CHAPTER 16 (Moran and Morgan, 1997) One of the primary focuses in the atmospheric sciences is the scientific forecasting of future weather events. An initial step in the forecast process is the systematic collection of weather data not only near the earth's surface, but at various levels of the atmosphere, using various types of instruments that may be mounted on such instrument platforms as balloons, rockets or weather satellites. These data are collected for use in numerical weather prediction models. Weather maps are often plotted and analyses made to permit a visual interpretation of spatial distribution of weather phenomena. STUDY NOTES CHAPTER 16 Figures 16.1 and 16.2 -- Take time to become acquainted with the differences between the orbits of the polar-orbiting and geosynchronous (also known as geostationary) satellite platforms used to provide coverage of the earth-atmosphere system for meteorologists and other earth scientists. As described in the text, the altitude of the polar-orbiting satellite is approximately 1000 km (600 mi) above the earth's surface, while that of the geosynchronous satellite is approximately 36,000 km. Table 16.2 -- Take a moment to compare the values of emissivity in the IR portion of the electromagnetic spectrum for selected materials. While you need not memorize these values, note that liquid water can be considered to be nearly an ideal radiator (also known as a "black body"). On the other hand other substances have lower emissivity values, signifying that these substances appear to behave less like a black body, especially in the infrared region of the spectrum. Figure 16.3 -- Compare the different brightness regions on the infrared satellite image. The darkest areas over the equatorial Pacific and the Gulf of Mexico represent the warmest regions. These areas are probably cloud-free and the satellite is essentially detecting the warm sea surface. Milky gray areas over the North Pacific and South Pacific near South America contain warm, low clouds that have cloud-top temperatures not significantly cooler than the surface temperature. The brightest white areas that appear across the central North Pacific, much of South American and portions of North America are cold, high clouds. Figure 16.4 -- Look at the sample water vapor channel satellite image. Figure 16.5 -- This figure provides a key to the standard weather station model used on surface weather maps produced by the National Weather Service. If you would like more detailed information, please consult Appendix III. Figure 16.7 -- Look at the national composite radar image. This diagram utilizes a color code to identify the six reflectivity levels that are related to the precipitation intensity rates. Similar types of diagrams are used on television weather broadcasts to identify regions of precipitation and the varying intensity levels. Figure 16.8 -- Look at the sample 500 mb level chart that meteorologists use to describe mid-tropospheric weather patterns. Since this chart depicts an undulating three-dimensional surface where all points have the same 500 mb pressure, a set of height contours is drawn to portray the topography of this constant pressure surface. These height contours are solid lines. Dashed lines are isotherms, or lines of equal temperature. Figure 16.9 -- An example of a display from a new three-dimensional visualization tool. By necessity, the vertical dimension is greatly exaggerated. Table 16.6 -- Take a moment to review the various types of event-specific weather watches, warnings and advisories that are issued by the National Weather Service. Some of these statements have been identified in previous chapters (on thunderstorms, tornadoes and hurricanes), while the rest are described in the accompanying text. Read Special Topic (Weather Proverbs: Fact or Fiction?) on pages 392 and 393 for interesting information. Read Weather Fact (Forecasting Tomorrow Morning's Low Temperature) on page 396 for information content. If possible, try your hand at short-term weather forecasting by making a forecast for your community for the next morning using the previous afternoon's dewpoint. Skim the Special Topic (Aviation Weather Hazards) pg. 398 and 399. This topic describes the three weather-related hazards to aviation interests. Skim the Mathematical Note (Some Orbital Characteristics of Weather Satellites) on page 400. This Note is intended for the mathematically inclined and derives a mathematical equation that expresses the speed of a satellite in terms of the altitude above the earth's surface. CHAPTER 16 (Moran and Morgan, 1997) WEATHER ANALYSIS AND FORECASTING With this chapter, we close our study of the basic concepts of meteorology with a description of the steps involved in the preparation of weather forecasts. First we consider how data are gathered via surface and upper-air observations with a special focus on weather satellites. We then show how weather data are plotted on maps for analysis by meteorologists and input into computerized numerical models of the atmosphere. Next, we describe forecast methodology and discuss forecast skill and the techniques and challenges of long-range forecasting. The chapter concludes with a discussion of how weather information reaches the public. CHAPTER OBJECTIVES After reading this chapter, the student should be able to: describe how weather data are obtained from the surface and upper atmosphere. compare and contrast geosynchronous and polar-orbiting weather satellites. list the advantages of weather satellite observations. explain the difference between surface weather maps and upper-air weather maps. describe the complementary roles of meteorologists and computers in weather data analysis and forecasting. explain how and why forecast skill changes with length of the forecast period. explain how meteorologists prepare long-range weather forecasts. describe the significance of teleconnections in seasonal weather forecasting. summarize how each of us can make reasonably accurate single-station weather forecasts. review prospects for future improvement in weather forecast skill. 16 Weather Analysis and Forecasting 374 World Meteorological Organization 375 Acquisition of Weather Data 376 Meteorology by Satellite 377 Data Depiction on Weather Maps 381 Weather Prediction 385 Communication and Dissemination 392 Conclusions 395 Weather Fact: Forecasting Tomorrow Morning's Low Temperature 393 Special Topic: Weather Proverbs: Fact or Fiction? 396 Special Topic: Aviation Weather Hazards 398 Mathematical Note: Some Orbital Characteristics of Weather Satellites 400 Key Terms 395 Summary Statements 397 Review Questions 399 Questions for Critical Thinking 401 Selected Readings 401 3