ࡱ>  UbjbjAA 4#y#yU&8x"#%%%%%%,йQQQ14111#1#11U`yɓS&\0hp1py1yNǦHQQ1p :   SCI-145: Introduction to Meteorology Lecture Note Packet 3 Chapter 9: WEATHER FORECASTING I. Introduction A. Knowing what the weather will be like in the future is vital to many human activities B. In some cases, such as with hurricanes and tornadoes weather forecasts can save lives C. Unfortunately, weather forecasting is not an exact science and there are significant limitations on forecast accuracy D. Fortunately, technology has improved forecast skill considerably in recent years II. Observations A. Weather forecasting entails predicting how the present state of the atmosphere will change over time B. Therefore, the first, and possibly the most important, part of a weather forecast requires accurately (or as accurately as possible) representing (observing) the present state of the atmosphere C. Over 10,000 land-based stations and hundreds of ships and buoys provide surface observations (at least 4x/day) D. Upper-air data is provided by radiosondes, aircraft and satellite III. Organizations A. The World Meteorological Organization (WMO) is responsible for the international exchange of weather data and observations as well as the standardization and certification of observation procedures B. The National Center for Environmental Prediction (NCEP), located near Washington, D.C., is responsible for the massive job of analyzing the data, preparing weather maps and charts, and utilizing massive computing power (super computers) to run forecast models utilized for predicting the weather on a global scale C. NCEP transfers the data, as well as the results of computer model runs, to both public and private agencies such as National Weather Service (NWS) offices that use the information to issue local and regional weather forecasts D. Both NCEP and NWS are part of the larger government agency NOAA, the National Oceanic and Atmospheric Administration E. These weather data and computer model results are, essentially, part of the public domain and can be obtained by anyone with a computer IV. The Public A. Although NWS forecasts are readily available, the public also receives weather forecasts from additional sources, including television and radio stations as well as private companies such as AccuWeather and The Weather Channel B. These organizations hire meteorologists to either interpret and transmit the NWS local forecast or, in most cases, to interpret the data and model results to create a somewhat modified (and hopefully better) forecast C. In the case of The Weather Channel, the local forecast (local on the 8s) is computer generated without human input V. Hazardous Weather A. Separate branches within NOAA are devoted to forecasting hazardous weather such as severe thunderstorms and tornadoes (Storm Prediction Center [SPC] in Norman, OK) and hurricanes (National Hurricane Center [NHC] in Miami, FL) B. When severe or hazardous weather is likely the NWS issues watches and warnings to alert the public C. A watch indicates that atmospheric conditions favor hazardous weather occurring over a particular region during a specified time period, but the actual location and time of occurrence is uncertain D. A warning, on the other hand, indicates that hazardous weather is imminent or actually occurring within the specified forecast area E. Advisories are issued to inform the public of less hazardous conditions caused by wind, dust, fog, frost, snow, sleet or freezing rain VI. Weather Forecasting Tools A. Radar 1. Radar can be utilized for short-term forecasting or for modifying computer model results based upon the real-time visualization of precipitation coverage, intensity and motion 2. As we will see, doppler radar is critical for severe thunderstorm and tornado forecasts due to their ability to identify rotating thunderstorms (which have the potential to spawn tornadoes) and their motion B. Upper-air Data 1. Observational tools such as soundings, a two-dimensional vertical profile of temperature, dew point and winds obtained from radiosondes and radar generated vertical wind profiles (wind profilers) can be useful for short-term forecasts of hazardous weather such as severe thunderstorms, tornadoes, fog, air pollution alerts, frozen (hazardous) precipitation type and to warn pilots of strong head winds and dangerous wind shear C. Satellite 1. Satellites provide information on the location, appearance and motion of clouds and the storms with which they are associated 2. This is particularly important over oceans (70% of earths surface) where there are no land-based cloud observations 3. For this reason, satellite observations are critical in tropical cyclone forecasting a. In fact, before satellite images became available routinely (1979) tropical cyclones were frequently undetected until they approached land 4. There are two primary types of weather satellites: a. Geostationary satellites 1. Orbit earth over the equator at the same rate the earth spins so remain above a fixed spot on earths surface 2. Nine of these are currently in operation, covering the entire surface of the earth 3. Can loop images in sequence to see movement and development of storm systems b. Polar-orbiting satellites 1. Parallel earths meridian lines passing over north and south pole with each revolution 2. Cover areas further west with each pass as the earth rotates to the east underneath them (image the entire surface of earth in a single day) 3. Complement geostationary satellites by: a. Covering polar regions which are not well seen b. Providing higher resolution images since they orbit at a lower altitude 5. Geostationary satellites have two independently operating components, an imager and a sounder a. The sounder utilizes a radiometer to calculate vertical temperature and moisture profiles in the troposphere 1. Although not as accurate as radiosondes it is extremely useful to fill in gaps in the data, particularly when initializing computer model runs b. Aside from providing images, the imager also uses the motion of clouds to calculate wind speed and direction at different levels of the atmosphere c. The imager also has multiple channels to detect different wavelengths of radiation to visualize different aspects of the clouds and atmosphere 1. Visible images detect visible light from the sun reflected by the earth and clouds a. Thick clouds (thunderstorms) reflect more than thin clouds (cirrus or stratus) and appear brighter in visible images b. However, the height of the cloud cannot be determined and this channel cannot be used at night when there is little visible light 2. Infrared images visualize infrared radiation being emitted by clouds and the earth and can thus be used at all times of day a. Since warm objects (low clouds) emit more radiation than cold objects (high clouds) we can ascertain the height of the cloud tops b. The image is reversed so that cold objects (high clouds) are bright and warm objects (low clouds) are dark c. When used in conjunction with visible satellite we can differentiate cloud thickness and height 3. Water vapor images are utilized to visualize the movement of air where there are no clouds a. Water vapor emits at a unique band in the infrared range so when the channel is set to that wavelength we can visualize middle and upper tropospheric circulations and jet streams VII. Weather Forecasting Methods: Computer Models A. Before the advance of computing power within the past several decades, weather analysis and forecasting was performed by drawing maps of the surface and upper-levels by hand from the available data obtained from surface stations and radiosondes and then using knowledge, experience and extrapolation techniques to project the state of the atmosphere into the future B. In the present day, computers can analyze large quantities of data extremely fast C. Twice each day, thousands of observations are transmitted to NCEP and fed into their high speed supercomputer which plots and draws lines