Weather part 2

Origin of Wind

Air is subject to forces which accelerate it into motion Newton's Laws: An object at rest will remain at rest and an object in motion will remain in motion as long as no force is executed on the object. The force exerted on an object equals its mass times the acceleration produced. Acceleration: speeding up, slowing down, change of direction of an object's motion. Pressure gradient forces drive the wind Other forces merely alter it What causes air pressure changes at the surface?

Ordinary Cell Thunderstorms

Also called Air-mass thunderstorms Simple storms They develop when air is conditionally unstable but has limited wind sheer Stages: cumulus, mature, dissipating Lifetime about an hour

NWP 2 Step Process

Analysis Apply observations to grid to obtain initial condition (giving them a weight dependent on an estimate of the size of their errors) Forecast Step the equations of motion ahead in time

Coriolis Force

Apparent deflection due to rotation of the Earth Deflection to the right in northern hemisphere and left in southern hemisphere Stronger wind = greater deflection No coriolis effect at the equator, greatest at poles. Only influences direction, not speed Only has significant impact over long distances

Role of Jet Streams

As it meanders, it controls the location of the troughs that can feed the development of cyclones: If the jet stream swings slightly south of a developing storm, it can position the area of upper-level divergence so that the storm can intensify When the storm moves northeastward and occludes, it gets cut off from the supporting upper-level divergence and begins to die out Upper level flow controls the movement of Low and High pressure areas and the fronts associated with them Lows often move to the northeast, following the divergent flow along the eastern flank of an upper-level trough Highs often move to the southeast, following the convergent flow on the other side of the trough

Storm Surge

At least five processes can be involved in altering tide levels during storms: the pressure effect, the direct wind effect, the Coriolis effect, the effect of waves, and the rainfall effect. The maximum potential storm surge for a particular location depends on a number of different factors: storm intensity, forward speed, size (radius of maximum winds), angle of approach to the coast, central pressure , and the shape and characteristics of coastal features

Seasonal Variation

Average Surface Wind and Pressure Semi-permanent high and lows Major features shift seasonally with the high sun North in July South in December

Tornado Formation

Basic requirements are an intense thunderstorm, conditional instability, and strong vertical wind sheer Supercell Tornadoes Recall that a supercell thunderstorm has a single rotating updraft that can last for hours Wind sheer causes spinning vortex tube that is pulled into thunderstorm by the updraft

NWP History

Bjerknes (1904) identified equations of motion for atmosphere Richardson (1922) first numerical integration of equations, by hand (6 months calc for 6 hr forecast) Charney & von Neuman (1950) successful 24 hr forecast, using electronic computer ENIAC First routine operation use of NWP: Sweden 1954, US 1955 Steady improvement in forecasting skill since then tied to growth in computing power permitting finer horiz resolution, more layers, more physical processes, more detailed parameterizations of clouds, turbulence, etc.

Fronts

Boundary that separates two air masses of different densities One air mass is usually warm and often contains more moisture than other, but fronts can form between any two contrasting air masses Boundary relatively narrow compared to size of air masses Identify on map with Sharp change in temperature Changes in moisture content Shift in wind direction Clouds and precipitation patterns

Forecasting Tornados

Cannot predict locations and times of particular tornados, but can predict when conditions will be favorable for the formation of tornados: First identify regions where supercell thunderstorms are likely (high values of CAPE lead to severe thunderstorms) Next, find regions of favorable wind shear (look at difference between 500-mb winds and surface winds: 35 kt diff means conditions favorable for mesocyclone development) Rotating updrafts occur when surface winds are rotated relative to upper-level winds (helicity)

Arctic Oscillation

Closely related to NAO Pressure changes between Arctic and adjacent southern areas causes changes in upper-level winds Positive = mild winter in US and W Europe Negative = cold US, cold dry Europe, wet Mediterranean

General Motion of Fronts

Cold Fronts Move to the east Move faster than warm fronts Warm Fronts Move to the north

Occluded Fronts

Cold front catches up to and overtakes a warm front Purple line with purple triangles and semi-circles Cold occlusion, warm occlusion

Cold Front

Cold, dry stable air replaces warm, moist unstable air Blue line with blue triangles (that point in direction of motion) Steeper vertically, sharper discontinuities Clouds of vertical development Thunderstorms, squall lines Usually move east, southeast, or south; backdoor out of Canada, moves southwest

Very Short Range Forecasts 0-12 hr

Considerable skill & utility, particularly for large and medium sized weather systems Forecasting small-scale, shorter-lived phenomena, like tornadoes, hail storms, is less mature Difficulty is due to insufficient computational ability, inadequate observational capabilities, and limited understanding of physical processes General areas where these phenomena are expected can be forecast 3 days ahead, but precise location cannot be reliably forecast with any lead time

Winds and Vertical Air Motion

Cyclone = rise Anticyclone = sink/subsidence

Southern Hemisphere

Cyclonic circulation reversed

Forecast by Rule-of-Thumb ProjectionsHow did textbook example forecasts do?

