Showing papers on "Convective available potential energy published in 1996"

Natural Convection as a Heat Engine: A Theory for CAPE

Abstract: On many planets there is a continuous heat supply to the surface and a continuous emission of infrared radiation to space by the atmosphere. Since the heat source is located at higher pressure than the heat sink, the system is capable of doing mechanical work. Atmospheric convection is a natural heat engine that might operate in this system. Based on the heat engine framework, a simple theory is presented for atmospheric convection that predicts the buoyancy, the vertical velocity, and the fractional area covered by either dry or moist convection in a state of statistical equilibrium. During one cycle of the convective heat engine, heat is taken from the surface layer (the hot source) and a portion of it is rejected to the free troposphere (the cold sink) from where it is radiated to space. The balance is transformed into mechanical work. The mechanical work is expended in the maintenance of the convective motions against mechanical dissipation. Ultimately, the energy dissipated by mechanical friction is transformed into heat. Then, a fraction of the dissipated energy is radiated to space while the remaining portion is recycled by the convecting air parcels. Increases in the fraction of energy dissipated at warmer temperature, at the expense of decreases in the fraction of energy dissipated at colder temperatures, lead to increases in the apparent efficiency of the convective heat engine. The volume integral of the work produced by the convective heat engine gives a measure of the statistical equilibrium amount of convective available potential energy (CAPE) that must be present in the planet's atmosphere so that the convective motions can be maintained against viscous dissipation. This integral is a fundamental global number qualifying the state of the planet in statistical equilibrium conditions. For the earth's present climate, the heat engine framework predicts a CAPE value of the order of 1000 J kg^−1 for the tropical atmosphere. This value is in agreement with observations. It also follows from our results that the total amount of CAPE present in a convecting atmosphere should increase with increases in the global surface temperature (or the atmosphere's opacity to infrared radiation).

Model for multiscale disaggregation of spatial rainfall based on coupling meteorological and scaling descriptions

Abstract: The precipitation output of a mesoscale atmospheric numerical model is usually interpreted as the average rainfall intensity over the grid cell of the model (typically 30 × 30 km to 60×60 km). However, rainfall exhibits considerable heterogeneity over subgrid scales (i.e., scales smaller than the grid cell), so it is necessary for hydrologic applications to recreate or simulate the small-scale rainfall variability given its large-scale average. Rainfall disaggregation is usually done statistically. In this paper, a new subgrid scale rainfall disaggregation model is developed. It has the ability to statistically reproduce the rainfall variability at scales unresolved by mesoscale models while being conditioned on large-scale rainfall averages and physical properties of the prestorm environment. The model is based on two extensively tested hypotheses for midlatitude mesoscale convective systems [Perica and Foufoula-Georgiou, 1996]: (1) standardized rainfall fluctuations (defined via a wavelet transform) exhibit simple scaling over the mesoscale, and (2) statistical scaling parameters of rainfall fluctuations relate to the convective available potential energy (CAPE), a measure of the convective instability of the prestorm environment. Preliminary evaluation of the model showed that the model is capable of reconstructing the small-scale statistical variability of rainfall as well as the fraction of area covered with rain at all analyzed subgrid scales. The performance evaluation was based on comparison of summary statistics and spatial pattern measures of simulated fields with those of known fields observed during the Oklahoma-Kansas Preliminary Regional Experiment for Storm-Central (PRE-STORM).

Tropical deep convection, convective available potential energy and sea surface temperature

Abstract: The convective available potential energy (CAPE) based on monthly mean sounding has been shown to be relevant to deep convection in the tropics. The variation of CAPE with SST has been found to be similar to the variation of the frequency of deep convection at one station each in the tropical Atlantic and W. Pacific oceans. This suggests a strong link between the frequency of tropical convection and CAPE. It has been shown that CAPE so derived can be interpreted as the work potential of the atmosphere above the boundary layer with ascent in the convective region and subsidence in the surrounding cloud-free region.