on surface and upper-air maps 1. Meteorologists review these maps to correct any errors 2. The final maps are referred to as an analysis D. The computer then forecasts how this analysis of the atmosphere will change over time by using mathematical equations that govern the behavior of the atmosphere 1. This is called numerical weather prediction 2. Meteorologists devise atmospheric models that consist of numerous mathematical equations that describe how atmospheric variables such as temperature, pressure, winds and moisture will change with time 3. Since not all aspects of atmospheric behavior are fully understood, these models are not exact but contain some approximations (parameterizations) 4. The models are programmed into the computer, and surface and upper- air observations of temperature, moisture, winds and air density are fed into the equations 5. Each equation is then solved for a short period of time (e.g. 5 minutes) for a large number of locations, called grid points, each situated a given distance apart, depending on the resolution of the model 6. The results of these computations are fed back into the equation as new data and the equations are solved for the next 5 minutes 7. This procedure is done repeatedly until a desired time into the future is reached (e.g. 3 or 6 hours) 8. The computer then redraws all the maps as a forecast (prognostic) chart 9. Prognostic charts are then drawn by the computer at this interval out to as much as 16 days into the future (384 hours) 10. The computer makes hundreds of billions of calculations during a single computer model run in a matter of a few hours E. Computer Model Guidance 1. These computer models are run twice a day (00Z and 12Z) from observations and then again at 06Z and 18Z with updated satellite and surface readings 2. There are many different types of computer models, some low resolution models which forecast for the entire globe, which tend to be better and forecasting large scale flow and synoptic systems (e.g. jet stream and highs and lows) and higher resolution models which cover smaller regions (e.g. Great Plains), which are better at forecasting mesoscale processes such as thunderstorms 3. The meteorologist compares the prognostic charts from different models and uses these results as guidance for his/her final forecast 4. It is up to the meteorologist to use their experience to understand which models tend to do better with different situations and which forecast scenario looks more realistic 5. The forecaster then puts this information together with their own observations (e.g. radar, satellite, etc.) and other forecasting tools (e.g. statistical models) to make a final forecast F. Where the Forecast Goes Wrong 1. There are three basic sources of errors in computer model forecasts: 1) Inherent model flaws 2) Inadequate/ inaccurate observations and 3) Chaos a. Inherent Model Flaws Approximations (Parameterizations) 1. Each model makes estimations or simplifications of complex atmospheric processes such as convection (thunderstorms), cloud physics, and atmospheric interactions with terrain and water features, that vary in accuracy between different weather scenarios & models b. Inadequate/inaccurate Observations 1. Even though the density of observations is improving (e.g. radar wind profilers and satellite sounders), there are still regions where data is sparse (e.g. oceans and polar regions) 2. In addition, these new methods, although they increase the quantity of observations are not yet as accurate as direct observations 3. Global models, with large spacing between grid points, may inadequately represent the observations in between grid points 4. High-resolution models, with small spacing between grid points, may better represent observations and small scale features (e.g. interactions with terrain features) a. However, these models do not have global coverage, as any decrease in grid spacing requires an enormous increase in computations and model run time b. Therefore, these models can run into problems with errors creeping in from outside the models domain 5. As a result of these differences between models, some are more accurate in certain weather scenarios and with certain weather features and it is up to the meteorologist to take into account these strengths and weaknesses in making a final forecast 6. For example, global models are better at forecasting large-scale features such as the broad areas of precipitation associated with midlatitude cyclones whereas high-resolutions models are better at forecasting smaller scale features such as thunderstorms c. Chaos 1. The atmosphere is a chaotic environment, which means that each atmospheric feature and variable influences other features and variables 2. Therefore, small errors in observations and forecasts tend to become amplified as the computer model projects further into the future 3. Therefore, short-range forecasts are inherently better than long-range forecasts and there is a limit to how far into the future a forecast can be made with any skill VIII. Other Weather Forecasting Methods A. Ensemble Forecasting 1. Because of the atmospheres chaotic nature and the uncertainty of the initial conditions, meteorologists have developed a forecast method to provide a level of confidence to a given forecast 2. This method, called ensemble forecasting, runs the same model with slightly different initial conditions, to provide numerous versions of the same forecast 3. If all of the results for a particular regions are similar this gives a high level of confidence to the forecast and if there are big differences, a low level of confidence B. Statistical Forecasting 1. Statistical models are now used to enhance the accuracy of local forecasts, particularly in regard to high and low temperatures 2. These models determine, statistically, the most likely weather conditions for any particular day by comparing numerous present atmospheric variables to historical data 3. These forecasts, known as model output statistics (MOS), have become quite good in regard to forecasting high and low temperatures and, in some cases (NWS), surface wind speeds C. Probability Forecasts 1. When precipitation is forecast it is usually reported as the Probability of Precipitation (POP) 2. This means, the probability that any given random location within the forecast area will receive measurable precipitation (0.01 in. of liquid equivalent) 3. The wording of these forecasts is: 20% POP Slight chance 30-50% Chance 60-70% Likely 80-100% Rain, snow, etc. IX. Wording for Cloud Cover A. Wording for cloud cover is frequently confusing B. For example, partly sunny, since it has sunny in it sounds like less cloud cover than partly cloudy, which has cloudy in it 1. Actually, the opposite is true C. Meteorologists can subjectively use what ever wording they feel will best communicate the forecast, however, the following is the official wording used by the NWS: 1. Sunny (0 5%); Mostly Sunny (5 25%); Partly Cloudy (25 50%); Partly Sunny (50 69%); Mostly Cloudy (69 87%); Cloudy/Overcast (87 100%) X. Forecasting Skill A. Although many different methods are utilized in an attempt to quantify forecast skill, what constitutes an accurate forecast is subjective and depends on what is being forecast B. Whatever method is used, however, reveals enormous improvement in the past several decades C. Forecasting of large scale processes is much better than small scale (e.g. thunderstorms) and short-range forecasts are better than long-range D. The bottom line, as a general rule, is that local weather forecasts are very accurate out to 24 hours 1. Forecast skill decreases gradually from 1 to 3 days but is still quite good 2. From 3 to 5 days the forecast is still fairly good, particularly in regard to general conditions 3. From 5 to 7 days only forecasting of general conditions has significant skill 4. Beyond 7 days forecast skill drops off rapidly 5. Beyond 10 days there is essentially no skill for a local forecast