M6 Mesoscale and Local Circulations

Topic

Mesoscale circulations are atmospheric phenomena on an intermediate scale—ranging from ten to one thousand kilometers—situated between the synoptic scale of weather systems and the microscale of small-scale turbulence. These circulations are driven by horizontal temperature contrasts between adjacent surfaces—such as the sea breeze (arising from the contrast between land and ocean), mountain and valley winds (resulting from the contrast between slopes and valley floors), and the urban heat island effect (caused by the contrast between city and countryside). It is precisely this horizontal temperature contrast that initiates each of these mesoscale circulations. Furthermore, these circulations play a fundamental role in the regional distribution of precipitation and temperature, particularly during the summer.

Among them, the sea breeze is the quintessential thermal circulation: during the day, the landmass heats up more rapidly than the sea, causing the air above the land to rise; this upward movement generates a surface circulation that draws air from the ocean toward the land, creating a breeze front that advances at speeds of five to ten meters per second and can penetrate twenty to fifty kilometers inland. At night, however, the thermal contrast between land and sea reverses—as the land cools faster than the sea—and this reversal of the contrast causes the sea breeze circulation itself to reverse, resulting in airflow from the land toward the sea. Over mountainous terrain, an alternation similar to the sea breeze occurs, though driven by the topography itself: during the day, a valley wind blows, rising up the slopes as they heat up faster than the valley floor; at night, a mountain wind blows, descending those same slopes as they cool more rapidly than the air in the valley. This daily cycle between valley and mountain winds causes the temperature on the valley floor to fluctuate between day and night with a range of five to twenty degrees.

The urban heat island represents another thermal contrast capable of generating its own mesoscale circulation; in this case, however, the contrast depends not on topography or the coastline, but on the city itself. The city stores and releases heat differently than the surrounding countryside, creating temperature anomalies of one to eight degrees between the urban area and its rural surroundings. On summer afternoons, this urban heat island not only warms the air over the city but also intensifies convection above it, as the warmer urban air rises more vigorously than the cooler air from the neighboring countryside.

Finally, föhn winds occur on the leeward slopes of mountains: the same air that rose on the windward side—shedding its moisture in the process—subsequently descends the leeward side. This descent adiabatically compresses the air, causing it to warm to a temperature ten to twenty degrees higher than it was before crossing the mountain. Thus, all these mesoscale circulations—sea breezes, mountain and valley winds, urban heat islands, and föhn winds—originate from the same type of horizontal temperature contrast, even though each arises in a different way. It is precisely at the sea-breeze front—where air arriving from the ocean collides with air already rising over land—that convection is most frequently triggered; this convection subsequently intensifies over cities and is distributed, day after day, between valley and mountain winds. It is this very daily alternation that causes the mesoscale—among all atmospheric scales—to most closely track the rhythm of day and night.

ID:4988