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Showing papers on "Convective available potential energy published in 1998"


Journal ArticleDOI
TL;DR: In this article, the authors examined all of the 0000 UTC soundings from the United States made during the year 1992 that have nonzero convective available potential energy (CAPE) and classified them as nonsupercell thunderstorms, supercells without significant tornadoes, and supercells with significant hurricanes.
Abstract: All of the 0000 UTC soundings from the United States made during the year 1992 that have nonzero convective available potential energy (CAPE) are examined. Soundings are classified as being associated with nonsupercell thunderstorms, supercells without significant tornadoes, and supercells with significant tornadoes. This classification is made by attempting to pair, based on the low-level sounding winds, an upstream sounding with each occurrence of a significant tornado, large hail, and/or 10 or more cloud-to-ground lightning flashes. Severe weather wind parameters (mean shear, 0–6-km shear, storm-relative helicity, and storm-relative anvil-level flow) and CAPE parameters (total CAPE and CAPE in the lowest 3000 m with buoyancy) are shown to discriminate weakly between the environments of the three classified types of storms. Combined parameters (energy–helicity index and vorticity generation parameter) discriminate strongly between the environments. The height of the lifting condensation level a...

722 citations


Journal ArticleDOI
TL;DR: In this paper, a cloud-resolving model has been run to a radiative-convective equilibrium state in three dimensions, and the total integration lasted 70 days, and a statistical equilibrium state was reached at all heights after 30 days of simulation in all model variables.
Abstract: A knowledge of radiative convective interactions is key to an understanding of the tropical climate. In an attempt to address this a cloud-resolving model has been run to a radiative-convective equilibrium state in three dimensions. The model includes a three-phase bulk microphysical scheme and a fully interactive two-stream broadband radiative-transfer scheme for both the infrared and solar radiation. The simulation is performed using a fixed sea surface temperature, and cyclic lateral boundary conditions. No ‘large-scale’ convergence, mean wind shear or background vorticity was imposed. The total integration lasted 70 days, and a statistical equilibrium state was reached at all heights after 30 days of simulation in all model variables. It is seen that some variables, such as vertical mass flux, adjust quickly to their equilibrium values while others, such as column-integrated water amount, domain-mean temperature and convective available potential energy (CAPE) display variation on a longer 30-day time-scale. The equilibrium state had a column-integrated vapour amount of 42.3 kg m−2, a mean temperature of 258.7 K and a pseudo-adiabatic CAPE value of 1900 J kg−1. The equilibrium-state statistics are consistent with tropical observations. The convection does not remain randomly distributed but instead becomes organized, aligning in a band structure associated with high moisture values in the boundary layer. This organization seems to result from interactions between radiation, convection and surface fluxes. The surface-flux feedback is due to higher boundary-layer winds, associated with convection, increasing surface fluxes of moisture locally. Horizontally inhomogeneous radiation can act to make clouds longer lasting and also increase convergence into cloudy region. Replacing the wind-sensitive surface-flux calculation with a linear relaxation to surface values appeared to largely destroy this organization, as did the use of an imposed horizontally uniform radiative-heating rate.

235 citations


Journal ArticleDOI
TL;DR: In this article, the authors proposed a prognostic cumulus kinetic energy (CKE) as a replacement for the quasi-equilibrium closure hypothesis of Arakawa and Schubert (AS).
Abstract: The paper describes the introduction of a prognostic cumulus kinetic energy (CKE) as a replacement for the quasi-equilibrium closure hypothesis of Arakawa and Schubert (AS). In the original version of the AS parameterization, the cloud work function, a measure of the convective available potential energy, is assumed to be maintained at ‘small’ values through a quasi-equilibrium between the cumulus convection and the ‘large-scale forcing’. It is argued here, however, that the distinction between the convective and large-scale processes is ambiguous and subjective. It is demonstrated that the need for such a distinction can be avoided by relaxing the quasi-equilibrium assumption, through the introduction of a prognostic CKE; referred to as prognostic closure. A dimensional parameter, α, is introduced to relate the CKE to the square of the cloud-base convective mass flux. It is shown that ‘adjustment time’ defined by AS is related to α, so that when the adjustment time approaches zero the prognostic closure reduces to quasi-equilibrium closure. A second dimensional parameter, τD, is used to determine the rate at which the CKE is dissipated. In the limit of small α and τD, the convective mass flux is formally independent of both α τD if the environmental sounding is assumed to be given, but in reality the results of a prognostic model do depend on these two parameters because they affect the time-dependent sounding. For simplicity, a single constant value of α is used for all cloud types in tests with a general-circulation model, and this gives reasonably good results. Larger values of α lead to more frequent shallow cumulus convection and a cooler and more humid troposphere, in which stratiform condensation is more active and more large-scale precipitation can reach the surface. A longer dissipation time-scale leads to a warmer tropical troposphere. The interactions between stratiform cloudiness and convection prove to be quite important, leading to the conclusion that the convection parametrization really cannot be evaluated independently of the stratiform cloud parametrization with which it interacts.

