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1. Exam 1 Review

   Weather Forecasting Handbook Ch. 1-5
2. Time Zones
3. Common Meteorological Variables The major meteorological variables of study in this course are Pressure Geopotential height Temperature Dew point temperature Relative humidity Wind speed and direction
4. Atmospheric Pressure Atmospheric pressure is the force per unit area of a column of air above you Atmospheric pressure arises from gravity acting on a column of air. Pressure is the weight of the air above you The units of pressure are given in hPa or millibars. 1 hPa = 1 mb = 100 Pa 1 inch of mercury = 33.86 mb
5. Pressure Field Isobars – Lines of constant pressure at fixed elevation. If pressure increases (decreases) radially outward, then the isobar is positively (negatively) curved. High pressure center – local maximum in the pressure field Low pressure center – local minimum in the pressure field
6. Geopotential Height The geopotentialis defined as the work required to raise a unit mass a distance dzabove sea level. Therefore, geopotential is given as The geopotential height is defined as Geopotential height is typically plotted on upper air maps, whereas pressure is typically plotted on surface maps
7. Troughs and Ridges Isopleths– Lines of constant geopotential height at a fixed pressure. Ridge axis: Locus of maximum negative curvature Trough axis: Locus of maximum positive curvature
8. Negatively Tilted Trough Tilted troughs are those whose axes are not meridionally oriented. Negatively tilted troughs lean toward the west with increasing latitude Negatively tilted troughs are usually indicators of potential severe weather
9. Positively Tilted Trough Positively tilted troughs lean toward the east with increasing latitude. A positively tilt trough often is a sign of a weakening weather system, and generally is less likely to result in severe weather than a negative-tilt trough.
10. Kinematics of the Pressure (Height) Field Troughs that have a component of motion toward the equator (poles) are said to be digging (lifting out). When the pressure in a high or along a ridge rises (falls), it is said that the high or ridge is building (weakening). When the pressure in a low or along a trough falls (rises), it is said that the low or trough is deepening (filling).
11. Pressure (Height) Tendency Lines of constant pressure tendency (height tendency) are called isallobars (isallohypses). Lows tend to move from an adjacent region of greatest pressure rises toward an adjacent region of greatest pressure falls
12. Pressure (Height) Tendency High tend to move from an adjacent region of greatest pressure falls toward a region of greatest pressure rises. Lows (highs) will form where pressures are falling (rising) most rapidly Troughs (ridges) will form along lines where pressures are falling (rising) most rapidly.
13. Temperature Temperature is a measure of the motion (mean kinetic energy) of air molecules. The units of temperature are usually given in Fahrenheit, Celsius, or Kelvin (9/5 x °C) + 32 = °F (°F – 32) x 5/9 = °C K = °C + 273.15 All of the weather in the atmosphere occurs within the troposphere where the temperature generally decreases with height
14. Temperature Field A line of constant temperature is called an isothermand an isallotherm is a line of constant temperature tendency. A ridge (trough) in the temperature field is sometimes called a warm tongue (cold tongue). A local minimum in the temperature field is sometimes referred to as a cold pool.
15. Humidity Humidity is the amount of water vapor in the air. The most common and useful way for meteorologists to measure humidity is the dew point. Dew point temperature is the temperature at which the air would become saturated (and dew would begin forming)
16. Dewpoint Temperature A line of constant dewpointtemperature is called an isodrosotherm. A ridge (trough) in the temperature field is sometimes called a moist tongue(dry tongue).
17. Other Moisture Variables Dewpoint depression: the difference between the actual air temperature and the dew point temperature Relative humidity: The ratio of the vapor pressure to the saturation vapor pressure Here, e is the vapor pressure and es is the saturation vapor pressure. The saturation vapor pressure is a function of temperature – saturation vapor pressure increases as temperature increases
18. Relative Humidity A line of constant humidity is called an isohume. A ridge in the water-vapor field is often called a moist tongue or moist axis A trough in the water-vapor field is sometimes called a dry tongueor dry slot.