Denver Cold High pressure area should be centered slightly south of Denver by forecast day Clear skies, low winds Strong radiational cooling at night But a downslope wind kept air mixed, not as cold as forecast (knowledge of local topography) Dallas Cold front passing (cloudy & mild, then chance of thunderstorms, clearing and much colder) Northerly winds bring cold temps to Dallas But cold front stalled, leaving cloudy weather: actions that could not be anticipated by rule-of-thumb projections Memphis First a warm front then a cold front passage Forecast basically correct except strength of storms not quite right Chicago Cold air north of low -> snow Forecast correct except more snow than expected: front occluded and slowed down, leading to more snowfall Augusta Cold, then warming prior to an approaching warn front Fog actually occurs due to cold ground Forecast didn't anticipate this because it couldn't take into account previous history of local conditions Washington Approaching low pressure area is bringing snow to points south of Washington, so it's reasonable to forecast snow there, too Instead, sleet falls Forecast didn't take into account the intensification of the storm, leading to stronger winds pulling in warmer air from the Atlantic

Acquisition of Weather Information

10,000 land-based stations, hundreds of ships and buoys; four times a day, airports hourly ASOS (Automated Surface Obs. System) Upper level: radiosonde, aircraft, satellites United Nations World Meteorological Organization, 175 countries World Meteorological Centers: Melbourne, Moscow, Washington D.C. NCEP, US NWS

Derecho

A derecho is a widespread, long-lived, straight-line windstorm that is associated with a fast-moving band of severe thunderstorms. Generally, derechos are convection-induced and take on a bow echo form of squall line, forming in an area of wind divergence in the upper levels of the troposphere, within a region of low-level warm air advection and rich low-level moisture. They travel quickly in the direction of movement of their associated storms, similar to an outflow boundary (gust front), except that the wind is sustained and increases in strength behind the front, generally exceeding hurricane-force. A warm-weather phenomenon, derechos occur mostly in summer, especially during June and July in the Northern Hemisphere, within areas of moderately strong instability and moderately strong vertical wind shear. They may occur at any time of the year and occur as frequently at night as during the daylight hours. June 29, 2012 Derecho

Anatomy of a Hurricane

A hurricane is characterized by a clear eye, towering clouds around the eye called the eye wall, and bands of rain that spiral outward from the eye wall. The strongest winds in a hurricane are in the eye wall.

Squall Lines

A line of thunderstorms Often seen preceding an advancing cold front (pre-frontal squall-line)

Butterfly Effect

A small initial change can result in a large difference in the outcome of a nonlinear system In 1961, Edward Lorenz was using a simple numerical computer model to rerun a weather prediction, when, as a shortcut, he entered the decimal .506 instead of entering the full .506127. The result was a completely different weather scenario. The implication for NWP is that because of the inevitable lack of knowledge of initial conditions for the global atmospheric system, our integrations of the equations will eventually diverge from the true state even if we have all the proper physics, limiting how far into the future we will be able to forecast

Thunderstorms

A storm containing lightning and thunder Convective storms (strong vertical transport) Severe thunderstorms: one with large hail, wind gusts greater than or equal to 50kts, or tornadoes

Proper Upper-Level Support

A trough of low pressure to the west of the developing Low

Short Range Forecasts 12-72 hrs

Accuracy has continued to increase Details of precipitation patterns are tied to smaller-scale structures such as fronts, thunderstorm boundaries, etc., that are still difficult for current models to simulate

Watches, Warnings, Advisories

Advisories: potential hazardous conditions; wind, wind chill, heat, urban and small stream, snow, dense fog Watch: atmospheric conditions favoring hazardous weather over a region in time, actual location and time not known; flash flood, severe thunderstorm, tornado, hurricane Warning: imminent or occurring hazardous weather over a region in time; high wind, heat, flash flood, severe storm, tornado, hurricane, winter storm, blizzard, gale, storm

Surface Convergence at Cyclone

Air flows into low-pressure area

Global Winds

Describes the average air flow - actual winds will vary considerably. (Average conditions help identify driving forces.) The basic cause of the general circulation is unequal heating of the Earth's surface. Warm air is transferred from the Tropics to the Poles Cool air is transferred from the Poles to the Tropics Single Cell Model Assume Uniform water surface Sun always directly overhead the Equator Earth does not rotate Result: global direct convection cell (Hadley, 1735!) Three Cell Model Allow earth to spin = three cells (Hadley, Ferrell, Polar) Alternating belts of pressure starting with Low at Equator Alternating belts of wind with NE trade winds just North of Equator, Westerlies at midlatitudes, Polar Easterlies Trade winds: subsidence from subtropic highs, return to equator, turned by Coriolis effect to easterly wind Mariner's Observations: Trade winds & westerlies made round-trip travel between Europe & New World possible Doldrums - where trade winds converge (Inter Tropic Convergence Zone (ITCZ) Horse Latitudes - Subsidence at subtropic Highs, divergence of surface flow Prevailing Westerlies - geostrophic flow B. Franklin noted storms move from west to east General flow has to prevail over passing weather systems Hadley & Polar cells simple, closed loops; Ferrel cell not Net result: winds in midlatitudes more variable than those in eq. & polar zones

