DOCSLIB.ORG

Clouds and Precipitation

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Clouds and Precipitation 28: Clouds and Precipitation GEORGE A ISAAC1 AND JOHN HALLETT2 1Cloud Physics and Severe Weather Research Division, Meteorological Service of Canada, Toronto, ON, Canada 2Division of Atmospheric Sciences, Desert Research Institute, Reno, NV, US Clouds and the precipitation that comes from them are important elements of the hydrological cycle. Clouds provide a blanket for our Earth, both shielding it from radiation from the Sun and trapping heat escaping from the surface. They also generate precipitation through the condensation-coalescence mechanism, which involves only liquid cloud drops, or through ice initiation leading to large ice particles and eventually snow or rain. Mean annual precipitation amounts reach a maximum near the equator, near 8 mm day−1 , and decrease poleward to about 1 mm day−1 . A good understanding of both cloud and precipitation processes is very important for climate and weather predictions. This paper outlines some of the most important processes and provides reference material where more detailed information can be obtained. INTRODUCTION the earth in the tropical regions is much greater than that at the poles, which also accounts for a greater amount of Earth is covered with clouds as any satellite photo of our precipitation in that region. At the poles, the temperatures planet will show. Figure 1 shows a cloud climatology from are low and the stratiform clouds that exist barely produce the GEWEX Surface Radiation Budget data set (Whitlock precipitation, less than 1 mm per day. et al., 1995). Almost everywhere, the annual mean cloud There are many uncertainties in our knowledge about cover is greater than 50% and in some large areas it is clouds and precipitation. However, much progress has been greater than 80%. These clouds literally provide a blanket made and this article will briefly summarize our current for our Earth, both shielding it from radiation from the Sun knowledge, and point to more comprehensive articles where and trapping heat escaping from the surface. Understanding additional information can be found. Earlier textbooks on clouds is extremely important for both climate and weather cloud physics which provide useful information include predictions because their presence or absence can strongly those written by Fletcher (1962) and Mason (1971). More affect surface temperatures. Figure 2 shows that for a recently, a general textbook on cloud physics has been station in northern Canada, the presence of cloud in the written by Rogers and Yau (1989) and a more detailed wintertime makes the surface warmer, while it provides a book on the microphysics of clouds and precipitation was cooling effect in the summertime. published by Pruppacher and Klett (1997). Life cannot exist without water and in most cases we get our water from precipitation. Figure 3 shows the annual mean precipitation rate as compiled by Xie and Arkin CLOUD FORMATION AND TYPES (1996, 1997) using gauge observations, satellite estimates and numerical model outputs. Precipitation amounts reach a Clouds can exist in many forms in the atmosphere. See maximum near the equator, near 8 mm day−1, and decrease the World Meteorological Organization International Cloud poleward to about 1 mm day−1. The higher temperatures in Atlas (WMO, 1975, 1987), the AMS Glossary (Glickman, the tropical regions produce strong convection and more 2000) and Scorer (1972) for a full description of cloud precipitation, with cloud base temperatures being greater types. Cirrus clouds form at temperatures below −40 ◦C, than 20 ◦C. It should also be mentioned that the area of generally occur above 5 km, and they cover wide areas in a Encyclopedia of Hydrological Sciences. Edited by M G Anderson. 2005 Canadian Crown Copyright 2 METEOROLOGY AND CLIMATOLOGY Annual mean cloud cover 60N 30N EQ 30S 60S 0 60E 120E 180 120W 60W 0 0.2 0.4 0.6 0.8 Figure 1 Mean annual cloud cover as seen from satellite as a function of latitude and longitude. From the GEWEX surface radiation budget data set (Whitlock et al., 1995. 1995 American Meteorological Society). A color version of this image is available at http://www.mrw.interscience.wiley.com/ehs Inuvik, January, 1961−1990 Inuvik, July, 1961−1990 −15 25 C) C) Scattered ° ° −20 Broken 20 Overcast All −25 15 −30 Scattered 10 −35 Broken Overcast Mean monthly temperature ( All Mean monthly temperature ( −40 5 N NEE SE SE SW W NWCalm All NNEE SESESWWNWCalm All (a)Surface wind direction (b) Surface wind direction Figure 2 January and July mean monthly temperature at Inuvik, Northwest Territories, Canada, as a function of cloud cover and surface wind direction. ‘‘Scattered’’ indicates 0–1 tenth sky, ‘‘broken’’ 2–8 tenths, and ‘‘overcast’’ 9–10 tenths sky coverage (Isaac and Stuart, 1996. 