19. Wind Wind is air’s response to pressure differences Wind speed can be measured in miles per hour (mph), knots (kt), or meters per second (m/s) 1 knot = 1.15 mph = 0.514 m/s Wind direction tells us FROM WHERE the wind is blowing Wind direction is measured in degrees (like a compass, not like the degrees in math).
20. Advection of Scalar Fields Since the atmosphere continuously evolves in time, the temperature and moisture fields also evolves in time. Advection described the rate of importation of any scalar variable. When warm (cold) air is imported into a region by the wind field, it is described as warm (cold) advection. When moist (dry) air is imported into a region by the wind field, it is described as moisture (dry) advection.
21. Horizontal Wind Field The horizontal wind field can be decomposed into three types of flow patterns: divergence, vorticity, and deformation. Divergence is composed of two components: Speed divergence Directional divergence (i.e. diffluence/confluence) Vorticity is composed of two components: Shear vorticity Curvature vorticity Total deformation is composed of two components: Shearing deformation Stretching deformation
22. Divergence Divergence/convergence can occur when There is a change in wind speed along a streamline (speed divergence) There is a change in wind direction perpendicular to the flow (directional divergence). When the flow spreads out (contracts) downstream, this is called diffluence(confluence).
23. Divergence Generally speaking, synoptic-scale values of divergence are very small. Thus, stretching of air parcels (speed divergence) is usually accompanied by confluence. Shrinking of air parcels (speed convergence) is usually accompanied by diffluence.
24. Vorticity The production of vorticity is due to A change in wind direction (curvature vorticity) A change in wind speed (shear vorticity) Cyclonic vorticity (or positive vorticity) can be produced by Increasing wind speed moving away from a trough Positive curvature in the wind flow Anticyclonicvorticity (or negative vorticity) can be produced by Decreasing wind speed moving away from a ridge Negative curvature in the wind flow
25. Deformation For a field of pure stretching deformation, the flow is stretched along the axis of dilatation, while it’s compressed along the axis of contraction. Stretching deformation deform the air parcel through speed divergence and confluence. For a field of pure stretching deformation, the flow is similar to the stretching deformation rotated counterclockwise by 45⁰ We are usually most interested in the total deformation field given by Deformation tends to be extremely important for intensifying surface fronts
26. Station Plot Format for Surface Map PPP = sea-level pressure pp = pressure tendency over the past 3 hours a = overall pressure tendency RR = precipitation TdTd = dewpoint temperature VV = visibility in miles ww = current weather TT = temperature dd = wind direction ff = wind speed N = cloud cover
27. Station Plot for Constant Pressure Map TT = Temperature (°C) DD = Dewpoint or Dewpoint Depression (°C) HHH = Geopotential Height in meters hhh = 24-hr change in height of the pressure surface
28. METAR METAR is a format for reporting weather information. Raw METAR is the most popular format in the world for the transmission of weather data. This METAR example is from Trenton-Mercer Airport near Trenton, New Jersey, and was taken on 5 December 2003 at 18:53 UTC. METAR KTTN 051853Z 04011KT 1/2SM VCTS SN FZFG BKN003 OVC010 M02/M02 A3006 RMK AO2 TSB40 SLP176 P0002 T10171017=
29. General Rules for Drawing Contours Contours pass through stations with the same value as the contour LABEL CONTOURS Remember to draw contours at the specific intervals Do not draw contours beyond where data exist Contours do not cross, fork or end in the middle of the map if data exists there Always keep in mind the physical nature of the variable you are analyzing (i.e. pressure vs. temperature)
30. Fundamental and Apparent Forces The collection of forces required to adequately represent Newton’s second law on the rotating Earth can be split into two broad categories: fundamental forces and apparent forces. The most important fundamental forcesare: The pressure gradient force The gravitational force The viscous force Two important apparent forces to be investigated are: The centrifugal force The Coriolisforce
31. Pressure Gradient Force Consider the pressure exerted by the atmosphere on sides A and B of the fluid element. The pressure exerted on sides A and B arises from the fact that random molecular motions compel molecules to strike the sides. Each time a molecule strikes the side of the fluid element, a certain amount of momentum is transferred to that side. By Newton’s 2nd law, the total momentum transferred each second defines the force exerted by the atmosphere on the side of the fluid element.
