Showing papers on "Convective available potential energy published in 1992"

A radar and electrical study of tropical hot towers

Abstract: Radar and electrical measurements for deep tropical convection are examined for both “break period” and “monsoonal” regimes in the vicinity of Darwin, Australia. Break period convection consists primarily of deep continental convection, whereas oceanic-based convection dominates during monsoonal periods, associated with the monsoon trough over Darwin. Order-of-magnitude enhancements in lightning flash rates for the “break period” regime are associated with 10–20-dB enhancements in radar reflectivity in the mixed-phase region of the convection compared with the monsoonal regime. The latter differences are attributed to the effect of convective available potential energy (CAPE) and its nonlinear influence on the growth and accumulation of ice particles aloft, which are believed to promote charge separation by differential particle motions. CAPE, in turn, is largely determined by the boundary-layer wet-bulb temperature. Modest differences (1°–3°C) in wet-bulb potential temperature between land and s...

The Role of Convectively Generated Rear-Inflow Jets in the Evolution of Long-Lived Mesoconvective Systems

Abstract: In this study, the structure of convectively generated rear-inflow jets and their role in the evolution of long-lived mesoconvective systems are investigated through an analysis of idealized three-dimensional simulations using a nonhydrostatic cloud model. Rear-inflow jets are generated within these systems in response to the upshear-tilting of the convective circulation, as the horizontal buoyancy gradients along the back edge of the expanding system create a circulation that draws midlevel air in from the rear. Within this framework, a wide range of rear-inflow strengths and structures are produced, depending on the magnitude of the ambient convective available potential energy (CAPE) and the vertical wind shear. In general, for environments characterized by weak-to-moderate vertical wind shear and weak-to moderate CAPE, the rear-inflow jet descends and spreads along the surface well behind the leading edge of the gust front, and the subsequent convective activity becomes weaker. However, for e...

A Preliminary Morphology of Precipitation Systems In Tropical Northern Australia

Abstract: A preliminary morphology of convective systems observed in the vicinity of Darwin, Australia is presented. Several types of tropical convection during monsoonal and break-season flow are identified using specific examples and compared with a range of systems observed worldwide in the general context of the bulk Richardson number. A significant spectrum of convective activity ranging from low shear and low Convective Available Potential Energy (CAPE), to high shear and moderate to high CAPE, typical of mid-latitude severe storms, is identified. Significant differences between monsoonal flow and break-season systems are apparent. the formation of convective lines perpendicular to the low-level shear and the apparent balance between advancing shallow cold pools and the shear are ubiquitous features. Convective-scale downdraughts and mesoscale descent appear to be responsible for redistribution of 700 mb momentum to the planetary boundary layer (PEL). The motion of the break-season squall lines appears to result from an equilibrium between the cold pool and the undisturbed environmental PEL flow. Deviations from this balance are hypothesized, to result in the observed ‘propagating’ and ‘non-propagating’ or slow-moving modes. Examples of these modes are given, showing the classical continuous development of new cells in a balanced state along the leading edge of long-lived squalls and, in the propagating case, the discontinuous development of new cells on the cold pool ahead of the squall.

Observations of record cold cloud-top temperatures in Tropical Cyclone Hilda (1990)

Abstract: A detailed analysis is made of the development of a region of cold cloud-top temperatures in Tropical Cyclone Hilda (1990) in the Coral Sea off eastern Australia. Observed temperatures of approximately 173 K (−100°C) from two independent satellite sources indicate that the convective turrets penetrated well into the stratosphere to an estimated height of around 19.2 km. The analytical parcel model of Schlesinger is used, together with available observations from the cyclone vicinity, to estimate the convective updrafts required to produce the observed stratosphere penetration. Under realistic assumptions of entrainment and hydrometeor drag, an updraft speed of between 15 and 38 m s−1 at tropopause level is required to provide the observed stratospheric penetration. Independent calculations using observed anvil expansion and environmental CAPE (convective available potential energy) support these updraft findings.

Structure of a midlatitude squall line formed in strong unidirectional shear

Abstract: Data from five Doppler radars, the surface mesonet, aircraft, and rawinsondes from the Cooperative Convective Precipitation Experiment (CCOPE) are used to document the structure and evolution of a squall line with unusually persistent cells and an anvil that spreads downwind in strong upper-level westerlies. The environmental sounding showed linear shear of ∼4 m s−1 km−1 through the troposphere, a convective available potential energy of 600 m2 s−2, and a convective Richardson number of 10, based on the wind in the lowest 6 km. The orientation of the squall line, comprised of high-reflectivity centers spaced 20–40 km apart, changed with time. Initially, the squall-line axis was normal to the environmental shear, but with time it became parallel to the shear vector, as the northeastern portion of the subcloud cold dome merged with cold air generated by individual storms that had formed ahead of the line. The intensity of the cells within the squads line diminished as its axis became more parallel ...

Clouds, Noncloudy Latent Heat Convection, Entrainment, and Horizontal Averaging

Abstract: Crucial observations describing the mechanisms active in the convective planetary boundary layer are reviewed. Attention is drawn to the observed transport of heat and moisture upward against vertical stratifications stable to cloud-free convection. Occasional saturated parcels carry air of a different composition through regions well below cloud base. These parcels do not form sustained or easily visible cloud and are observed to penetrate even where no sustained cloud forms higher up. This mechanism is likely to be of major importance. The possibility of erosion into the warm overlying air at the top of the convective layer due to a negative heat flux is discussed. One must conclude from the observations discussed here that it does not actually occur. Despite claims to the contrary, the air above appears to enter the convecting layer only when the convection has effectively reached its temperature. When stratus cloud is present, mixtures with the overlying unsaturated air often have the same density as the cloudy air below. Both these phenomena illustrate the inadequacy of conceptual thinking based on averaged horizontal uniformity in the fluid, without recognizing that convective transport is a process where isolated thermal elements in the fluid carry practically all of the quantitiesmore » such as heat and moisture from one level to another. Gradient turbulent diffusion usually has no role to play and is not a satisfactory concept in describing the planetary boundary layer. 30 refs.« less