Showing papers on "Convective available potential energy published in 1994"

Three-Dimensional Evolution of Simulated Long-Lived Squall Lines

Abstract: Simulations of squall lines, using nonhydrostatic convection-resolving models, have been limited to two dimensions or three dimensions with the assumption of along-line periodicity. The authors present 3D nonhydrostatic convection-resolving simulations, produced using an adaptive grid model, where the lines are finite in length and the restriction to along-line periodicity is removed. The base state for the simulations is characterized by weak, shallow shear and high convective available potential energy (CAPE), an environment in which longlived midlatitude mesoscale convective systems (MCSs) are observed. The simulated systems bear strong resemblance to many observed systems, suggesting that large-scale forcing, absent in the horizontally homogeneous environment, is not needed to produce many of the distinguishing features of midlatitude MCSs. In simulations without Coriolis forcing, the presence of line ends leads to mature symmetric systems characterized by a central region of strong convectio...

Influence of Ocean Surface Conditions on Atmospheric Vertical Thermodynamic Structure and Deep Convection

Abstract: The authors analyze the influence of Sea Surface Temperature (SST) and surface wind divergence on atmospheric thermodynamic structure and the resulting effects on the occurrence of deep convection using National Meteorological Center radiosonde data and International Satellite Cloud Climatology Program data for July 1983-July 1985. The onset of deep convection requires not only the existence of positive convective available potential energy (CAPE), but also an unstable planetary boundary layer (PBL). A stable PBL is observed to suppress deep convection even when CAPE is positive. Variations of SST have a major effect on CAPE, but surface wind divergence can also affect deep convection by changing the lapse rate in the lower troposphere and humidity in the PBL. Specifically, when SST is greater than or equal to 28 C, CAPE is always positive, and surface wind divergence does not qualitatively change the buoyancy profile above the PBL. Strong surface wind divergence, however, stabilizes the PBL so as to suppress the initiation of deep convection. In warm SST regions, CAPE is greater than 0 regardless of assumptions about condensate loading, although the pseudoadiabatic limit is more consistent with the observed deep convection than the reversible moist-adiabatic limit under these circumstances. When SST is less than 27 C, CAPE is usually negative and inhibits convection, but strong surface wind convergence can destabilize the inversion layer and moisten the PBL enough to make the atmosphere neutrally stable in the mean. As a result, deep convection is generally enhanced either when SST is greater than or equal to 28 C in the absence of strong surface wind divergence or when strong surface wind convergence occurs even if SST is less than 27 C. The anomalous suppression of deep convection in the warm area of the equatorial west Pacific lying between the intertropical convergence zone (ITCZ) and south Pacific convergence zone (SPCZ) is probably caused by dryness in the PBL and an inversion in that area. The seasonal cycles of deep convection and surface wind divergence are in phase with the maximum solar radiation and lead SST for one to three months in the central Pacific. The change of PBL relative humidity plays a critical role in the changeover to convective instability in this case. The seasonal change of deep convection and associated clouds seems not to have important effects on the seasonal change of local SST in the central Pacific.

Convective Available Potential Energy in the Environment of Oceanic and Continental Clouds: Correction and Comments

Abstract: In 1980, Zipser and LeMone estimated the convective available potential energy (CAPE) for the Thunderstorm Project cumulonimbus environment to be about 3000 J per kg. By assuming the most adiabat reported by Byers and Braham (1949) to be that of an undilute parcel rather than a reference moist adiabat, a significant error was introduced. On the basis of recent calculations made under similar conditions in Oklahoma and Florida, CAPE is now estimated to be considerably lower. These lower CAPE estimates shed doubt on the suggestion that differences in CAPE account for differences in the vertical velocities in convective updrafts over land and over the ocean.

Predictability of Precipitation Patterns: An Operational Approach

Abstract: Predictability, defined as the ability to forecast precipitation over an area by Lagrangian persistence, is studied for 11 radar precipitation patterns. After a time ranging between 40 and 112 min, depending on individual cases, all forecast skill is lost. Attempts at relating this range of predictability to larger-scale meteorological parameters lead to positive results when the convective available potential energy is considered alone or in combination with wind shear energy. It appears from this study that the limited range of scales properly sampled by a single radar severely hampers the possibility of establishing a solid empirical relationship between mesoscale predictability and synoptic-scale meteorological parameters.

The Moist Available Energy of a Conditionally Unstable Atmosphere. Part II: Further Analysis of GATE Data

Abstract: The generalized convective available potential energy (GCAPE) observed during GARP Atlantic Tropical Experiment (GATE) has been analyzed using the Lagrangian algorithm of Lorenz, as modified by Randall and Wang. The effects of ice are included and are discussed in an Appendix. A high positive correlation is found between the rate of GCAPE production by large-scale processes and the observed precipitation rate, and a negative correlation between the GCAPE itself and the precipitation rate. The observed time rate of change of the GCAPE is much smaller than the rate of GCAPE production by large-scale processes.

A Modeling Study on the Early Electrical Development of Tropical Convection: Continental and Oceanic (Monsoon) Storms

Abstract: Numerical modeling studies of continental tropical and maritime tropical convection were conducted using the two-dimensional, nonhydrostatic, cloud electrification model developed at the South Dakota School of Mines and Technology. The model contains six classes of water (water vapor, cloud water, cloud ice, rain, snow, and graupel) and a full set of ion equations. All hydrometeors are permitted to exchange charge. Charge transfer between microphysical species is accomplished through a noninductive charging parameterization following Takahashi. The goal of the numerical experiments was to examine the kinematic and microphysical differences that lead to marked differences in observed electrification between the break (continental) and monsoon (oceanic) convective regimes observed near Darwin, Australia. The break regime is associated with deep, intense convection that forms in high-CAPE (convective available potential energy) environments. Normally, copious amounts of lightning accompany break per...

A Numerical Study of Nonlinear Nonhydrostatic Conditional Symmetric Instability in a Convectively Unstable Atmosphere

Abstract: Nonlinear nonhydrostatic conditional symmetric instability (CSI) is studied as an initial value problem using a two-dimensional (y, z)nonlinear, nonhydrostatic numerical mesoscale/cloud model. The initial atmosphere for the rotating, baroclinic (BCF) simulation contains large convective available potential energy (CAPE). Analytical theory, various model output diagnostics, and a companion nonrotating barotropic (BTNF) simulation are used to interpret the results from the BCF simulation. A single warm moist thermal initiates convection for the two 8-h simulations. The BCF simulation exhibited a very intricate life cycle. Following the initial convection, a series of discrete convective cells developed within a growing mesoscale circulation. Between hours 4 and 8, the circulation grew upscale into a structure resembling that of a squall-line mesoscale convective system (MCS). The mesoscale updrafts were nearly vertical and the circulation was strongest on the baroclinically cool side of the initial...

Water and Climate

Abstract: The vertical profiles and temperature and moisture in convective regimes were investigated, using moist available energy as a guide. The generalized convective available potential energy observed during the Global Atmosphere Research Program's Atlantic Tropical Experiment (GATE) phase 3 was analyzed. Ice effects were included. The results have been used to develop an improved cumulus parameterization. Several reprints from the Journal of Atmospheric Sciences are appended.