Lecture5_092211 - FORECASTING TECHNIQUES AND UPPER AIR AND...

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Unformatted text preview: FORECASTING TECHNIQUES AND UPPER AIR AND MSC 243 Lecture #5, 9/22/11 Heads-up: Mid-Term on Thursday October 6th Decoding short-range MOS forecasts Decoding METAR code, plotting station models Drawing isobars and isotherms (contours) from Drawing station models station Interpreting Satellite and Radar Imagery Finding surface weather features such as fronts, high Finding and low pressure systems and Upper air dynamics 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 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 Pressure obs on surface maps Pressure 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 WxChallenge WxChallenge http://www.wxchallenge.com Starts on Monday 26th. First City: Miami! First day forecast: for the period of 06 UTC First Tue 27th through to 06 UTC Wed 28th Tue (2am Tue morning to 2am Wed morning) (2am On Tuesday, we will have our first forecast lab Check whether you can enter a forecast. 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. 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 type / 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 Light Winds Enhanced radiational cooling, ground cools fast, longwave Enhanced radiation is not absorbed by clouds radiation Surface decoupling (less advection and mixing) Low Dew Points Water Vapor good absorber of IR radiation, i.e. less Water radiation is absorbed in the atmosphere if dew points are low low Low Temperature Forecasting Low Factors that promote warm minima: Clouds / Fog Strong Winds Keep the boundary layer mixed Urban Heat Island Absorb IR radiation emitted from ground etc. High heat capacity of city versus countryside High Dew Points Water Vapor good absorber of infrared radiation 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 (e.g. height in meters of the 500 mb pressure surface) Thickness is the difference in height between 2 pressure sfcs. It is directly proportional to the mean temperature of the layer (e.g. 1000 - 500 mb). Thickness is useful in determining precipitation type. Ridges and Troughs Aloft Ridges Mountains and valleys of warm and cool air The height of the pressure level depends on the The temperature of the column of air below it temperature Increasing Height Ridge Ridge 500 mb 500 mb Trough 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 Typical Height 1500 m / 5000 feet 3000 m / 10000 feet 5500 m / 18000 feet 9000 m / 30000 feet 12000 m / 39000 feet Height on a pressure surface is analogous to pressure on a height surface! Surface Chart Surface Typically look at mean sea level pressure (isobars), precipitation, and winds and temperatures to identify surface weather such as fronts. These are the conditions that affect us directly. The behavior of these systems is largely governed by what is going on in the upper troposphere. 850 mb Chart 850 The 850 mb chart is good for estimating surface temperatures, low level moisture, and determining precipitation type (rain, snow, sleet). 850 mb Chart 850 The 850 mb chart is good for 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 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 chart RIDGE TROUGH The 500 mb chart is the forecasters’ favorite for depicting the motion of weather systems. It shows the largescale flow (long waves) and jet streams, and also the small-scale flow (shortwaves, low level storm systems) 500 mb chart RIDGE TROUGH The 500 mb chart is the forecasters’ favorite for depicting the motion of weather systems. It shows the largescale flow (long waves) and jet streams, and also the small-scale flow (shortwaves, low level storm systems) 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. 5400m contour = 5400m first approx for rain/snow border rain/snow Thickness (yellow lines): what is it related to? related Hydrostatic Approximation Hydrostatic Mass = density x Volume Newton’s Second Law pressure = force per unit area Rearrange last equation to yield hydrostatic approximation Thickness and Temperature Thickness Hydrostatic approximation for the atmosphere: (p is pressure, z is height, g is gravity, and The ideal gas law is: (R is a constant, T is temperature) Rearranging terms: is density) Thickness and Temperature Thickness Equation from before (hydrostatic & ideal gas law): p2 thickness p1 Integrating through a layer with average temp Tm yields Integrating yields Thus, the thickness of a layer is proportional to the average temperature in that layer. ...
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