Mass divergence and mass convergence involve the density field as well as the velocity field. However, the mass divergence and mass convergence of the atmosphere are believed to be largely stratified into two layers as follows:

. Below about 600 hPa, velocity divergence and convergence occur chiefly in the friction layer, which is about one-eighth of the weight of the 1,000-to 600-hPa advection stratum, and may be disregarded in comparison with density transport in estimating the contribution to the pressure change by the advection stratum.

. Above 600 hPa, mass divergence and convergence largely result from horizontal divergence and convergence of velocity. However, on occasion, stratospheric advection of density may be a modifying factor.

The stratum below the 400-hPa level may be regarded as the ADVECTION stratum, while the stratum above the 400-mb level maybe regarded as the significant horizontal divergence or convergence stratum. Also, the advection stratum maybe thought of as the zone in which compensation of the dynamic effects of the upper stratum occurs.

At about 8km (26,000 ft) the density is nearly constant. This level, which is near the 350-hPa pressure surface, is called the isopycnic level. This level is the location of constant density, with mass variations above and below.

Since the density at 200 hPa is only four-sevenths the density at the isopycnic level, the height change at 200 hPa would have to be twice that at the isopycnic level (350 hPa) for the same pressure/height change to occur. Thus, height changes in the lower stratosphere tend to be a maximum even though pressure changes are a maximum at the isopycnic level.

Pressure changes occur at the isopycnic level, and in order to maintain constant density a corresponding temperature change must also occur. Since the density is nearly constant at this level, the required temperature variations must result from vertical motions. When the pressures are rising at this level, the temperature must also rise to keep the density constant. A temperature rise can be produced by descending motion. Similarly falling pressures at this level require falling temperatures to keep the density constant. Falling temperatures in the absence of advection can be produced by ascent through this level.

Thus, rising heights at the isopycnic level are associated with subsidence, and falling heights at the isopycnic level are associated with convection.

Subsidence at 350 hPa can result from horizontal convergence above this level, while convection here would result from horizontal divergence above this level.

Since rising heights in the upper troposphere result in a rising of the tropopause and the lower stratosphere, the maximum horizontal convergence must occur between the isopycnic level (350 hPa) and the average level of the tropopause (about 250 hPa). This is due to the reversal of the vertical motion between the tropopause and the isopycnic level. Thus, the level of maximum horizontal velocity convergence must be between 300 hPa and 200 hPa and is the primary mechanism for pressure or height rises in the upper air.

Similarly, upper height falls are produced by horizontal velocity divergence with a maximum at the same level. The maximum divergence occurs near or slightly above the tropopause and closer to 200 hPa than to 300 hPa.

Therefore, it is more realistic to define a layer of maximum divergence and convergence as occurring between the 300- and 200-hPa pressure surfaces. The 300- to 200-hPa stratum is also the layer in which the core of the jet stream is usually located. It is also at this level that the cumulative effects of the mean temperature field of the troposphere produce the sharpest horizontal contrasts in the wind field.

The level best suited for determination of convergence and divergence is the 300-hPa level. Because of the sparsity of reports at the 300-hPa level, it is frequently advantageous to determine the presence of convergence and divergence at the 500-hPa level.

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