Convective available potential energy

Abstract: Large-scale modification of the environment by cumulus clouds is discussed in terms of entrainment, detrainment, evaporation, and subsidence. Drying, warming, and condensation by vertical displacement of air are considered as well as budget equations for mass, static energy, water vapor, and liquid water.

Abstract: A new one-dimensional cloud model, specifically designed for application in mesoscale convective parameterization schemes (CPSs), is introduced. The model is unique in its representation of environmental entrainment and updraft detrainment rates. In particular, the two-way exchange of mass between clouds and their environment is modulated at each vertical level by a buoyancy sorting mechanism at the interface of clear and cloudy air. The new entrainment/detrainment scheme allows vertical profiles of both updraft moisture detrainment and updraft vertical mass flux to vary in a physically realistic way as a function of the cloud-scale environment. These performance characteristics allow the parameterized vertical distribution of convective heating and drying to be much more responsive to environmental conditions than is possible with a traditional one-dimensional entraining plume model. The sensitivities of the new model to variations in environmental convective available potential energy and verti...

Abstract: The Kain-Fritsch (KF) convective parameterization scheme (CPS) is based on the same fundamental closure assumption as the Fritsch-Chappell (FC) (1980) scheme—convective effects are assumed to remove convective available potential energy in a grid element within an advective time period. Its development was motivated by ongoing observational and numerical investigations of mesoscale convective systems that have revealed the potentially significant impact of certain physical processes that were not represented in the FC scheme. For example, in the FC scheme, detrainment from convective clouds to their environment occurs over a limited vertical depth near cloud top. Yet, it has become evident from diagnostic studies (e.g., Leary and Houze 1980; Gamache and Houze 1983) that midlevel detrainment of mass and moisture from deep convective clouds plays an important role in the development of some mesoscale convective systems.

Abstract: A simplified cumulus parameterization scheme, suitable for use in GCMs, is presented. This parameterization is based on a plume ensemble concept similar to that originally proposed by Arakawa and Schubert (1974). However, it employs three assumptions which significantly simplify the formulation and implementation of the scheme. It is assumed that an ensemble of convective‐scale updrafts with associated saturated downdrafts may exist when the atmosphere is locally conditionally unstable in the lower troposphere. However, the updraft ensemble is comprised only of those plumes which are sufficiently buoyant to penetrate through this unstable layer. It is assumed that all such plumes have the same upward mass flux at the base of the convective layer. The third assumption is that moist convection, which occurs only when there is convective available potential energy (CAPE) for reversible ascent of an undiluted parcel from the sub‐cloud layer, acts to remove CAPE at an exponential rate with a specified...

Abstract: Observations of individual convective clouds reveal an extraordinary degree of inhomogeneity, with much of the vertical transport accomplished by subcloud-scale drafts. In view of these observations, a representation of moist convective transports for use in large-scale models is constructed, in which the fundamental entities are these subcloud-scale drafts rather than the clouds themselves. The transport by these small-scale drafts is idealized as follows. Air from the subcloud layer is lifted to each level i between cloud base and the level of neutral buoyancy for undilute air. A fraction (ϵi) of the condensed water is then converted to precipitation, which falls and partially or completely evaporates in an unsaturated downdraft. The remaining cloudy air is then assumed to form a uniform spectrum of mixtures with environmental air at level i; these mixtures ascend or descend according to their buoyancy. The updraft mass fluxes Mi are represented as vertical velocities determined by the amount o...