G304 – Physical Meteorology and Climatology Chapter 10 Weather forecasting and analysis By Vu Thanh Hang, Department of Meteorology, HUS 10.1 Forecasting methods • There is no single “correct” way to forecast the weather • Forecasts based on long-term information in the absence of any data about current weather are called climatological forecasts • The reliability of a climatological forecast depends on year-toyear variability in weather conditions for the forecast day • A persistence forecast relies completely on current conditions with no reference to climatology • This simple procedure might work for a little while but will eventually fail to catch changes in weather 10.1 Forecasting methods (cont.) • The analog approach tries to recognize similarities between current conditions and similar well-studied patterns from before, assuming that what happened sometime in the past provides a clue about the future • Numerical weather forecasting is based on computer programs that attempt to mimic the actual behavior of the atmosphere • The numerical models typically used in weather forecasting are very large and can only be run on the most powerful computers, so-called supercomputers 10.2 Types of forecasts • The product (or result) of a forecast method can take a variety of forms Æ type of forecast • Quantitative forecasts specify the “amount” of the forecast variable Ex.: “an cm of rain is expected” • Qualitative forecasts provide only a categorical value for the predicted variable Ex.: “rain/norain” • In a probability forecast, the chance of some event is stated Ex.: the probability-of-precipitation forecast (PoP forecast) 10.3 Assessing forecasts • Forecast quality refers to the agreement between forecasts and observations • Forecast value refers to the utility of a forecast and necessarily depends on the application of a forecast to a particular problem or decision • The quality of a forecast concerns forecast accuracy • That is, on average, how close is the forecast value to the true value? 10.3 Assessing forecasts (cont.) • Forecast bias concerns systematic over- or under-prediction • A biased forecast method is one whose average forecast is above or below the true average • The mean absolute error (MAE), ignores the sign (positive or negative) of the errors Æ over- and under-predictions are treated the same • Forecast skill is defined as the improvement a method provides over what can be obtained using climatology, persistence, or some other “no-skill” standard 10.4 Data acquisition and dissemination • The starting point for almost all weather forecasting is information about the current state of the atmosphere • The first process in operational weather forecasting is acquiring the necessary data • The WMO collects data from land observation stations, ship stations, moored and drifting buoys with automatic weather sensors, weather satellites • Upper-air data from weather balloon sites, instruments aboard wide-bodied commercial aircraft • The data from all countries are sent to three World Meteorological Centers at Washington, Moscow, Melbourne, which in turn disseminate the data to all member countries 10.4 Data acquisition and dissemination (cont.) • The member nations of the WMO maintain their own meteorological agencies that obtain and process the data and issue regional and national forecasts • Hydrogen-filled balloons carry weather instrument packages called radiosondes • Radiosondes tracked by radar are called rawinsondes 10.5 Forecast procedures and products • The general procedure for all numerical models includes the following three phases: • The analysis phase, in which observations are used to supply values corresponding to the starting state of the atmosphere for all the variables carried in the model • Part of what analysis accomplishes is converting those irregular observations into “uniform” initial values • Observed values also need to be consistent with the particular model being used • All the models are approximations to the real atmosphere Æ initial field not contain features that can’t be represented by the model 10.5 Forecast procedures and products (cont.) • The prediction phase begins with values delivered by the analysis phase using governing equations to obtain new values a few minutes into the future • The process is then repeated, using the output from the first step as input for the next set of calculations • This procedure is performed over and over as many times as necessary to reach the end of the forecast period (24hr, 48hr, …) • Huge computational resources are needed for this • The governing equations can’t be solved directly but must be broken down into simple operations that computers can perform 10.5 Forecast procedures and products (cont.) • In the post-processing phase, conditions forecast by the model at regular intervals are represented in grid form for mapping and other display purposes • Forecasters study maps for each period and interpret the conditions that would probably be associated with such patterns • In addition to gridded fields of model variables, forecasts for a number of secondary variables are produced • The output products are called model output statistics (MOS) and are designed to capture the effect of topography and other factors that influence local weather conditions 10.5 Forecast procedures and products (cont.) Ensemble forecasting involves a number of different model runs performed for the same forecast period starting with slightly different initial values 10.5 Forecast procedures and products (cont.) • 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, and is referred to as chaos • Forecast at still longer lead times are called long-range forecasts, ranging from a week to the limits of technical feasibility • Another product is the “seasonal outlook”, a kind of forecast for an entire season • In contrast to long-range forecasts that predict conditions for particular days, seasonal outlooks predict average conditions for an entire season 10.6 Weather maps • Although computers play a critical role in weather analysis, ultimately meteorologists apply their knowledge to produce the forecast that issued to the general public • Probably no tool is as valuable to a forecaster as a weather map • Weather analysis requires using a series representing different layers of the atmosphere of maps 10.6 Weather maps (cont.) • Surface maps of prevailing conditions present a general depiction of sea level pressure distribution 10.6 Weather maps (cont.) Station models offer detailed knowledge of the conditions at a particular location with over a dozen weather elements represented on each station model 10.6 Weather maps (cont.) A 850mb map resides at an average height of 1.5km above sea level Heights of the 850mb level are plotted with solid lines (height contours in decameters) 10.6 Weather maps (cont.) A 700mb map (~ 3km) 10.6 Weather maps (cont.) A 500mb map (~ 5.6km) Visible images view the atmosphere by registering the intensity of reflected shortwave radiation and are available only during the daytime Infrared images are based on measurements of longwave radiation emitted (not reflected) from below Water vapor image obtained from GOES satellite Radar images observe the internal cloud conditions by measuring the amount of radiation backscattered by precipitation (both liquid and solid) [...]... pressure distribution 10. 6 Weather maps (cont.) Station models offer detailed knowledge of the conditions at a particular location with over a dozen weather elements represented on each station model 10. 6 Weather maps (cont.) A 850mb map resides at an average height of 1.5km above sea level Heights of the 850mb level are plotted with solid lines (height contours in decameters) 10. 6 Weather maps (cont.)... conditions for an entire season 10. 6 Weather maps • Although computers play a critical role in weather analysis, ultimately meteorologists apply their knowledge to produce the forecast that issued to the general public • Probably no tool is as valuable to a forecaster as a weather map • Weather analysis requires using a series representing different layers of the atmosphere of maps 10. 6 Weather maps (cont.)... designed to capture the effect of topography and other factors that influence local weather conditions 10. 5 Forecast procedures and products (cont.) Ensemble forecasting involves a number of different model runs performed for the same forecast period starting with slightly different initial values 10. 5 Forecast procedures and products (cont.) • If two model runs are made with slightly different initial.. .10. 5 Forecast procedures and products (cont.) • In the post-processing phase, conditions forecast by the model at regular intervals are represented in grid form for mapping and other display purposes •... A 850mb map resides at an average height of 1.5km above sea level Heights of the 850mb level are plotted with solid lines (height contours in decameters) 10. 6 Weather maps (cont.) A 700mb map (~ 3km) 10. 6 Weather maps (cont.) A 500mb map (~ 5.6km) Visible images view the atmosphere by registering the intensity of reflected shortwave radiation and are available only during the daytime Infrared images