The Atmosphere and Weather
The Atmosphere and Weather CLIM 2000
Utah State University
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CLIM2000 The Atmosphere and Weather Lecture Notes Instructor Dr Esmaiel Malek Associate Professor Chapter Ten Mid latitude Cyclones Bjerknes a Norwegian scientist 1862 1951 the founder of the Bergen school of meteorology developed polar front theory to describe interactions between unlike air masses and related aspects of the midlatitude cyclone The life cycle of a mid latitude cyclone What is cyclogenesis Cyclogenesis is the beginning of cyclone formation The life cycle of a mid latitude cyclone is composed the following stages a A polar stationary front separates the cold easterlies and the warm westerlies b Cold air north of the front begins disruption of the linear frontal boundary This creates a counterclockwise rotation in the Northern Hemisphere around a weakly developed low pressure system c With further intensi cation the low pressure deepens even further and distinct warm and cold fronts emerge from the original polar front d As the cyclone matures warm and cold fronts extend from the low pressure system Further development creates an occlusion e The occlusion occurs as the center of the low pulls back from the warm and cold fronts This stage represents the end of the cyclone s life cycle dissipation How are the weather conditions around a mature cvclone Cloud patterns wind uplift processes and precipitation patterns associated with a mature cyclone are illustrated in the following gure Do all mature cyclones have an inverted V shape extending toward southwest and southeast The above condition does not apply to all mid latitude cyclones The following gure depicts this exception Processes of the middle and upper atmosphere During World War 11 British and US pilots ying missions over Europe and Japan observed winds with speeds up to 400 km 250 mph This nding stirred an interest among meteorologists in the upper tropospheric ow and how it might be related to weather conditions on the surface Why do Rossby waves meander oscillate The concept of Rossby waves was introduced in a previous chapter To answer this question we need to know what is vorticg39 g The turning of an object such as an air parcel usually with respect to the vertical direction is called vorticity vorticity can also refer to rotation around a horizontal axis for instance a roll of paper towels Vorticity is important to meteorology because of its association with the oscillation of the Rossby waves and areas of divergence and convergence Earth itself has vorticity called Earth vorticity because of its rotation An object rotating on Earth has relative vorticity Absolute vorticity is the summation of these two Relative vorticity depends on air rotations with respect to the Earth s surface while 47 Earth vorticity is solely a function of latitude the greater the latitude the greater the vorticity with zero vorticity at the equator Earth vorticity is a function of the sinus of the latitude Over the mid latitudes where Rossby waves are most likely to occur Earth vorticity changes fairly small with latitude What are positive and negative v01ticities If the ow of air relative to the surface is in the same direction as the rotation of the Earth itself counterclockwise in the NH it is called positive vorticity Relative vorticity has two components a Shear variation of the speed of uid across the direction of ow b Curvature of the ow Absolute v01ticity sum of relative and Earth v01ticities like absolute angular momentum is conserved that is in the absence of intervening forces it remains constant This means when the Rossby wave ows southward toward the lower latitudes its Earth vorticity decreases But because absolute vorticity is conserved an increase in relative v01ticity compensates for the decrease in Earth vorticity causing the air to turn counterclockwise Then as it starts to ow poleward EaIth vorticity increases Thus the air turns back to its light and once again exhibits negative vorticity This yields meandering waves What are convergence and divergence Convergence is horizontal motions of air which results in a net in ow of air with more imp01ted than exported causing air to rise near the surface and sink aloft Divergence is horizontal motions of air which results in a net out ow of air with more exported than imported causing air to sink near the surface and rise aloft Let s elaborate on the air s vorticity around a trough In segments 1 and 3 the air undergoes little change in direction and thus has no relative vorticity In transition A as the horizontal area occupied by an air parcel decreases by convergence then its vorticity 0r spin must increase In segment 2 it turns counterclockwise and thus has positive vorticity Decreasing vorticity as in transition zone B likewise leads to divergence What are dif uence and con uence Dif nence occurs when air diverges horizontally due to a variation in wind direction In case a a certain amount of air is contained in the shaded area between points 1 and 3 Importance of vorticity Vorticity changes happen in the horizontal areas occupied by air parcels aloft but their real importance lies in their effects on veItical motions and pressure changes near the surface Let s see how this happens Divergence in the upper atmosphere caused by decreasing v01ticity draws air upward from the surface and provides a lifting mechanism for the intervening 48 column of air This in turn can initiate and maintain a low pressure system at the surface Surface low pressure systems resulting from upper tropospheric motions are referred to as dynamic lows also called cold core low distinct from the thermal warm core lows caused by localized heating of the air from below The interconnectedness between surface patterns and those aloft provides the true 1 39 for understandin the li e cycle of midlatitude cvclones Furthermore we know that upper level patterns are in uenced in turn by temperature conditions near the surface Examples of a mid latitude cyclone are illustrated below Flow Patterns and Large Scale Weather Zonal wind ow A wind that has a predominate west to east component a Meridional wind ow A type of atmospheric circulation pattern in which the north south component of the wind is pronounced b What are barotropic and baroclinic conditions