232 citations


Journal ArticleDOI
TL;DR: In this paper, the relationship between mid-tropospheric dryness and supercell morphology and evolution using a three-dimensional, non-hydrostatic cloud model was studied using a 3D model.
Abstract: This work studies the relationship between midtropospheric dryness and supercell thunderstorm morphology and evolution using a three-dimensional, nonhydrostatic cloud model Environments that differ only in midtropospheric dryness are found to produce supercells having different low-level outflow and rotational characteristics Thunderstorms forming in environments with moderate vertical wind shear, large instability, and very dry midtropospheric air produce strong low-level outflow When this low-level outflow propagates faster than the midlevel mesocyclone, the storm updraft and low-level mesocyclone weaken However, in environments with larger vertical wind shear or with higher-altitude dry midtropospheric air, the low-level outflow is not as detrimental to the supercell This provides a possible explanation for why some environments that appear favorable for the development of strong low-level mesocyclones in supercells fail to do so Downdraft convective available potential energy (DCAPE) is

169 citations


Journal ArticleDOI
TL;DR: In this paper, the authors compared the convective available potential energy (CAPE) with standard instability indices (LI) for evaluating convective potential of the atmosphere such as the lifted index (LI), revealing only moderate correlations.
Abstract: Comparisons of convective available potential energy (CAPE) with standard instability indices for evaluating the convective potential of the atmosphere such as the lifted index (LI) reveal only moderate correlations. This is because the LI is a measure of single-level buoyancy while CAPE is a measure of both integration depth and the buoyancy. Normalizing the CAPE values by the depth over which the integration takes place provides an index (NCAPE) that is independent of the depth and is a convenient measure of the mean parcel buoyancy. This normalization effectively distinguishes between environments with similar CAPE but exhibiting different buoyancy and integration depth. Also, because the vertical distribution of CAPE can have an important effect on convective updraft strength, it is advantageous to vertically partition CAPE and NCAPE into multiple layers. NCAPE may provide a more useful indicator of buoyancy in environments in which the depth of free convection is shallow and total CAPE is sm...

129 citations


01 Jan 1998
TL;DR: In this article, the authors proposed to normalize the convective available potential energy (CAPE) values by the depth over which the integration takes place to provide an index (NCAPE) that is independent of the depth and is a convenient measure of the mean parcel buoyancy.
Abstract: Comparisons of convective available potential energy (CAPE) with standard instability indices for evaluating the convective potential of the atmosphere such as the lifted index (LI) reveal only moderate correlations. This is because the LI is a measure of single-level buoyancy while CAPE is a measure of both integration depth and the buoyancy. Normalizing the CAPE values by the depth over which the integration takes place provides an index (NCAPE) that is independent of the depth and is a convenient measure of the mean parcel buoyancy. This normalization effectively distinguishes between environments with similar CAPE but exhibiting different buoyancy and integration depth. Also, because the vertical distribution of CAPE can have an important effect on convective updraft strength, it is advantageous to vertically partition CAPE and NCAPE into multiple layers. NCAPE may provide a more useful indicator of buoyancy in environments in which the depth of free convection is shallow and total CAPE is small. It is suggested that NCAPE computations be used in combination with CAPE for evaluation of convective potential.