1996 American Meteorological Society) sheet form, often with a fibrous aspect (Figure 4a). Below vapor trails caused by high-flying jet aircraft (Figure 4b). this temperature, ice crystals form even in the absence of These clouds generally do not create precipitation that insoluble nuclei in cloud drops and a few degrees lower reaches the ground and they are primarily formed of ice in diluted haze droplets. Cirrus can form from broadscale crystals. However, they can start to precipitate (Figure 4c) uplift, in the outflow of thunderstorms, or even from the and the falling ice crystals can “seed” lower layers of CLOUDS AND PRECIPITATION 3 Annual mean precipitation (mm day−1) 60 N 30 N EQ 30 S 60 S 0 60 E 120 E 180 120 W 60 W 0 Figure 3 Annual mean precipitation (mm day−1) as compiled by Xie and Arkin (1996, 1997) using gauge observations, satellite estimates and numerical model outputs (Xie and Arkin, 1996. 1996 American Meteorological Society). A color version of this image is available at http://www.mrw.interscience.wiley.com/ehs cloud and initiate precipitation. Mid-level clouds such as sublimation. The liquid to solid phase change involves altostratus or altocumulus are also formed by broadscale the freezing process, while melting occurs when solid lifting, often produced by frontal systems. They can be water changes to liquid. Phase changes occur with the composed of either ice crystals or liquid water, but they corresponding release or uptake of latent heat. For example, do not account for much of our precipitation. Lower-level in order for water to change from the liquid to vapor state, cloud types such as stratus, and stratocumulus (Figure 4d, latent heat is required to break the hydrogen bonds between e) are associated with precipitation, either snow or rain. In water molecules in the liquid. This is called the latent midlatitudes, especially in winter, these cloud types account heat of vaporization. Similarly, thelatentheatoffusionis for most of our precipitation. Cumulus clouds (Figure 4f) required to change from the solid to liquid state. The latent and especially thunderstorms (Figure 4e) create most of our heat of sublimation is released when vapor changes directly precipitation in the summer at mid- and lower-latitudes. into ice. Normally, phase changes from liquid or solid to Clouds are generally formed in circulation around low vapor occur when the air is subsaturated with respect to pressure cyclonic storm systems, with precipitation tak- liquid water or ice, and the reverse happens when the air is ing place at fronts where higher temperature, moist air if supersaturated. lifted above cooler air. Mesoscale complexes and hurri- The Clausius–Clapeyron equation, one of the most canes form towards the tropics and easterly waves form important in cloud physics, relates the saturation vapor convergence zones for precipitation in low-latitude tropical pressure with respect to water (es)orice(ei) to the latent regions. heat of either vaporization (Lv) and or sublimation (Ls) to temperature (T). For the saturation vapor pressure over water, the equation may be written: WATER PHASES, LATENT HEAT des = Lves Water in the atmosphere exists in three phases: vapor, dT RvT solid, and liquid. Cloud and precipitation formation involve −1 transforming vapor into the other two phases. A vapor where Rv is the gas constant for water vapor (461.5 J kg to liquid phase change occurs through condensation and K−1). the reverse process involves evaporation. A vapor to solid Table 1 shows the saturation vapor pressure over water phase change is called deposition, while the reverse is called and ice, and the latent heats of vaporization (condensation) 4 METEOROLOGY AND CLIMATOLOGY (a) (b) (c) (d) (e) (f) (g) (h) Figure 4 Some examples of cloud types such as: (a) cirrus cloud, (b) cirrus clouds produced by high-flying aircraft (contrails), (c) cirrus clouds with falling ice crystal streaks, (d) stratus clouds below an inversion, (e) stratocumulus deck, (f) cumulus cloud, (g) thunderstorm, and (h) mammatus. For full definitions see American Meteorological Society (AMS) Glossary (Glickman, 2000), World Meteorological Organization Cloud Atlas (WMO, 1975, 1987), and Scorer (1972). A color version of this image is available at http://www.mrw.interscience.wiley.com/ehs ( John Hallett) and sublimation as a function of temperature. The latent water is approximately 1.8, 3.8, 7.8 and 15.0 g of vapor heat of fusion, which is released when ice changes to liquid, per kg of air, respectively (List, 1968). This helps explain is the difference between Lv and Ls. The high values of why there is more precipitation in the Tropics than in the the latent heat of vaporization of ice and water, compared Arctic. with the latent heat of fusion (melting/freezing) show that water molecules are mostly bonded in both liquid and solid, ATMOSPHERIC STABILITY AND CLOUD and the bonding only changes by about 12% on melting. FORMATION It is clear from Table 1 that warm air can hold much more water than cold air. For example, at −10 ◦C, 0 ◦C, +10 ◦C In order to consider how clouds form in the atmosphere, and +20 ◦C and 1000 mb, the saturation mixing ratio over it is necessary to understand atmospheric stability and the CLOUDS AND PRECIPITATION 5 Table 1 The saturation vapor pressure over water and ice, about 1.5 km (a saturated parcel would keep rising through and the latent heats of vaporization (condensation) and buoyancy), showing the potential for convection.