The following gure describes these two conditions Migration of surface cyclones relative to Rossby waves For mid latitude cyclone to form there must be upper level divergence If there is more divergence aloft than convergence near the surface the surface low deepens and a cyclone form If the convergence at the surface exceeds the divergence aloft the low gradually lls until it ceases to exit Although the optimal place for mid latitude cyclone to develop is just below the zone of decreasing vorticity aloft they don t usually remain in a xed position relative to the upper level trough Instead they are usually along so that they migrate in the same direction and at about half of the speed as the winds at the 700 mb level typically about 3 km 2 miles near the surface The modern view Mid latitude cyclones and conveyor belts A three dimensional view of a mid latitude cyclone aloft is illustrated in the following gure How about anticyclones in mid latitude Anticyclones clockwise rotations are as much in uenced by upperlevel conditions as are cyclones and they exert an important impact on weather While cyclones can bring heavy precipitation and strong winds anticyclones foster clear skies and calm conditions because the cool air within them slowly sinks toward the surface Anticyclones are not always associated with wonderful weather Examples Outbreaks of continental polar cP air over the eastern US are associated with quot 39 behind quot d or quot d moving cold fronts Anticyclones often tend to remain over a region for an extended period of time which can lead to drought Anticyclones over the Rocky Mountains can lead to Santa Anna wind conditions over the West Coast and to the development of katabatic winds 49 CLIM2000 The Atmosphere and Weather Lecture Notes Instructor Dr Esmaiel Malek Associate Professor Chapter Thirteen Weather Forecasting and Analysis Knowing the weather in the future is vital to many human activities A summer forecast of extended heavy rain and cool weather a Construction under protective cover a Advertising of umbrellas rather than bathing suits a Alert farmer to harvest their crops before their elds become too soggy a To support the heavy machinery for the job a Flooding A forecast calling for extended high temperature with low humidity a Ice cream makers prepare for record sales a Dairy farmers anticipate a decrease in milk and egg production a Fire danger in parched timber and grassland Why is weather forecasting imperfect We ve all had careful plans upset by a bad weather forecast and are understandably quick to nd fault when actual conditions depart from the forecast So whv are forecasts often so far from correct After all with powerful computers satellites weather radar and global communication networks it seems as if making a good forecast ought to be easy But however as much as the public might think so this is de nitely not the case in fact accurate weather forecasting is extremely difficult Why Imagine that you want to forecast tomorrow s temperatures T and think about just a few of factors that you must consider 1 First the temperature structure of the atmosphere depends in part on Absorption and emission of radiation shortwave and longwave which itself depends on the vertical and horizontal distribution of atmospheric gases clouds and so on So to compute the temperature at a point in the air you need to begin with detailed information about the composition of the atmosphere in three dimensions 2 The constantly changing atmospheric water phases ice liquid and vapor also affect the temperature by removing or adding latent heat to the air This means we need to keep track of that as well as radiation transfer But these phase changes are in uenced by vertical and horizontal motions in the atmosphere close to the ground and aloft 3 Another consideration is the continual interaction among the weather elements So even though you re only interested in temperature you can t pretend the winds are unchangeable but instead you are forced into the business of forecasting atmospheric motion Unfortunately this is very dif cult because the atmosphere is dynamically unstable By this we mean that small disturbances often grow into large features and eventually dominate the eld of motion So small and large scales should be considered too Obviously weather forecasting involves a set of interlocking problems each dif cult to solve in isolation let alone in combination 64 Weather forecasting in the US began in the 1870s by the National Weather Service NWS NWS was renamed into the National Weather Bureau The National Oceanic and Atmospheric Administration NOAA was established in 1970 to include the reverted NWS and other environmental agencies Forecasting Methods There is no single correct way to forecast the weather depending on the length of forecast the type information desired and how much is known about the current state of the atmosphere One can even attempt a forecast in the absence of any data about the current weather provided that long term information is available Forecasting approaches Climatological forecasts Persistence forecasts Analog approach Numerical weather forecasting Climatological forecasts They depend on the longterm averages yeartoyear variability in weather conditions for the forecast day For instance a forecast of hot muggy conditions with a chance of afternoon thunderstorms in Orlando Florida in mid August has a reasonably good chance of proving to be correct Such prognoses based on long term averages are known as climatological forecasts Persistence forecasts They rely completely on current conditions with no reference to climatology A special case of persistence forecasting is used by all of us in everyday life When we see clear skies and leave the umbrella behind we re betting that the prevailing conditions will continue and are making a short term forecast on that basis it might work for a little while but will eventually fail In other words one could assume persistence in a trend to make a guess regarding changes in weather Analog approach In this method one tries to recognize similarities between current conditions and similar well established patterns from before Some subjective depending on the forecaster s expertise and objective depending on statistical relations matters should be considered in this approach Numerical weather