Convective instability

Sci-Tech Dictionary: convective instability

(kən′vek·div in·stə′bil·əd·ē)

(meteorology) The state of an unsaturated layer or column of air in the atmosphere whose wet-bulb potential temperature (or equivalent potential temperature) decreases with elevation. Also known as potential instability.

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Sci-Tech Encyclopedia: Convective instability

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A state of fluid flow in which the distribution of body forces along the direction of the net body force is unstable and will thus break down. Fluid flows are subject to a variety of instabilities, which may be broadly viewed as the means by which relatively simple flows become more complex. Instabilities are an important step in the transition between smooth and turbulent flow, and in the atmosphere they are responsible for phenomena ranging from thunderstorms to low- and high-pressure systems. Meteorologists and oceanographers divide instabilities into two broad classes: convective and dynamic. See also Dynamic instability.

In the broadest terms, convective instabilities arise when the displacement of a small parcel of fluid causes a force on that parcel which is in the same direction as the displacement. The parcel of fluid will then continue to accelerate away from its initial position, and the fluid is said to be unstable. In most geophysical flows, the convective motions that result from convective instabilities operate very quickly compared with the processes acting to destabilize the fluid; the result is that such fluids seem to be nearly neutrally stable to convection.

The simplest type of convective instability arises when a fluid is heated from below or cooled from above. Warm air rises and cold air sinks; thus a fluid whose temperature decreases with altitude is convectively unstable, while one in which the temperature increases with height is convectively stable. A fluid at constant temperature is said to be convectively neutral.

The above description assumes that density depends on temperature alone and that density is conserved in parcels of fluid, so that when the parcels are displaced their density does not change. However, in the Earth's atmosphere, neither of these assumptions is true. In the first place, the density of air depends on pressure and on the amount of water vapor in the air, in addition to temperature. Second, the density will change when the parcel is displaced because both its pressure and its temperature will change. Because of these conditions it is convenient to define a quantity known as virtual potential temperature (θ_v) that both is conserved and reflects the actual density of air. This quantity is given by the equation below, where T is the $\theta_v \equiv T\left(\frac{1+(r/0.622)}{1+r}\right)\left(\frac{1000}{p}\right)^{0.287}$ temperature in kelvins, p is the pressure in millibars, and r is the number of grams of water vapor in each gram of dry air. When θ_v decreases with height in the atmosphere, it is convectively unstable, while θ_v increasing with height denotes stability.

In the oceans, density is a function of pressure, temperature, and salinity; convection there is driven by cooling of the ocean surface by evaporation of water into the atmosphere and by direct loss of heat when the air is colder than the water. It is also driven by salinity changes resulting from precipitation and evaporation. In many regions of the ocean, a convectively driven layer exists near the surface in analogy with the atmospheric convective layer. This oceanic mixed layer is also nearly neutral to convection.

Convective instabilities are also responsible for convection in the Earth's mantle, which among other things drives the motion of the plates, and for many of the motions of gases within other planets and in stars.

Wikipedia: Convective instability

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For a more general discussion of the same phenomenon, see Convective available potential energy.

This article incorporates public domain text created by the US government.

In meteorology, convective instability or stability of an airmass refers to its ability to resist vertical motion. A stable atmosphere makes vertical movement difficult, and small vertical disturbances dampen out and disappear. In an unstable atmosphere, vertical air movements (such as in orographic lifting, where an airmass is displaced upwards as it is blown by wind up the rising slope of a mountain range) tend to become larger, resulting in turbulent airflow and convective activity. Instability can lead to significant turbulence, extensive vertical clouds, and severe weather such as thunderstorms.^[1]

Adiabatic cooling and heating are phenomena of rising or descending air. Rising air expands and cools due to the decrease in air pressure as altitude increases. The opposite is true of descending air; as atmospheric pressure increases, the temperature of descending air increases as it is compressed. Adiabatic heating and adiabatic cooling are terms used to describe this temperature change.

The adiabatic lapse rate is the rate at which a rising or falling airmass lowers or increases per distance of vertical displacement. The ambient lapse rate is the temperature change in the (non-displaced) air per vertical distance. Instability results from difference between the adiabatic lapse rate of an airmass and the ambient lapse rate in the atmosphere.

If the adiabatic lapse rate is lower than the ambient lapse rate, an airmass displaced upward cools less rapidly than the air in which it is moving. Hence, such an airmass becomes warmer relative to the atmosphere. As warmer air is less dense, such an airmass would tend to continue to rise.

Conversely, if the adiabatic lapse rate is higher than the ambient lapse rate, an airmass displaced upward cools more rapidly than the air in which it is moving. Hence, such an airmass becomes cooler relative to the atmosphere. As cooler air is more dense, the rise of such an airmass would tend to be resisted.

Moist air cools when rising at a lower rate (given the same vertical movement) than dry air, and hence has a relatively low adiabatic lapse rate. Thus, moist air is generally less stable than dry air. The dry adiabatic lapse rate (for unsaturated air) is 3 °C (5.4 °F) per 1,000 vertical feet. The moist adiabatic lapse rate varies from 1.1 °C to 2.8 °C (2 °F to 5 °F) per 1,000 vertical feet.

The combination of moisture and temperature determine the stability of the air and the resulting weather. Cool, dry air is very stable and resists vertical movement, which leads to good and generally clear weather. The greatest instability occurs when the air is moist and warm, as it is in the tropical regions in the summer. Typically, thunderstorms appear on a daily basis in these regions due to the instability of the surrounding air.

The ambient lapse rate differs in different meteorological conditions, but, on average, is 2 °C (3.5 °F) per 1,000 vertical feet.

References

^ "Weather Theory". Pilot's Handbook of Aeronautical Knowledge. United States Department of Transportation, Federal Aviation Administration. 2008. p. 11-12 – 11-13. http://www.faa.gov/library/manuals/aviation/pilot_handbook/media/PHAK%20-%20Chapter%2011.pdf.

	severe local storm
	instability
	Thunderstorm (meteorology and climatology)

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