Chapter 10: THUNDERSTORMS I. Introduction A. A thunderstorm is defined as a storm that contains lightning and thunder B. Thunderstorms can take many forms, from single cumulonimbus clouds, to clusters of lines of thunderstorms that can extend for hundreds of miles II. Thunderstorm Formation A. They are convective storms in which warm, moist air rises in a conditionally unstable environment 1. As long as a rising parcel remains warmer (lighter) than the air surrounding it, there is an upward-directed buoyant force acting on it 2. The warmer the parcel compared to the air surrounding it, the greater the buoyant force and the stronger the convection 3. The potential energy created by this difference in temperature is called Convective Available Potential Energy (CAPE) B. A trigger (forcing/lifting mechanism) is needed to start the air parcel moving upward and may be one or more of the following: 1. Unequal heating at the surface 2. Lifting of air along shallow boundaries of different air density 3. Lifting by terrain 4. Diverging upper-level winds, coupled with converging surface winds 5. Warm air being lifted along a front C. The combination of this lifting and instability create a large upward- directed force that can generate very strong updrafts that can exceed 50 mph D. This kinetic energy is utilized to generate rain, gusty wind, thunder and lightning and occasionally hail and tornadoes III. Severe Thunderstorms A. The majority of storms, despite all this energy do not reach the status of truly damaging storms, defined as severe thunderstorms by the NWS, as having at least one of the following: 1. Hail at least in diameter 2. Wind gusts of at least 50 kts (58 mph) 3. Tornado B. Severe thunderstorms are usually the result of a particular type of storm, called a supercell thunderstorm (rotating) which we will discuss later IV. Ordinary (Air Mass/Single-Cell) Thunderstorms A. Introduction 1. These are the scattered thunderstorms, sometimes called pop-up thunderstorms, that typically form on hot, humid days 2. They are called air mass thunderstorms because they tend to form in warm, humid air masses (e.g. mT) away from significant low pressure or fronts 3. These storms are single cumulonimbus clouds that do not become severe and go through their entire life-cycle in less than an hour B. Formation 1. The lifting mechanism for these storms is usually simple convection (uneven heating of the surface), terrain, or shallow boundaries separating air of differing density, such as when the cooler air from the downdraft of one thunderstorm plows into warmer surface air 2. These storms form in a low vertical wind shear environment a. This means that wind speed and direction change little with height above the surface C. Life Cycle: These storms go through a well-defined life cycle 1. Cumulus (Growth) Stage a. The updraft develops as the warm, humid surface air is lifted and then accelerates upward due to the unstable environment b. The rising air cools and condenses to form a cumulus cloud c. Released latent heat accelerates upward motion further d. The developing cumulus cloud builds upward toward the tropopause but has yet to develop rain or lightning and thunder 2. Mature Stage a. During this stage the cloud becomes a cumulonimbus and forms the characteristic anvil cloud as the updraft spreads horizontally when it hits the stable tropopause b. Rainfall and thunder and lightning develop c. A downdraft develops as the falling rain drags air downward d. This downdraft is cooler than the air around it due to evaporation of raindrops as dry air is pulled in (entrained) from outside the cloud 3. Dissipation Stage a. The storm dies relatively quickly because of the low vertical wind shear environment b. The downdraft becomes superimposed on the updraft and squelches it like water putting out a fire and the storm dies out less than an hour after it began c. However, as the downdraft hits the ground and spreads out horizontally as a gust front the cooler air can lift surrounding warmer air to begin this process anew with another ordinary thunderstorm V. Multicell Thunderstorms A. When storms form in higher vertical wind shear environment, in which wind increases in speed with height, they can become more intense multicell thunderstorms B. The wind shear permits the updraft and downdraft to tilt and separate with the updraft riding up and over the downdraft C. This allows these storms to survive for a long time, enabling them to become more intense while the gust front is free to enhance the updraft and develop adjacent, coexisting cells D. The more intense updraft with these storms may actually shoot past the tropopause into the stable stratosphere, resulting in an overshooting top that extends beyond the anvil E. As the air sinks back into the anvil it can extend beyond the inferior margin as mammatus clouds F. The intense updrafts and downdrafts in these storms can create tremendous turbulence making flying into one of these storms extremely dangerous VI. The Gust Front A. When the cold downdraft of a thunderstorm reaches earths surface it pushes outward, horizontally in all directions B. The leading edge of this cold, outflowing air is called a gust front C. On the ground, the passage of a gust front is similar to a cold front with dropping temperatures and strong, gusty winds D. These winds, called straight-line winds (to distinguish them from the rotating winds of a tornado) can exceed 60 mph and create extensive damage as they bring the momentum of strong winds aloft down to the surface E. A low, dark, ominous looking cloud can develop along the forward edge of a thunderstorm called a shelf cloud F. This cloud forms as the warm, moist air rises over the leading edge of the gust front G. An elongated, horizontally spinning cloud sometimes develops just behind the gust front called a roll cloud H. Outflow Boundaries 1. When a complex of multicell thunderstorms form, their gust fronts may merge to form a single, huge gust front called an outflow boundary 2. These boundaries can often be seen on radar as a thin line of echoes as they kick up dirt, dust and insects 3. Outflow boundaries frequently generate new thunderstorms as the lift surrounding warm, humid conditionally unstable air VII. Downbursts/Microbursts A. Before it develops into a gust front, a strong downdraft can plunge to the ground beneath the thunderstorm generating a strong radial burst of wind as it hits the ground B. This is called a downburst, or if less than 4 km in diameter, a microburst C. Straight-line wind damage can be immense D. These winds have also been responsible for multiple airline crashes in the past E. This occurs as the nose of the plane is tilted upward by the leading edge of the downburst and, as the pilot adjusts by tilting the nose down, the plane dives downward to the ground as it flies through the downburst F. However, doppler radar as well as detection and adjustments by the onboard computer system of passenger jets, however, has reduced the likelihood of these accidents VIII. Squall-Line Thunderstorms A. Multicell thunderstorms may form as a line of thunderstorms, most frequently along a cold front B. As the cold air advances, updrafts are continuously being regenerated by the advancing cold front and gust front, with new cells being regenerated to replace decaying cells 1. In this way, the squall line can maintain itself for hours on end C. A strong downdraft trails behind, associated with the band of heavy rainfall, separated from the updraft 1. This downdraft can become quite intense causing damaging straight- line winds, as it brings strong upper-level winds down to the surface D. As this wind rushes forward it can cause a bowing forward of the radar echoes, called a bow echo 1. This radar signature alerts the meteorologist of possible damaging straight-line winds 2. When this line of wind (sometimes in excess of 90 knots) is maintained for a prolonged period it is called a derecho E. Pre-frontal