124 citations


Journal ArticleDOI
TL;DR: In this paper, the authors used the Penn State-NCAR Mesoscale Model Version 5 (MM5) with a horizontal resolution of 54 km to simulate the successive development and eastward propagation of a series of convective rainstorms.
Abstract: On 12‐13 June 1991, a series of convective rainstorms (defined as mesoscale precipitation systems with rainfall rates exceeding 10 mm h21) developed successively along the Mei-Yu front. During this event, new rainstorms formed to the east of preceding storms at an interval of approximately 300‐400 km. The successive development and eastward propagation of these rainstorms produced heavy rainfall over the Jiang-Huai Basin in eastern China, with a maximum 24-h accumulation of 234 mm. This study presents the results of a numerical simulation of this heavy rainfall event using the Penn State‐NCAR Mesoscale Model Version 5 (MM5) with a horizontal resolution of 54 km. Despite the relatively coarse horizontal resolution, the MM5, using a moist physics package comprising an explicit scheme and the Grell cumulus parameterization, simulated the successive development of the rainstorms. The simulated rainstorms compared favorably with the observed systems in terms of size and intensity. An additional sensitivity experiment showed that latent heat release is crucial for the development of the rainstorms, the mesoscale low-level jet, the mesolow, the rapid spinup of vorticity, and the Mei-Yu frontogenesis. Without latent heat release, the maximum vertical motion associated with the rainstorm is reduced from 70 to 6 cm s 21. Additional model sensitivity experiments using the Kain‐Fritsch cumulus parameterization with grid sizes of 54 and 18 km produced results very similar to the 54-km control experiment with the Grell scheme. This suggests that the simulation of Mei-Yu rainstorms, the mesoscale low-level jet, and the mesolow is not highly sensitive to convective parameterization and grid resolution. In all the full-physics experiments, the model rainfall was dominated by the resolvable-scale precipitation. This is attributed to the high relative humidity and low convective available potential energy environment in the vicinity of the Mei-Yu front. The modeling results suggest that there is strong interaction and positive feedback between the convective rainstorms embedded within the Mei-Yu front and the Mei-Yu front itself. The front provides a favorable environment for such rainstorms to develop, and the rainstorms intensify the Mei-Yu front.

101 citations


Journal ArticleDOI
TL;DR: In this paper, it was found that frequency distributions of tropical Pacific convective available potential energy (CAPE), as well as the dependence of CAPE on surface wet-bulb potential temperature (Θw) simulated by the Goddard Institute for Space Studies's GCM, agree with that observed during the Australian Monsoon Experiment period.
Abstract: Observed variations of convective available potential energy (CAPE) in the current climate provide one useful test of the performance of cumulus parameterizations used in general circulation models (GCMs). It is found that frequency distributions of tropical Pacific CAPE, as well as the dependence of CAPE on surface wet-bulb potential temperature (Θw) simulated by the Goddard Institute for Space Studies’s GCM, agree well with that observed during the Australian Monsoon Experiment period. CAPE variability in the current climate greatly overestimates climatic changes in basinwide CAPE in the tropical Pacific in response to a 2°C increase in sea surface temperature (SST) in the GCM because of the different physics involved. In the current climate, CAPE variations in space and time are dominated by regional changes in boundary layer temperature and moisture, which in turn are controlled by SST patterns and large-scale motions. Geographical thermodynamic structure variations in the middle and upper tr...

75 citations


Journal ArticleDOI
TL;DR: In this paper, the authors investigated the diurnal cycle of cloud and thermodynamic structure and found that diurnal variations in cumulus development are modulated by the stability in the transition layer.
Abstract: Radiosonde, in situ, and surface-based remote sensor data from the Atlantic Stratocumulus Transition Experiment are used to study the diurnal cycle of cloud and thermodynamic structure. A cloud layer and decoupled subcloud layer separated by a stable transition layer, often observed in the vicinity of cumulus cloud base, characterizes the thermodynamic structure during the study period. The mode of cloud structure is cumulus with bases below decoupled stratus. Data are presented that support the hypothesis that diurnal variations in cumulus development are modulated by the stability in the transition layer. The frequency of cumulus convection decreases during the afternoon, but mesoscale regions of vigorous cumulus with cloud tops overshooting the base of the trade inversion and increased surface drizzle rates are present during the late afternoon and early evening, when the transition layer is the most stable. It is postulated that mesoscale organization may be required to accumulate enough water vapor in the subcloud layer to produce the convective available potential energy needed for developing cumulus to overcome transition layer stability. The mesoscale regions appear to fit the description of cyclic cumulus convection proposed in a previous study, and this theory is expanded to account for diurnal variations in the stability of the transition layer. The occurrence of these mesoscale clusters of vigorous convection makes it difficult to determine if the latent heat flux in the cloud layer has actually decreased in the late afternoon and early evening, when the transition layer is the most stable. Liquid water structure was examined and no pronounced diurnal signal was found. Results showed that clouds thicker than approximately 450 m tended to have subadiabatic integrated liquid water contents, presumably due to evaporation of drizzle in the subcloud layer, removal of liquid water at the surface, and the evaporation of cloud water at cloud top. A significant fraction of clouds less than 450 m thick produced liquid water contents that were greater than adiabatic, and there may be a physical mechanism that could produce such values in this cloud system (i.e., lateral detrainment of cloud water from convective elements mixing with existing liquid water in decoupled stratus or with liquid water detrained by nearby convective elements). Unfortunately, instrument limitations may have also produced these greater-than-adiabatic values and the extent of instrument artifacts in these results is unclear.

