (LW) band.

[Edit this paragraph] The difference between long wave and short wave.

Long wave (including ultra-long wave) refers to radio wave with frequency below 300kHz.

Medium wave refers to radio waves with a frequency of 300 kHz ~ 3 MHz.

Short wave refers to radio waves with a frequency of 3 ~ 30 MHz.

Ultrashort wave refers to radio wave with wavelength of 1 ~ 10m (frequency is 30 ~ 300mhz).

[Edit this paragraph] 2. Meteorological long wave

Long wave refers to the fluctuation with the wavelength of 3000 ~ 8000 km in the atmospheric circulation of the westerlies in the middle and upper troposphere. It is caused by the effect of Coriolis force changing with latitude when the westerly airflow is disturbed north and south (see atmospheric fluctuation). Long wave has a life of about one week, which is a large-scale disturbance of westerlies and belongs to a planetary scale weather system, also known as planetary wave or Rossby wave. Since the radiosonde was applied to high-altitude meteorological detection in the 1920s, people have discovered the westerlies and their fluctuations moving at high altitude in the polar regions. J Pierre Knies first realized that long wave is a hemisphere phenomenon in 1937, and C-G Rossby theoretically studied the characteristics of long wave in 1939, and established the long wave theory. This theory laid the foundation for the later development of numerical weather forecast. The occurrence, development, movement and change of long waves play a very important role in the intensity, direction and speed of weather scale systems (such as cyclones and fronts) and future weather changes. Therefore, it is one of the important systems of weather situation forecast research.

[Edit this paragraph] Meteorological long wave characteristics

Long wave has the structure of cold trough and warm ridge. On the high altitude isobaric surface map (see weather map), the phases of temperature wave and altitude wave are similar. The intensity of long wave increases with height, and it is the strongest at the tropopause. The developing long wave, its temperature wave is often slightly behind the height wave, and its phase is generally nearly a quarter wavelength behind. There is warm advection behind the long wave ridge and cold advection behind the long wave trough, which is the main reason for the development of the long wave trough and ridge (see atmospheric dynamic instability). Because the long wave is a dispersive wave, the energy of the long wave propagates downstream at a speed greater than the wave motion in the upstream of the westerlies. Therefore, this feature can be used to predict the position and intensity of long waves from upstream to downstream. When the long wave trough and ridge develop strongly, the amplitude increases continuously, and the high pressure center appearing on the long wave ridge sometimes cuts off from the ridge, forming blocking high pressure.

Long wave and short wave can be converted to each other. When the temperature field and pressure field are properly configured (cold advection behind the trough and warm advection behind the ridge), short wave can gradually develop into long wave; On the contrary, long waves can be weakened and split into short waves, and then disappear eastward.

[Edit this paragraph] Meteorological long wave velocity

In the upper troposphere, according to Rossby's long wave theory, the wave velocity c of long wave is C =υ-βλ2/4π2.

WheRE ω is the basic zonal wind speed, β =2ωcosφ/re is the Rossby parameter, λ is the wavelength, Re is the radius of the earth, ω is the angular velocity of the earth rotation, and φ is the latitude. The above formula shows: ① The stronger the west wind, the faster the long wave moves, but the moving speed is always less than the west wind speed. ② At a certain westerly wind speed, the longer the wavelength of long wave, the slower the moving speed; The shorter the wavelength, the faster the moving speed. (3) When the westerly wind speed and long wave reach a certain value, c =0. At this time, the long wave stops, which is called standing wave, and the wavelength at this time is called standing wave or critical wavelength; When the wavelength is greater than other wavelengths, C.

[Edit this paragraph] Relationship with atmospheric circulation and cyclone

Long wave trough and ridge play an important role in maintaining atmospheric circulation. Long wave grooves and ridges of true sinusoidal curves are extremely rare. At the early stage of the development of the ridge, the trend of the groove line and ridge line is mostly northeast-southwest With the development of trough and ridge, the center of strong westerly wind (jet center) generally moves from the back of trough to the front of trough. Because the southwest wind in front of the trough is much stronger than the northwest wind behind the trough, it is beneficial to transport the excess angular momentum at low latitudes to the middle and high latitudes, thus maintaining the westerly angular momentum at the middle and high latitudes (see atmospheric angular momentum balance). At the same time, because there is warm advection in front of the trough and cold advection behind the trough, it is beneficial to transport heat from low latitude to high latitude and maintain the global heat balance. Therefore, the activity of long wave trough and ridge is an important mechanism to maintain atmospheric circulation.