Load more

Recommended publications
  • Ice Ic” Werner F
    Extent and relevance of stacking disorder in “ice Ic” Werner F. Kuhsa,1, Christian Sippela,b, Andrzej Falentya, and Thomas C. Hansenb aGeoZentrumGöttingen Abteilung Kristallographie (GZG Abt. Kristallographie), Universität Göttingen, 37077 Göttingen, Germany; and bInstitut Laue-Langevin, 38000 Grenoble, France Edited by Russell J. Hemley, Carnegie Institution of Washington, Washington, DC, and approved November 15, 2012 (received for review June 16, 2012) “ ” “ ” A solid water phase commonly known as cubic ice or ice Ic is perfectly cubic ice Ic, as manifested in the diffraction pattern, in frequently encountered in various transitions between the solid, terms of stacking faults. Other authors took up the idea and liquid, and gaseous phases of the water substance. It may form, attempted to quantify the stacking disorder (7, 8). The most e.g., by water freezing or vapor deposition in the Earth’s atmo- general approach to stacking disorder so far has been proposed by sphere or in extraterrestrial environments, and plays a central role Hansen et al. (9, 10), who defined hexagonal (H) and cubic in various cryopreservation techniques; its formation is observed stacking (K) and considered interactions beyond next-nearest over a wide temperature range from about 120 K up to the melt- H-orK sequences. We shall discuss which interaction range ing point of ice. There was multiple and compelling evidence in the needs to be considered for a proper description of the various past that this phase is not truly cubic but composed of disordered forms of “ice Ic” encountered. cubic and hexagonal stacking sequences. The complexity of the König identified what he called cubic ice 70 y ago (11) by stacking disorder, however, appears to have been largely over- condensing water vapor to a cold support in the electron mi- looked in most of the literature.
    [Show full text]
  • A Field Guide to Falling Snow
    Basic Snowflake Forms (from SnowCrystals.com) Although no two snowflakes are exactly alike, snow crystal forms usually fall into several broad categories. You can find a more descriptive guide in the book – The Snowflake: Winter’s Secret Beauty. Stellar Dendrites Dendrite means "tree-like", which describes the multi-branched appearance of these snow crystals. Stellar dendrites have six symmetrical main branches and a large number of randomly placed sidebranches. They can also be large, perhaps 5mm in diameter. Although they have complex shapes, each stellar dendrite is a single crystal of ice. The molecular ordering of the water molecules is the same from one side of the crystal to the other. Sectored Plates What identifies these crystals are the numerous ice ridges that seem to divide the plate-like arms into sectors -- hence the name. Like the stellar dendrites, sectored plates are flat, thin slivers of ice that grow into in a stunning diversity of complex shapes. Hollow Columns Plate-like snow crystals get the most attention, but columnar crystals are the main constituents of many snowfalls. The columns are hexagonal, like a wooden pencil, and they often form with conical hollow features in their ends. Needles Columnar crystals can grow so long and thin that they look like ice needles. Sometimes the needles contain thin hollow regions, and sometimes the ends split into additional needle branches. Spatial Dendrites Not all snowflakes form as thin flat plates or slender columns. Spatial dendrites are made from many individual ice crystals jumbled together. Each branch is like one arm of a stellar crystal, but the different branches are oriented randomly.
