Atmospheric convection
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Atmospheric convection is the process by which air parcels vertically transfers heat between layers of the atmosphere. It is often responsible for the formation of clouds, precipitation, and even severe weather.
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[edit] Initiation
Atmospheric convection initiates at the level of free convection (LFC). This happens when there is sufficient heating of an atmospheric layer to allow air in that layer to rise freely or, the air is dynamically lifted. This dynamic and forced lift can be from upper level divergence, low level convergence, jet streak dynamics, frontal lifting, etc. and brings air to the LFC. The LFC is determined when the lapse rate of the rising air parcel is lower than that of the environment. The heating required for the convection usually comes from insolation or warm air advection (WAA). Moisture also aids convection by the release of latent heat. This happens if the rising air hits the lifted condensation level (LCL) or convective condensation level (CCL) - depending on lift source - where saturation and condensation for cloud formation occurs.
[edit] Types of moist convection
Moist convection is categorized into two different types, surface based convection and elevated convection. Surface based convection is caused by surface heating from insolation and can result in cumulus clouds and air-mass thunderstorms. Elevated convection is the second type where convection takes place above the planetary boundary layer (PBL). It is caused by the PBL being stable from either cold air, dry air, or a high lapse rate. For convection to occur in this situation, there needs to be a dynamic lifting source taking place. If these dynamic lifting sources take place along with surface based convection, then there is a higher chance for strong to severe thunderstorms.
[edit] 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.
[edit] Inhibition
Atmospheric convection is inhibited by the ambient temperature being higher than that of a parcel of rising air. An atmospheric inversion layer, or cap, can cause or strengthen this inhibition. The cap is an increase in temperature with height and is measured in degrees Celsius. Convective inhibition (CINH) is the index used to measure the overall inhibition and is measured in J/kg necessary to get the rising air to the LFC.
[edit] 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 (TTI), 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.