In winter, the westerly circulation around the northern hemisphere generally has 4 ~ 5 long waves. The typical relationship between long-wave trough and surface cyclone family can be expressed by idealized long-wave flow pattern and schematic diagram of low-level circulation system. The cyclone family is located behind the long wave ridge, and each cyclone corresponds to the short wave in the upper atmosphere. It can be seen from the figure that the short wave disturbance is superimposed on the long wave. Because of their short wave length, they move faster than long waves, and the ground cyclones corresponding to short waves move forward relative to long waves, and are generally dragged by long-wave airflow. Because of the close relationship between long wave and surface cyclone and front, the prediction of long wave movement and flow pattern change is of great significance to weather forecast.

Application: Multi-quantum well integrated waveguide photodetector

3. Medium wave: Generally speaking, medium wave broadcasting (MW: medium wave) adopts amplitude modulation, and unconsciously, an equal sign is drawn between MW and AM. In fact, medium wave is just a kind of broadcasting that uses amplitude modulation. For example, the modulation mode used by international short-wave broadcasting at high frequency (3-30MHz) is AM, and even aviation navigation communication at higher frequency than FM broadcasting (116-136 MHz) uses AM, but the AM band we refer to in our daily life refers to medium-wave broadcasting (MW

The way of medium wave propagation mainly depends on ground waves, and only a small part of it propagates in the form of sky waves. When a radio wave touches a conductor, it will induce a current in the conductor, thus losing some energy. This phenomenon of weakening radio wave energy is called radio wave absorption. The earth is a conductor and absorbs medium waves very strongly, so medium waves propagating in the form of ground waves will not travel very far (about two or three hundred kilometers). During the day, due to sunlight, the density of the ionosphere increases, making the ionosphere a good conductor. A small part of the medium wave propagating in the form of sky wave is strongly absorbed when it enters the ionosphere, and it is difficult to return to the ground. In addition, the medium wave propagating in the form of ground wave is absorbed by the earth, and the propagation distance is not far, which makes it difficult to receive the distant medium wave radio station during the day. At night, the atmosphere is no longer exposed to sunlight, and the electrons and ions in the ionosphere recombine with each other and increase significantly, so the ionosphere becomes thinner, the density becomes smaller, the conductivity becomes worse, and the absorption of radio waves is greatly weakened. At this time, the medium wave can spread far away by means of sky wave. So I received more medium-wave radio stations at night.

China's medium-wave band range: 526.5kHz~ 1606.5kHz, and medium-wave emission bandwidth: 9kHz.

Application: medium wave photographic broadcasting

4. Short wave: short wave transmission mode

There are two basic modes of short wave propagation: one is ground wave and the other is sky wave.

As mentioned above, the ground wave propagates along the earth's surface, and its propagation distance depends on the characteristics of the surface medium. The conductivity of the sea surface medium is most conducive to the propagation of electric waves, and the short-wave ground wave signal can travel about 1000 km along the sea surface; The conductivity of land medium is poor, and the radio wave attenuation is large. The radio wave attenuation of different land medium is different (the attenuation of wet land surface is small, and the attenuation of dry sand surface is large). Short-wave signals can only travel along the ground for dozens of kilometers at most. Ground wave propagation does not need to change the working frequency frequently, but it is different from sky wave propagation because of the obstruction.

The main transmission route of short wave is sky wave. After the short-wave signal is sent by the antenna, it is reflected back to the ground through the ionosphere, and then reflected back to the ionosphere from the ground. It can be reflected many times, so the propagation distance is far (hundreds to tens of thousands of kilometers) and will not be blocked by obstacles on the ground. But the sky wave is very unstable. In the process of sky wave propagation, factors such as path attenuation, time delay, atmospheric noise, multipath effect and ionospheric fading will cause signal attenuation and distortion, which will affect the effect of short-wave communication.

Application: broadcasting

5. Ultrashort wave: ultrashort wave, also known as VHF wave and meter wave (wavelength range is 1 m to 10 m), is a radio wave with a frequency of 30 MHz to 300M MHz, with a wide frequency band, and short-distance propagation depends on electromagnetic radiation characteristics. For TV broadcasting and wireless microphone to transmit audio signals, the sharp directional antenna can compensate the attenuation in the transmission process. In the field of professional audio, the frequency stability of V-band wireless microphone is slightly poor, and the price is relatively low, but it is prone to frequency drift. Through various technical measures, the frequency stability can reach the required level.

Ultrashort wave is also called "meter wave". Refers to the radio band with wavelength from 1m to 10m (corresponding frequency from 300 MHz to 30 MHz). Unlike short-wave propagation, there is no ionospheric reflection, and long-distance propagation is realized by ionospheric scattering. It is used in navigation, television, FM broadcasting, radar, ionospheric scattering communication, fixed and mobile communication services, etc.

Application: communication, medical treatment

Landlord: Radio broadcasting mainly uses radio waves.

Radio waves are electromagnetic waves with a wavelength greater than 1mm and a frequency less than 300GHz.

Belong to ultrashort wave