Squall-Line Thunderstorms 1. A squall-line does not always form along the advancing cold front but may form up to 150 miles in advance of the front 2. These thunderstorms may be severe as they form in the warm, unstable air mass in the warm sector of the cyclone 3. The origin of these squall lines is uncertain but they most likely form in the upward portion of a wave (gravity wave) generated by the front IX. Mesoscale Convective Complexes A. Where conditions are favorable for convection, (e.g. Great Plains and Midwest, Sub-Saharan Africa) multicell thunderstorms may occasionally organize into a large circular convective system called a Mesoscale Convective Complex B. They can be 1000 times the size of an ordinary thunderstorm C. The numerous thunderstorms work together to generate new thunderstorms as well as a region of widespread precipitation D. These systems are critical as they provide a significant portion of the growing season rainfall in agricultural regions X. Lightning and Thunder A. Introduction 1. Lightning is a discharge of electricity which usually occurs in mature thunderstorms 2. However, it can also occur in other types of storms with enough friction generated to separate charge (e.g. gas cloud of an erupting volcano, duststorms, and in snowstorms with strong instability and updraft) 3. The lightning stroke can heat the air to an incredible 54,000F (5 times hotter than the sun), causing the air to expand explosively, initiating a booming shock/sound wave called thunder 4. Lightning strikes may take place within a cloud, between clouds, from a cloud to the surrounding air, or from a cloud to the ground 5. The majority occur within the cloud and only about 20% occur between a cloud and the ground 6. Sound waves travel 1 mile in 5 seconds so, if you start counting from the moment lightning is seen, for each 5 seconds the lightning strike is another mile away 7. Because of the temperature structure of the atmosphere, thunder is refracted up, so that thunder is not usually heard if the lightning strike is more than 5 10 miles away B. What Causes Lightning? 1. For lightning to occur separate regions with opposite electrical charges must exist within a cumulonimbus cloud 2. How this occurs is not well understood 3. One theory is that smaller ice crystals collide with larger ice particles called graupel with the smaller ice crystals becoming positively charged and the larger graupel becoming negatively charged 4. As the updraft lifts the smaller particles, the upper part of the cloud becomes positively charged and the middle and bottom part of the cloud becomes negatively charged C. The Lightning Stroke 1. As the tip of the stepped leader approaches the ground, positive charge begins to flow up from the ground to meet it 2. As they meet, large numbers of electrons flow to the ground 3. The stepped leader is faint and is usually invisible to the human eye 4. However, a channel of low resistance has now been established and a much brighter return stroke (several centimeters in diameter) of current follows the same path back upwards to the cloud base 5. Usually this process is repeated 3 or 4 times along the same ionized channel at intervals of 4/100ths of a second 6. The new leader is called a dart leader and the subsequent return strokes are less luminous 7. This whole process happens so quickly that to the human eye it appears as a single bright flash that flickers 8. This type of lightning is called negative cloud-to-ground lightning because the stroke carries negative charge from the cloud to the ground 9. About 90% of all cloud to ground lightning is negative 10. Positive cloud-to-ground lightning is relatively rare but more dangerous a. This type of lightning occurs when the positively charged anvil discharges to a negatively charged ground beneath it b. Since the stroke must travel a much larger distance it requires a greater charge potential and thus discharges a much larger current c. This type of lightning is more common with severe supercell type thunderstorms d. Since the discharge is from the anvil it can occur several miles before the storm arrives or after it leaves D. Lightning Safety 1. About 10% of people struck by lightning are killed with about 100 fatalities per year in the U.S. 2. Most victims are in open places, where they are the highest object 3. Some are killed while standing under trees 4. Buildings and vehicles are usually safe since the current is conducted through the walls of that structure into the ground 5. It is recommended that people get under cover of a building or vehicle as soon as thunder can be heard (positive lightning) 6. If outside, the best position is crouched low with as little contact with the ground as possible 7. Lightning rods are used to protect structures from lightning damage a. The metal rod extends well above the roof of the building so that it will accumulate positive charge and is most likely to be struck b. The lightning current then follows an attached insulated conducting wire into the ground where the other end of the rod is buried deeply XI. Distribution of Thunderstorms A. 50,000 thunderstorms occur each day globally (18 million/year) B. As we would expect, thunderstorms are most common over equatorial landmasses which have the warmth and moisture at the surface necessary to create instability, with the lifting (forcing) of the ITCZ C. In the U.S., thunderstorms become more prevalent as one gets closer to the warm, moist surface waters of the Gulf and southern Atlantic coasts D. Clockwise flow around the Bermuda High brings this warm, moist, conditionally unstable air inland into the Great Plains and Midwest XII. Supercell Thunderstorms A. These are (by far) the most intense thunderstorms and are responsible for most of the damaging hail and straight line winds and virtually all damaging tornadoes B. They form where all of the components necessary for thunderstorm formation are present and pronounced: warm, moist surface air, conditional instability and mechanisms which lift the air 1. Therefore, these storms have incredibly strong updrafts with vertical velocities up to 100 mph C. In addition, they form in an environment with vertical speed and directional wind shear an increase in wind speed and a change in wind direction with increasing height above the surface 1. This environment causes these storms to tilt vertically, which separates the updraft from the downdraft, enabling them to last a long time and intensify 2. This environment also causes these storms to rotate which enables them to, possibly, produce tornadoes D. These storms are by far the most common in the Great Plains and Midwest of the U.S. where all of these conditions tend to develop in the Spring (March June) E. These storms form in the warmest, moistest, and most unstable area of an extratropical cyclone, the warm sector, which lies between the cold front (or dry line) and the warm front F. Supercell Structure 1. The rotating updraft of these storms, called the mesocyclone contains no precipitation 2. The downdraft, containing precipitation rotates around the precipitation free updraft creating a hook echo pattern typical of supercells on radar 3. The heaviest rain and hail fall just to the northeast of the mesocyclone 4. A more rapidly rotating cloud may descend from the cloud base below the mesocyclone a. This cloud is called the wall cloud b. If a tornado forms it will be below this cloud 5. However, only about 15% of supercells ever produce a tornado XIII. Tornadoes A. Introduction 1. Tornadoes are violently rotating columns of air that extend from a thunderstorm cloud to the ground 2. Exceptionally strong tornadoes can destroy steel-reinforced structures, throw automobiles over 100 feet and sweep trains off their tracks 3. Range in diameter from 150 ft. to mile but, in rare instances can be as large as 2 miles in diameter 4. Windspeeds range from 65 to 300mph 5. Most are short-lived but can remain on ground for as much as one hour 6. 