forecasting This dominant method forecasts the weather based on computer programs that attempt to mimic the actual behavior of the atmosphere That is numerical weather models explicitly compute the evolution of wind pressure temperature and other elements over time By examining the output for a given point in time one obtains a depiction of a three dimensional state of the atmosphere for that moment The numerical models typically used in weather forecasting are very large and can only be run on the most powerful computers so called supercomputers A short discussion about numerical forecast models is presented below 65 Numerical forecast models Over the last 4 decades weather forecasters have relied on several generations of computers and a variety of different models for guidance As computers have increased in speed and capacity the models have become increasingly complex always straining the limits of computer power in an effort to achieve greater realism and accuracy But even with today s computers numerous compromises and approximations are necessary In fact the models use almost the same theory but rather different approaches to the basic problems of abstraction and simpli cation The result is a tremendous variety in both details and gross features of numerical models We rst discuss the major features of numerical models using the primary National Center for Environmental Prediction NCEP operational models as examples then describe some models for assessing forecast quality Model characteristics Major models used at the present time are 1 The Global Spectral Model 2 The Nested Grid Model NGM 3 The Eta the Greek letter H 11 model uses pressure scaled in a particular way relative to surface pressure Scale Considerations Horizontal Representation Physical Processes are involved with all of these major models A Scale considerations At this step the model domain and spatial resolution will be considered i The model domain The domain is the region of the globe to be presented entire globe Europe North America etc A larger domain is preferable but requires more computer resources which means some other aspect of the model must be compromised Of the three NCEP models mentioned above only the Global Spectral Model has a global domain the others have domains centered on the US and cover only part of the N01thern Hemisphere ii Another issue is spatial resolution The fundamental equations governing atmospheric behavior are continuous in space meaning that they describe the evolution of the atmosphere everywhere If the governing equations were solved directly they would yield a solution for an infmite number of points we could find forecast values at every location within the domain But the equations are far too complex to solve directly by analytical means no such solution exists Instead the equations are written and solved in approximate form with the result that forecast values are available only at widely spaced locations there is some minimum size below which explicit representation is impossible Current models are limited horizontally to a few tens of kilometers and larger for example nothing as small as an individual thunderstorm could possibly appear 66 If such sub scale phenomena are to be considered an error prone process called parameterization is required Obviously one wants high resolution so that small scale processes and phenomena can be modeled and appear in the forecast but this can come only at the cost of more computation Roughly speaking doubling the resolution leads to eight times the computation The resolution issue applies to the vertical coordinates as well as the horizontal coordinates The horizontal resolution of the Global Spectral Model is about 10 in latitude and longitude about 60 miles It has 28 levels in the vertical ranging from the surface to the 27 mb level It has 8 levels below 800 mb with increasing spacing to lower pressure higher altitudes Operational versions of the Eta the Greek letter H 11 model have been run with increasing resolution since its inception ranging from 80 km initially to 29 km with 50 levels in the vertical at the present time The Nested Grid Model NGM has 16 layers and two grids The smaller inner grid centered over the US and Canada has a resolution of about 80 km It lies completely within a larger coarser outer grid that extends through the domain of much of the Northern Hemisphere it has the advantage of confining most of the computation to the region of interest B Horizontal representation Another major difference among the models is the horizontal representation Many models adopt a grid representation in which the domain is subdivided into a lattice of grid points C Physical processes Numerical models physics package includes i Atmospheric processes such as condensation ii Atmosphere surface interaction such as friction between the atmosphere and ground iii Purely surface processes such as soil moisture or depth of snow Parameterization 1radiation convection clouds and precipitation and surface properties and processes is heavily used in the physics package Different models not only include different processes but also employ different parameterizations for the same processes The governing equations are solved only at the grid points The ner the grid the higher is the model s resolution Implicit in this is the idea that the grid captures horizontal variation in the atmosphere and that intermediate values can be inferred knowing values at nearby grid nodes In spectral rQresentatitm variables are represented as a series of waves in space each having a characteristic wavelength Global Spectral Model uses 126 waves Advantages of the Spectral models are 1 Horizontal resolution is determined by the smallest of the harmonics or wavelengths present so there is no escaping the problem of how to represent sub scale processes 67 11 Not all of the variables can be represented in spectral terms the advective quantities such as heat and moisture are treated this way Other variables such as radiation must be computed on a point by point basis III The spectral representation applies only to the horizontal Spectral models is layered in the vertical Types of forecasts Quantitative forecasts Amount of the forecasted variable is speci ed for instance 3 inches of rain is expected Qualitative forecasts Provide only a categorical value for