Dimensional analysis of a convecting atmosphere in equilibrium with external forcing

Abstract: It has recently been suggested by Renno and Ingersoll that certain properties of a convecting atmosphere in equilibrium with external forcing can be predicted by considering the convecting atmosphere as a heat engine. Their argument is modified to express the forcing of convection in terms of a specified cooling of the convective layer. It is found that convective available potential energy and mean convective vertical velocity are independent of the forcing, while the fractional area of the convection increases in proportion with the forcing. These predictions are consistent with recent numerical results from cloud-ensemble models. Finally, it is shown by a formal dimensional analysis that these conclusions are independent of the details of the thermodynamic formulation.

Evolution of the Vertical Mass Flux and Diagnosed Net Lateral Mixing in Isolated Convective Clouds

Abstract: The evolution of the vertical mass flux in isolated cumulus and cumulus congestus clouds is documented using two King Airs during the Convection and Precipitation/Electrification Experiment (CaPE,), conducted in east-central Florida during the summer of 1991. These clouds develop over the sea-breeze convergence in an environment characterized by low shear and moderate convective available potential energy. The aircraft, separated vertically by 600–1000 m, commence flying near-simultaneous penetrations as the cloud top passes through the altitude of the upper aircraft and continue sampling until the cloud disappears. The net vertical mass flux for each level is estimated; the difference in the mass flux between the two levels leads to a diagnosis of the net entrainment or detrainment that occurs laterally in the intervening cloud layer. This kinematic technique relies on determination of cloud edge based on liquid water measurements, a cloud shape factor, and the vertical velocity. The technique i...

The meteorology of high‐intensity rainfall events over the west Mediterranean region

Abstract: The meteorological situations in which heavy rainfall events occur over the western Mediterranean region are presented and discussed. The study of such situations has been made looking for mechanisms able to maintain convection during larger intervals than the life cycle of a single convective cell. These mechanisms have been analysed firstly from a thermodynamic standpoint, involving the determination of stability, precipitable water mass, convective available potential energy (CAPE), and bulk Richardson number (bRi). This permits definition of some features of these events and establishment of some thresholds for different variables. Secondly, resort has been made to objective analysis focused on the diagnosis of spatial and temporal distributions of quasi‐geostrophic forcing for vertical motion, convective instability, water vapour divergence/convergence at low levels and CAPE. Following this analysis compound maps were obtained, showing the zones of superimposition of the above four variables. These z...

A Cumulus Parameterization Based on the Generalized Convective Available Potential Energy

Abstract: This paper reports tests of a cumulus parameterization in which the reference state associated with the generalized convective available potential energy (GCAPE) is chosen as the end-state of the convective adjustment. The GCAPE is defined as the enthalpy difference between the given state and the reference state and represents the total potential energy available for conversion into convective kinetic energy in a given sounding. The reference state is the unique state in which the system enthalpy is minimized; it is also a statically neutral or stable state. By assuming that convection drives the atmosphere from the given state toward the reference state, we can use Nitta's diagnostic method to determine the cloudbase mass flux in a prognostic model. The adjustment timescale is finite and varies, depending on the intensity of the large-scale forcing. so this is a “relaxed” scheme. The effects of detrainment are parameterized in a very simple way. After the cloudbase mass flux has been obtained, ...

Numerical Simulation of the Effects of Mesoscale Convergence on Convective Rain Showers

Abstract: A nonhydrostatic axisymmetric cloud model is used to quantify the effects of persistent mesoscale convergence on cumulus development and convective rainfall. The model was initialized by environmental conditions adopted from sounding and Doppler radar velocity data sampled on 19 August 1992 in central Alberta. The sounding showed a moist warm air mass with a moderate amount of convective available potential energy and the wind field had boundary layer convergence but almost no vertical shear in the lowest 5 km. The simulated rainfall intensity and accumulation compared well with radar observations. The dependence of the convective rainfall on the characteristics of the convergence zone is investigated by intercomparing model simulators with different convergence magnitudes, convergence depths, and convergence profiles. Increasing the magnitude or the depth of convergence causes stronger convection and more precipitation. Rainfall increases monotonically (but nonstrictly linearly) with the converg...