Tropical Weather

Different from mid-latitude weather Noon sun is always high, seasonal temperature changes small Seasons defined instead by precipitation: greatest precip. in high-sun period, when ITCZ is overhead Daily heating and humidity = cumulus clouds and afternoon thunderstorms (Indiv. storms, non-squall clusters, or tropical squall line)

Forecasting Thunderstorms

Difficult to forecast specific thunderstorms very far in advance But we can predict the propensity for thunderstorms to occur by considering the stability of the air for convection Calculate CAPE (convectively available potential energy) by summing (Tparcel - Tenv)/Tenv over the altitude range from LCL to EL CAPE = a measure of the buoyancy of the atmosphere

Convergence and Divergence

Directional Divergence Road: new lanes opening up Speed Divergence Road: stoplight turning green

Weather Information

Doppler radar Satellite imagery of clouds Soundings: altitude profiles of temp, moisture content Wind profiles High speed data modeling systems (AWIPS): communication, storage, processing, and display

Observing Tornadoes and Severe Weather

Doppler radar measures the speed of precipitation toward and away radar unit Two Doppler radars can provide a 3D view doppler lidar NEXRAD

Winds

Driven by horizontal PGF Sea breeze example Coriolis force Geostrophic wind (straightline, parallel to isobars) Gradient winds: circulation around Lows and Highs Upper level vs surface winds (effect of friction

Flow Structure

Eddies - rotational flow Vorticity - measure of the spin of small air parcels Cyclonic - in same direction as Earth's spin (counterclockwise in northern hemisphere) Anticyclonic - in opposite direction (clockwise in NH) Divergence - measure of the outflow/inflow from/to a source/sink

Atmospheric Circulation

Eddies Big and Small - the scales of atmospheric circulations Flow Geometries Waves and Turbulence Wind shear and flow around obstacles Local Winds Thermal circulation - sea/land breezes Mountain/valley breezes Katabatic/Chinook winds Global Winds 3-cell model Trade winds/Prevailing Westerlies Climate patterns Monsoon Jet streams Ocean Interactions El Nino/Southern Oscillation Teleconnections Other Large-scale Oscillations

Eddies

Eddies on leeward side of solid object Roll eddies, mountain wave eddy (clear air turbulence) Increase wind speed/shear deforms layer into wave and air pocket

Eddies- Big and Small

Eddy: swirling mass of fluid with an identity of its own Imbalance of Forces creates Circulation Sources of Circulation Thermal Differences Wind Shear - Wind Differences Surface friction Circulation relaxes (tends to reduce) differences e.g., convection mixes hot air from below with colder air above to warm it and reduce temperature difference

El Nino and the Southern Oscillation

El Nino: a warm south-flowing countercurrent replaces Peruvian current (named for the timing of its appearance) At irregular intervals (3-7 years), this countercurrent becomes stronger than usual Modifies climate & affects fishing economy: warm water blocks upwelling of nutrient-rich cold water Change in Phase of Southern Oscillation - reversal of Walker Circulation El nino phase: rise in pressure over W Pacific, fall in the E Pacific Weakens trade winds Equatorial countercurrent flows eastward Teleconnection: ENSO affects weather worldwide (displaces paths of subtropical & polar jet streams)

Conveyor Belt THeory

Elaboration of polar-front theory based on 3-D observations Helps visualize flow of air through cyclone system Explains comma-shaped pattern of clouds & precipitation Warm conveyor belt carries warm moist air from Gulf of Mexico into the warm sector of cyclone; convergence causes this air to ascend Cold conveyor belt starts at surface north of warm front, travels through center of cyclone, then rotate cyclonically Dry conveyor belt cold & dry air from upper levels, produces clear conditions behind storm

Downdrafts

Entrainment of adjacent drier air causes some raindrops to evaporate, cooling air Cooler air, no longer buoyant, starts to descend as downdraft Downdraft enhanced as falling raindrops drag air with them Downdrafts cut off fuel for storm (upflowing humid air), limiting its lifetime

Why Forecasts go Awry

Equation assumptions are wrong Models not global, boundary conditions wrong Regions with few observations spoil analysis Cannot model small-scale features All phenomena, all spatial & time scales, cannot be modeled (clouds, turbulence) Models are nonlinear, sometimes very sensitive to smaller changes in parameters or initial conditions than the bounds on their errors