the predicted variable examples include rain no rain hurricane no hurricane above below normal cloudy partly cloudy mostly cloudy Probability forecasts Chance of some event is stated for instance Examples include probability of precipitation PoP as the rain chance today is 70 percent there is a 60 percent chance of afternoon showers How is the weather data acguired The start point for almost all weather forecasting is information about the current state of the atmosphere To know th e future we begin with information about the present The World Meteorological Organization W MO under the auspices of the United Nations oversees the collection of the weather data across the globe from 179 nations The WMO collects data from about 10000 land stations 7000 ship stations 300 moored and drifting buoys Land ship and buoy stations have automated weather sensors Several weather satellites a continuous basis from instruments aboard wide bodied commercial aircrafts Weather radar and rockets How is the weather data disseminated The data from all these sources are sent to the three World Meteorological Centers W MC at Washington D C Moscow Russia and Melbourne Australia which in turn disseminate the data to all members of the WMO The member nations of the WMO maintain their own meteorological agencies that obtain and process the data and issue regional and nation forecasts In the US the National Center for Environmental Prediction NCEP of the Weather Service performs these tasks while in Canada they are handled by the Canadian Meteorological Center of the Atmospheric Environment Service AES Of the approximately 10000 relatively dense networks of surface observations in the US About 120 are National Weather Service Offices The rest are Federal Aviation Administration FAA airport sites The Canadian AES operates about 270 surface stations 68 What weather data are recorded temperature humidity pressure cloud conditions including 1 cloud type 2 cloud height 3 percent of sky obscured by cloud Wind speed and direction visibility the presence of signi cant weather such as fog or rain accumulated precipitation reading at the ground As part of an on going modernization program the FAA and NWS have installed 1 A network of more than 800 automated sensors called Automated Surface Observation Systems ASOS and Automated Weather Observation Systems AWOS in Canada with 100 data observation platforms for measuring and recording these variables The following gure shows a typical ASOS unit 2 The NWS launches hydrogen lled balloons carrying weather instrument packages called radiosondes Twice daily at 0000 and 1200 UTC about 750 radiosondes are launched worldwide about 80 within the US and Canada UTC Universal Coordinated Time also called Greenwich Mean Time GMT which is an international reference for time keeping used for weather observations satellite imaging etc While ascending in the air these radiosondes observe and transmit pressure and dry and wet bulb temperatures to ground recording stations every 5 seconds If these radiosondes are tracked by radar wind speed and direction are added Radiosondes tracked by radar are called rawinsondes Forecasters not only have a tremendous amount of data available to them but they also have the ability to easily display and manipulate the information to suit the immediate need The Advanced Weather Interactive Processing System A WIPSZ allows forecasters to display maps of current weather conditions output of computer forecast models displayed in map form satellite and radiation images forecasting advisories and discussions from other weather facilities etc A typical NWS office A work station The AWIPS graphical display monitors Steps in weather forecasting Numerical models are the preeminent tool of modern weather forecasting There are many numerical models used by weather agencies around the world Despite large differences between models the general procedure is the same for all numerical models Steps in weather forecasting are 1 Analysis phase 69 2 Prediction phase 3 Post processing phase 1 Analysis phase In this step three dimensional observations are used to supply values corresponding to the starting current state of the atmosphere for all variables carried in the model Unfortunately the network of weather stations and radiosonde launches is highly irregular and doesn t come close to providing even coverage This step converts those irregular observations into uniform initial values Though only a preparatory step this is a dif cult task There are millions of data values from a variety of sources satellites ships and so on representing various moments in time None of the measurements is completely free of error and many are subject to large error It is necessary to remove as much error as possible while at the same time producing elds that are self consistent for instance assigned wind velocities must satisfy the conservation of mass in the resulting wind eld 2 Prediction phase Fundamentally the job of a numerical model is to solve the governing equations as the equation of motion the equation of continuity the equation of energy etc Beginning with values delivered by the analysis phase the model uses the governing equations to obtain new values a few minutes into the future The process is then repeated using the output from the rst step as input for the next set of calculations This process is then performed over and over as many times as necessary to reach the end of the forecast period 24 48 or whatever hours This is called the prediction phase of the model run This results in many billions of calculations for each time step despite the fact that there are just a handful of fundamental atmospheric variables temperature pressure wind velocity vector density and moisture 3 Post processing phase The weather conditions forecasted by the model at regular intervals for example every 12 hours are presented in the form of mapping The following series of maps are depicted for the forecast distributions a Sea level pressure and 1000 to 500 mb thicknesses b 850 mb heights and temperature c 700 mb heights and vertical velocities ascent and descent d 500 mb heights and absolute vorticity values e Precipitation amounts A 24 h precipitation forecast from three numerical models along with the nal manual and observed rainfall for June 2 1992 is shown below Regardless of