32. Pressure Gradient Force Physically, the pressure gradient force demonstrates that any spatial change of pressure in the atmosphere leads to the acceleration of air parcels The pressure gradient results in a net force that is directed from high to low pressure. The pressure gradient force is responsible for triggering the initial movement of air
33. Gravitational Force Newton’s law of universal gravitation says that any two elements of mass in the universe attract each other with a force proportional to their masses and inversely proportional to the distance between their centers of mass. This is represented mathematically as For an air parcel in the atmosphere, M is the mass of the Earth and m is the mass of the air parcel.
34. Gravitational Force We can express the gravitational force per unit mass as For synoptic-scale motions, air parcels remain in the troposphere (lowest 10-12 km of the atmosphere). Therefore, we can use an approximation of the gravitational force per unit mass
35. The Viscous Force The effects of friction dominate near the surface where the momentum of air parcels is lost due to frictional dissipation with the surface. The region of the atmosphere where viscous forces become important is called the boundary layer. Surface friction tends to weaken the wind field and cause the wind field to cross isobars.
36. Centrifugal Force Consider an object moving in uniform circular motion. From the perspective of the object, there is a centripetal force acting on the object. From the perspective of a person rotating with the ball, the ball is stationary. In order for a person to apply Newton’s laws under this condition, an apparent force that exactly balances the true centripetal force must be included. This force is called the centrifugal force and is directed outward along the radius of rotation and its magnitude is given by
37. Centrifugal Force On a rotating Earth, the centrifugal force affects the vertical force balance. When the centrifugal force and the gravitational force are added, the result is called effective gravity and is given by Here, is the rotation rate of the Earth, and is the position vector from the axis of rotation to the object in question.
38. Coriolis Force Consider a moving object on a rotating Earth. The Coriolis force deflects objects to the right of its original path in the northern hemisphere. We see that given an eastward (westward) directed impulse, the Coriolis force will deflect the object to the south (north). The Coriolisforce is proportional to the velocity and to the latitude.
39. Coriolis Force Equatorward motion in the northern hemisphere will induce a westward-directed zonal motion. Here, the Coriolis force compels an object to the right of its intended path. Since the Coriolis force always acts perpendicular to the velocity vector, it can do no work on the moving parcel. Thus, the Coriolis force can only change the direction of motion but cannot initiate motion in an object at rest.
40. Thickness Meteorologists often think of the atmosphere in layers, that is, from one height to another, or from one pressure level to another. The vertical distance between two pressure levels is called that layer's thickness.
41. Thickness Less (more) dense air will correspond to a greater (smaller) thickness. Less (more) dense air will correspond to a higher (lower) average temperature. Therefore, the temperature should have a bearing on the thickness between two isobaric levels.
42. Thickness Equation The thickness equation relates the thickness between two isobaric layers with the average temperature The thickness equation can also be written in terms of the geopotential
43. Applications of the Thickness Equation The thickness equation allows us to make a general relationship between temperature and geopotential heights on isobaric maps. As a rule, warmer temperatures in the lower troposphere imply higher geopotential heights at upper levels. Since warm air leads to a greater thickness than cold air, thickness is another easy way to diagnose where warm and cold air masses are present in the atmosphere. Thickness can also be used to forecast the precipitation type by determining the rain-snow line.
44. Applications of the Thickness Equation The thickness equation allows us to calculate a reduced sea-level pressure, which is an estimate of what the sea-level pressure would be were the surface elevation 0 m. This allows us to compare the sea-level pressures for surface observation stations at different elevations. Rearranging the thickness equation to solve for the reduced sea-level pressure gives the altimeter equation.
