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Atmospheric Convection

Atmospheric convection is a broad topic area generally covering transport of heat by diffusion and advection in the atmosphere when using the general ideas of heat transfer (see convection). Thus, convection can be broken down into forced and free convection. For more information concerning forced convection (i.e. temperature advection) in the atmosphere see topics discussed in synoptic meteorology (i.e. extratropical cyclone, weather front, etc). This article will focus more specifically on free convection. Free convection in the atmosphere can be considered the advection of heat through the release of an instability in some direction, on a small scale (i.e. mesoscale or smaller).

Types of Convection

There are two general classifications of convection in the atmosphere, gravitational and symmetric (or slantwise).^[1] Some have argued that a distinction between the two is artificial and that there is generally just a continuum of the two types.^[2] Convection can also be classified as activated (triggered) or statistical-equilibrium convection. Active convection is when a buildup of potential energy or instability occurs and then convection is initiated by some trigger. Statistical-equilibrium type convection essentially uses instability as it is generated by the flow. In this case CAPE would be nearly constant in time.^[3] Both gravitational and symmetric convection can occur as activated or statistical-equilibrium type. ^[1]

Gravitational Convection

Gravitational Convection, or upright convection is the process by which an air parcel vertically transfers heat (both latent and sensible) between layers of the atmosphere through the release of buoyant instability. This type of convection can be either dry or moist. An example of dry upright convection is a thermal (without cloud) and a cumulus cloud is an example of moist upright convection.

Dry Gravitational Convection

Dry convection occurs in regions of the atmosphere when a dry air parcel (no water condensate) is warmer than its surroundings. This parcel is therefore less dense (assuming equal specific humidities) than its surroundings. The atmospheric lapse rate will determine how far the parcel will rise given the initial temperature difference. As an example, consider a parcel with a 3 K positive temperature perturbation sitting in a layer with a lapse rate of 7.8 K/km. The parcel will rise and cool at the dry adiabatic lapse rate (9.8 K/km) because it is dry. This parcel will be only 1 K warmer than the environment at 1 km in height and it will reach its equilibrium level (EL) at 1.5 km because it loses 2 K relative to the environment every 1 km. It will accelerate until it reaches its EL when it will begin to decelerate. The period of oscillation can be described by using the Brunt-Väisälä frequency. If the atmospheric lapse rate were equal to the dry adiabatic lapse rate (DALR) the parcel would accelerate through the entirety of the atmosphere (an unrealistic example) because it would always be less dense than the atmosphere.

The phenomena of dry convection explains why the atmospheric lapse rate rarely exceeds the DALR. When the atmospheric lapse rate exceeds 9.8 K/km (a superadiabatic lapse rate), the atmosphere is absolutely unstable and any minute perturbation will cause convection. This acts to restore the atmosphere to the dry adiabatic lapse rate when no water processes are considered. On sunny days it is possible to have superadiabatic lapse rates (see lapse rate) when the surface radiative forcing overwhelms the ability of dry convection to redistribute the heat gain at the surface throughout the atmosphere through the process of convective adjustment.^[4]

Moist Gravitational Convection

Moist convection occurs when a parcel is again warmer than its surroundings, but moist processes (condensation, freezing, etc) are occurring. This is the type of convection traditionally coined convection and is associated with cumulus clouds and thunderstorms. More specifically thunderstorms are a form of moist deep convection, because they extend through a large portion of the atmosphere. The same basic principles as dry convection apply here as well with the complication of the moist processes. A parcel that is positively buoyant will continue to accelerate until it reaches its EL. Again, an oscillation will ensue, this time described by the moist Brunt-Väisälä frequency.^[5] The initial rise past the EL explains the phenomena of an overshooting top seen in strong thunderstorms. The large amount of kinetic energy accumulated by a parcel in situations of high CAPE (updraft velocity is proportional to the square root of CAPE) results in the cloud protruding above the anvil, which is located near the EL.

Three ingredients are necessary to produce moist deep convection and they are: lift, moisture and instability.^[6] Without all three of these ingredients present, the process occurring should not be considered moist deep convection. Lift is included because the free troposphere is generally conditionally unstable. The lapse rate of the free troposphere is generally between the dry adibatic lapse rate and approximately 4-5 K/km, which is some extreme of the moist adibatic lapse rate (MALR). Thus as a parcel is lifted initially it is dry, cools at the DALR and is colder (more dense) than its surroundings. When it reaches its lifted condenstation level (LCL) the parcel begins to cool at some MALR determined by its mixing ratio. For the parcel to experience moist deep convection its MALR needs to be less than the atmospheric lapse rate so it can become positively buoyant at its LFC. Therefore in general terms, some type of initial lift must be present to lift the parcel to its level of free convection (LFC).^[6] Moisture is required because without it the process would be dry. Instability (buoyant instability specifically) is required for the parcel to experience free convection.

As is the case with dry convection, moist convection explains why the free troposphere typically has a lapse rate between the DALR and some extreme of the MALR. Moist convection also adjusts the atmosphere toward the MALR for a given convective element.^[7] See convective adjustment for more details.

Symmetric Convection

Symmetric convection, or slantwise convection distributes heat through the atmosphere on a slant path. Symmetric convection occurs by the release of symmetric instability (SI) which occurs on the mesoscale (O(100 km)).^[8] In that regard it is different from gravitational convection, which occurs on smaller scales (O(10 km)).^[9] Also, slantwise convection generally has less available energy, referred to as slantwise convective available potential energy (SCAPE) and is generally overwhelmed when gravitational convection is also present.^[1] Slantwise convection can be thought of as gravitational convection occurring along a line of constant geostrophic absolute momentum (M_g). ^[1] Slantwise convection can also occur in dry and moist conditions.