TRMM: The Tropical Rainfall Measuring Mission

Abstract: Precipitation is one of the most important parts of both the global water and energy cycles, yet it is one of the most poorly observed climate parameters. Globally, the net latent heating associated with precipitation accounts for about 80% of the total diabatic heating required to balance radiative cooling to space. The spatial pattern and temporal variability of this heating serves as the primary driver for the tropical general circulation. The local balance between precipitation and evaporation over ocean helps determine the thermohaline component of the ocean circulation and may influence both tropical El Nino-Southern Oscillation (ENSO) events and high latitude deep water formation. Over land, the precipitation-evaporation balance controls the tendency for drought to occur and directly influences the surface temperature by altering the relative contributions of sensible and latent heat fluxes to the surface energy budget. Within clouds, precipitation formation is thought to be the major sink of cloud water and thus affects the reflectance of bright tropical cumulus anvils. Drizzle formation in low-level marine stratocumulus may help detach those clouds from surface moisture fluxes as the drizzle evaporates in the subcloud layer, thereby contributing to the stratocumulus-trade cumulus transition. Land- ocean differences in precipitation efficiency, associated with different concentrations of nucleating aerosols, may cause cloud feedback in a changing climate to differ for marine and continental clouds.

Transport of radioactive materials in convective clouds

Abstract: Publisher Summary After the Chernobyl accident, highly radioactive particles were found at longer distances than expected. It was because either the maximum effective release height was considerably larger than previously reported or the convective currents of warm air lifted the particles upwards to layers, where different wind conditions prevail. During the early stages of the accident, convective uplift may have transported radioactive materials from lower air layers, originally transported to Sweden, to higher air layers in which materials were transported to Finland. The radiological consequences might have been different had the accident occurred in the day time, when convective clouds would have been present near Chernobyl. The present long-range transport and dispersion models developed for radioactive substances are not necessarily adequate to describe the radiological hazards, as long as convective updraft in cumulonimbus clouds is not included in the models.

Catastrophic rainfalls in the West Mediterranean area

Abstract: Meteorological and thermodynamic factors associated with catastrophic flood events affecting the West Mediterranean Area have been studied. First of all, the usually assumed causal connection between cut-off lows aloft and severe storms has been thoroughly investigated. Once identified the general meteorological features, the synoptic- scale pattern and forcing are studied by using objective techniques. The thermodynamic analysis took specific form in the study of stability, precipitable water mass, convective available potential energy, vertical wind shear and bulk Richardson number. The combination of the synoptic and thermodynamic results show a strong quasi-geostrophic forcing for vertical motion at low levels while only a weak forcing at 500 hPa, high values of precipitable water mass produced by a strong convergence of water vapour near surface and convective and latent instability. Composite charts delineating the area covered by each of the indicated factors seem to be an effective tool for delimiting the are where mesoscale focusing mechanisms become effective.

Impact of environmental conditions on the mesoscale characteristics of squall-line systems: Toward the development of anvil cirrus parameterization for GCMs

Abstract: Our earlier studies indicated that a strong coupling exists in the mesoscale convective systems (MCSs) between deep convection and its related anvil cloud through the interaction among dynamical, thermodynamical and radiative processes. They also showed that the tilting structure of MCSs makes an important contribution to the water budget of anvil clouds, particularly the tropical anvil due to the jetlike wind profile. However, most earlier GCMs did not include a direct and physically consistent representation of this coupling. To this end, Randall et al. suggested a more realistic anvil parameterization by adding prognostic cloud water (or ice) variables to account for the formation of anvil clouds from cumulus detrainment. In addition to this effort, our recent studies further suggest the need to parameterize the tilting structure of MCSs in GCMs. The objective of this work is to parameterize the large-scale effects of this tilting structure. Our primary interest focuses on MCSs in an environment with substantial wind shear, such as squall-line systems, since they have longer lifetimes and wider coverage to affect the earth-atmosphere radiation budget and climate. Using varied convective available potential energy (CAPE), wind shear intensity, shear depth, and the pattern of shear profile over a wide range of bulk Richardson number (Ri), a sensitivity study is performed in a cloud resolving model to link its resulting mesoscale ascent/descent with GCM-resolvable variables. The ultimate goal of this research is to develop an anvil cirrus parameterization (ACP), that will couple with cumulus parameterizations in GCMs to improve the cloud-radiation feedback on large-scale climate.