CHAPTER 9 (Moran and Morgan, 1997) Wind, another weather element, represents a large quantity of air moving essentially in a horizontal direction with respect to the earth's surface. Meteorologists measure both the wind speed and wind direction with various instruments. Conceptual models have been developed based upon the laws of motion to explain atmospheric motion, especially the observed wind. Considering the forces, a simple model of the hydrostatic balance can be formulated, then the geostrophic wind and finally wind moving around a high and low pressure system. One can link the horizontal and vertical motions in the atmosphere to explain why clouds are often found in low pressure systems and clear skies in high pressure systems. STUDY NOTES CHAPTER 9 In Figure 9.1, check the differences in the horizontal pressure gradient for two different cases appearing in panels A and B. Each panel represents a map (or floor plan view) of the sea level pressure distribution, with isobars drawn using the traditional 4 mb spacing. In these examples, a horizontal pressure gradient would be directed perpendicularly across the isobars from the higher-value isobars toward the lower-value isobars. As mentioned in the legend, since the isobars are closer together in A than in B, the pressure gradient is greater at A than B. Therefore, the pressure gradient force associated with A would be also greater. Figure 9.2 -- Read the description of an analogue to how the atmosphere would respond to a pressure gradient, with fluid moving from high to low pressure. Figure 9.3 -- Note that the centripetal force is directed inward at each point in the rock's circular orbit. Figure 9.4 -- Visualize a counterclockwise curl in the clouds about the low pressure system marked by the red "L" off the Middle Atlantic Coast. In essence, the clouds serve as tracers for the wind flow around the low. Figure 9.5 -- This figure is an attempt to demonstrate how the Coriolis effect works using the positions of the path of a moving object at two different times. You will have to visualize how the outline map of the North American continent moves from its position as shown in the initial position (left panel) to a new position at a later time. This motion results from the earth's rotation from west to east (or in a counterclockwise direction when viewed from a perspective above the North Pole). Note that the path of the moving object in a fixed reference frame with respect to the stars will remain unchanging in direction or speed as indicated by the parallel red arrows. However, when compared with the 100 degree West meridian that is fixed with respect to the rotating earth's surface, the motion would appear to turn to the right. Figure 9.6 -- Spend some time verifying how moving objects are deflected in both hemispheres as a result of the earth's rotation. This figure provides a summarization of what happens in either hemisphere for any combination of intended motions. You should note that regardless of orientation, the moving object is deflected to the right of its intended motion in the Northern Hemisphere, while in the Southern Hemisphere, the deflection is to the left. Also note that no deflection as a result of the Coriolis effect will occur on an object moving along the equator. Figure 9.8 -- Notice the example of mechanically induced turbulence. Figure 9.10 -- In this figure, the successively increasing length of the arrows from bottom to top of the diagram shows the increase in wind speed with height through the lowest thousand meters of the atmosphere as a result of frictional effects. Figure 9.11 -- Study the schematic of the hydrostatic balance shown in a vertical view of the atmosphere. Note that the upward pointed arrow symbolizing the vertical pressure gradient force acting upon an air parcel is equal and opposite to the downward pointing arrow associated with the force of gravity acting upon the parcel. Table 9.1 -- Study this Summary Table that lists the various combinations of force (or acceleration) components for several different models. For example, in the first column marked Hydrostatic Equilibrium, only two forces, the pressure gradient and gravitational forces (or accelerations) are operative. In this case, only vertical components are involved, while in the other cases in the next three columns, the horizontal components are considered. Figure 9.12 -- Trace how an air parcel attains geostrophic equilibrium in the Northern Hemisphere on a simplified surface weather map with parallel and straight-line isobars. Let us assume that the air parcel is put into the prescribed pressure field, then accelerates initially from high toward low pressure. You should realize how the air parcel would accelerate and then become