Numerical Weather Predictions

Equations solved on a grid so that only a finite number of points need to be calculated This prevents resolution of finer features Horizontal resolution & vertical resolution are separate issues The larger the domain, the coarser the resolution will typically be Global Regional (mesoscale) Boundary conditions must be imposed at edges of bounded domain Atmosphere interacts with land, sea, & ice at earth's surface; solar radiation

Jet Streams

Established by steep temperature and pressure gradients between circulation cells 100-200 kt winds at 10-15km, thousands of km long, several 100 km wide and a few km thick (polar and subtropical) Jet Stream meanderings caused by Rossby waves along the polar front Jet Stream Animation Tabletop Experiment Jet Stream meanderings help control the variability of Midlatitude Weather

Air MAsses

Extremely large body of air whose temperature and humidity are similar horizontally and vertically Weather Forecasting Determining air-mass characteristics How they will be modified Where the air masses will move Source Regions: area where air mass originates, usually flat and uniform terrain with light surface winds, dominated by high pressure (not midlatitudes)

Floods and Flashfloods

Flash floods rise rapidly with little or no advance warning; many times caused by stalled or slow thunderstorm Large floods can be created by training (a sequence of storms passing over same area) of storm systems Great Flood of 1993 in upper Midwest

Monthly and Seasonal Forecasts

Forecasts of mean temperature and precipitation now useful for specialized applications in major economic sectors, if utilized over long period Utility of these forecasts should improve No verifiable skill exists or is likely to exist for forecasting day-to-day weather changes beyond two weeks

Surface Winds

Friction force acts in direction opposite to wind Friction reduces the wind speed which in turn decreases the Coriolis effect Winds cross the isobars at about 30° into low pressure and out of high pressure

Boundary Layer Near Ground

Friction important Irregular turbulent motion instead of laminar flow above Depth of planetary boundary layer about 1000 m

Satellite Soundings

From the strength of the longwave emissions in various wavelength bands, the temperature profile in the atmosphere can be inferred The advantage is that the profiles can be obtained over broad areas instead of single points like the balloon sounding; important for providing data for forecast models

Stationary Front

Front with little or no movement Alternating red and blue line with blue triangles and red semi-circles (semi-circles face colder air) Winds parallel to front but opposite directions on either side of front Variable weather

Satellite Observations

Geostationary or polar orbiting Visible light provides a black and white picture of clouds, yields cloud thickness Infrared allows cloud temperature, and thus height, to be inferred Satellites measure many other variables: longwave radiation, sea surface temperatures, ozone, upper level features, snow cover, land cover

Infared Water-vapor Image

Geostationary or polar orbiting Visible light provides a black and white picture of clouds, yields cloud thickness Infrared allows cloud temperature, and thus height, to be inferred Satellites measure many other variables: longwave radiation, sea surface temperatures, ozone, upper level features, snow cover, land cover The brighter the gray, the more moisture is the air in the middle troposphere

Electrification of Clouds

Graupel and hailstones fall through supercooled water Net transfer of + charge from warmer to colder object Smaller ice crystals become positively charged & lifted up Upper part of cloud becomes positive, bottom part, negative

Nonsupercell Tornadoes

Gustnadoes Land spout Cold-air funnels

Feeding the Hurricane

Heat Potential: combination of warm sea surface temperatures and depth of warm water

Thermal Circulation

Heating and cooling of the atmosphere above the ground create cold core high and warm core low pressure cells. Wind travels from high to low and rises until it cools and begins to sink

Devastating Wind, Storm Surge, and Flooding

Highest winds on the eastern side of storm (wind + speed of storm) Swell Storm surge on north side of storm Coastal flooding (+tide) River flooding Hurricane spawned tornadoes Saffir-Simpson scale (1 weakest, 5 strongest)

Hurricane Formation

Hurricane Stages of Development Tropical Disturbance (group of t-storms, only slight circ.) Tropical Depression (22-34kts) Tropical Storm (35-64kts) (storm gets a name) Hurricane (> 65kts) Hurricane Stages of Dissipation Same categories, in reverse order

Seasonal Variations

Hurricane formation requires a combination of conditions Atlantic Hurricanes 2005 Abnormally warm ocean water and weak vertical sheer allowed for high frequency of hurricanes Atlantic Hurricanes 2007 Low frequency of hurricanes

Hurricane Movement

Hurricanes tend to form along the edges of the ITCZ. Initially they move toward the west as they are carried along by the tropical easterly winds. However, the subtropical highs cause the storms to eventually curve poleward. Then they get caught up in the mid-latitude westerly winds and move toward the east.