    [Show full text]
  • ESSENTIALS of METEOROLOGY (7Th Ed.) GLOSSARY
    ESSENTIALS OF METEOROLOGY (7th ed.) GLOSSARY Chapter 1 Aerosols Tiny suspended solid particles (dust, smoke, etc.) or liquid droplets that enter the atmosphere from either natural or human (anthropogenic) sources, such as the burning of fossil fuels. Sulfur-containing fossil fuels, such as coal, produce sulfate aerosols. Air density The ratio of the mass of a substance to the volume occupied by it. Air density is usually expressed as g/cm3 or kg/m3. Also See Density. Air pressure The pressure exerted by the mass of air above a given point, usually expressed in millibars (mb), inches of (atmospheric mercury (Hg) or in hectopascals (hPa). pressure) Atmosphere The envelope of gases that surround a planet and are held to it by the planet's gravitational attraction. The earth's atmosphere is mainly nitrogen and oxygen. Carbon dioxide (CO2) A colorless, odorless gas whose concentration is about 0.039 percent (390 ppm) in a volume of air near sea level. It is a selective absorber of infrared radiation and, consequently, it is important in the earth's atmospheric greenhouse effect. Solid CO2 is called dry ice. Climate The accumulation of daily and seasonal weather events over a long period of time. Front The transition zone between two distinct air masses. Hurricane A tropical cyclone having winds in excess of 64 knots (74 mi/hr). Ionosphere An electrified region of the upper atmosphere where fairly large concentrations of ions and free electrons exist. Lapse rate The rate at which an atmospheric variable (usually temperature) decreases with height. (See Environmental lapse rate.) Mesosphere The atmospheric layer between the stratosphere and the thermosphere.
    [Show full text]
  • Properties of Diamond Dust Type Ice Crystals Observed in Summer Season at Amundsen-Scott South Pole Station, Antarctica
    180 JournaloftheMeteorological SocietyofJapanVol 57,No.2 Properties of Diamond Dust Type Ice Crystals Observed in Summer Season at Amundsen-Scott South Pole Station, Antarctica By Katsuhiro Kikuchi Department of Geophysics, Hokkaido University, Sapporo and Austin W. Hogan Atmospheric Sciences Research Center, State University of New York at Albany, Albany, New York (Manuscript received 17 November 1977, in revised form 20 May 1978) Abstract The properties of diamond dust type ice crystals were studied from replicas obtained during the 1975 austral summer at South Pole Station, Antarctica. The time variation of the number concentration and shapes of crystals, and the length of the c-axis, the axial ratio (c/a) and the growth mode of columnar type crystal were examined at an air tempera- ture of -35*. Columnar type crystals prevailed, but occasionally more than half the number of ice crystals were plate types, including hexagonal, scalene hexagonal, pentagonal, rhombic, trapezoidal and triangular plates. A time variation of two hour periodicity was found in the number concentration of columnar and plate type crystals. When the number con- centration of columnar type crystals decreased, the length of the c-axis of columnar type crystals also decreased. When the number concentration of columnar type crystals increased, the length of the c-axis of the crystals also increased. There was sufficient water vapor to grow these ice crystals in a supersaturation layer several tens to several hundred meters above the surface. The growth mode of columnar type crystals was different from that of warm and cold region columns reported by Ono (1969). The mass growth rate was 6.0* 10-10 gr*sec-1, and was similar to that obtained in cold room experiments by Mason (1953), but less than that found in field experiments by Isono, et al.
    [Show full text]
  • Exercise 10: Cumulus Cloud with Bulk Cloud Physics
    Exercise Hints Tasks Results Exercise 10: Cumulus Cloud With Bulk Cloud Physics PALM group Institute of Meteorology and Climatology, Leibniz Universität Hannover last update: 21st September 2015 PALM group PALM Seminar 1 / 16 I Initialize the simulation with a marine, cumulus-topped, trade-wind region boundary layer. I Trigger the cloud by a bubble of rising warm air. I Parameterize condensation using a simple bulk cloud physics scheme. I Learn how to carry out conditional averages. Exercise Hints Tasks Results Exercise Exercise 10: Cumulus Cloud With Bulk Cloud Physics Simulate a cumulus cloud: PALM group PALM Seminar 2 / 16 I Trigger the cloud by a bubble of rising warm air. I Parameterize condensation using a simple bulk cloud physics scheme. I Learn how to carry out conditional averages. Exercise Hints Tasks Results Exercise Exercise 10: Cumulus Cloud With Bulk Cloud Physics Simulate a cumulus cloud: I Initialize the simulation with a marine, cumulus-topped, trade-wind region boundary layer. PALM group PALM Seminar 2 / 16 I Parameterize condensation using a simple bulk cloud physics scheme. I Learn how to carry out conditional averages. Exercise Hints Tasks Results Exercise Exercise 10: Cumulus Cloud With Bulk Cloud Physics Simulate a cumulus cloud: I Initialize the simulation with a marine, cumulus-topped, trade-wind region boundary layer. I Trigger the cloud by a bubble of rising warm air. PALM group PALM Seminar 2 / 16 I Learn how to carry out conditional averages. Exercise Hints Tasks Results Exercise Exercise 10: Cumulus Cloud With Bulk Cloud Physics Simulate a cumulus cloud: I Initialize the simulation with a marine, cumulus-topped, trade-wind region boundary layer.