75% of all tornadoes, and the vast majority of significant tornadoes (EF2 or greater) occur in the U.S., particularly in tornado alley, the southern Great Plains of the U.S. (bulls eye in Oklahoma) 7. Approximately 1,000 tornadoes per year in the U.S. kill, on average, 56 people and injure 975 with $855 million in property damage B. Tornado Formation 1. The vast majority of all tornadoes, and virtually all destructive tornadoes (F2 or higher) are associated with supercell thunderstorms 2. The rotating updraft, called the mesocyclone (surface low pressure/counterclockwise circulation) develops its rotation as vertical wind shear creates a horizontal, rolling tube of air parallel to the ground which gets tilted vertically by the updraft a. This mesocyclone is not the tornado, it is about 3 miles wide (tornadoes rarely are wider that 0.5 miles) and wind speeds are much slower than a tornado 3. Tornadoes form when the two downdrafts form an occlusion (like an occluded front) and surround the updraft and choke it, isolating it from its warm, low level air source 4. The updraft decreases at low-levels but continues at robust speeds at mid and high levels 5. This causes the mesocyclone to stretch vertically, becoming much narrower 6. By the law of conservation of angular momentum the central part of the mesocyclone begins to spin much faster, forming the narrower, more rapidly rotating wall cloud and, eventually, the tornado C. Tornado Classification Scale 1. Fujita (F) scale developed in 1971 to categorize tornadoes by severity, based upon the damage incurred by the winds 2. Due to an over-estimation of wind speeds with the initial scale the Enhanced Fujita (EF) scale was designed in 2007 as an attempt to more accurately characterize tornado severity 3. Tornado categorized by the worst damage along its path (only small regions of an EF5 tornado will incur that degree of damage 4. 2.3% of all tornadoes (EF4 and EF5) are responsible for roughly 70% of tornado deaths 5. 56 F5/EF5 tornadoes since 1950, all in the Great Plains and Midwest D. Tornado Occurrence 1. Tornado occurrence peaks in Spring when the polar front is still strong and surface heating is great enough to generate large amounts of instability but differs regionally because of seasonal migration of polar front and jet stream 2. Tornadoes peak in late afternoon to early evening when instability is greatest E. Tornado Detection 1. Hook echo on standard radar is suggestive of a supercell thunderstorm 2. Doppler radar provides more definitive evidence of a supercell thunderstorm since it can detect rotation in a thunderstorm a. Doppler radar can identify rotation in the storm by measuring the frequency shift of the microwaves between outgoing and reflected radiation to determine where wind is blowing the precipitation toward the radar and where it is blowing it away from the radar b. Therefore, doppler radar detects the mesocyclone, the rotating updraft of a supercell, which is identified by its counterclockwise rotation c. However, this only indicates a supercell thunderstorm, not a tornado, which is usually too small for the resolution of the radar d. Must keep in mind that a tornado will only form in about 15% of supercells e. Still, with doppler radar, forecasters can now warn people, on average, 12 minutes before a tornado forms F. Tornado Forecasting 1. Storm Prediction Center (SPC) [  HYPERLINK "http://www.spc.noaa.gov" http://www.spc.noaa.gov ] located in Norman, Oklahoma a. Meteorologists analyze conditions (low-level moisture, instability, lift, vertical wind shear, etc.) for the next 3 days to ascertain where supercell thunderstorms and tornadoes are most likely to form and what the probability is that they will form 2. Tornado Watches and Warnings a. Tornado Watch 1. Issued when: environmental conditions are conducive for tornado formation b. Tornado Warning 1. Issued when: a rotating thunderstorm (supercell) is identified on Doppler radar a. Only 15% chance that tornado will actually occur 2. A tornado has been spotted G. Tornado Safety 1. Seek Shelter a. Take shelter in a well constructed building if possible (not a mobile home) b. Go to basement if possible, otherwise the safest place is usually a small room on the lowest floor near the center of the building 2. Roofs, Walls and Windows a. Pressure in the center of a tornado can be 100 mb lower than its surroundings 1. Can lift roofs off buildings 2. Dont open windows a. Actually increases pressure on walls of structure and increases risk from flying debris b. Windows will most likely shatter anyway 3. Outdoors or On The Road a. Do not try to outrun a tornado 1. Can move up to 80 mph and have erratic paths b. Look for ravine, ditch or streambed and lie flat with head covered c. Do not take cover under a freeway overpass 1. Winds are actually strengthened as they are funneled by the overpass Chapter 11: HURRICANES I. Introduction A. A hurricane is an intense storm of tropical origin with sustained winds exceeding 74 mph B. The same type of storm is called a typhoon in the western North Pacific Ocean, a cyclone in India and a tropical cyclone in Australia C. They are all the same thing, a cyclone (surface low pressure system) that forms in the tropics D. Therefore, by convention, they are all referred to as tropical cyclones E. Tropical cyclones are similar to midlatitude/extratropical cyclones in that they both have surface low pressure with a circulation that is counterclockwise and into the center of the low (in the NH), and they are both associated with stormy weather 1. However, the similarity ends there F. Tropical cyclones form without a jet stream nearby (no temperature contrast in the tropics) and derive their energy from the warm ocean water instead 1. As a result tropical cyclones also have a very different structure II. Structure A. Hurricanes have an eye at their center where the skies tend to be clear, the air is warm and winds are light 1. Strangely, this is where the surface air pressure is lowest, sometimes extremely low B. Surface winds blow counterclockwise and in toward the eye, increasing in strength to a maximum in the ring of intense thunderstorms adjacent to the eye, called the eyewall C. Bands of showers and thunderstorms, called spiral rain bands, alternate with bands of drier, sinking air and this pattern spirals in toward the eyewall D. The enormous amount of latent heat that is released in the eyewall creates a warm core in the center of the storm that results in low pressure at the surface but high pressure aloft 1. This high pressure aloft generates a clockwise outflow from the top of the storm 2. As the outflow cools (radiational cooling) it sinks, warms and dries at the periphery of the storm, which is why it is very clear surrounding a hurricane (calm before the storm) 3. The air also sinks into the eye where it warms and dries, creating the fair weather previously described III. Formation A. There are still many unanswered questions about how hurricanes form B. However, it is know that certain ingredients are required for hurricane formation 1. Warm sea surface temperature of at least 80F a. The warm water provides the latent heat which provides the energy for these storms b. Therefore, the Atlantic hurricane season runs from June through November with a peak in mid-September 2. Hurricanes must be at least 5 latitude north or south of the equator a. There needs to be enough Coriolis force to generate a circulation b. In fact, two-thirds of all tropical cyclones form between 10 and 20 latitude 3. Weak Vertical Wind Shear a. When there is vertical wind shear (wind speed increases with height) this tilts the hurricane 1. In order for the storm to strengthen, latent heat must be focused vertically over a small area b. Wind shear also tends to disrupt the symmetrical circulation C. Hurricanes develop from clusters of thunderstorms within the ITCZ that move away from the equator and develop surface low pressure and a counterclockwise