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Atmospheric convection
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Atmospheric convection is the result of a parcel-environment instability, or temperature difference, layer in the atmosphere. It is often responsible for adverse weather throughout the world.
e are a few general archetypes of atmospheric instability that correspond to convection and lack thereof. Steeper and/or positive lapse rates (environmental air cools quickly with height) suggests atmospheric convection is more likely, while weaker and/or negative environmental lapse rates would suggest it is less likely.
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Encyclopedia
Atmospheric convection is the result of a parcel-environment instability, or temperature difference, layer in the atmosphere. It is often responsible for adverse weather throughout the world.
Overview
There are a few general archetypes of atmospheric instability that correspond to convection and lack thereof. Steeper and/or positive lapse rates (environmental air cools quickly with height) suggests atmospheric convection is more likely, while weaker and/or negative environmental lapse rates would suggest it is less likely. This is due to the fact that any displaced air parcels will become more (less) buoyant, given their sign of adiabatic temperature change, in the steep (weak) lapse rate environments.
Convection begins at the Level of Free Convection (LFC), where it begins its ascent through the Free Convective Layer (FCL), and then stops at the equilibrium level. The rising parcel, if having enough momentum, will continue to rise to the Maximum Parcel Level (MPL) until negative buoyancy decelerates the parcel to a stop.
As long as a parcel is convecting, it theoretically will be accelerating. The exception to this is if drag produced by the updraft creates an equal, albeit opposite force to counter that from the buoyancy.. This could be thought of as similar to the terminal velocity of a falling object. This force from buoyancy can be measured by Convective Available Potential Energy (CAPE), or the joules of energy available per kilogram of potentially buoyant air. A theoretical updraft velocity can be derived from this value via substitution into the kinetic energy equation. Although this value will be an underestimation given the aforementioned drag or entrainment effects holding back further acceleration at some point. See the CAPE, buoyancy, and parcel links for a more in depth mathematical explanation of these processes.
Initiation
The initiation of the convection requires the onset of certain parameters of the atmospheric profile. The driving force behind convection is instability, which requires, as said before, a difference in temperature between some parcel of air and the environmental air surrounding it. Certain atmospheric conditions and setups are known to be conducive to this temperature difference. The idea behind convection, especially that which leads to vertically growing cumulus towers (I.E.
cumulus congestus), and cumuloform precipitation (Deep Moist Convection (DMC)), is that latent heat release supplies the necessary energy to keep the rising air buoyant. The lapse rate of rising air, before it can reach the condensation level, is such that it is almost always cooler than the environment. The latent heat release from condensation is the determinate between significant convection and almost no convection at all. The fact that air is generally cooler during winter months, and therefore can not hold as much water vapor and associated latent heat, is why significant convection (thunderstorms) are infrequent in cooler areas during that period. Thundersnow is one situation where forcing mechanisms provide support for very steep environmental lapse rates, which as mentioned before is an archetype for favored convection. The small amount of latent heat released from air rising and condensing moisture in a thundersnow also serves to increase this convective potential, although minimally.
Boundaries and Forcing
Despite the fact that there might be a layer in the atmosphere that has positive values of CAPE, if the parcel does not reach or begin rising to that level, the most significant convection that occurs in the FCL will not be realized. This can occur for numerous reasons. Primarily, it is the result of a cap, or convective inhibition (CIN/CINH). Processes that can erode this inhibition are heating of the earth's surface and forcing. Such forcing mechanisms encourage upward vertical
velocity, characterized by a speed that is relatively low to what you find in a thunderstorm updraft. Because of this, it is not the actual air being pushed to its LFC that "breaks through" the inhibition, but rather the forcing cools the inhibition adiabatically. This would counter, or "erode" the increase of temperature with height that is present during a capping inversion.
Forcing mechanisms that can lead to the eroding of inhibition are ones that create some sort of evacuation of mass in the upper parts of the atmosphere, or a surplus of mass in the low levels of the atmosphere, which would lead to upper level divergence or lower level convergence, respectively. Upward vertical motion will often follow. Specifically, a cold front, sea/lake breeze, outflow boundary, or forcing through vorticity dynamics (differential positive vorticity advection) of the atmosphere such as with troughs, both shortwave and longwave. Jet streak dynamics through the imbalance of Coriolis and pressure gradient forces, causing subgeostrophic and supergeostrophic flows, can also create upward vertical velocities.There are numerous other atmospheric setups in which upward vertical velocities can be created.