45. Fundamental Conservation Laws in the Atmosphere There are three fundamental conservation laws that govern the evolution and motion of the atmosphere: conservation of momentum, mass, and energy. Conservation of momentum leads to the balanced flow conditions in the atmosphere Conservation of mass leads to the relationship between divergence and vertical motion Conservation of energy leads to the relationship between temperature advection and vertical motion
46. Conservation of Momentum Newton’s 2nd law is a statement of the conservation of momentum A scaling analysis of the momentum equation for synoptic-scale motions show The two dominant vertical forces are the vertical pressure gradient force and gravitational force, leading to hydrostatic balance The two dominant horizontal forces are the horizontal pressure gradient force and the Coriolis force, leading to geostrophic balance
47. Hydrostatic Balance For sufficiently small vertical accelerations, the vertical momentum equation reduces to the hydrostatic equation Hydrostatic balance represents the fundamental vertical balance condition in the Earth’s atmosphere. Hydrostatic balance is obeyed to great accuracy under nearly all conditions in the Earth’s atmosphere.
48. Hydrostatic Balance Recall that mass can be described as the measure of the substance of an object. Hydrostatic balance implies that the pressure exerted by Earth’s atmosphere decreases with increasing distance away from the surface. This implies that the mass of the atmosphere also decreases with height.
49. Geostrophic Balance For sufficiently small horizontal accelerations, the dominant balance will be between the horizontal pressure gradient force and the Coriolis force. To first order, the atmosphere is in geostrophic balance horizontally. PGF is always directed from high to low pressure For balance, CF must be equal and opposite to the PGF as depicted The CF displaces parcels to the right of its motion, causing the air parcels to flow parallel to isobars.
50. Geostrophic Wind The wind field associated with geostrophic balance is called the geostrophic wind. The geostrophic wind can be determined from the expression of geostrophic balance in isobaric coordinates Writing the above expression in component form gives
51. Gradient Balance Gradient balance exists in regions of curved isobars or curved isopleths Gradient balance represents the force balance between the pressure gradient force, Coriolis force, and centrifugal force.
52. Mass Continuity Equation Physically, the mass continuity equation states that regions of local convergence (divergence) leads to an increase (decrease) in mass. The mass continuity equation also demonstrates that horizontal convergence (divergence) leads to rising (sinking) motion in the atmosphere.
53. Air Masses Continental Polar, “cold and dry” Originates closer to the Poles over land-locked regions. Continental Tropical, “warm and dry” Originates closer to the Tropics over land-locked regions. Maritime Polar, “cold and damp” Originates closer to the Poles over water. Maritime Tropical, “warm and humid” Originates closer to the Tropics over water. Arctic, “very cold” Originates in the very cold land-locked areas
54. Characteristics of Fronts Fronts are sloping zones of pronounced transition in the thermal and wind fields typically characterized by Horizontal temperature gradients Vertical wind shear Static stability Cyclonic vertical vorticity Ascending air, clouds, and precipitation near the front Greatest intensity near the surface, weakening with height
55. Potential Temperature An additional variable of meteorological consequence which arise from further consideration of the thermodynamic energy equation is the potential temperature It is the temperature a parcel of air would have it were adiabatically compressed (or expanded) from its original pressure, p, to a reference pressure, (usually 1000 hPa).
56. Uses of Potential Temperature Potential temperature can be used to compare the temperature of air parcels that are at different levels in the atmosphere and can be used to predict temperature advection. If the potential temperature of an air parcel at one pressure level is colder than air parcels at other pressure levels, a forecaster can infer cold advection at the pressure level with the lowest potential temperature.