that forecast s lack of success the model output is coupled with other information in producing of cial forecasts The old rules must be constantly reevaluated in light of new model behavior 70 Forecasts for a number of secondary variables such as maximum and minimum temperature dew point wind conditions and probability of precipitation are produced using the statistical relationships between model output and observed surface conditions from the past The output products are called model output statistics MOS and are designed to capture the effect of topography and other factors that in uence local weather conditions Numerical models have only limited ability to represent processes occurring near the surface and they provide a rather generalized picture of the atmosphere How good are today s forecasts There s no single answer to this question It depends very highly on the variable in question the forecast lead time the model used the place and season For example there s no doubt that temperature wind and pressure distributions are forecasted far better than precipitation Some statistical terms M The average error or bias is mostly def39med as the difference between the average forecast value and the average observed value Example Given The following errors occurred in daily temperature in 0 F forecast over three days 5 4 and 00 Find The bias Solution 5400 3 033 0 F Mean absolute error MAE 1 A mathematical equivalent de nition of bias is simply the mean absolute error In the other words for each forecast we nd the absolute ie disregard the sign of individual error departure from observed and compute the average of those errors Compute the MAE in the previous example MAE5400033 0F Root mean sguare error RMSE RMSE is just the square root of average squared error which is calculated by rst summing the square of each individual error in the series then dividing by the total number of observations then taking the square root Compute the RMSE in the previous example RMSE 52 42 002 3 5 1367 37 F Measures of forecast accuracy and skill Accuracy Number of correct forecasts total number of forecasts Example In Vancouver Canada the skies are cloudy 327 days each year the average then the accuracy is 327 365 90 What is forecast skill equitable threat score Skill measures forecast improvement above the climate average Hypothetical distributions of observed and forecasted precipitation are illustrated below 71 In Figure a below precipitation was forecasted for 40 of the area while 30 of the area received precipitation observed As shown only 10 of the forecast was correct A measure of change in skill is expresses as the threat score TS TS correct forecast observed correct TS 10 40 30 10 017 for the above case A perfect forecast has a skill score of 10 An example of the 24 h forecast skill equivalent thread score for various NCEP models and precipitation amounts is shown in the following gure The cool season October March maximum temperature and precipitation forecast scores are shown below Medium range forecasts Going beyond short term forecasts 72 hours or less there is considerable attention directed to so called medium range forecasts MRF For example the European Center for Medium range Weather Forecasting ECMWF has a model that generates forecasts up to 7 days In the US the Global Spectral Model GSM is used at NCEP to prepare 15 day forecasts The procedure is fundamentally the same in medium and short term forecasting Rather than making just a single forecast ensemble forecasting is widely employed in which a number of different runs are performed for the same forecast period If two model runs are made with slightly different initial values the results might be very different after a week or so This behavior is now known to be typical of many natural and human systems called chaotic behavior The ten day ensemble forecast from the NCEP MRF model is shown below This gure shows the 5700 m contour 500 mb height and the locations of troughs and ridges Long range forecasts Forecasts are also produced at still longer lead times so called long range forecasts In the U S the Climate Prediction Center CPC of NCEP is charged with preparing forecasts for periods ranging from a week to the limits of technical feasibility The methods used include climatology statistics numerical models ocean and atmosphere are coupled subjective judgment For example because of its important role in the global climate system sea surface temperature SST in the Tropics is routinely forecasted for up to a year in advance What are station models What are station models More knowledge of the conditions at a particular location can be obtained from station models The arrangement of a surface station model along with some important symbols is shown in the following gure 72 Weather maps and images Although computers play a critical role in the analysis of weather ultimately the meteorologist applies her or his knowledge to produce the forecast that is issued to the general public A typical surface weather map is illustrated below The 850 mb weather map is depicted below typically found about 15 km 1 mi above equivalent sea level The 700 mb weather map is shown below The 500 mb weather maps are depicted below with an omega H The 300 mb map along the lines of equal wind velocity isotachs contoured at 20 knot 1 knot 184 km h intervals Shaded areas have winds in excess of 70 knots Open areas within the shaded regions have winds greater than 110 knots A radar composite map is revealed below Thermodynamic diagrams The maps and images previously described provide two dimensional views of atmospheric conditions but they fail to provide detailed vertical information Vertical profiles of temperature and dew point observed by radiosondes are plotted on thermodynamic diagrams also called pseudo adiabatic charts or Stuve thermodynamic diagrams An example of sounding on a Stuve diagram is depicted below What are lifted and K indices Lifted index The lifted index combines the average humidity in the lowest kilometer of the atmosphere the predicted maximum temperature for the day and the temperature at the 500 mb level into a single number The magnitude and sign of the values together indicate the potential for thunderstorms For instance negative values indicate sufficient water vapor and instability to trigger thunderstorms More speci