Concerns regarding severe deep moist convection
Buoyancy is key to thunderstorm growth and is necessary for any of the severe threats within a thunderstorm. It should be noted that there are other processes, not necessarily thermodynamic, that can increase updraft strength. These include updraft rotation, low level convergence, and evacuation of mass out of the top of the updraft via strong upper level winds and the jet stream.
An updraft that realizes a larger CAPE profile will theoretically have an updraft with a higher velocity (which will be referred to as a "stronger updraft" from this point on). A stronger updraft will increase the residence time of any condensation nuclei and their continued growth into hail stones (accretion). If this process occurs in the favored hail growth zone, which is above the freezing level/around the -20C level and can be achieved by a stronger updraft, then the larger hail potential increases. Thus, a stronger updraft can lead to larger hail.
The amount of hail within an updraft can be increased by a stagnation point just upstream the updraft. If the updraft is strong enough, the upper level flow will have to divert to either side of it, creating an area of little flow just behind the updraft. This area is favored for collection of condensation nuclei and thus can result in more hailstones if more of these nuclei make it into the updraft's hail growth area.
Precipitation loading of the updraft can result from increased updraft strength too. Given the precipitation falls from a decent height, this can have the effect of dragging air to the surface, increasing strong wind (downburst) potential. To add to the downburst potential, a stronger updraft can more effectively entrain environmental dry air into the thunderstorm. This air would undergo evaporative cooling and become negatively buoyant, then would proceed to the earth's surface and create strong winds as they spread out along the surface. In more organized severe winds, the stronger updraft serves as the blockade to a rear inflow jet (RIJ), and can divert it to the surface to potentially create severe winds.
With regard to the thermodynamics of an updraft parcel, a downdraft parcel undergoes very similar changes. Both updraft and downdraft parcels experience temperature change at the dry adiabatic lapse rate of 9.8 C/KM, except decreasing for the former and increasing for the latter. The updraft parcel will have its dry adiabatic lapse rate moderated by the release of latent heat from moisture (adds heat to the dry adiabatic cooling), to the moist adiabatic lapse rate. This rate varies with moisture content, but will be warmer than its dry adiabatic counterpart. A downdraft, experiencing a temperature increase at the dry adiabatic lapse rate, will have that warming moderated by the absorption of latent heat from the rain it is accompanied by (just as the updraft does except for moderating the cooling with heat). This is only fully true for wet downbursts. Dry downbursts only experience this before hitting the ground.
An updraft begins convection at the Level of Free Convection (LFC), while a downdraft begins negatively convecting at the Level of Free Sink (LFS).
There is still uncertainty regarding how tornadoes form, although it has been discovered that low level CAPE might increase tornado potential, especially in environments that wouldn't seem to be supportive of tornadoes. A parameter called Three CAPE (3CAPE), measures the CAPE in the lowest three kilometers of the atmosphere. Overall CAPE and a stronger updraft is important to maintaining a deep and persistent mid-level mesocyclone, which would be the parent mesocyclone to the tornado.
Measurement
The potential for convection in the atmosphere is often measured by an atmospheric temperature/dewpoint profile with height. This is often displayed on a Skew-T chart or other similar thermodynamic diagram. These can be plotted by a measured sounding analysis, which is the sending of a radiosonde attached to a balloon into the atmosphere to take the measurements with height. Forecast models can also create these diagrams, but are less accurate due to model uncertainties and biases, and have lower spatial resolution. Although, the temporal resolution of forecast model soundings is greater then the direct measurements, where the former can have plots for intervals of up to every 3 hours, and the latter as having only 2 per day (although when a convective event is expected a special sounding might be taken outside of the normal schedule of 00Z and then 12Z.).
Other forecasting concerns
Atmospheric convection can also be responsible for and have implications on a number of other weather conditions. A few examples on the smaller scale would include: Convection mixing the planetary boundary layer (PBL) and allowing drier air aloft to the surface thereby decreasing dewpoints, creating cumulus type clouds which can limit a small amount of sunshine, increasing surface winds, making outflow boundaries/and other smaller boundaries more diffuse, and the eastward propagation of the dryline during the day. On the larger scale, rising of air can lead to warm core surface lows, often found in the desert southwest.
See also
External links
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