atmospheric convergence and divergence

atmospheric convergence and divergence Atmospheric convergence refers to accumulation of an air mass at a point. This process usually occurs at the surface in the centre of well-developed low-pressure systems, and, in the upper air, along the western limb of Rossby waves. Mass convergence is difficult to measure at the cloud scale, based on updraughts, but values are locally 50 times that of synoptic-scale convergence. Convergence is measured in s⁻¹, with values of 10⁻⁵ s⁻¹ typical for a mid-latitude cyclone. This is achieved through a horizontal gradient of wind speed of roughly 1 m s⁻¹ per 100 km, a rate of change that is difficult to measure given the observation network and instrument accuracy. If values of 10⁻⁵ s⁻¹ were maintained throughout the lowest half of the troposphere, an uplift near 5 m s⁻¹ would result.

Convergence also results from frictional effects at the surface. If a geostrophic wind, with pressure gradient force balanced by the Coriolis force, were to encounter a rough surface, the wind speed would decrease. Since Coriolis force is proportional to wind speed, this deflecting force would diminish, while the pressure gradient would remain unaltered. The balance of forces is lost and the flow crosses the isobars (lines of equal air pressure) towards the low pressure. This frictional inflow leads to an additional component of convergence.

Atmospheric divergence typically occurs where airflow (mass) is moving away from the centre of a pressure system. The air is thus being spread out, stretched, and expanded. This stretching may, however, assume many forms, each at a characteristic scale. It may result from cross-isobar (ageostrophic) flow under frictional effects, from the divergence of streamlines (lines of instantaneous air motion), from deceleration of flow, or from the interference of barriers such as mountains.

Divergence typically occurs on a large scale at the surface of large, high-pressure systems such as the Azores High. The spreading out of surface air is compensated by descending air in the core of the pressure system, since outflow must be balanced by vertical motion, a requirement of mass conservation. Subsidence, with typical values of a few centimetres a second, leads to adiabatic warming and cloudless conditions. This link between horizontal divergence and vertical motion is a major cause of weather, atmospheric stability playing a secondary role. Divergence may result from friction with the Earth's surface. Offshore winds, for example, experience a reduction in friction, resulting in acceleration which leads to localized divergence. Divergence may also occur in the upper air as a result of acceleration in a Rossby wave. This promotes uplift and may initiate a surface cyclone. Generally, divergence is well marked either at the surface or upper atmosphere (near 10 km altitude). At a height of roughly 5 km (500 hPa), values of divergence reach a minimum. This is known as the level of non-divergence.

R. Washington

Bibliography

Meteorological Office (1991) Meteorological glossary. HMSO, London.