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Download lecture 11 precipitation (rain, snow, hail, etc.)
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Precipitation and Soundings Precipitation comes in many different types including snow, rain, drizzle, ice pellets, freezing rain and hail. In this Presentation we begin by looking at the forms and shapes of the particles that make the precipitation. We then look at the different types of precipitation on the ground. Finally, because the form of the particles depends on the temperature and humidity conditions that they encountered in the air above, we learn about soundings (the vertical profiles of temperature, humidity and wind in the atmosphere). And since soundings are taken during clear weather as well as storms, we learn to relate each sounding to the type of weather it indicates. A weather balloon about to lift a radiosonde (not the meteorologist) through the atmosphere to measure T, p, RH, and wind speed and direction. Snow and Ice Crystals Note: Spikes on each branch are parallel to the next branch. Crystal Shapes Snow crystals are built on a hexagonal plan but assume many different shapes including flat plates, long columns or needles, clusters, bullets with pyramidal endings, capped columns, and the classic 6-sided stars (i. e., dendrites). The next slide shows that crystal shape tells the temperature and degree of supersaturation at which the crystals form. Stars form only at high humidity at T -15C. At T 0C, snow crystals get wet. They then stick together when they collide and can grow into giant clusters or colonies. A blizzard (heavy snowstorm with cold temperatures and strong winds) with huge snowflakes, Upper Saddle River, NJ In the Good Old Days When Winter Was Really Winter Great Lakes Effect Snowstorm, New York State, January 2007 Raindrops Small raindrops and cloud droplets are spheres, not tears my dears. (Surface tension pulls them into the most compact shape). Large raindrops flatten on bottom (as they fall) and resemble hamburger buns. Tear-shape drops only occur when the drops slide down surfaces that can get wet (like windows or skin on our faces). Most raindrops (except those that form in clouds over tropical oceans) begin their lives as snow crystals or snowflakes several km above sea level and melt when they fall into warmer air near the ground. Some winter storms produce truly unBEARable weather. In addition to snow, there can be freezing rain, which is rain that freezes on contact with the ground. This is also called black ice, the most dangerous, paralyzing form of precipitation from winter storms. Freezing rain creates a world without friction. When T < 0C at ground level, how can we tell whether precipitation will come in the form of snow, freezing rain, or ice pellets? The answer is that we must look at the SOUNDINGS, vertical profiles of T, Td, and winds. http://severInewx.atmos.uiuc. edu/06/online.6.1.html Ice Pellets Ice pellets (also called sleet) are refrozen snowflakes. They occur when a layer of air aloft is warm enough (> 0C) to melt snowflakes into raindrops but the air near the ground is cold enough to freeze the drops. (The refrozen crystals never regain their virginal form.) Ice pellets occur in winter storms and sting when they hit your face. Ice pellets are not hailstones, which are much larger accretions of frozen raindrops and cloud droplets. Hailstones only form in severe thunderstorms because they need large updrafts to suspend them long enough to grow large. FORMATION OF PRECIPITATION To produce precipitation, cloud droplets and crystals must form and then grow large enough to fall to the ground. There are three stages in the growth process. 1. Nucleation. Vapor molecules collide and stick to aerosols to form an embryo droplet or crystal. This begins to happen when RH < 100%. For example, salt particles get wet and expand (deliquesce) when RH > 75%. 2. Diffusion. When air is slightly supersaturated with water vapor (RH > 100%), vapor molecules drift or diffuse toward the droplet or crystal and stick. When both supercooled liquid droplets and ice crystals are present the crystals grow at the expense of the droplets, which shrink and often evaporate. 3. Collision and Collection. (Coalescence for drops and Accretion for snow and hail.) Large particles fall faster than small particles, so they collide and the large particles collect or engulf the smaller particles. At this stage, raindrops grow like planets. COALESCE Snowflake Growth by Accretion (Riming) As snowflakes fall through a cloud full of tiny supercooled droplets, they collect the droplets in their path, which then freeze on them. In this accretion or riming process they grow and fall to the ground. If air near the ground is warm they will melt to rain drops. During the accretion process the original pristine crystal form is covered and destroyed. This specimen resembles an animal that has been engulfed by parasites. Planets form and grow by the same accretion process (and may melt for a time). Hailstones Coffeyville, NB Record Hailstone Hailstones only form in thunderstorms. Large updrafts loft the stones, allowing time to grow by accretion as raindrops and cloud droplets collide with them and freeze. Hailstones often have alternating rings of cloudy and clear ice showing that they have moved up and down within the storm. Cloudy ice has trapped air bubbles due to rapid freezing in cold air. Clear ice forms in air just below 0C, so it freezes slowly enough for bubbles to escape. The largest recorded hailstone in the United States by diameter (8 inches) and