    [Show full text]
  • Exam 2: Cloud Physics April 16, 2008 Physical Meteorology 3440
    Name ____________________________ Exam 2: Cloud Physics April 16, 2008 Physical Meteorology 3440 Questions 1-10 are worth 5 points each. Questions 11-15 are worth 10 points each. 1. Rank the concentrations of the following from lowest (1) to highest (3): cloud condensation nuclei (3) cloud droplets (2) raindrops (1) 2. Match the following particles with their typical size cloud condensation nuclei 10 μm cloud droplets 0.1 μm raindrops 1000 μm 3. Why do ice crystals grow at the expense of supercooled water droplets? The saturation vapor pressure over liquid is higher than the saturation vapor pressure over ice. Therefore, the environment will be more supersaturated with respect to ice than with respect to liquid, and the ice crystals will grow more quickly. At some point, the ice crystals may bring S (with respect to ice) down to 1, in which case Sl (with respect to liquid) is less than 1, causing the liquid drops to evaporate. 4. Match the following particles with their most likely means of growth 5 μm cloud drop accretion 5 μm ice crystal aggregation 100 μm dendritic ice crystals in ice cloud depositional growth 500 μm ice crystal in mixed-phase cloud condensational growth 100 μm cloud drop collision-coalescence 5. Fill in the blanks: Not all clouds with temperatures below the freezing point of water contain ice, because of the scarcity of ice nuclei in the atmosphere, and the temperature at which they nucleate ice. As the cloud temperature decreases, the probability of ice increases to the 1 temperature of -40 °C, at which point homogeneous freezing occurs.
    [Show full text]
  • Lakes in Winter
    NORTH AMERICAN LAKE NONPROFIT ORG. MANAGEMENT SOCIETY US POSTAGE 1315 E. Tenth Street PAID Bloomington, IN 47405-1701 Bloomington, IN Permit No. 171 Lakes in Winter in Lakes L L INE Volume 34, No. 4 • Winter 2014 Winter • 4 No. 34, Volume AKE A publication of the North American Lake Management Society Society Management Lake American North the of publication A AKE INE Contents L L Published quarterly by the North American Lake Management Society (NALMS) as a medium for exchange and communication among all those Volume 34, No. 4 / Winter 2014 interested in lake management. Points of view expressed and products advertised herein do not necessarily reflect the views or policies of NALMS or its Affiliates. Mention of trade names and commercial products shall not constitute 4 From the Editor an endorsement of their use. All rights reserved. Standard postage is paid at Bloomington, IN and From the President additional mailing offices. 5 NALMS Officers 6 NALMS 2014 Symposium Highlights President 11 2014 NALMS Awards Reed Green Immediate Past-President 15 2014 NALMS Photo Contest Winners Terry McNabb President-Elect 16 2014 NALMS Election Results Julie Chambers Secretary Sara Peel Lakes in Winter Treasurer Michael Perry 18 Lake Ice: Winter, Beauty, Value, Changes, and a Threatened NALMS Regional Directors Future Region 1 Wendy Gendron 28 Fish in Winter – Changes in Latitudes, Changes in Attitudes Region 2 Chris Mikolajczyk Region 3 Imad Hannoun Region 4 Jason Yarbrough 32 A Winter’s Tale: Aquatic Plants Under Ice Region 5 Melissa Clark Region 6 Julie Chambers 38 A Winter Wonderland . of Algae Region 7 George Antoniou Region 8 Craig Wolf 44 Water Monitoring Region 9 Todd Tietjen Region 10 Frank Wilhelm 48 Winter Time Fishery at Lake Pyhäjärvi Region 11 Anna DeSellas Region 12 Ron Zurawell At-Large Nicki Bellezza Student At-Large Ted Harris 51 Literature Search LakeLine Staff Editor: William W.