cally lifted index values between 2 and 6 indicate a high potential for thunderstorms whereas less than 6 suggests a threat of severe thunderstorms K index The K index uses values of temperature and dew point at the surface and the 850 700 and 500 mb levels to translate the probability of heavy rains and thunderstorms In general K values less than 15 indicate no potential for thunderstorms values above 40 suggest that they are highly likely 73 CLIM2000 The Atmosphere and Weather Lecture Notes Instructor Dr Esmaiel Malek Associate Professor Chapter Six Cloud Development and Forms What is a cloud An area of the atmosphere containing sufficient concentration of water droplets andor ice crystals to be visible Clouds are formed due to the upward motion of air Mechanisms that lift the air Four mechanisms lift air so that condensation and cloud formation can occur 1 Orographic lifting The forcing of air above a mountain barrier 2 Frontal lifting The displacement of one air mass over another 3 Convergence The horizontal movement of air into an area of low pressure levels 4 Localized convective lifting due to buoyancy 1 Orographic lifting Air owing toward a hill or mountain will be de ected around and over the barrier The upward displacement of the air leads to adiabatic cooling This mechanism is called orographic uplift orographic effect The height at which these clouds can rise is not limited to the height of the hill or mountain the top of orographic clouds can extend many 100s of meters higher and even into the lower stratosphere What is a rain shadow Downwind of a mountain ridge on its leeward side air descends the slope and warms adiabatically by compression to create a rain shadow effect an area of lower precipitation The Sierra Nevada mountain range higher than 3500 m 11500 ft provides a dramatic illustration of this effect While precipitation on the western windward side is being higher than 250 cm 100 in the ascending air on the leeward slope creates one of the strongest rain shadow effects on Earth The Sierra Nevada windward side 2 Frontal lifting When two air masses warm and cold approach each other they may push each other When cold air advances toward warm air cold front the denser cold air displaces the lighter warm air ahead of it When warm air ows toward a wedge of cold air warm front the warm air if forced upward like an orographic effect 3 Convergence When a low pressure cell is near the Earth s surface winds in the lower atmosphere tend to converge on the center of the low pressure from all directions This process is called horizontal convergence or just convergence In the case of low level convergence rising air will be the result 4 Localized convective lifting due to buoyancy Buoyancy is the tendency for a lighter uid to oat upward through a denser air Free convection mentioned in Chapter 3 arises from buoyancy This process can produce updrafts strong enough to form clouds and precipitation Hot air balloons When an air parcel is less dense warmer than the air around it it has a positive 29 buoyancy and oats upward In the case of negative buoyancy the air parcel is denser cooler than the surrounding air and it sinks if not subject to continued updrafts Static stability and the envi 39 lapse rate Static stability air The condition of the atmosphere that inhibits or favors vertical displacement of air parcels Statically unstable air The air parcel becomes buoyant when lifted and continues to rise if given an initial upward push Absolutely Unstable air Statically stable air The air parcel resists upward displacement and descends back to its original level when the lifting mechanism ceases Absolutely stable air Statically neutral air The air parcel neither rises on its own following an initial lift nor sinks back to its original level it simply comes to rest at the height at which it was displaced Conditionally unstable air Q When the ELR is between the DALR and the SALR DALR gt ELR gt SALR Initially the atmosphere resists vertical motions Q If a parcel is forced to rise and saturation occurs parcel cooling at the lesser SALR will eventually create a situation where the parcel temperature will exceed that of the ambient air Q The parcel will accelerate upward under a positive buoyancy situation Parcel buoyancy is dependent on lifting to the level of free convection Conditionally unstable air Factors affecting the environmental lapse rate ELR 1 Heating or cooling of the lower atmosphere Frontal up and subsidence inversion an extremely stable air Taking advantage of the radiation inversion to prevent frost 2 Advection horizontal wind of cold and warm air at different levels 3 Advection of an air mass with a different ELR Finding stability from thermodynamic diagrams A complete thermodynamic diagram for Detroit Michigan on 27 June 2002 right and left solid lines are the air and dew point temperatures respectively What is the entrainment Talking about a rising or falling air parcel we mean a small mass that undergoes motions distinct from the surrounding atmosphere Q We can imagine such a parcel contained within a balloon But uner a balloon which has a rubber film to isolate the air within an air parcel has no barrier to prevent it from mixing with its surroundings Q In fact as air rises considerable turbulence is generated which causes ambient air to be drawn into the parcel This process called entrainment is especially important along the edge of growing clouds Entrainment 30 6 Ambient air intrusions into parcels which limits vertical cloud development 6 Causes evaporation along cloud boundaries 6 The evaporation process uses latent heat which cools the cloud margins and reduces buoyancy What is a cloud Cloud is an area the atmosphere containing suf cient concentration of water droplets and or ice crystals to be visible Cloud types Abbreviations High clouds Bases above 6000 m 19000 ft composed of ice crystals Cirrus Ci Cirrostratus Cs Cirrocumulus Cc Middle Clouds Bases between 2000 and 6000 m 6000 19000 ft largely composed of liquid drops Altostratus As Altocumulus Ac Low clouds Bases below 2000 m 6000 ft normally composed of liquid water Stratus St Nimbostratus Ns Stratocumulus Sc Clouds with vertical development Cumulus Cu Cumulonimbus Cb High vertical velocities in air that is unstable or conditionally unstable Why do clouds have clearly defined boundaries 1 