57. General Rules for Locating Fronts Rule 1: Look for a strong temperature gradient Rule 2: Look for a strong dewpoint gradient Rule 3: Look for strong pressure gradient Rule 4: Look for a strong wind shift Rule 5: Check cloud and precipitation patterns
58. Frontal Location and Upper Air Maps Upper air maps can also be useful for identifying fronts 850 mb map: Used for identifying regions of temperature and moisture advection 500 mb map: Fronts are usually guided by the geostrophic winds at upper levels 1000-500 mb thickness: Thickness advection is related to temperature advection 300 mb map: Jet streaks are usually located in the vicinity of synoptic fronts
59. Location of Cold Fronts Zone between warmer, moist, unstable air being replaced by colder, drier, more stable air. Location of cold front: Leading edge of sharp temperature change Moisture content (dew point) changes dramatically Wind shift (direction and speed) Pressure trough (pressure tendency is useful) Often cloudy with showers or thunderstorms
60. Passage of Cold Front
61. General Structure of Cold Fronts Warm air ahead of front is lifted up Cs and Ci clouds are blown ahead of the front by upper level winds Cloud base is generally lower behind the front Steep frontal boundary, slopes backward into cold air
62. Katafront Structure The katafront is one where the component of flow perpendicular to the frontal zone is faster than the frontal speed and is downward toward the surface. The downward flow leads to compression and warming and subsequent evaporation of any moisture and cloud droplets. This results in a long, narrow band of precipitation oriented along and parallel to the surface front
63. Katafront Structure The ascending warm conveyor belt is overrun by the dry intrusion. The dry air originates from upper levels of the troposphere and crosses the cold front from behind. The warm conveyor belt acquires a component which is inclined forwards relative to the movement of the cold front. The cloud tops in the area of the dry airstream are relatively low, whereas on the leading edge of this area the cloud tops are higher.
64. Anafront Structure A cold front at which the warm air is ascending the frontal surface up to high altitudes is called an anafront. The cold air moves rapidly against warm air, creating convergence and upward motion along the frontal surface. This upward motion is not as vigorous and occurs over a broader area, generating an expanded precipitation shield. A cold anafront is akin to a warm front in reverse. The developing cloud band is inclined rearward with height and the main zone of precipitation is located behind the surface front.
65. Anafront Structure The frontal cloud band and precipitation are related to an ascending warm conveyor belt, causing the frontal cloud band and precipitation to appear behind the surface front. Parallel to the warm conveyor belt there is a dry intrusion. The sharp rear cloud edge of frontal cloudiness marks the transition between the two relative streams.
66. Identifying Warm Fronts Location of warm front: Leading edge of moderate temperature change Moisture content (dew point) changes dramatically Wind shift (direction and speed) Pressure trough (pressure tendency is useful) Often cloudy with weak precipitation Note that isobars kink along the frontal boundary
67. Passage of Warm Front
68. General Structure of Warm Fronts Zone between advancing warmer, moist air and cooler, drier air Frontal surface has a much smaller slope than for cold fronts Often produces widespread Ns precipitation near front
69. Conveyor Belt Model of Warm Fronts Frontal cloud band and precipitation are in general determined by the ascending warm conveyor belt, which has its greatest upward motion between 700 and 500 hPa. The warm conveyor belt starts behind the frontal surface in the lower levels of the troposphere, crosses the surface front and rises to the upper levels of the troposphere. There the warm conveyor belt turns to the right (anticyclonically) and stops rising, when the relative wind turns to a direction parallel to the front. If there is enough humidity in the atmosphere, the result of this ascending warm conveyor belt is condensation and more and more higher cloudiness.
70. Conveyor Belt Model of Warm Fronts The cold conveyor belt in the lower layers, approaching the warm front perpendicularly in a descending motion, turns immediately in front of the surface warm front parallel to the surface front line. From there on the cold conveyor belt ascends parallel to the warm front below the warm conveyor belt. Due to the evaporation of the precipitation from the warm conveyor belt within the dry air of the cold conveyor belt, the latter quickly becomes moister and saturation may occur with the consequence of a possible merging of the cloud systems of warm and cold conveyor belt to form a dense nimbostratus.