    [Show full text]
  • Overview of Icing Research at NASA Glenn
    Overview of Icing Research at NASA Glenn Eric Kreeger NASA Glenn Research Center Icing Branch 25 February, 2013 Glenn Research Center at Lewis Field Outline • The Icing Problem • Types of Ice • Icing Effects on Aircraft Performance • Icing Research Facilities • Icing Codes Glenn Research Center at Lewis Field 2 Aircraft Icing Ground Icing Ice build-up results in significant changes to the aerodynamics of the vehicle This degrades the performance and controllability of the aircraft In-Flight Icing Glenn Research Center at Lewis Field 3 3 Aircraft Icing During an in-flight encounter with icing conditions, ice can build up on all unprotected surfaces. Glenn Research Center at Lewis Field 4 Recent Commercial Aircraft Accidents • ATR-72: Roselawn, IN; October 1994 – 68 fatalities, hull loss – NTSB findings: probable cause of accident was aileron hinge moment reversal due to an ice ridge that formed aft of the protected areas • EMB-120: Monroe, MI; January 1997 – 29 fatalities, hull loss – NTSB findings: probable cause of accident was loss-of-control due to ice contaminated wing stall • EMB-120: West Palm Beach, FL; March 2001 – 0 fatalities, no hull loss, significant damage to wing control surfaces – NTSB findings: probable cause was loss-of-control due to increased stall speeds while operating in icing conditions (8K feet altitude loss prior to recovery) • Bombardier DHC-8-400: Clarence Center, NY; February 2009 – 50 fatalities, hull loss – NTSB findings: probable cause was captain’s inappropriate response to icing condition 5 Glenn Research
    [Show full text]
  • Twelve Lectures on Cloud Physics
    Twelve Lectures on Cloud Physics Bjorn Stevens Winter Semester 2010-2011 Contents 1 Lecture 1: Clouds–An Overview3 1.1 Organization...........................................3 1.2 What is a cloud?.........................................3 1.3 Why are we interested in clouds?................................4 1.4 Cloud classification schemes..................................5 2 Lecture 2: Thermodynamic Basics6 2.1 Thermodynamics: A brief review................................6 2.2 Variables............................................8 2.3 Intensive, Extensive, and specific variables...........................8 2.3.1 Thermodynamic Coordinates..............................8 2.3.2 Composite Systems...................................8 2.3.3 The many variables of atmospheric thermodynamics.................8 2.4 Processes............................................ 10 2.5 Saturation............................................ 10 3 Lecture 3: Droplet Activation 11 3.1 Supersaturation over curved surfaces.............................. 12 3.2 Solute effects.......................................... 14 3.3 The Kohler¨ equation and its properties............................. 15 4 Lecture 4: Further Properties of an Isolated Drop 16 4.1 Diffusional growth....................................... 16 4.1.1 Temperature corrections................................ 18 4.1.2 Drop size effects on droplet growth.......................... 20 4.2 Terminal fall speeds of drops and droplets........................... 20 5 Lecture 5: Populations of Particles 22 5.1
    [Show full text]
  • Lightning Dr
    ESCI 340 - Cloud Physics and Precipitation Processes Lesson 12 - Lightning Dr. DeCaria References: The Lightning Discharge, Uman The Electrical Nature of Storms, MacGorman and Rust Fundamentals of Lightning, Rakov `Runaway Breakdown and the Mysteries of Lightning', Gurevich and Zybin, Physics Today, 2005 Mechanisms of Charge Separation • The top of a thunderstorm (cumulonimbus) cloud becomes positively charged, while the middle-to-lower portions of the cloud becomes negatively charged. • Often there is also a smaller pocket of positive charge near the bottom of the cloud. • The reason for this charge separation is not completely understood, but some of the more prominent theories are described below. • Those mechanisms requiring a preexisting electric field are called inductive charging mechanisms, while those not requiring a preexisting electric field are called nonin- ductive charging mechanisms. Inductive ion capture: In a preexisting electrical field, there will be a separation of charge across a hydrometeor. As the hydrometeor falls, gaseous ions will either be attracted or repelled from the underside of the hydrometeor, depending on their sign. Thus, the hydrometeor will gain an increasing charge of whatever sign it has on its topside. In Fig. 1 the positive ions are repelled as the hydrometeor falls, but the negative ions are attracted. Thus, the hydrometeor gains a net negative charge as it falls. Figure 1: Inductive ion capture. 1 Ion capture may play a role in weakly electrified storms, but cannot be used to explain the amount of charge separation seen in a thunderstorm without the presence of other mechanisms. Inductive particle rebound: Two hydrometeors in a preexisting electric field, falling at different speeds, will exchange charge during collision as shown in Fig.