circulation 1. In the Atlantic Ocean basin, these clusters of thunderstorms frequently originate from equatorial waves (vertically oriented atmospheric waves) that form over Africa, called African Easterly Waves (since they move east to west) D. The energy for a hurricane comes from the direct transfer of sensible and latent heat from the warm ocean surface 1. The latent heat released due to the large amount of condensation within the cluster of thunderstorms generates a warm column of air, relative to the surrounding environment, within the core of the thunderstorms 2. As we have noted before, a warm column of air will create high pressure aloft, low pressure at the surface, and a vertical circulation 3. A positive feedback process then develops which enables the storm to develop and strengthen: a. The winds circulating in toward the surface low cause evaporation of moisture from the warm surface waters b. The release of latent heat as this moisture rises and condenses causes the surface low to deepen c. As the low deepens, wind speeds increase which causes an increase in evaporation rates d. This provides more moisture for latent heat release and further deepening of the surface low. IV. Intensification A. The warmer the sea surface temperatures, the greater energy that can be derived from the ocean 1. Therefore, as a general rule, all else being equal, the warmer the ocean surface, the deeper the surface low and the stronger the winds 2. If hurricanes remain over warm water they can maintain hurricane force winds indefinitely 3. However, most hurricanes last for less than a week V. Weakening and Demise A. Three mechanisms disrupt the positive feedback cycle which maintains hurricane strength 1. Cool Sea Surface Temperatures a. This results is a loss of the hurricanes energy source b. Studies show that if the water cools by 4.5F the storm will dissipate c. In addition, if the storm is moving slowly, and the warm water is relatively shallow, turbulence generated by the hurricanes winds will bring cooler water to the surface and weaken the storm 2. Strong Vertical Wind Shear a. As discussed previously 3. Land a. Loss of energy source (warm ocean surface) b. Friction 1. Slows winds 2. Causes increased convergence into the center of the surface low which leads to a filling of the low pressure center VI. Life Cycle A. All hurricanes go through well-defined stages of development 1. Tropical Disturbance a. Any cluster of thunderstorms without a well-defined low pressure center or cyclonic circulation 2. Tropical Depression a. When a disturbance develops a low pressure center and cyclonic circulation and wind speeds are between 23 and 39 mph b. Only 1 in 10 disturbances become depressions 3. Tropical Storm a. When sustained wind speeds increase to between 39 and 74 mph b. This is when the tropical cyclone is given a name 4. Hurricane a. When sustained winds are 74 mph or greater b. About half of all tropical storms become hurricanes VII. Movement A. Tropical cyclones in the North Atlantic and western North Pacific tend to be directed westward by the easterly trade winds initially and then turn northwestward, northward and, eventually, northeastward as they come under the influence of the clockwise flow of the subtropical highs as they reach the western Ocean basins B. As they reach the jet stream in midlatitudes they can accelerate and, occaisionally, transition to powerful midlatitude cyclones (extratropical transition) C. Most hurricanes in the North Atlantic curve away from the U.S. and do not make landfall 1. There are approximately 11 named storms (tropical cyclones), 6 hurricanes, and 2 major hurricanes per year 2. However, only about 25% of hurricanes make landfall and only a small percentage of these are Category 4 or 5 D. Hurricanes in the eastern North Pacific have a relatively small pool of warm water to work with because of the cool California Current to the north 1. They tend to drift slowly westward or northwestward and usually weaken and dissipate as they move over cooler water 2. These hurricanes can occasionally drift northward and make landfall in Mexico (only about 10%) but will not make landfall along the western U.S. coast because of the California Current E. Hurricane formation and paths vary with the position of the ITCZ and the warmest water 1. In August and September, most hurricanes originate off the coast of Africa, developing from African Easterly Waves 2. In fact, 85% of all major hurricanes form in this manner, needing the long fetch of warm ocean water to strengthen 3. In October, as the ITCZ shifts south, most hurricanes form in the Gulf of Mexico and Caribbean Sea and tend to be weaker, as the sea surface cools VIII. Naming Tropical Storms and Hurricanes A. Tropical cyclones are named when they reach tropical storm strength B. The storms are given alternately male and female English, Spanish and French names C. The names are recycled every six years D. If a major hurricane causes great damage, its name will be retired for at least 10 years E. If the number of named storms should exceed the names on the list, as occurred in 2005, when there were 27 named storms, the tropical storms are given names from the Greek alphabet (e.g. Alpha, Beta, Gamma) F. Zeta, the latest named Atlantic storm ever, formed in January 2006 IX. Damage A. Wind 1. The strongest winds in a hurricane are to the right (relative to storm motion) of the eye a. This is because the actual winds are a combination of rotational motion and translational motion b. For example, if the sustained winds in a hurricane are 100 knots (rotational motion air flowing counterclockwise at 100 knots), and the storm is moving at 25 knots (translational motion) the wind will be 125 knots to the right of the eye (100 + 25) and only 75 knots to the left of the eye (100 25) 2. Wind can cause significant damage, however, it is not the direct effects of the wind that cause the most damage but the indirect effect, flooding from the storm surge a. For example, most of the damage from Hurricane Andrew, which made landfall in southern Florida in 1992 as only the third Category 5 hurricane to make landfall in the U.S. with wind gusts close to 200 mph, was direct wind damage B. Storm Surge 1. However, there were only 15 deaths from Andrew, whereas Hurricane Katrina (2005), which was only a Category 3 hurricane at landfall, had close to 1500 deaths due to the storm surge related flooding 2. The storm surge is an abnormal rise in the sea level which inundates low lying areas as the hurricane makes landfall 3. The storm surge is responsible for 90% of all hurricane related deaths 4. This is, essentially, a tidal wave caused by the hurricane winds pushing a wall of water forward in advance of the storm 5. The sea level rise is enhanced by the low pressure center itself (like sucking on a straw) 6. Due to the wind difference around the hurricane, the greatest surge is to the right of the eye 7. The surge is enhanced by narrow inland bays and if landfall occurs near high tide C. Hurricane Intensity 1. Because wind and storm surge damage are both related to wind speed, hurricane intensity is based upon the one-minute sustained wind speed a. This scale, called the Saffir-Simpson scale has five categories: 1. 74 95 mph 2. 96 110 mph 3. 111 130 mph 4. 