71. Occluded Fronts Occluded fronts develop when the cold front of a midlatitude cyclone overtakes the warm front. Cold occlusion: Cold front undercuts the warm front Warm occlusion: Warm front undercuts the cold front
72. Frontal Motion Fronts are generally guided by upper level winds – therefore, frontal movement is governed by the Coriolis force and the pressure gradient force. Rising (falling) pressure on the cold (warm) side of a cold front causes the front to move toward the warm air. The best method to use to predict frontal movement is the Continuity/Extrapolation method Trend based on the entire history of past movements of the front Front will stall or accelerate if the low stalls or accelerates
73. Frontal Motion: Cold Front Cold fronts most often extend from the SW quadrant of a low, but may extend from the W or NW quadrant. Active cold fronts move toward the SE at an average of speed of 15-25 kts. Depending on the length of the cold front, portions of the front may move toward the E, while other portions move S. The portion nearest the low may be elongated to move NE with the low’s movement.
74. Frontal Motion: Warm Front Warm fronts most often extend from the NE through SE quadrant of a low-pressure system. They generally move toward the NE at an average speed of 10 kts. Warm fronts speed up (slow down) during the day (night) due to heating (cooling). In the day, mixing occurs on both sides of the front. At night, radiational cooling creates cool dense surface air behind the front  inhibiting lifting and forward progress
75. Kinematics of Frontogenesis Frontogenesis: Increase in the magnitude of the horizontal density gradient Frontolysis: Decrease in the magnitude of the horizontal density gradient This is typically determined through the analysis of horizontal θ gradients in dry environments. Frontogenesis is usually caused by three forcing mechanisms Convergence and deformation Differential heating Horizontal shear
76. Horizontal Shear Shear frontogenesis describes the change in front strength due to differential temperature advection by the front-parallel wind component Frontogenesis occurs when there is significant cold (warm)-air advection in the cold (warm) air. t=0 t=+24 Example: positive contribution to F along the cold front: shearing frontogenesis
77. Confluence/Diffluence Confluence frontogenesis describes the change in front strength due to stretching. If the isotherms are stretching (spreading out), there is frontolysis. If they are compacting, frontogenesis is occurring.
78. Deformation Frontogenesis occurs when the isotherms are aligned closely parallel to the axis of dilatation Frontolysis occurs when the isotherms are aligned closely perpendicular to the axis of dilatation
79. Differential Heating The differentialdiabatic heating term takes into account all diabatic processes together: Differential solar radiation, differential surface heating due to soil characteristics, differential heat surface flux, condensation, and evaporation Whenever the diabatic heating rate is greater in the warm air than in the cold air, frontogenesis occurs. Whenever the diabatic cooling rate is greater in the cold air than in the warm air, frontogenesis occurs. Example: positive contribution to F along a front: differential diabatic heating
80. Frontogenesis in Forecasting Frontogenesis generally accompanies cyclogenesis For a typical developing midlatitude cyclone, three main frontogenesis areas are: Deformation frontogenesis with cold front Deformation (confluence) frontogenesis with warm front Convergence frontogenesis near low center (generally on portion of warm front closest to the low center)
81. Type I: Significant Surface Low Pattern Frontogenesis is common in the developing and mature stages of a low pressure system, but not in the dissipating stage when precipitation rates decrease. In mature systems, frontogenesis and potentially heavy “wrap around precipitation” can occur to the NW of the surface cyclone.
82. Type II: Frontal/ Weak Surface Low Pattern In these situations look for Confluent flow around 700mb in advance of a positively tilted trough Deformation and convergence creating frontogenetical forcing N of the front, resulting in a band of precipitation Adequate moisture and sufficient low-level baroclinicity
83. Coastal Fronts Coastal fronts are common along the Gulf Coast and the East Coast of the US. Coastal fronts are low-level baroclinic zones prevalent in the late fall and winter that separate relatively warm maritime air originating over the ocean from cold continental air originating from an anticyclone over land. Coastal fronts are not synoptic fronts since they usually occur in the absence of synoptic-scale frontogenesis.
84. Coastal Fronts Coastal front formation depends on a land-sea temperature contrast and large onshore flow Other important factors are: Surface friction Orography Coastal configuration