Hadley circulation

The first global CH4 budgets were compiled by Ehhalt (1974) and Ehhalt and Schmidt (1978), who used available published information to estimate emissions of CH4 to the atmosphere. They considered paddy fields, freshwater sources (lakes, swamps, and marshes), upland fields and forests, tundra, the ocean, and enteric fermentation by animals as biogenic sources. Anthropogenic sources included industrial natural gas losses and emission from coal mining, and were considered to be free. Observations of CH4 placed an upper limit on anthropogenic sources. Oxidation by the OH radical, as well as loss to the stratosphere by eddy diffusion and Hadley circulation, were presumed to be methane sinks. In spite of lack of data, this work correctly identified the major atmospheric sources and did... 

Transport from the troposphere to the stratosphere occurs primarily in the tropics and is associated with the upward branch of the Brewer-Dobson circulation. Folkins et al. (1999) argue that the top of the tropospheric Hadley circulation in the tropics occurs at 14 km, he., well below the tropopause, and that a barrier to vertical mixing therefore exists in the tropical tropopause layer (TTL) 2 or 3 km below the thermal tropopause. Air injected above this barrier subsequently participates in generally slow vertical ascent into the stratosphere. Small-scale exchanges also take place at mid-latitudes through filamentary structures that are drawn poleward in relation with anticyclonic circulation in the upper troposphere (Chen, 1995). 

Mean motions in the lower stratosphere also are driven by the Hadley circulation, because its ascending branch causes tropospheric air to enter the stratosphere. The direction of motion in the lower stratosphere is northward in December-February, southward in July-August. The maximum value of the integrated mass flux associated with the Hadley cell is 200Tg/s in the middle troposphere and 6Tg/s in the stratosphere. 

Mohnen (1977), based on earlier work of Danielsen (1964, 1968) and Reiter (1975) 320 -470 Stratospheric-tropospheric air mass exchange tropopause folding events plus large-scale Hadley circulation m(03) = 1.3 pg/g assumed. 

As we discussed earlier, in the equatorial region, warm air expands upward and creates a poleward pressure gradient force at the upper altitudes, where air flows poleward from the equator. This air, as it moves poleward, cools and sinks in the subtropical high-pressure belts ( — 30°) and returns to the equator at the surface. This thermally driven circulation between the equator and the subtropics is referred to as the Hadley cell. In the polar regions, a similar thermally driven circulation occurs. An airflow exists at upper levels toward the equator and at lower levels toward the poles, producing a Hadley circulation between the poles and the subpolar low-pressure regions. 

The motions on the largest spatial scales amount to the aggregate of the world s synoptic weather systems, often called the general circulation. Both with respect to substances that have atmospheric lifetimes of a day or more and with regard to the advection of water, it is useful to depict the nature of this general circulation. The mean circulation is described to some extent in terms of the Hadley and Ferrell cells shown in Fig. 7-4. They describe a coupled circulation... 

The austral wintertime upper-level circulation (Fig. 2.1b) over South America is characterized by weak winds over the tropics, while the subtropical westerly jet is stronger and located more equatorward than in summer, in agreement with the descending branch of the Hadley-type circulation over that area. At lower levels, a northward-displaced near-equatorial low-pressure trough and SST maximum characterize the circulation (Fig. 2. Id). A northward cross-equatorial flow turning clockwise is found over the tropical eastern Pacific and Atlantic Oceans (Fig. 2.1b). 

These three cells of convective circulation occur within the troposphere, contributing to relatively rapid and complete mixing of this layer of the atmosphere. The tropical cells, located north and south of the equator, often are called Hadley cells. The circulation patterns immediately north and south... 

The descending branch of the Hadley cell gives rise to a high-pressure belt at about 30° latitude. Further poleward, the meridional circulation is opposite in direction to that required for thermally driven motion. These features, which are called Ferrel cells, must be indirect circulation systems. Yet another pair of cells is evident in the polar regions. They are very weak systems. 

A direct thermally-driven and zonally symmetric large- scale atmospheric circulation first proposed by George Hadley in 1735 as an explanation for the trade winds. It carries momentum, sensible heat, and potential heat from the tropics to the mid-latitudes (30 degrees). The poleward transport aloft is complemented by subsidence in the subtropical high pressure ridge and a surface return flow. The variability of this cell and the Walker cell is hypothesized to be a major factor in short-term climatic change, halocline... 

Hadley ceU Circulation cell with updrafts near the Equator and downdrafts near 30° latitude. 

Evangehsta Torricelh, an Itahan physicist, mathematician, and secretary to Gahleo, invented the barometer, which measures barometric pressure, in 1643. The first attempt to explain the circulation of the global atmosphere was made in 1686 by Edmond Halley, an Enghsh astronomer and mathematician. In 1735, George Hadley, an English optician, described... 

---