131 155 mph 5. > 155 mph D. Hurricane Hunters 1. Since most hurricanes are categorized while over the ocean, the surface wind speed is estimated from the flight-level wind speed measured by hurricane hunter aircraft which fly through the storm E. Rainwater Flooding 1. Sometimes the worst flooding from a hurricane is not caused by the storm surge but, instead, by inland rainwater flooding 2. The voluminous amounts of moisture associated with these tropical systems can result in rainfall measured in feet instead of inches 3. This is particularly true in Central America where the mountainous terrain enhances the rainfall and can result in devastating landslides 4. In fact, the most deaths worldwide from all tropical cyclones (tropical storms and hurricanes) are caused by rainwater flooding a. For example, Hurricane Mitch in 1998 weakened to a tropical storm after making landfall but moved slowly and dropped up to 6 feet of rainfall on Honduras and Nicaragua X. Forecasting A. National Hurricane Center (NHC) integrates numerous computer models, including ensemble forecasts, to generate a best track forecast 1. These forecasts have dramatically improved over the past several decades, especially for longer range forecasts 2. The typical error at 72 hours was 440 miles in the 1970s but this has dropped to 173 miles 3. Unfortunately, hurricane intensity forecasting is still fairly poor Chapter 13: CLIMATE CHANGE I. Introduction A. Earths climate has undergone fairly profound changes over the past hundreds of millions of years as the composition of earths atmosphere has evolved due to the decline of volcanic activity, the condensation of water vapor to form oceans and the ascension of plant and animal life B. However, the composition of earths atmosphere and, consequently, surface and atmospheric temperatures have remained relatively stable over the past 500 thousand years C. Despite the relative stability of atmospheric composition, there have been alternating relatively cold and warm periods, with a 100 thousand year cycle 1. These are called glacial and interglacial periods and are caused by variations in Earths orbit which cause differences in the amount of solar radiation reaching the earth 2. We are presently in a warmer interglacial period (thankfully) a. Temperatures have been quite stable during this period, which we have been in for approximately the past 10 thousand years D. However, in the past 100 years the composition of earths atmosphere has abruptly changed due to the advent of the industrial age 1. Concurrently, a rapid warming of the atmosphere has occurred that is unprecedented in recent history II. Reconstructing Past Climates A. Before we analyze this recent warming, and attribute the recent change to human activities, lets take a look at how we learn about past climates and climate change over history B. Glaciers/Ice Sheets 1. Much of the information comes from ice cores taken from glaciers 2. Glaciers, which include the Greenland and Antarctic ice sheets, form when snow falls year round and does not melt 3. The snow piles up, becomes compacted, and slowly turns to ice 4. Ice cores taken from the Greenland and Antarctic Ice Sheets, which are thousands of feet thick, can sample snow that fell hundreds of thousands of years ago 5. By taking samples from different levels, atmospheric composition and climate for the past several hundred thousand years can be analyzed C. Gaseous Composition of the Atmosphere of Past Climates 1. Analysis of the gaseous composition of the atmosphere is obtained by analyzing air bubbles trapped in the ice 2. Complimented by direct sampling in recent years provides us with a fairly direct and accurate assessment that the concentration of certain greenhouse gases (e.g. carbon dioxide, methane) have increased during the past 100 years to unprecedented levels relative to the natural variation over the past 650 thousand years, due primarily to the burning of fossil fuels D. Temperatures of Past Climates 1. Temperature evidence is less direct and therefore there is greater uncertainty in the data 2. Oxygen and hydrogen isotope ratios in the same ice cores change depending on the atmospheric temperatures at the time the snow formed and fell a. Heavier isotopes contain extra neutrons and tend to be less prevalent in snow forming in colder air 3. Calcium carbonate shells from organisms that once lived near the surface can be evaluated from core samples taken from ocean floor sediments a. Oxygen isotope ratios reveal a higher proportion of heavier isotopes when the ocean surface is colder b. Distribution and type of organisms provide information since certain organisms can only live within a narrow range of temperature 4. Tree rings (dendrochronology), the study of fossils, oxygen-isotope ratios in corals, the study of pollen deposition in caves and soil, boreholes taken from earths crust and historical documents (and other methods) provide additional data and less uncertainty in the data over the past 1000 years 5. Direct thermometer data, which is even more accurate, has been available since about 1880 6. From this data it can be determined with a fairly high degree of certainty that temperatures are higher now than they have been at any time within the past 1000 years a. In fact, the Intergovernmental Panel on Climate Change (IPCC) in its latest evaluation (2007) stated: Warming of the climate system is unequivocal III. Intergovernmental Panel on Climate Change (IPCC) A. Scientific body whose reports are widely regarded as the most authoritative statements of scientific knowledge on climate change B. Established in 1988 by the World Meteorological Organization (WMO) and the United Nations Environment Programme (UNEP) to deal with the issue of global warming C. Climate experts from around the world evaluate and synthesize the most recent climate science findings every 5 to 7 years (fourth assessment in 2007) D. Experts from more than 130 countries with 450 lead authors that received input from 800 contributing authors E. An additional 2500 experts reviewed and made changes to the draft documents F. The IPCC bases its assessments on published and peer reviewed scientific technical literature only IV. Causes of Climate Change A. Introduction 1. There are three external causes of climate change a. Changes in incoming solar radiation b. Changes in the composition of the atmosphere c. Changes in earths surface 2. There is also natural internal variability, oscillations in the circulation patterns of the ocean and atmosphere like ENSO 3. The time scale over which these changes occur will be critical in attributing a cause to our recent abrupt increase in global temperatures B. Natural Causes 1. Plate Tectonics a. Falls into the category of changes in earths surface b. Earths outer shell is composed of plates, with embedded continents, that slide over a partially molten layer underneath c. Hundreds of millions of years ago, earths continents were joined into a single continent (Pangaea) centered close to the equator d. Gradually, the plates have shifted and the continents drifted to their current location e. This shift has caused a general cooling of earths atmosphere over time as the continents are now located closer to the poles where they can accumulate ice and snow that reflects sunlight 2. Life on Earth a. The decrease in volcanic activity, the dissolving of carbon dioxide in the oceans and the ascendency of plant life, in the same time frame, has decreased the amount of greenhouse gases (carbon dioxide) to the relatively stable levels of the past 500 thousand years (200 260 ppm) b. This cause falls under the category of change in the composition of earths atmosphere c. Both of these causes (life on earth and plate tectonics) have produced a pronounced cooling of earths atmosphere and over millions of years, a time frame irrelevant to our recent rapid warming 3. Variation in Earths Orbit: Milankovitch Cycles a. Variations in earths orbit, called Milankovitch cycles, produce global temperature changes due to variations in incoming solar radiation b. There are three types of variation: 1. Changes in the shape of the orbit (eccentricity) 2. Precession (wobbling) of earths axis of rotation 3. Changes in the tilt (obliquity) of earths axis c. These three oscillations occur with a different period but, when combined, result in alternate cooling and warming periods that occur with a 100 thousand year cycle that are responsible for the alternating ice ages and interglacial periods apparent in the geologic record d. We are presently in the early stages of an interglacial period that is expected to last for at least another 10 thousand years