Dry Slantwise Convection

Here again the basic principles of dry convection apply. However, with this type of convection one must consider the slant path along the M_g surface.^[1]

Moist Slantwise Convection

Moist slantwise convection can be associated with precipitation bands in synoptic scale systems.^[1] ^[8] However, it is difficult to distinguish the exact cause of banding features in many cases due to the simultaneous presence of frontogenesis. ^[1] For ways to assess if moist slantwise convection could occur, see (SI). The occurrence of this type of convection can be important in precipitation forecasts in winter storms, as the release of SI can result in large precipitation accumulation and gradients due to the banded nature of precipitation when this occurs. ^[1]

Measurement

The "strength" of atmospheric convection can be measured by convective available potential energy (CAPE). CAPE is the amount of energy available to a rising parcel of air between the LFC and the equilibrium level (EL), the latter being where convection stops due to the air parcel reaching the temperature of the environment, which is due to environmental warming at the tropopause (although the air continues rising due to its momentum until the maximum parcel level (MPL)). CAPE is in joules per kilogram of air (J/kg). Other variations of CAPE include SBCAPE (surface-based), MLCAPE (mixed layer), MUCAPE (most unstable), and DCAPE (downdraft). Values of CAPE around 500 J/kg are usually considered weak, while a value around 4000 J/kg is considered very strong and could lead to severe thunderstorms. The lifted index (LI) is the temperature at some level, usually 500 millibars (50 MPa), minus the temperature of an air parcel at the same level when raised from the surface. This shows the temperature difference between the two and how much buoyancy the parcel of air has. A negative number for the LI shows instability.

Forecasting atmospheric convection

Atmospheric convection is best forecast using the sounding analysis. The sounding analysis is the dewpoint and temperature profile plotted with height on a thermodynamic diagram. This is done with a weather balloon and radiosonde. In addition to the current sounding analysis, there are also model forecast soundings. This data can be interpreted visually or with different indices such as CAPE, the total totals index (TT), the energy-helicity index (EHI), the severe weather threat index (SWEAT), and many others. These indices can help determine the strength of the convection and storm type. It is also forecasted by determining if conditions will be met for the different types of instability. For example, convective instability can be forecasted by determining if an area will experience large amounts of moisture in the low levels and dry air in the mid levels, along with a forced lift source.

References

^ ^a ^b ^c ^d ^e ^f ^g ^h Schultz, D.; Schumacher, P. N. (1999), "The use and misuse of conditional symmetric instability" (PDF), Mon. Wea. Review, 127
^ Xu, Q.; Clark, J.H.E. (1985), "The nature of symmetric instability and its similarity to convective and inertial instability" (PDF), J. Atmos. Sci., 42
^ Emanuel, K. (1994). Atmospheric Convection. New York, NY: Oxford University Press.
^ Manabe, S.; Strickler, R. F. (1964), "Thermal Equilibrium of the Atmosphere with a Convective Adjustment" (PDF), J. Atmos. Sci., 21
^ Durran, D. R.; Klemp, J. B. (1982), "The Effects of Moisture on the Brunt-Väisälä Frequency." (PDF), J. Atmos. Sci., 39
^ ^a ^b Doswell, C.; Brooks, H.E.; Maddox, R. A. (1996), "Flash flood forecasting: An ingredients-based methodology" (PDF), Wea. Forecasting, 11
^ Arakawa, A.; Schubert, W. H. (1974), "Interaction of a Cumulus Cloud Ensemble with the Large-Scale Environment, Part I" (PDF), J. Atmos. Sci., 31
^ ^a ^b Seltzer, M. A.; Passarelli, R. E.; Emanuel, K. A. (1985), "The possible role of symmetric instability in the formation of precipitation bands" (PDF), J. Atmos. Sci., 42
^ Fujita, T. T. (1981), "Tornadoes and downbursts in the context of generalized planetary scales" (PDF), J. Atmos. Sci., 38

[ss1999-1] ^ ^a ^b ^c ^d ^e ^f ^g ^h Schultz, D.; Schumacher, P. N. (1999), "The use and misuse of conditional symmetric instability" (PDF), Mon. Wea. Review, 127

[2] Xu, Q.; Clark, J.H.E. (1985), "The nature of symmetric instability and its similarity to convective and inertial instability" (PDF), J. Atmos. Sci., 42

[3] Emanuel, K. (1994). Atmospheric Convection. New York, NY: Oxford University Press.

[4] Manabe, S.; Strickler, R. F. (1964), "Thermal Equilibrium of the Atmosphere with a Convective Adjustment" (PDF), J. Atmos. Sci., 21

[5] Durran, D. R.; Klemp, J. B. (1982), "The Effects of Moisture on the Brunt-Väisälä Frequency." (PDF), J. Atmos. Sci., 39

[cd1996-6] Doswell, C.; Brooks, H.E.; Maddox, R. A. (1996), "Flash flood forecasting: An ingredients-based methodology" (PDF), Wea. Forecasting, 11

[7] Arakawa, A.; Schubert, W. H. (1974), "Interaction of a Cumulus Cloud Ensemble with the Large-Scale Environment, Part I" (PDF), J. Atmos. Sci., 31

[spe1985-8] Seltzer, M. A.; Passarelli, R. E.; Emanuel, K. A. (1985), "The possible role of symmetric instability in the formation of precipitation bands" (PDF), J. Atmos. Sci., 42

[9] Fujita, T. T. (1981), "Tornadoes and downbursts in the context of generalized planetary scales" (PDF), J. Atmos. Sci., 38

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