weight (1.93 pounds) fell in Vivian, South Dakota on July 23, 2010. PLANETARY ACCRETION Clouds (Nebula) of gases and particles surround newly forming stars. The particles collide and often stick (if they have velcro-like facets like snowflakes). Eventually they grow large enough so that gravity holds them together. When this happens they gather particles more efficiently and even capture gases. In this way all the planets and moons have formed. Accretion Although some material may escape, each meteor adds its mass to the planet or moon it crashes into. Meteor Crater, Arizona The meteorite, about 1/40th the width of the crater, slammed into Earth about 50,000 years ago. Although it disintegrated and produced what appears to be a great hole, it added its mass to that of the Earth. The Cratered Moon has no atmosphere, wind, or rain to wash away ancient craters But how did the Moon form? The larger Earth should have gathered most of the meteors. In the early days of the solar system, a planetoid about the size of Mars, crashed into Earth. Debris from the impact circled the Earth and accreted to form the Moon. http://en.wikipedia.org/wiki/Giant_impact_hypothesis ATMOSPHERIC SOUNDINGS AND WEATHER Introduction to Atmospheric Soundings A sounding is a vertical profile of temperature, humidity and wind in the atmosphere. It helps to tell the weather in the air directly above, the vertical extent of clouds, the type of precipitation and the likelihood of thunderstorms or air pollution. How Soundings are Obtained Every 12 hours at several hundred stations around the world, instrumented weather balloons are released into the atmosphere. These balloons measure temperature, pressure, and humidity as they ascend through the atmosphere, and radio the readings to the ground-based weather station. The wind above the ground is determined by tracking the balloon by a radar as it rises. The height of the balloon is determined from the pressure and the temperature mathematically by the hydrostatic equation, which equates changes of pressure to the weight of a column of air. Units for the Weather Variables Temperature, T, for soundings is expressed in Centigrade. Dew Point Temperature, Td, is expressed in Centigrade and is a measure of Humidity. Pressure, p, is expressed in millibars (mb) or hectoPascals. 1 mb = 100 Pa (Pascals) Height, h, is expressed in meters. Web sources: http://weather.uwyo.edu/upperair/sounding.html http://vortex.plymouth.edu/u-make.html The dew point temperature is the temperature at which the air becomes saturated with water vapor. Dew and clouds first appear when the air is cooled to the dew point. Further cooling can produce precipitation. Td is an indirect measure of the amount of water vapor in the air. For example, when Td = 0C, each kilogram of air at a pressure of 1000 millibars (mb) is holding 3.84 grams of water vapor. In rough terms, The amount of water vapor that the air can hold doubles for each 10C increase of T. The amount of water vapor the air actually holds doubles for each 10C increase of Td. Thus, when Td = 10C, 1 kg of air at 1000 mb is holding 7.76 g (nearly double 3.84), while when Td = 20C, 1 kg of air at 1000 mb pressure is holding 14.95 g (nearly 4 times 3.84). Plotting a Sounding Soundings are profiles of both T and Td. They are often plotted on a graph known as an adiabatic chart. For most adiabatic charts, the x-axis of this chart is proportional to T (or Td) while the y axis is related to pressure or height. T and Td are plotted in the same manner as you would plot any graph. For each data point of T make a large dot and then connect the dots with a solid line. For each data point of Td, make a small x and connect x’s with a dashed line. Data for Sounding z(m) 0 1000 2000 4000 6000 T 10 15 10 -9 -23 Td 3 4 10 -9 -40 Illustrative Sounding Clouds and Precipitation Clouds and precipitation occur when the air is saturated, or when T Td. In the illustrative sounding, the cloud layer occurs from about 2000 to 4000 m. Many clouds are too thin or have bases that are too high to produce precipitation at the ground. In order to produce precipitation a cloud base should be within about 3000 m of the ground and should in general be at least 2000 m thick. The thicker the cloud and the lower its base, the more likely it will produce precipitation. Clouds often occur even where the sounding suggests that T is up to about 5C higher than Td! This occurs in layers in which there are breaks in the clouds. As mentioned, this is common with thunderstorms. It also occurs when T is much below 0C because of the difference between saturated mixing ratios of water and ice. Temperature Inversions An inversion is a layer in which T increases with height. In the illustrative sounding, the inversion occurs from 0 m to 1000 m. Inversions are most common just above the ground on clear nights and early mornings. Pronounced inversions also occur in polar lands during winter. Inversions also occur at elevated surfaces of fronts. Inversions are associated with pollution episodes because they act like lids, suppressing vertical motions in the atmosphere. Ironically, weak inversions often occur several hours before severe thunderstorms, where they help intensify the thunderstorms by delaying them until the atmosphere becomes extremely unstable, and by restricting the area over which they form. Instability will be discussed below. Thunderstorms Thunderstorms, which rise to heights of 8 – 17 km are exceptional because they pump warm, humid air to great heights and form in the midst of soundings that are colder and may be dry at most altitudes. Therefore, thunderstorms do not match the sounding of the surrounding atmosphere. Few soundings are actually taken within thunderstorms, because they are so narrow, so short-lived, and because their weather is so severe they shred the balloons. But the thunderstorm sounding can be estimated from the sounding of the surrounding atmosphere. Soundings for Classical Weather Situations Very often it is possible to tell the type of weather to expect from the sounding. Soundings for different types of weather conditions are described and graphed below. 