e. This cause of climate change is well-defined and occurs over a time frame much too long to be responsible for earths recent abrupt warming 4. Variations in Solar Output a. Huge magnetic storms on the sun show up as darker regions on the surface called sunspots b. They occur in cycles with a maximum every 11 years c. During periods of maximum sunspots the sun emits about 0.1 % more energy d. However, the first decade of the 21st century has been the warmest on record with 2010 the warmest year, despite a decrease in sunspot activity to a minimum over the course of the decade e. In addition, actual spacecraft measurement indicates that solar activity has held steady or decreased in the past 50 years and could not account for the rapid increase in temperatures over the past few decades 5. Atmospheric Particles a. The addition of solid particles to the atmosphere results in a net cooling of earths surface due to reflection of sunlight b. Volcanic eruptions produce highly reflective sulfuric acid particles that can reside in the stratosphere for several years c. Studies indicate that global temperatures can cool by approximately 0.5C following a large volcanic eruption 1. This occurred following the eruption of Mt. Pinatubo in 1991 d. The eruption of Mt. Tambora in Indonesia in 1815 is probably responsible for the year without a summer (1816) in which it snowed in every month in New England e. There is evidence that greater than normal volcanic activity may be responsible for what is referred to as the Little Ice Age between about 1400 and 1850 A.D. f. It is unlikely that volcanic activity is related to recent warming since there has been an increase in global volcanic activity in the past 50 years 6. Natural (Internal) Variability a. There are natural oscillations in atmospheric and oceanic circulations b. El Nio/Southern Oscillation (ENSO) is the most notable example although there are others (e.g. AO, NAO, PNA) c. Can lead to changes in global temperatures but on an inter- annual time scale (from year to year, not over an entire century) C. Human (Anthropogenic) Activities 1. Greenhouse Gases a. Without greenhouse gases, namely water vapor and carbon dioxide, earths average surface temperature would be 0F instead of 59F (greenhouse effect) b. Even the most basic studies confirm a strong correlation between the concentration of greenhouse gases and surface and tropospheric temperatures c. Due primarily to the burning of fossil fuels the concentration of several greenhouse gases (e.g. carbon dioxide, methane, nitrous oxide) have increased to unprecedented levels over the past century d. Carbon dioxide, which makes the greatest contribution to the greenhouse effect of all of the anthropogenic greenhouse gases, has increased in concentration from 280 parts per million (ppm) to 400 ppm, an increase of a whopping 40% in just 100 years, a veritable microsecond of geologic time e. The radiative forcing (additional radiation added to the atmosphere) provided by these extra gases is about 3 watts/meter2 (power/area) of which about 60% is contributed by carbon dioxide 2. Aerosols a. These calculations alone suggest that if the greenhouse effect was the only influence on global climate the warming should be somewhat more than the 1.0C (2F) that was observed over the past century b. However, as we have already noted, there are multiple contributions to the complex climate system c. Why has the warming, although significant, been less than expected? d. One reason is the net cooling which has been contributed by the addition of reflective solid particles (e.g. sulfate aerosols) to the atmosphere, also by the burning of fossil fuels 1. This may be the reason that the warming took a pause in the 1950s and 1960s as automobiles and factories exploded but then the warming accelerated after passage of clean air acts in industrialized nations in the 1960s and 1970s e. Another reason the warming has been less than expected is likely the increased volcanic activity (particularly over the past 50 yrs) V. Global Warming: Natural vs. Anthropogenic A. In order to differentiate natural climate forcing from anthropogenic forcing, numerous climate modeling studies have been done using natural forcing alone and comparing the results to models run with both natural and anthropogenic forcing 1. The results indicate that with only natural forcing there would have been a slight cooling of earths temperatures over the past 50 years 2. When the models are run with both natural and anthropogenic forcing the temperatures approximate the actual recorded temperatures over the past century fairly closely B. Because the climate system is complex, there is not absolute proof that the increase in anthropogenic greenhouse gases are the cause of global warming C. However, the evidence is strong enough for the IPCC to state: Most of the observed increase in globally-averaged temperatures since the mid-20th century is very likely (>90% probability) due to the observed increase in anthropogenic greenhouse gas concentrations. VI. Global Warming: Projections for the Future A. The IPCC utilizing results from numerous climate models with each model using different scenarios for how greenhouse gas emissions will likely change with time, and how society will utilize energy in the future, estimates an increase of global temperatures ranging from 1.8 to 4.0C (3.5 to 7.0F) by the year 2100 1. Most scientists agree that a rise of 2.0C may be a threshold for multiple profound negative effects on plant and animal life (humans as well) including species extinction and significant melting of the ice sheets VII. Global Warming Uncertainty A. Feedback Mechanisms 1. Much uncertainty exists for all of these projections due at least in part to the effect of positive and negative feedbacks 2. Positive feedback mechanisms are when a change occurs that initiates other changes that feedback to enhance the original change a. Increase the likelihood of greater than expected warming 1. Water vapor greenhouse gas effect a. Warming leads to increased water vapor due to increased evaporation of water from oceans and higher saturation vapor pressure ! increased greenhouse effect ! further warming..... 2. Snow/ice albedo feedback a. Warming ! melting snow and ice sheets ! less albedo ! further warming.... 3. Thawing tundra a. Warming ! thawing of the arctic tundra ! release of carbon dioxide and methane from decay of buried plants and peat moss ! further warming...... 3. Negative feedback mechanisms are when a change occurs that initiates changes that  feedback to inhibit the original change a. Increase the likelihood of less than expected warming 1. Increased cloudiness a. Warming leads to increased water vapor ! possible increased clouds which reflect solar radiation ! cooling 4. Evidence suggests that positive feedbacks dominate negative B. Oceans 1. The impact provided by oceans contributes additional uncertainty 2. Much of the emitted carbon dioxide dissolves in the ocean 3. The oceans also have a large capacity for storing heat energy 4. Therefore, the overall contribution by the oceans is to lessen the degree of warming 5. However, there is considerable uncertainty in regard to the oceans capacity to continue to absorb carbon dioxide and heat at the present rate VIII. Global Warming: Consequences A. Warming Distribution 1. The warming has not been uniformly distributed, and models project this non-uniform distribution to continue into the future 2. The greatest warming will be over higher latitudes, particularly over the Northern Hemisphere a. This is due mostly to the snow/ice albedo feedback 3. The continents have warmed and are projected to continue to have greater warming than the oceans due to the greater heat capacity of the oceans B. Precipitation 1. Changes in precipitation amounts and patterns have already shown change and are projected to continue to change in a non-uniform distribution 2. 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