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Unformatted text preview: FRONTS, FORECASTING TECHNIQUES AND UPPER AIR TECHNIQUES MSC 243 Lecture #6, 10/1/09 Heads-up: Mid-Term next Thursday: October 8th Decoding short-range MOS forecasts Decoding METAR code, plotting station models Drawing isobars and isotherms (contours) from Drawing station models station Interpreting Satellite Imagery Interpreting Radar Imagery Finding surface weather features such as fronts, high Finding and low pressure systems and Reading assignment Vasquez: “Weather Forecasting Handbook” Vasquez: 1.5: Pressure Coordinate System 3.1: Chart Analysis (esp. 3.1.3-3.1.8) 5.1-5.7: Air Masses and Fronts FRONTS FRONTS The interface or transition zone between two air masses of different temperature. Air Masses Air Air masses are large semi-permanent volumes of air that exist in a specific area. Air masses are named for their source region and characteristics. Maritime or Continental Arctic, Polar, or Tropical When air-masses collide, fronts form. What is a front? What The boundary between two air masses Named for the advancing air mass: Cold Front Warm Front Stationary Front Occluded Front In any collision of air-masses, the denser air In wins! wins! Colder Drier Structure of a Cold Front Structure A “lifting mechanism” causes moist air to rise and form clouds that produce precipitation. For a cold front, moist warm air is forced by the denser cold air to rise above the cold dome. NW SE Structure of a Warm Front South North A Cold Frontal Passage Before a cold front approaches, winds are often from Before the S/SW/SE. Air is relatively warm and moist. the As front approaches, pressure drops, it gets cloudier As (cirrus then cumulus/cumulo-nimbus) and it often rains or snows heavily. rains After front has passed through, winds are from the N/ NW, air is relatively cold and dry, pressure starts to NW, increase as a high builds over the region. Sunny days and cold, calm nights. and A Warm Frontal Passage Passage Before a warm front approaches, air is relatively cool Before and dry. and As front approaches, pressure does not change As significantly, low-level stratus appears with accompanying drizzle / light rain or snow. accompanying After front has passed through, winds are from the After S/SW/SE, air is relatively warm and moist, Cloudy, relatively humid days and mild nights due to clouds absorbing IR radiation from Earth. absorbing Stationary Front Stationary Neither the cold nor warm air masses move much in any direction. Layered, stratus-type clouds, sometimes accompanied by light precipitation of long duration Occluded Front Occluded Cold Front catches up with Warm Front Types of fronts Types Cold Front Cold air advances Steep slope (vertically) Convective Clouds, sometimes heavy rain and snow Cold air retreats Gradual Slope (vertically) Stratiform clouds, light rain or snow Neither air mass making much headway When a cold front catches a warm front A gradient in moisture, rather than temperature Warm Front Stationary Front Occluded Front Dry Line (not technically a “front”) Characteristics of Fronts Characteristics Can discern differences in: Temperature Dew Point Low surface pressure Wind direction Clouds and precipitation Find the Front Front Fronts can be located by looking for: •A Wind Shift •Temperature differences •Dew Point Temperature differences •Clouds or Precipitation •Lower surface pressures Surface Weather Map Surface All stations are not at the same elevation. All Pressure decreases with height. Hence, all pressure readings at the surface are Hence, adjusted to sea level This allows accurate comparison of horizontal This differences in pressure and eliminates vertical differences in pressures due to elevation. differences Isobar - line of equal pressure Isobar Units: millibars or Pascals (Pa) (mb = 100 Pa) Units: Standard atmospheric pressure = 1013.25 mb Surface Pressure and Winds Surface Surface charts show surface high and low pressure centers, fronts, wind directions, and are good for determining temperature advection. Surface Weather: Advection Surface Red circle: Warm advection. Winds bringing warmer air from the south. Blue circle: Cold advection. Winds bringing cooler air from the north. Surface Temperature: Fronts Surface Cold front is easy to discern: strong temperature gradient with warmer temperatures to the south. Forecasting Summary Guide Forecasting 1. NORTH AMERICA SURFACE MAP: What are the major weather features? What is the big picture? What air masses will I be concerned with? Any pending frontal passages? 2. SATELLITE: How are these major weather features moving? This gives you a quick impression of speed of these systems. 3. RADAR: Where is precipitation occurring? How intense are these systems? Forecasting Summary Guide Forecasting 4. METAR OBSERVATIONS: What is happening locally? What happened last night and today? Link with what you see here with what you would expect to happen based on your conceptual models developed for the weather features you have identified in items #1,#2, and #3. 5. CLIMATOLOGY: What is normal? What are the extremes/records typically experienced? Will any of the weather features identified in the previous questions allow the approach of extremes or will they be typically normal? 