Hurricane Dissipation

Hurricanes weaken rapidly when they travel over colder water and lose their energy source Also weaken if layer of warm water is shallow, because waves stirred up by storm mix cooler water up to surface Landfall: lose energy source and suffer friction Vertical wind shear will rapidly dissipate system

Atmospheric pressure

Ideal Gas Law P is proportional to T x ρ P = pressure T = temperature ρ = density = N / V N = number of molecules V = volume Thus Increase T or ρ → increase P With fixed N, decrease V → increase P

Pressure Gradient Force

Imagine a box with a partition in the middle The left side is filled with gas, the right side is empty The partition will feel force from left side On other hand, If both sides were filled with gas at same T and ρ, pressure on both sides would balance, thus no force Pressure Gradient =Difference in Pressure/Difference in Distance Pressure Gradient Force = - Pressure Gradient / Density

Movement of Weather Systems Rules of Thumb

In making these forecasts, we assume pressure systems move at constant rate We know there are several reasons why this might not occur: Lows tend to accelerate until they become occluded and slow Direction of motion may change due to "blocking" highs or lows in their path, or due to shifting upper-level wind patterns

Winds Aloft in both Hemispheres

In the middle and high latitudes of both hemispheres, we generally find westerly winds aloft.

Tropical Wave

In tropics, map streamlines instead of isobars Streamlines show where flow converges or diverges Why is the general direction of flow westward? Trough of low pressure: tropical wave Trough moves westward Fair weather on west side, rain on east side Converging air rises, cools, releases latent heat & produces showers & t-storms Hurricanes are tropical waves that intensify

Pressure Readings

Instrument error: temperature, surface tension affect mercury barometer Altitude corrections: add 10mb per 100m to reading to get sea level pressure

Surface Charts

Isobaric Maps A constant pressure (isobaric) surface where the atmospheric pressure at all points along the surface represents a specific atmospheric pressure Sea-level pressure chart: pressure variations at constant height

Horizontal Pressure Variations

It takes a shorter column of dense, cold air to exert the same pressure as a taller column of less dense, warm air. Warm air aloft is normally associated with high atmospheric pressure and cold air aloft with low atmospheric pressure. At surface, horizontal difference in temperature → horizontal difference in pressure → wind

Supercell Thunderstorms

Large, long-lasting thunderstorm with a single rotating updraft Strong vertical wind shear Outflow never undercuts updraft Wind shear creates rotation Classic, high precipitation and low precipitation supercells Updraft so strong, rain can't fall

Medium Range Forecasts 3-7 days

Large-scale events are now forecasts days in advance Temperature forecasts better than precipitation; they show considerable skill at day 3, decreasing to only marginal skill by day 7 Expect skillful day-7 forecasts in future

Lightning

Lightning Detection and Suppression Lightning direction finder detects radiowaves produced by lightning (spherics) National Lightning Detection Network Suppression: seed clouds with aluminum Lightning safety DO NOT seek shelter during a thunderstorm under an isolated tree. Lightning distance Sound of thunder travels at about 5 seconds per mile

Lightning and Thunder

Lightning: discharge of electricity in mature storms (within cloud, cloud to cloud, cloud to ground) Thunder: explosive expansion of air due to heating by lightning

More Road Analogies

Low-pressure circulation convergence Road: rotary with no exit ramps Low-pressure circulation with upper-level support Road: spiral parking-garage ramp

Tornado Winds

Measurement based upon damage after storm or Doppler radar Add the tornado velocity along track to the circulation velocity to find the maximum For southwest approaching storms, winds strongest in the northeast of the storm

Pressure Measurements

Mercury Barometer Aneroid barometers Altimeter, barograph

Products

Meteograms Advisories, watches, & warnings

Northeasters

Mid-latitude cyclones that develop or intensify off the eastern seaboard of North America then move NE along coast

Multicell Thunderstorms

Moderate to strong wind shear helps separate updraft and downdraft regions, permitting storm to persist for longer time Thunderstorms that contain a number of convection cells, each in a different stage of development Tilt, over-shooting top, gust front, shelf cloud, roll cloud, outflow boundary Micro-bursts: localized downdraft that hits the ground and spreads horizontally in a radial burst of wind; distinguish their damage from tornado damage by seeing evidence of straight-line winds

Ocean Currents and Climate

Moderating effect on temperature of poleward moving currents (e.g., Gulf stream on Britain) Cold currents stabilize air, lead to west-coast deserts (e.g., Atacama in Peru/Chile) Earth's heat balance: transport heat from tropics to poles (75% winds, 25% ocean currents)

Gas in Box

Molecules bumping into walls exert a force on them Pressure = Force / Area

Asian Monsoons

Monsoon - Arabic for seasonal Development of Siberian High, Shift of ITCZ Winds change direction seasonably causing extreme dry and wet season Eastern and southern Asia, North America

Backdoor Cold Front

Most cold fronts move to the east, southeast, or south Backdoor cold fronts move to the west or southwest

