Convective available potential energy

The change in density of a hypothetical parcel of air as it rises relative to the density of the surrounding air determines where in the atmosphere it can continue rising by buoyancy. The density of an air parcel is based on its temperature, pressure, and water vapor content, provided that its chemical makeup is otherwise constant. CAPE is a measure of the maximum kinetic energy per unit mass that an air parcel can acquire by remaining less dense than its surroundings. This requires an approximation of the change in the parcel's density as it rises. For calculations of CAPE, the hypothetical air parcel is assumed to initially rise and cool at the dry adiabatic lapse rate: the rate at which air cools as it expands without any release of latent heat. Once the parcel cools to the point of saturation, it is then assumed to cool at the moist adiabatic lapse rate: the rate at which air cools adjusted for the release of latent heat from water vapor condensing within the saturated air parcel. For typical daytime atmospheric conditions, accounting for moisture produces larger and more accurate estimates of CAPE. The parcel begins with temperature and moisture characteristics of its surroundings but then deviates from those conditions as it rises. MUCAPE may be a more appropriate measure of the buoyant energy available to a thunderstorm with inflow originating well above the surface. While CAPE quantifies instability in the context of air moving directly upward, slantwise CAPE (SCAPE) can be computed in situations where buoyant ascent can be realized if parcels move in some combination of both the horizontal and vertical. thermodynamically representing a reversible moist adiabatic process. This calculation is better suited for humid tropical environments such as within tropical cyclones. The numerical methodology underlying CAPE can also be performed for portions of the atmosphere where an air parcel would be denser and cooler than its surroundings. These areas have negative buoyancy, resulting in the force of buoyancy acting downwards. Integrating within these areas, typically between the surface and the LFC, results in a negative value also known as convective inhibition (CIN). Additional upward forces are required for an air parcel to rise against negative buoyancy, with CIN providing a measure of the work required to overcome negative buoyancy and reach a freely buoyant height. A similar quantity is downdraft CAPE (DCAPE), which integrates the negative buoyancy potentially imparted on an initially saturated parcel as it descends from some arbitrary height to the ground. This measure is used to quantify the potential for downbursts. == Applications and limitations ==

On thermodynamic diagrams, CAPE is proportional to the area swept out by the varying temperature of a hypothetical rising air parcel warmer than the surrounding atmosphere. Parcel trajectories that take large and extended excursions away from the environmental air temperature thus indicate large amounts of CAPE. In these areas, the 95th percentile of CAPE annually as analyzed between 1979 and 2019 was over  J/kg. The highest CAPE values over land are found over the Congo Basin with a 95th percentile value around  J/kg. Two notable days for severe weather exhibited CAPE values over 5 kJ/kg. Two hours before the 1999 Oklahoma tornado outbreak occurred on May 3, 1999, the CAPE value sounding at Oklahoma City was at 5.89 kJ/kg. A few hours later, an F5 tornado ripped through the southern suburbs of the city. Also on May 4, 2007, CAPE values of 5.5 kJ/kg were reached and an EF5 tornado tore through Greensburg, Kansas. On these days, it was apparent that conditions were ripe for tornadoes and CAPE wasn't a crucial factor. However, extreme CAPE, by modulating the updraft (and downdraft), can allow for exceptional events, such as the deadly F5 tornadoes that hit Plainfield, Illinois on August 28, 1990, and Jarrell, Texas on May 27, 1997, on days which weren't readily apparent as conducive to large tornadoes. CAPE was estimated to exceed 8 kJ/kg in the environment of the Plainfield storm and was around 7 kJ/kg for the Jarrell storm. Severe weather and tornadoes can develop in an area of low CAPE values. The surprise severe weather event that occurred in Illinois and Indiana on April 20, 2004, is a good example. Importantly in that case, was that although overall CAPE was weak, there was strong CAPE in the lowest levels of the troposphere which enabled an outbreak of minisupercells producing large, long-track, intense tornadoes. == See also ==