The clear definition of the cloud base is partly due to the rapid growth of droplets as they form above the lifting condensation level LCL At the beginning these droplets are very small but they quickly attain diameters about one micron which makes them effective at scattering visible light hence a clearly visible base 2 The sharp boundaries along the sides of cumulus clouds are the result of entrainment When saturated air just outside the margins of the cloud is drawn into the cloud by turbulence some of the water droplets evaporate The rapid evaporation produces a well de ned sharp boundary separating the unsaturated ambient air and the cloud Unusual Clouds Lenticular clouds Form as a result of turbulence downwind of mountain ranges 9 Exhibit a lens shape Banner clouds Similar to lenticular but are anchored to individual mountain peaks Mammatus Sack like protrusions from the base of a cloud indicate low level turbulence common in cumulonimbus clouds Downdraft below the cloud base Nacreous clouds Composed of supercooled water or ice are stratosphere clouds sometimes called mother of pearl clouds Noctilucent clouds Form in the mesosphere and are typically illuminated after sunset 31 Cloud coverage When clouds comprise more than 910th of the sky overcast When coverage is between 610th and 910th broken When coverage is between 110th and 610th scattered Cloud coverage less than 110th M 32 CLIM2000 The Atmosphere and Weather Lecture Notes Instructor Dr Esmaiel Malek Associate Professor Chapter Twelve Tropical Storms and Hurricanes How do tropical storms differ from hurricanes Tropical storms originate in tropical regions and have wind speeds between 60 to 120 kmh 37 to 75 mph Hurricanes are intense tropical cyclones warm core low with sustained winds of at least 120 kmh 75 mph Modi ed satellite image of the progression of hurricane Andrew in August 1992 is shown below Hurricanes around the world Extremely strong tropical storms have different names around the world They are called Typhoons over the extreme western Paci c Cyclones over the Indian Ocean and Australia Hurricanes over the Atlantic and eastern Paci c The distribution of hurricanes around the globe is illustrated below The tropical setting We ve learned that during much of the year air spirals out of massive high pressure cells that occupy large parts of the Atlantic and Paci c oceans Mid and upper level air along the eastern side of these anticyclones sinks as it approaches the west coasts of adjacent continents Because the air does not descend all the way to the surface a subsidence inversion forms above the surface This particular subsidence inversion is called the trade wind inversion The air below the inversion called the marine layer is cool and relatively moist The thickness of the marine layer and the height of the inversion base vary across the tropical oceans The inversion is lowest a few hundred meters above the surface along the eastern margins of the oceans where upwelling and cold ocean currents maintain a relatively cool marine layer The low inversion inhibits vertical cloud growth and low clouds often occupy the region Farther to the west the warmer surface waters heat the marine layer and cause it to expand to a greater height Greater inversion height in this region allows for more convection and deep cumulus clouds are more likely to form For this reason more hurricanes occur along the western portion of the ocean basin Hurricane characteristics Hurricanes are the most powerful of all storms The energy unleashed by just a single hurricane can exceed the annual electrical consumption of the U S and Canada Despite having wind speeds lower than tornadoes hurricanes are very much larger and have far longer life spans The standard sea level pressure of 1013 mb can reach around 950 mb near the center of a typical hurricane but pressures as low as 870 mb have been observed for extremely powerful hurricanes In contrast to tornadoes whose diameters are typically measured in tens of meters hurricanes are typically about 600 kilometers 350 mi wide Because hurricanes obtain most of their energy from the latent heat released by condensation they are more common where a deep layer of warm water can fuel them Given that tropical oceans have their highest surface temperatures and evaporation rates in late summer and early fall it is not surprising that August and September are the prime hurricane months in the Northern Hemisphere January to March in the Southern Hemisphere A cross section of a typical hurricane is illustrated below Hurricanes do not consist of only one uniform convective cell Instead they contain a large number of thunderstorms arranged in a pinwheel formation with bands of thick clouds and thundershowers spiraling counterclockwise in the N H around the storm center or eye Both the wind speed and the intensity of precipitation increase toward the center of the system reaching a maximum 10 to 20 km away from the center at what is called the eye wall Hurricanes are warm cored cyclones compared to cold cored mid latitude ones where air ascends and due to expansion cools down Why does hurricane have warm cores As air ow toward lower pressure the warm ocean surface supplies large amounts of latent and sensible heat to the overlying air Because pressure within the moving air decreases as it ows toward the low adiabatic expansion keeps the temperature from increasing dramatically with the result that there is little temperature difference across the base of the storm Nevertheless much thermal energy is added resulting in a warm central core Aloft after condensation and release of latent heat the warmth is re ected in temperature so that temperatures near the center are much higher than those of the surrounding air See the gure below As a warm core low pressure within a hurricane increases relatively slowly with increasing altitude Thus the horizontal pressure within the storm gradually decreases with altitude At about 76 km 25000 ft about the 400 mb level air pressure is the same as that of the immediate area outside the storm From this height to the lower stratosphere the hurricane has relatively high pressure So uner the lower part of the hurricane in which the air rotates cyclonically the air in its upper portion spirals anticyclonically from the center clockwise in the Northern Hemisphere In the