    [Show full text]
  • Hexagonal Ice (Ice Ih)
    Hexagonal Ice (Ice Ih) Some physical properties Ice nucleation and growth Is ice slippery? Hexagonal ice (ice Ih) [1969]. is the form of all natural snow and ice on Earth (see Phase Diagram), as evidenced in the six-fold symmetry in ice crystals grown from water vapor (that is, snow flakes). Hexagonal ice (Space group P63/mmc, 194; symmetry D6h, Laue class symmetry 6/mmm; analogous to β-tridymite silica or lonsdaleite) possesses a fairly open low-density structure, where the packing efficiency is low (~1/3) compared with simple cubic (~1/2) or face centered cubic (~3/4) structuresa (and in contrast to face centered cubic close packed solid hydrogen sulfide). The crystals may be thought of as consisting of sheets lying on top of each other. The basic structure consists of a hexameric box where planes consist of chair-form hexamers (the two horizontal planes, opposite) or boat-form hexamers (the three vertical planes, opposite). In this diagram the hydrogen bonding is shown ordered whereas in reality it is random,b as protons can move between (ice) water molecules at temperatures above about 130 K [1504]. The water molecules have a staggered arrangement of hydrogen bonding with respect to three of their neighbors, in the plane of the chair-form hexamers. The fourth neighbor (shown as vertical links opposite) has an eclipsed arrangement of hydrogen bonding. There is a small deviation from ideal hexagonal symmetry as the unit cellc is 0.3 % shorter in the c- direction (in the direction of the eclipsed hydrogen bonding, shown as vertical links in the figures).
    [Show full text]
  • Cloud Microphysics
    Cloud microphysics Claudia Emde Meteorological Institute, LMU, Munich, Germany WS 2011/2012 Overview of cloud physics lecture Atmospheric thermodynamics gas laws, hydrostatic equation 1st law of thermodynamics moisture parameters adiabatic / pseudoadiabatic processes stability criteria / cloud formation Microphysics of warm clouds nucleation of water vapor by condensation growth of cloud droplets in warm clouds (condensation, fall speed of droplets, collection, coalescence) formation of rain, stochastical coalescence Microphysics of cold clouds homogeneous nucleation heterogeneous nucleation contact nucleation crystal growth (from water phase, riming, aggregation) formation of precipitation Observation of cloud microphysical properties Parameterization of clouds in climate and NWP models Cloud microphysics November 24, 2011 2 / 35 Growth rate and size distribution growing droplets consume water vapor faster than it is made available by cooling and supersaturation decreases haze droplets evaporate, activated droplets continue to grow by condensation growth rate of water droplet dr 1 = G S dt l r smaller droplets grow faster than larger droplets sizes of droplets in cloud become increasingly uniform, approach monodisperse distribution Figure from Wallace and Hobbs Cloud microphysics November 24, 2011 3 / 35 Size distribution evolution q 2 r = r0 + 2Gl St 0.8 1.4 t=0 t=0 0.7 t=10 t=10 1.2 t=30 t=30 0.6 t=50 t=50 1.0 0.5 0.8 0.4 0.6 0.3 n [normalized] n [normalized] 0.4 0.2 0.2 0.1 0.0 0.0 0 2 4 6 8 10 12 14 16 0 2 4 6 8 10 12 14 16 radius [arbitrary units] radius [arbitrary units] Cloud microphysics November 24, 2011 4 / 35 Growth by collection growth by condensation alone does not explain formation of larger drops other mechanism: growth by collection Cloud microphysics November 24, 2011 5 / 35 Terminal fall speed r 40µ .
    [Show full text]