CHAPTER 12 (Moran and Morgan, 1997) Regional circulation systems are between the planetary and the synoptic (or macro-) scale circulation systems. The best example is the monsoon circulation regimes of the Indian subcontinent and equatorial Africa. These monsoon regimes result from a seasonal change in the prevailing winds, and also result in dry winters and wet summers. Local circulation regimes are smaller than the synoptic (macro-) scale circulation regimes. Since these systems are relatively small -- lasting for less than a day and extending no more than several kilometers -- the Coriolis effect is not a significant factor. Examples include sea (or lake) and land breezes, lake-effect snows, katabatic winds, chinook winds, desert winds, and mountain and valley breezes. STUDY NOTES Figure 12.1 -- This diagram shows the seasonal differences in the monsoon circulation regime around the globe, with particular emphasis on this circulation regime over south Asia and central Africa. Take time to consider the seasonal changes in the prevailing winds over this region. You may want to look back to Figure 10.8 (pg. 234) to see the seasonal variations in the Intertropical Convergence Zone (ITCZ). In Figure 12.1 the ITCZ is marked as the "Wind Convergence Zone". In January (left panel), the low sun season in the Northern Hemisphere, a large high pressure system is located over Siberia. A northeasterly or offshore flow is found across India. At this time, the ITCZ is located far to the south. This season is also dry over South Asia. However, in July (right panel), the high sun season, a broad onshore flow is found across much of south Asia. The ITCZ has moved north over the continents of the Northern Hemisphere and allows Southern Hemisphere air to flow across the equator. The onshore flow, coupled with the orographic lifting along the Himalayan Massif is responsible for the rainy season occurring at this time of year. Figures 12.2 and 12.3 -- Inspect the panels in Figure 12.2 associated with the sea (lake)/land breeze circulation. This vertical cross-section is shown to extend to an altitude of approximately 300 m (or 1000 feet). The left panel is during the afternoon, while the right panel is the nighttime case. With the text, see where the flow develops. Figure 12.3 contains a simplified weather map over the western Great Lakes during a day when a mesoscale high develops over Lake Michigan and produces a lake breeze along the shore line. The dashed red line would represent a lake breeze front, which would penetrate inland during the afternoon and produce effects similar to that of a cold front. Figure 12.6 -- Take a moment to inspect the map showing the annual average snowfall across the Great Lakes. As mentioned in the legend, the greatest snowfall depths occur downwind of the lakes. Since lake-effect snow events typically occur when a cold polar air mass moves across the relatively warm lakes on northwesterly winds, the greatest snow amounts are found to the southeast of the lakes. Topographic features, such as the highlands along Michigan's Upper Peninsula and portions of Upstate New York, can also enhance the snowfall by orographic lifting. Figure 12.8 -- Inspect the diagram of the average minimum temperatures for winter in the Washington, DC area. The region surrounded by the 31 degree isotherm near the center of the map includes part of downtown Washington, DC and portions of neighboring northern Virginia with a concentration of major highways and buildings. Figure 12.10 -- Visualize the general wind flow that produces the chinook case. For simplicity, you are looking toward the eastern slopes of the Rockies, with a westerly wind flow. This westerly wind is the result of a synoptic-scale pressure gradient produced by a large region of high pressure in the foreground and a low pressure cell located to the north. Consequently, winds passing over the crest of the Rockies would descend and produce the chinook wind case. Figure 12.12 -- Inspect this simplified surface weather map for a Santa Ana case. From inspection of the isobar pattern, you should realize that the Santa Ana winds over Southern California are easterly. These dry winds have originated along the south flank of the large high pressure center located over the Great Basin. These winds would move across the mountains of Southern California and descend to near sea level in the Los Angeles Basin, producing the Santa Ana wind episode described in the text. Figure 12.14 -- Compare the differences between the valley breeze that is formed during daylight (top panel) with the nighttime mountain breeze. Note the direction that the wind flows in each case. Also note that clouds may often form over the mountain tops during the day as a result of the upslope valley winds. Read the Special Topic (Monsoon Failure and Drought in Sub-Saharan Africa) on pages 290 to 292 for background information. Read the Special Topic (Santa Ana Winds and California Fire Weather) on pages 300 and 301. Read the Weather Fact (Desert Heat) on page 302 for informational content. CHAPTER 12 (Moran and Morgan, 1997) LOCAL AND REGIONAL CIRCULATION SYSTEMS The previous two chapters focused on planetary-scale and synoptic-scale circulation systems. Here we consider the genesis and characteristics of local and regional circulation systems most of which are mesoscale or smaller. Land and sea (or lake) breezes, lake-effect snows, katabatic winds, chinook winds, desert winds, and mountain and valley breezes are small-scale systems that are not significantly influenced by the Coriolis effect. Of much larger scale are monsoon winds that cause dry winters and wet summers. Note that planetary and synoptic scale winds set the boundary conditions for local and regional circulation systems. CHAPTER OBJECTIVES After reading this chapter, the student should be able to: identify the various factors that contribute to monsoon circulations. distinguish between monsoon active phase and monsoon dormant phase. compare and contrast sea (or lake) breezes with land breezes. describe the conditions that favor lake-effect snowfall. list the factors that contribute to the development of an urban heat island. compare katabatic winds with chinook winds. explain the diurnal variation of winds in deserts and mountainous areas. 12 Local and Regional Circulation Systems 286 Monsoons 287 Land and Sea (or Lake) Breezes 288 Lake-Effect Snows 292 Heat Island Circulation 295 Katabatic Winds 297 Chinook Winds 297 Desert Winds 300 Mountain and Valley Breezes 302 Conclusions 304 Special Topic: Monsoon Failure and Drought in Sub-Saharan Africa 290 Special Topic: Santa Ana Winds and California Fire Weather 300 Weather Fact: Desert Heat 302 Key Terms 304 Summary Statements 304 Review Questions 304 Questions for Critical Thinking 305 Selected Readings 305 2 2