1. Fair weather, with possible fog and pollution. Here the air near the ground is almost saturated (T - Td 0) so that fog is likely, particularly when the air cools at night. The air at 800 m is also cooler (and denser) than the air at 1300 m. This inversion will trap any pollution near the ground if the winds are too light to blow it away. By contrast, the air aloft is very warm and dry, (T - Td > 10) so that no clouds can form in it and no humid but cooler and denser air from the ground can rise into it. Soundings like this are very common along the coast of California, where fog is frequent, and they also resemble the soundings that occurred during some of the worst pollution episodes in London. In these cases, and on most clear nights, the coldest air is right at ground level and the inversion typically extends several hundred meters above ground. 2. Unstable air with a possible thunderstorm. Here air near the ground is humid and much warmer than air above. Temperature decreases rapidly with height (i. e., the sounding line slopes sharply to the left). For these soundings, thunderstorms are possible whenever the sounding temperature decreases more rapidly with height than the cooling rate for rising saturated air (discussed below). This situation is called unstable. 3. Warm front above with snow. Here, a mass of humid, tropical air ascends a dome of cold air at the surface. Clouds and precipitation usually form in the warm air above the frontal surface (i. e. the inversion). The type of precipitation reaching the ground is determined by the temperature structure of the sounding. Because temperatures remain below 0C at all heights in sounding #3, snow is most likely. If temperatures near the ground are well above 0C, rain will occur. You should be able to draw a sounding with a frontal inversion that produces rain. 4. Warm front above with freezing precipitation. This is the same general situation as sounding #3, but now, T < 0C near the ground, while T > 0C in some layer aloft. Snow will form near the top of this sounding, melt or partly melt as it falls through the zone with T > 0C and then, as it falls through the cold air near the ground, either refreeze to ice pellets in mid-air or freeze on contact with the ground or any other object as freezing rain. ___________________________ The red dotted line in panel 2 indicates T inside the thunderstorm or the adiabatic cooling rate of rising, saturated air. It shows that a rising parcel (balloon) of saturated air will be buoyant because it is warmer than the surrounding air. T of surrounding atmosphere T in thunderstorm 2000 Topeka Sounding 12Z 20 Jan 2010 1600 1400 Height (m) From the afternoon of 19 Jan 2010 to the morning of 20 Jan skies aloft were clear over Topeka and winds blew from the East. As the ground radiated heat to space it cooled and fog formed. The sounding for the morning of 20 Jan shows that an inversion has formed in the lowest 150 m and that the the air is saturated (T = Td), while air aloft is dry. 1800 T(oC) 1200 1000 800 T(oC) 600 400 Inversion and Ground Fog 200 0 -15 -10 -5 0 T (oC) 5 10 15 Vandenberg Air Force Base, CA. Sounding 12Z 26 Jul 2009 The cold Pacific Ocean along the coast of California chilled the air just above sea level producing stratus and fog. Air aloft is much warmer. Its extreme dryness (note that T >> Td) indicates it had sunk. If you lived on the hills 1000 m above sea level, you would be hot and dry and look down on a sea of fog. 3000 2500 2000 1500 1000 Subsidence Inversion 500 Fog or Low Stratus 0 -5 0 5 10 15 20 25 30 Cross Sections Soundings provide a one-dimensional view of the vertical structure of the atmosphere above one point on earth. Cross sections provide a two-dimensional view of the atmosphere along a line on earth. Cross sections are "slices through the pie" that reveal the structure of fronts and the associated forms of precipitation. This is done by contouring the cross section for temperature. Cross sections show the sequence of precipitation form. With warm fronts there is a sequence of rain, freezing rain, sleet (ice pellets), and snow as you move from the front to the cold air. However, freezing rain and sleet only occur near sharp fronts with large temperature contrasts (as in the diagram below to the left), because only then is there band where a refreezing zone lies below a melting zone. This does not occur with a weak front (as in the diagram below to the right). Even when freezing rain and ice pellets do occur, they generally form in a narrower zone than either rain or snow. Global Precipitation Precipitation averages just about 1 meter per year over Earth but, like wealth, varies widely from place to place and over time. Rainfall is generally greatest near the Equator where the air rises, small in the subtropics where the air sinks, and tiny in the polar regions where the frigid air has a tiny vapor capacity. The next slide shows how precipitation varies over the year, as the main rain belts follow the Sun north and south with the seasons.