Weather Forecasting Using Surface Charts

Movement of Weather Systems Rules of Thumb Mid-lat cyclones move in same direction and speed as previous 6 hrs Lows move in direction parallel the isobars in the warm air ahead of the cold front Lows move toward region of greatest surface pressure drop; highs toward region of greatest pressure rise Surface pressure systems tend to move in same direction as wind at 500 mb pressure level (surface speed about half that of upper level wind)

Hurricanes in a Warmer World

No clear answer whether more common - offsetting factors Frequency of intense storms most likely to be impacted

North American Air Masses

North American cP and cA Source region: N Canada, Alaska (long,clear nights permit strong radiational cooling) Dry, cold, stable (A - more extreme) North American mP Source region: North Pacific, North Atlantic Cool, moist, unstable North American mT Source region: Gulf of Mexico, Caribbean, SE Pacific Wet, warm, unstable Pineapple Express, Bermuda High North American cT Source Region: SW US, Mexican Plateau summer Hot, dry, stable

Other LArge Scale Oscillations

North Atlantic Oscillation Arctic Oscillation Pacific Decadal Oscillation

Types of Forecasts

Now cast <6 hrs Short range 12-65 hrs Medium range 3-8.5 days Long Range >8.5 days

Winds and Ocean Currents

Ocean surface dragged by wind, basins react to circulation around high-pressure areas, forming gyres Cold current, flowing north to south, on west side of continent Warm current, flowing south to north, on east side of continent Wind along coast causes current away from coast Due to Coriolis effect Ekman spiral, Ekman transport Upwelling Water moving away from the coast causes upwelling

Addition of Forces

PGF and CF forces have magnitude and direction Their net result is the sum of their vectors

Straightline Geostrophic Winds

PGF and Coriolis forces Wind velocity parallel to isobars (remarkable!) Spacing of isobars indicates speed: close = fast flow spread out isobars = slow flow

Curved Isobars Aloft

PGF and Coriolis forces don't balance Net force = third force: centripedal Gradient wind parallel to curved isobars Around Low pressure area (Cyclonic): counterclockwise Around High pressure area (Anticyclonic): clockwise

NWP Equations

Partial differential equations that describe how various quantities vary as a function of time and space, starting from an initial state Continuity equation for conservation of mass (density) Momentum equations (winds) Equation of state (pressure) Conservation of energy (temperature) Conservation of water mass Time step determined by how fast flows are; 24 hour forecast requires many steps Mass Conservation Equation Change in mass within cell = Flux from every boundary Plus sources, minus losses Flux from one cell in a direction = flux into next cell Calculate change for every cell to update densities for next time step Iterate over time steps Treat momentum and energy equations simultaneously in similar way

Other Forecasting Techniques

Persistence Trend Analogue Statistical Weather type Climatological

Mid-latitude Cyclonic Storms

Polar Front Theory Bjerknes, shortly after WW I Polar front is a semi-continuous boundary separating cold, polar air from more moderate mid-latitude air Mid-latitude cyclone (wave cyclone) starts as a small wavelike form along the polar front and then grows into storm structure

Maritime Polar

Polar air from Asia Carried eastward and southward by circulation around Aleutian Low Modified by ocean with warmth & moisture At coast: cool, moist and conditionally unstable Coastal mountains force it to rise, leading to precipitation

The Lightning Stroke

Positive charge on ground, cloud to ground lightning Stepped leader, return stroke ground stroke, forked lightning, ribbon lightning, bead lightning, corona discharge

Extended Range Forecasts wk 2

Predictability of day-to-day weather for periods beyond day 7 is usually small Operationally, forecasts at these time ranges: mean temperatures and departures of precipitation from normal patterns Advances in observing systems, computer models, and statistical techniques may allow forecasts of mean conditions in week 2 in near future

Why Wind Blows

Pressure Gradient Force: difference in pressure over distance Directed perpendicular to isobars from high to low Large change in pressure over a short distance is a strong pressure gradient The force that causes the wind to blow

Prevailing Winds

Prevailing most frequently observed direction during a given time period Impact human and natural landscape Wind rose

Atlantic Hurricane 2013 Season

Prior to the season, NOAA predicted a season more active than usual Tropical waters were warm; No El Nino event to create strong upper-level winds Didn't turn out that way: one of the quietest seasons on record Explanations Madden-Julian oscillation Larger than expected volumes of subsiding air Stronger wind shear Dry air from the Sahara

Naming Hurricanes and Tropical Storms

Process has changed over the years: Latitude and longitude Letters of the alphabet Alphabetical female names Alphabetical, alternating female and male names Retirement (Katrina, Camille)

Air Pressure

Proportional to density x temperature Pressure gradient force (PGF) Vertical PGF balanced by gravity: hydrostatic equilibrium Isobar charts: sea level vs upper level

General Precipitation Patterns

Rain where air rises (low pressure) Less rain where air sinks (high pressure)

Tornadoes

Rapidly rotating column of air that circulates around a small area of intense low pressure with a circulation that reaches the ground. Tornado life cycle Organizing, mature, shrinking, decay stages Funnel cloud: rotating column of air that doesn't reach the ground

True Weather Lores

Red sky at night, sailor's delight Red sky at morning, sailors take warning. When the wind is blowing from the north, No fisherman should set forth. No weather is ill, if the wind is still. When halo rings the moon or sun, rain's approaching on the run. A cow with its tail to the West makes the weather best A cow with its tail to the East makes the weather least. A summer fog for fair A winter fog for rain.

