Jon Ahlquist 11/17/2006MET1010 Intro to the Atmosphere 1Chapter 13:Weather ForecastingForecasting is like driving a car down the road where you are only allowed to look out the back window. - Source unknownUtility of weather forecastsWeather observations and analysis at surface and aloftWeather forecasting methodsAccuracy versus SkillProbability forecastsNumerical (i.e., computer) forecastsEnsemble forecastingYou can forecastWatches, warnings, and advisoriesUtility of Weather Forecasts Meteorologists can’t change the weather,but forecasts can help people prepare for it. What to wear Travel (driving, airlines, shipping)) US National Weather Service is in Dept of Commerce Construction Agriculture Forest fires Tourism Good forecasts are based on good observations and a good understanding of the underlying science.ASOS: Automated Surface Observing System Most surface observations come from ASOS. Roughly 1000 ASOS stations in US, operated by National Weather Service, FAA, and military. ASOS automatically and electronically measures temperature, relative humidity, pressure, wind speed and direction, rainfall, visibility, height of cloud ceiling (but not cloud type). Also determines whether precipitation is rain or snow and whether there is freezing rain. At airports, people are contracted to take hourly observations of cloud type and weather that may be missed by ASOS (high clouds & weather in area)US Radiosonde Network Roughly 92 stations launching twice daily over US. N Hemisphere coverage is good for observing mid-latitude cyclones over land but inadequate in space and time to provide good observations of thunderstorms Can reach 100,000 ft during 90 minute ascent Equipment: about $100 per launch. Few recovered. Next generation sondes with GPS will cost about $160 each, so desirable to increase recovery rate. One possibility: glidersonde (glider the size of model airplane equipped with weather instruments). Takes observations as it is lifted by weather balloon. Released from balloon at preset altitude and glides back to launch point. Upper air obs info: http://www.ua.nws.noaa.gov/US Radiosonde StationsOther Observing Tools in US 2 geosynchronous satellitesfor E & W of US: www.goes.noaa.gov 164 Doppler radars covering US; see www.nws.noaa.gov/ost/88dtech.html 35 profilers, most in middle third of US, measure wind speed and direction as a function of height, reporting hourly (www.profiler.noaa.gov)Jon Ahlquist 11/17/2006MET1010 Intro to the Atmosphere 2Other Observing Tools in US Roughly 500 commercial aircraft carry weather sensors & report 150,000 observations per day, mostly in the upper troposphere (called ACARS, MDCRS, and AMDAR. http://aviationweather.gov/ general/pubs/front/docs/jul-03.pdf, http://acweb.fsl.noaa.gov/) Anchored buoys in Caribbean & Atlantic; also in the Pacific Ocean along & near the equator to monitor El Nino.See http://www.ndbc.noaa.gov/Worldwide Weather Network Over 1500 radiosonde observations per day Northern Hemisphere land covered fairly well Oceans, South America, and Africa not well covered. Over 10,000 land-based stations reporting surface conditions Hundreds of ships help with weather observationsWorld Meteorological Org. (WMO) World Meteorological Org (WMO)branch of the United Nationsover 175 nation members (http://www.wmo.ch)facilitates data exchange through “World Meteorological Centers” in Melbourne, Moscow, and Washington, DC and over 25 regional data centers around the worldeducates meteorologists in developing countries, e.g., workshops at FSUWeather Analysis:What’s the weather like now? Wait a few hours after each observation time to collect as many observations as possible before starting the analysis for that time Quality control: Discard readings if they are too different from climatology or nearby observations. Start analysis using a forecast for the analysis time, so every place has weather values. Blend in the new observations, making weighted averages of the forecast with observations to construct a 3-D grid of weather values. Adjust values so that they will work smoothly with the equations in the computer forecasting models.Weather Forecasting Methods(p. 368, 373) Forecasting technique depends on how far into the future you want to forecast Short range (a few minutes to next day): Persistence: weather will continue “as is” with no change. If it is dry/raining now, it will continue dry/raining forever. Tomorrow’s high and low temperatures will be the same as today’s. A persistence forecast is often good in tropics, which change little. Trend method (also called “steady-state” method in textbook): present trends will continue, e.g., fronts, lines of thunderstorms, hurricanes, etc., will continue to move, bringing in their associated weather. Often used for "nowcasting": forecast for next few minutes to at most a few hours.Forecasting 1 or more seasons into future Example: What would the weather be like for an outdoor wed-ding in Jacksonville at 10 am on a day months from now? Seasonal forecasting methods Forecast the seasonal average, i.e., climatology.(E.g., the Farmer’s Almanac seems to be climatological.) Simple statistical models involving correlations between past and future anomalies (i.e., deviations from the mean).Can be run on a personal computer or fancy pocket calculator. Very complex coupled ocean-atmosphere models.Requires a supercomputer. The simple statistical models are currently about as accurate asthe supercomputer forecasts. Supercomputer forecasts are improving, such as advances in ensemble forecasting by FSU’s Dr. T. N. Krishnamurti. More about this later in the lecture.Jon Ahlquist 11/17/2006MET1010 Intro to the Atmosphere 3Forecasting 1 day to 2 weeks into the futureHistorical ordering of techniques: Look at the sky. In middle latitudes, cirrus clouds are often a sign that a front is coming within 12 to 24 hours. See table 13.3, p. 348, and p. 351. Study daily weather maps year after year and gain an intuitive understanding how the weather typically evolves: analogue forecasting, weather types. Write a computer program using the equations that describe the atmosphere’s motion (dynamics) and temperature (thermodynamics): numerical weather prediction. This has become the dominant technique supplemented by human and
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