6. LOCAL STATION MODEL OBSERVATIONS: Begin to focus more on the small scale? How fast are nearby fronts moving? Any other interesting temperature, wind, or moisture patterns they will impact your forecast? Forecasting Summary Guide Forecasting 7. (LECTURES 7 and 8) UPPER LEVEL FLOW: Now we need to link the upper air weather flow pattern to the surface weather features? Is the upper air flow zonal or meridional? Is your location downstream or upstream of a upper air trof or ridge? Is it likely that there will be upper air divergence or convergence? What does all this mean for the strengthening or weakening of any surface features? Moreover, how will this intensity change, if any, alter the magnitude and timing of the surface forecast parameters? Identify large scale temperature and moisture advection? Jet stream position? Forecasting Summary Guide Forecasting 8. LOCAL EFFECTS: How will topography affect this forecast? Near mountains, big lakes, the ocean, in a valley, downtown? Downsloping vs Upsloping? 9. NUMERICAL MODEL OUTPUT: Now that you have an idea of the future weather for a forecast region after looking at a number of tools, what do the numerical models say will happen? Are you close? If so, great! If not, decide what you are missing by focusing on each forecasted parameter that is in question and what factors in turn affect that parameter. After thoroughly checking your forecast, if in your own mind you still can not rationalize the difference, stick to your guns! Models are often wrong! 10. MOS NUMBERS: Look at more specific model information from MOS and grid point data. Adjust these numbers depending on how your assessment of things compared with what the models had to say. High Temperature Forecasting High Three primary factors: ADVECTION Warm advection results in temperature rises Cold advection results in temperature falls Even advection above the surface can affect surface Even temperatures. temperatures. ADIABATIC WARMING / COOLING (will leave until later) DIABATIC WARMING / COOLING … Diabatic Effects Diabatic Factors affecting incoming solar radiation: Cloud cover Type (thickness) Duration Time of Day Ground Moisture / Vegetation If dew points are lower than the air temperature, falling If precipitation will cool temperatures to the wet-bulb temperature (in between temperature and dewpoint) temperature Low Temperature Forecasting Low Factors that promote cool minima: Clear Skies Enhanced radiational cooling Surface decoupling Enhanced radiational cooling / insulates surface from Enhanced ground below (reduces heat flux below it) ground Water Vapor good absorber of IR radiation, i.e. less Water radiation is absorbed in the atmosphere if dew points are low low Light Winds Snow Cover Low Dew Points Low Temperature Forecasting Low Factors that promote warm minima: Clouds / Fog Absorb IR radiation emitted from ground etc. Keep the boundary layer mixed High heat capacity of city versus country Water Vapor good absorber of infrared radiation Strong Winds Urban Heat Island High Dew Points Upper Levels Upper So far, we have only looked at surface weather So features. features. However, the upper levels are crucially However, important for the development of weather systems, and hence their forecasts. systems, Pressure Levels Pressure Pressure is the force exerted on an object by all air molecules that impinge on a surface area – in general, the weight of a column of air per unit area Pressure decreases with height. Meteorologists concentrate on a few standard pressure levels, plus the surface Each of these levels are important in weather forecasting for different reasons Upper Level Weather Maps Upper Upper level weather maps are plotted on a constant pressure surface Contours of equal geopotential height are plotted (height in meters of the "x" mb pressure surface) Thickness determines temperature of the layer (e.g. 1000 - 500 mb). Thickness is useful in determining precipitation type Aloft Ridges and Troughs Aloft Mountains and valleys of warm and cool air The height of the pressure level depends on the relative The temperature of the column temperature Increasing Height Ridge 500 mb Ridge Trough 500 mb Trough 700 mb 850 mb Surface Very warm column Cool column Warm column Very cool column Height of Pressure Surfaces Height Pressure Surface 850 mb 700 mb 500 mb 300 mb 200 mb feet feet Average Height 1460 m / 5000 feet 3000 m / 10000 feet 5600 m / 16000 feet 9180 m / 30000 feet 11800 m / 35000 11800 850 mb Chart 850 The 850 mb chart is good for identifying warm and cold advection, estimating surface temperatures, low level moisture, and determining precipitation type (rain, snow, sleet). 700 mb chart 700 The 700 mb chart is used to determine cloud cover or rainfall, using the relative humidity field and the vertical motion field. 700 mb chart 700 The 700 mb chart is also used to determine short-wave disturbances via the geopotential height field. 500 mb geopotential height 500 The 500 mb chart is the forecasters’ favourite for depicting the motion of weather systems. It shows the large-scale flow (long waves) and jet streams, and also the smallscale flow (shortwaves, low level storm systems) RIDGE TROUGH 250 mb Chart 250 The 250 mb chart is used to locate the jet stream. Strong upperlevel winds help develop surface low pressure in mid-latitudes. ...
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This note was uploaded on 10/08/2010 for the course GEOG 1172 taught by Professor Snyder during the Summer '10 term at Minnesota Colleges.

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