Polar Cell

Relatively warm air at 60 latitude rises, driving thermal loop Return surface flow turned by Coriolis effect: Polar Easterlies Edge of polar cell: permanent polar front, modulated by Rossby waves

Sandy

Remarkable for its size, leading to strong storm surges over a wide area due to the long fetch of its winds over the ocean Transition from tropical (warm core) cyclone Energy source: release of latent heat from warm ocean waters To extra-tropical (cold core) cyclone Energy source: temp difference across front Intensification just prior to coming onshore as it crossed the Gulf stream current with temps warmer than usual for this time of year

Pacific Decadal Oscillation

Reversal in Pacific Ocean temperatures Warm = more Pacific storms Cool = cool, wet NW North America, wetter over the Great Lakes, salmon fisheries decline

NAO

Reversal of pressure in North Atlantic Ocean affecting weather in Europe and eastern coast of North America Positive = strong Westerlies, storms in N Europe, wet and mild in eastern US Negative = wet southern Europe and Mediterranean, cold and dry in eastern US

Water spouts

Rotating column of air that is connected to a cummuliform cloud over a large body of water Tornadic waterspout Fair-weather waterspout

Madden-Julian Oscillation

SO, NAO, ... are standing waves: don't move MJO is a propagating wave: travels around the globe (with a 60 -90 day period), affecting equatorial rainfall The timing of the phase of MJO that suppresses rainfall may explain the quiet tropical-storm picture this year

Why pop culture loves the 'butterfly effect,' and gets it totally wrong By Peter Dizikes, June 8, 2008

SOME SCIENTISTS SEE their work make headlines. But MIT meteorologist Edward Lorenz watched his work become a catch phrase. Lorenz created one of the most beguiling and evocative notions ever to leap from the lab into popular culture: the "butterfly effect," the concept that small events can have large, widespread consequences. The name stems from Lorenz's suggestion that a massive storm might have its roots in the faraway flapping of a tiny butterfly's wings. In the 2004 movie "The Butterfly Effect" - we watched it so you don't have to - Ashton Kutcher travels back in time, altering his troubled childhood in order to influence the present, though with dismal results. In 1990's "Havana," Robert Redford, a math-wise gambler, tells Lena Olin, "A butterfly can flutter its wings over a flower in China and cause a hurricane in the Caribbean. They can even calculate the odds." Such borrowings of Lorenz's idea might seem authoritative to unsuspecting viewers, but they share one major problem: They get his insight precisely backwards. The larger meaning of the butterfly effect is not that we can readily track such connections, but that we can't. To claim a butterfly's wings can cause a storm, after all, is to raise the question: How can we definitively say what caused any storm, if it could be something as slight as a butterfly? Lorenz's work gives us a fresh way to think about cause and effect, but does not offer easy answers.

Local Winds

Sea and Land Breeze Uneven heating of land and water Day: land hot, water cold = sea breeze Night: water hot, land cold = land breeze Sea breeze convergence Local Winds and Water Local winds will change speed and direction as they cross a large body of water due to less friction; greater speed and greater Coriolis effect. Mountain and Valley Breeze On mountain slopes, warm air rises during the day creating a valley breeze; during night nocturnal drainage of cool air creating a mountain breeze Associated with cumulus clouds in the afternoon Katabatic winds Cold wind rushes down elevated slopes, usually 10 kts or less but can reach hurricane strength if funneled through a narrow mountain gap Chinook/Foehn Winds Dry warm descending on the leeward side of a orographic barrier Eastern slope of Rockies (chinook), Europe (foehn), Argentina (zonda) Strong westerly winds aloft flow over a N-S mountain range, creating a low-pressure trough on eastern side of mountains Descending air experiences strong compressional heating Santa Ana Winds Warm dry wind that blows from east or northeast down canyons into S. California Develops when a dome of high pressure builds over Great Basin - clockwise circulation forces air downslope from plateau Air undergoes strong compressional heating Very fast, desiccates vegetation, providing fuel for fires Desert winds Dust storms, sand storms, dust devil, haboob Mars

Estimating Pressure Pattern

Stand with your back to the direction from which the high-level clouds are moving Then lower pressure aloft will always be to your left and higher pressure to your right