deflected to the right of the motion to finally move parallel to the isobars after attaining a geostrophic balance, where the pressure gradient force (vector PH) and the Coriolis effect (C) are equal and opposite. This diagram is simplified. In reality, a process of adjustment would cause the air parcel to undergo an oscillatory path. Note that the notation in this figure is not completely correct in that the flow marked by the blue dashed arrow should not be identified as geostrophic until the parcel has reached an equilibrium state and moves parallel to the isobars. Figures 9.13 and 9.14 -- Study the two map view diagrams that show the wind flow around high and low pressure cells in the Northern Hemisphere. (These cases are also called "anticyclonic" and "cyclonic" flow, respectively.) Look at the direction of the force components in each diagram. Realize that the excess or net inward force (vector arrows C and Ce in Figure 9.13) or PH and Ce (in Figure 9.14) corresponds to the centripetal force. Figure 9.15 -- Study this diagram, which is a map view that shows the balance of horizontal forces in the friction layer. Visualize the orientation of these individual force components. The point to remember from this diagram is that near-surface friction deflects the actual wind at a small angle across the isobars toward lower pressure. Figure 9.16 -- Inspect this figure, which represents an oblique view of a stack of three horizontal slices through the atmosphere in the friction layer (approximately the lowest 1000 m or 3000 feet). The lowest plane is near the surface, while the top slice is essentially at the level were the actual wind becomes geostrophic as friction becomes zero. Visualize the flow at each level with respect to the isobars (dashed lines found on each plane). Notice how the wind changes with height. Specifically, notice how the wind arrow is turned at the largest angle across the isobars on the lowest level, but at the next level the angle between the wind arrow and isobars is less. Finally at the top of the layer the wind parallels the isobars as in the geostrophic case. The length of the arrow increases from the bottom to top of the layer, signifying an increase of actual wind speed with height. Figures 9.17 and 9.18 -- Study these two map diagrams, which describe the observed flow around a Northern Hemisphere surface anticyclone and cyclone, respectively. Reflect back to Figure I.3 and the "hand twist model" that you tried at the beginning of the course. Figure 9.19 -- Pause a moment to look at a sample surface weather map with isobars, a frontal analysis, and observed precipitation regions. What is the general wind direction across the Mississippi Valley or New England, based upon the isobar pattern? In other words, you should be able to tell that air would have been found flowing from the Gulf of Mexico to as far north as Iowa on southerly winds. Over New England, northerly winds would transport air southward from eastern Canada. Also note that the precipitation surrounds the low pressure system over the central Plains states, while the eastern third of the country is precipitation-free because of the influence of a high pressure system. Figures 9.20 and 9.21 -- Study these vertical cross-sections of the troposphere through the high and low pressure systems in these two diagrams. The upper arrows would represent winds that would include part of the upper tropospheric jet stream at about 10,000 m (30,000 feet). In each diagram think of the compensating processes operating in both the lower and upper troposphere. These processes ultimately provide links between the horizontal and vertical motions within the lower atmosphere. Convince yourself that in the cross-section through a surface anticyclone (Figure 9.20), the upper-level horizontal convergence (the piling of air) would increase the amount of air in the column, thereby causing a sinking motion and horizontal divergence at the surface. Likewise, in the cross-section through a surface cyclone (Figure 9.21), confirm that the upper tropospheric divergence would cause rising motion as air is evacuated out the top of the column, resulting in compensating surface convergence. Figure 9.22 -- Consider this realistic example appearing in this vertical cross-section diagram where a low-level wind moves from left to right. On the upwind (or left-hand) side of a large water body, a region of clearing would be produced immediately off the coastline as the wind flow accelerates once it moves over the smoother water surface, causing sinking motion. Along the other windward coastline where the wind slows, horizontal convergence occurs which produces upward motion and clouds. Table 9.2 -- Look at this table that lists the various scales of motion