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Вы можете ознакомиться и скачать презентацию на тему Precipitation. Доклад-сообщение содержит 31 слайдов. Презентации для любого класса можно скачать бесплатно. Если материал и наш сайт презентаций Mypresentation Вам понравились – поделитесь им с друзьями с помощью социальных кнопок и добавьте в закладки в своем браузере.

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Version 1 Give the definition of the solar constant? Write the dimension of the eddy coefficient, please. Write down the humid air state equation and give notions to all terms. What does convection process mean? Write down statics’ equation and give notions to all terms. What is the potential temperature?

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Lecture 26 Precipitation

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Water drops and ice crystals falling out or any product of the condensation of atmospheric water vapor that is pulled down by gravity and deposited on the Earth's surface is called Precipitation. Amount of precipitation is measured by the thickness of the liquid water layer that could be formed after precipitation fall out on a horizontal surface. 1 mm= 1 kg/m2

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Hydrometeor The term meteor describes an object from outer space which has entered the Earth's atmosphere and produces a light phenomenon

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The amount of Precipitation falling out per a unit of time is called The amount of Precipitation falling out per a unit of time is called Precipitation intensity mm/s (rare), mm/h (forecasts), mm/day, mm/ month (climatologically texts)

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Liquid drops formed on the surfaces after water vapor condensation against chilled surfaces. Example: Over thin blades of grass and leaves at night

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All precipitation types are called hydrometeors, of which additional forms are clouds, fog, wet haze, mist, blowing snow, and spray. Whenever rain or drizzle freezes on contact with the ground to form a solid coating of ice, it is called freezing rain, freezing drizzle, or glazed frost; it is also called an ice storm or a glaze storm, and sometimes is popularly known as silver thaw (оттепель) or erroneously (ошибочно) as a sleet storm.

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Duration of precipitation Brief -Short duration. Intermittent -Precipitation which ceases at times. Occasional -Precipitation which while not frequent, is recurrent. Frequent -Showers occurring regularly and often. Continuous -Precipitation which does not cease, or ceases only briefly. Periods of rain -Rain is expected to fall most of the times, but there will be breaks. Distribution of showers and precipitation Few- Indicating timing and not area. Isolated -Showers which are well separated in space during a given period. Local -Restricted to relatively small areas. Patchy- Occurring irregularly distributed over an area. Scattered -Irregularly distributed over an area. Showers, which while not widespread, can occur anywhere in an area. Implies a slightly greater incidence than isolated. Widespread -Occurring extensively throughout an area.

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Can you guess the shape of raindrops? They are actually shaped like hamburgers! As they fall, the air pushes on the bottoms of the drops, causing the bottoms to flatten out while the tops remain round.

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1- avearge values of cloud thickness 2- downpour precipitation is not included

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Precipitation size and speed

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Virga In meteorology, virga is an observable streak or shaft of precipitation that falls from a cloud but evaporates before reaching the ground. At high altitudes the precipitation falls mainly as ice crystals before melting and finally evaporating; this is usually due to compressional heating, because the air pressure increases closer to the ground. It is very common in the desert and in temperate climates.

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Virga Virga can cause varying weather effects, because as rain is changed from liquid to vapor form, it removes heat from the air due to the high heat of vaporization of water. In some instances, these pockets of colder air can descend rapidly, creating a dry microburst which can be extremely hazardous to aviation. Conversely, precipitation evaporating at high altitude can compressionally heat as it falls, and result in a gusty downburst which may substantially and rapidly warm the surface temperature. This fairly rare phenomenon, a heat burst, also tends to be of exceedingly dry air. Virga also has a role in seeding storm cells whereby small particles from one cloud are blown into neighboring supersaturated air and act as nucleation particles for the next thunderhead cloud to begin forming Virga can produce dramatic and beautiful scenes, especially during a red sunset. The red light can be caught by the streamers of falling precipitation, and winds may push the bottom ends of the virga so it falls at an angle, making the clouds appear to have commas attached.

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Virga The word virga is derived from Latin, twig or branch. A backronym sometimes found in amateur discussions of meteorology is "Variable Intensity Rain Gradient Aloft." Extraterrestrial occurrences Sulfuric acid rain in the atmosphere of Venus evaporates before reaching the ground due to the high heat near the surface. Similarly, virga happens on gas giant planets such as Jupiter. In September 2008 NASA's Phoenix lander discovered a snow variety of virga falling from Martian clouds.

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Virga

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Processes of cloud element enlargement and precipitation formation 1- w.v. supersaturation with respect to cloud droplet surface 2- w.v. condensation process 3- if ice crystal appears  distillation 4- coalescence /coagulation begins as droplet r= 20…60 micrometers result of different downward W)  gravitational coagulation; eddy and Brownian movements  … coagulation;

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Coagulation  rain drop, ice crystal grow up to few mm, snowflakes, hailstones –up to few cm Rate of droplet enlargement due to coagulation ~ r Rate of droplet enlargement due to condensation ~ 1/r

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Precipitation from stratiform clouds. Scheme of formation

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The rate of fall for solid and liquid particles in the atmosphere Atm. Particle is affected by: external forces (gravity, electrostatic, buoyancy…) air resistance force force of interaction between particles According to the 2 nd Newton’s law

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According to Stocks law for ball like particles According to Stocks law for ball like particles (26.2) is valid for low Reynolds number Re*

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dimensionless number, gives a measure of the ratio of inertial forces to viscous forces and consequently quantifies the relative importance of these two types of forces for given flow conditions. dimensionless number, gives a measure of the ratio of inertial forces to viscous forces and consequently quantifies the relative importance of these two types of forces for given flow conditions. laminar flow occurs at low Reynolds numbers, where viscous forces are dominant, and is characterized by smooth, constant fluid motion; while turbulent flow occurs at high Reynolds numbers and is dominated by inertial forces, which tend to produce chaotic eddies, vortices and other flow instabilities.

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Ball-like particle fall under influence of gravity, small Re Ball-like particle fall under influence of gravity, small Re External force= gravity + buoyancy forces Acting downward acting upward air density

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Owing to the fact that F and G are directed along vertical line, Owing to the fact that F and G are directed along vertical line, Using (26.2) and (26.5) :

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Steady state motion  dV/dt=0 Steady state motion  dV/dt=0 Vs is a speed of steady state motion So, Vs ~ r2 at small Re (or small r)

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Unsteady motion  dV/dt ≠ 0 let’s use an eq-on of the particle motion Since V=dx/dt, after integrating (26.10)

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At t=τ V ↓ in e times as compared with V0  physical meaning of relaxation time At t=τ V ↓ in e times as compared with V0  physical meaning of relaxation time Assuming All above is true for ball-like particle