Bergeron Classification

Temperature T = tropical P = polar A = arctic (like polar, but colder) Source/Humidity m = maritime (moist) c = continental (dry)

The Developing Hurricane

The Developing Storm Energy comes from transfer of sensible and latent heat from warm ocean surface Air aloft is warmed, creating high pressure, leading to upper-level wind divergence Surface low pressure forms Surface flow is in towards Low (cyclonic spiral) As air moves over ocean, picks up more heat & moisture The faster the winds, the greater the transfer of heat & moisture from ocean, leading to strengthening of storm

Waves on Polar Front Develop into Cyclones

The ones that get proper support from upper-level atmosphere

Global Circulation

Three-Cell Model Describes surface winds, pressure, and precipitation patterns well Discrepancies at upper levels (e.g., Ferrel cell suggests an east wind aloft, but upper-level winds aloft flow from west to east)

Hydrostatic Equilibrium

Tremendous vertical Pressure Gradients Balanced by gravity Weight of air in column = Pressure at base The warmer the atmosphere, the slower the fall off of density with altitude

Low-Latitude Satellite Observations: TRMM

Tropical Rainfall Measuring Mission Visible & Infrared Scanning Imagers Microwave Imager Precipitation Radar

The Right Environment

Tropical waters with light wind warm sea surface temperatures (June-November) Humidity high throughout troposphere Triggered by converging surface winds (tropical wave) Spin needs Coriolis effect: form > 5º latitude Trade-wind inversion can inhibit hurricane formation Strong upper-level winds (-> wind shear) inhibit

Wind Power

Turbines need moderate, steady winds

Tornado Occurrence

US experiences most tornadoes Tornado Alley (warm, humid surface; cold dry air aloft) Highest in spring, lowest in winter

Weather Forecasting Methods

Up to mid1950s maps & charts were plotted and analyzed by hand For short-term forecasts, surface weather systems were moved along at a steady rate Upper-air charts were used to predict where surface storms would develop & where cyclones would intensify or weaken Much an art, experience played major role in making an accurate forecast Numerical weather prediction (NWP) Solves equations of motion for atmosphere using gridded data 24 hr forecast for the N Hemisphere requires millions of calculations

Upper Level Charts

Upper level or isobaric chart: height variations at constant pressure surface (i.e. 500mb) Higher heights correspond to higher than normal pressures at a given latitude and vice versa

Warm Fronts

Warm, moist unstable air overrides cold, dry stable air Red line with red semi-circles Not as steep vertically as cold front Horizontal cloud development with steady rain Often move northward or northeastward

Hurricane Watches, Warnings, and Forecasts

Watch issued 24-48 hours before hurricane expected to make landfall Warning issued when storm expected to strike coast within 24 hours and probability of strike in a given location provided

Weather Forecasting

Weather Forecasting is predicting how current state of atmosphere will change Two steps: Analysis - description of current state Prognosis - description of how it will evolve Thus we first need good description of present state of atmosphere - weather information, obs, data

Accuracy and Skills

What constitutes a right or wrong forecast? Forecast can be accurate but not demonstrate skill Forecast exhibits skill if it is more accurate than persistence of climatology (if it can predict when the weather will be different from climatology, e.g., dry days in rainy season) NWP shows skill over forecast intervals of a few days; 12-24 hrs most accurate, 2-5 days good

Wind-driven Water Motion

Wind blowing across a water surface exerts a force on the water, depending on: Wind speed Distance of open water that the wind has blown over (called the fetch) Width of area affected by fetch Time duration the wind has blown over a given area Water depth Waves: ripples, ..., white caps

Determining Wind Speed and Direction

Wind characterized by direction, speed, and gustiness Wind direction describes the direction from which it is blowing e.g., a north wind blows from the north

Surface Boundary Layer

Wind flow around buildings and other obstructions

Wind Measurements

Wind vane Pressure plate anenometer Cup anenometer Aerovane Rawisonde Wind soundings QuickScat: scatterometer

Winds on Upper Level Charts

Winds parallel to contour lines and flow west to east: zonal flow Heights decrease from north to south: temp gradient with latitude

Lake Effect Snow

cP air mass flowing over body of warm water air warmed, less stable sweeps up moisture, becomes saturated lifts & forms Cu clouds further lifting over land - precipitation

Air pressures and winds

differences in pressure leads to winds

Atmospheric Turbulence

eddies get so strong that they interact with each other cascade of eddy strength through different scale sizes create a broad spectrum of eddies results in fluctuations on many different time scales and spatial scales hard to predict: many degrees of freedom, nonlinear, unstable, extreme sensitivity to small changes in initial conditions or parameters Turbulence is one of the grand challenges still facing physics

NWP Ensemble Forecasts

run several models with different assumptions or parameters, look for consensus More robust than single model Spaghetti plots show areas where forecast uncertainty will be largest