upper reaches of the storm the low temperatures cause water droplets to freeze into ice crystals As the ice crystals spiral out of the storm center they create a blanket of cirrostratus clouds that overlies and obscures the pinwheel like structure of the storm That explains why hurricane on satellite images often appears to have a uniform thickness and intensity when in fact they are strongly banded 60 What are the eye and the eye wall in a hurricane The M is one of the most distinctive characteristics of a hurricane It is a region of relatively clear skies slowly descending air and light winds Hurricane eye diameters vary from 20 to 50 km average about 25 km Generally a shrinking eye indicates an intensifying hurricane Whv are there almost calm conditions in the eve Remember the concept of conservation of angular momentum expressed as Mangular mrv Conservation of angular momentum means by approaching the center of the hurricane eye or reduction of the radius the speed should increase tremendously at the center Instead we have quiet conditions Why Because the hurricane doesn t have an in nite amount of energy to support such huge speeds So in order not to avoid the conservation of angular momentum the air must stop short of reaching the center The Eye wall is the margin of the 5 It is the zone of most intense storm activity and contains the strongest winds thickest cloud cover and most intense precipitation directly beneath the eye wall of the entire hurricane 2500 mmd 100 ind are not uncommon What happens when a hurricane makes a direct hit on a small island As the hurricane eye approaches the island the intensity of rain and wind steadily increase becoming most intense as the eye wall arrives When the eye approaches the island the storm seems to suddenly dissipate as blue skies and calm conditions resume Assuming that the average hurricane eye is about 20 km in diameter and travels at about 20 kmh the calm associated with the passage of the eye lasts about an hour The other half of the storm will arrive in about an hour with intense rain and wind at the beginning which disappear over time Hurricane formation It starts from small clusters of small thunderstorms called tropical disturbances in the eastern portion of the ocean A few of them undergo a process in which they grow in size join together and rotate around a common center Easterly waves trade winds move these hurricanes toward the western portion of the oceans When a tropical disturbance develops to the point where there is at least one closed isobar on the weather map the disturbance is classi ed as a tropical storm with wind speeds up to 120 kmh 74 mph A further increase in sustained wind speed creates a true hurricane Conditions necessapy for hurricane formation They form over the oceans latitudes of about 30 11S to 30 11N with a deep surface layer several tens of meters in depth with temperatures above 27 DC 81 DF In the Northern Hemisphere June through December is the ideal months for having the high ocean water temperatures 61 Hurricane formation also depends on a strong Coriolis force The absence of a Coriolis Effect at the equator prohibits hurricane formation between about 5 DS to 5 0N Hurricane ent and quot Tropical storms and hurricanes have a tendency to move north to northeast out of the Tropics along the southeast coast of North America The gure below shows their erratic paths The paths of Hurricanes Mitch Floyd and Lili are depicted below After making landfall the wind intensity decreases to tropical storm level A tropical storm may die out completely within a few days Even as the storm weakens though it can still import huge amounts of water and bring heavy rainfall hundreds of kilometers inland Hurricane destruction and fatalities Hurricane winds exceed 120 kmh and many are much faster The hurricane force wind can cause extensive damage and destroy even well built homes Many hurricanes also contain clusters of tornadoes which tend to have a shorter life span than tornadoes in the central US most often in the right forward quadrant see the gure below In addition to the threat of heavy rain strong winds and tornadoes coastal regions are also vulnerable to a special problem called storm surge a rise in water level induced by a hurricane A storm surge of 7 m 23 ft was reported in the case of Hurricane Camille along the coast of Mississippi in 1968 The naming of hurricanes The World Meteorological Organization WMO has created several lists of names for hurricanes over each of the oceans See the tables below The following gure shows why hurricane winds and storm surges are most intense on the right hand side of the storm Hurricane forecast and advisories The National Hurricane Center NHC in Miami Florida is responsible for tracking and predicting Atlantic and east Paci c hurricanes They use sophisticated numerical models on a supercomputer to predict the formation growth and movement of tropical storms and hurricanes When active hurricanes approach land specially equipped aircraft y into the storms and provide additional reconnaissance data from airborne radar and dropsondes packages containing temperature pressure and moisture sensors and transmitters released from the plane into the storm The NHC also uses standard computer models including statistical dynamical and hybrid models for conventional weather forecasting Statistical models apply information on past hurricane tracks and use them as predictors for current storms 62 Dynamic models take information on current atmospheric and sea surface conditions and apply the governing laws of physics to the current data Hybrid models combine elements of statistical and dynamical models The National Oceanic and Atmospheric Administration NOAA has recently deployed new geostationary satellites GOES 10 and GOES 12 to provide improved data acquisition So far the position of hurricanes at about 160 km 100 mi can be reported by complex models When forecasters at the NHC predict that an approaching hurricane will reach land in more than 24 hours they issue a hurricane watch If it is expected to make land fall over the US within 24 hours they issue a hurricane warning Hurricane intensity scale Like the Fuji scale for tornadoes the Saffir Simpson Scale is used to rate hurricane damage see the table below 63
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