in the atmosphere. You will not need to memorize this table for these numbers are meant only as guides for transitions in both time and space. However, you should become familiar with the relative ranking, ranging from the largest scale of motion called the planetary scale to the smallest scale called the micro-scale. You should also use this table for reference when reading about the various sized weather systems in subsequent chapters. Some meteorologists prefer to use the term "macro-scale" than synoptic scale. Table 9.3 -- While you need not memorize the entries in this tabulation, you should look at this table over the Beaufort Scale of Wind Force to see how increased wind speed affects the sea state, tree branches, flags and so forth. You may find that with a little practice, you should be able to use this Beaufort Scale as an expedient means for estimating wind speed. Figures 9.24, 9.25, and 9.26 -- Look at these pictures of the various wind- measuring instruments, and compare these with the description in the text. Figure 9.27 -- Note that these sample traces of a recording anemometer (top panel) and a wind vane (lower panel) show that both the wind speed and wind direction are rarely constant, but show a reasonably large minute-to-minute variability that fluctuates even over a 6 hour interval. Read the Special Topic (Wind Power) on page 204. Note that wind power, a renewable resource, requires large wind turbines. However, because of the unreliable winds of sufficient speed, large-scale wind farms are limited to several areas of the country. Read the Special Topic (Wind Gusts, wind shear and atmospheric stability) on page 214. Take time to reflect upon how the information presented in this topic corresponds to the daily variations in winds that you may have experienced. Read Weather Fact (The Windiest Place on Earth) on page 223 for background information. Skim the Mathematical Note (Geostrophic and Gradient Winds) on page 224. This note is for the mathematically inclined and presents the mathematical relationships for the horizontal components of the pressure gradient acceleration and the Coriolis effect, which together can be used to arrive at mathematical expressions for the geostrophic wind and gradient wind speeds. CHAPTER 9 (Moran and Morgan, 1997) THE WIND With this chapter, we begin our discussion of atmospheric circulation and weather systems. We are concerned here with forces that initiate and shape the wind. The individual forces (pressure gradient, centripetal, Coriolis, friction, and gravity) are first described separately with special emphasis on the conditions under which each force operates. Individual forces are then combined in hydrostatic equilibrium, the geostrophic wind, the gradient wind, and surface winds. The air circulation within pressure cells (Highs and Lows) is linked to the general type of weather associated with each of these systems. In a High (or anticyclone), surface winds diverge and winds aloft converge so that air descends, the relative humidity decreases, and clouds dissipate. In a Low (or cyclone), surface winds converge and winds aloft diverge so that air ascends, the relative humidity increases, and clouds develop. Note how in this discussion we are applying (and hence, reinforcing) concepts that were developed in the previous three chapters on moisture in the atmosphere. CHAPTER OBJECTIVES After reading this chapter, the student should be able to: describe the various forces involved in initiating and governing the circulation of air. describe the origins of air pressure gradients. describe how the pressure gradient force affects the motion of air. explain the source of the centripetal force. explain why the Coriolis effect varies with latitude and is important only in large-scale circulation systems. explain why gravity influences vertical motion and not horizontal motion. present Newton's first and second laws of motion. describe the balance of forces in hydrostatic equilibrium. describe the interaction of forces in geostrophic winds and gradient winds. explain how surface roughness influences the speed and direction of surface winds. describe the circulation in cyclones and anticyclones. explain why stormy weather is associated with cyclones and fair weather with anticyclones. 9 The Wind 200 The Forces 201 Joining Forces 210 Continuity of Wind 217 Scales of Weather Systems 218 Wind Pressure 219 Wind Measurement 219 Conclusions 222 Special Topic: Wind Power 204 Special Topic: Wind Gusts, Wind Shear, and Atmospheric Stability 214 Weather Fact: The Windiest Place on Earth 223 Mathematical Note: Geostrophic and Gradient Winds 224 Key Terms 222 Summary Statements 223 Review Questions 224 Questions for Critical Thinking 224 Selected Readings 224