49 citations


Journal ArticleDOI
TL;DR: The influence of large-scale advective cooling and/or moistening on the quasi-equilibrium behavior of simulated, tropical oceanic cumulus ensembles is examined in this paper.
Abstract: The influence of large-scale advective cooling and/or moistening on the quasi-equilibrium behavior of simulated, tropical oceanic cumulus ensembles is examined in this study. Two sensitivity simulations are performed by imposing time varying/invariant large-scale advective cooling effects and time invariant/varying large-scale advective moistening effects. The results are compared with a control simulation performed with both large-scale advective cooling and moistening effects that are time varying. It is found that the generalized convective available potential energy (GCAPE) tendency is almost one order of magnitude smaller than the GCAPE production in all simulations. This indicates that the quasi-equilibrium assumption of Arakawa and Schubert is well justified. The higher-order behavior of quasi-equilibrium cumulus ensemble is then examined. It is found that the GCAPE variations are nearly equally contributed by temperature and water vapor variations in the control simulation. In the sensiti...

37 citations


Journal ArticleDOI
TL;DR: In this article, two case studies in which elevated thunderstorms played an important role in enhancing precipitation totals are discussed, and the case studies suggest that convective instability aloft released by isentropic ascent, rather than frontogenetical forcing in the presence of weak symmetric stability, can result in heavy precipitation.
Abstract: Two case studies in which elevated thunderstorms played an important role in enhancing precipitation totals are discussed. During the period 20–22 February 1993, elevated thunderstorms over Iowa produced a mesoscale band of snow with amounts in excess of 25 cm across north central Iowa. Diagnosis of the environment revealed an elevated layer of convective instability between 700 and 540 hPa above a well-defined frontal zone. Elevated, upright convection resulted from the release of the convective instability as air parcels, ascending isentropically over the frontal zone, reached saturation. A second case, in which heavy rain (greater than 50 mm in 24 h) fell over parts of Oklahoma, Kansas and Missouri during the period 27–28 April 1994, is also examined. Once again, elevated thunderstorms resulted from the ascent of an elevated layer of convective instability over a strong baroclinic zone. Positive CAPE values are found by lifting air parcels having the maximum equivalent potential temperature in the lower portion of the troposphere, whereas no available energy is diagnosed for surface parcels. While both cases strongly resemble the climatology of elevated thunderstorms, these case studies suggest that convective instability aloft released by isentropic ascent, rather than frontogenetical forcing in the presence of weak symmetric stability, can result in heavy precipitation. Copyright © 1998 Royal Meteorological Society

Journal ArticleDOI
TL;DR: The sensitivity of the potential for convective precipitation to land surface characteristics in the West African Sudano-Sahelian zone is studied in this article, where simulations are performed with a one-dimensional version of a mesoscale atmospheric model coupled to a detailed land surface model to investigate the influence of the land surface on rain infiltration, surface evapotranspiration, and their impact on the convective available potential energy.
Abstract: The sensitivity of the potential for convective precipitation to land surface characteristics in the West African Sudano-Sahelian zone is studied. Simulations are performed with a one-dimensional version of a mesoscale atmospheric model coupled to a detailed land surface model to investigate the influence of the land surface on rain infiltration, surface evapotranspiration, and their impact on the convective available potential energy. The simulations show that the presence of a densely vegetated surface acts as a catalyst in the hydrological cycle, creating a positive feedback and enhancing precipitation recycling. It is argued that this is due to the relation between the characteristic drying-out time of the soil and the return frequency of the rain-triggering African easterly waves.

Journal ArticleDOI
TL;DR: It is found that by combining the CAPE (convective available potential energy) and low-level relative humidity diagnostics one can delineate quite large areas where convective triggering is possible and outside which convection will not develop.
Abstract: A mesoscale numerical weather prediction model and its associated diagnostics are evaluated to gauge their ability to forecast convection and the convective environment for ten case-study days. The diagnostic indices and parameters evaluated are those used to assess atmospheric instability, mesoscale forcing, inhibition and low-level moisture supply. In addition, bulk Richardson number and helicity diagnostics are examined for convection organisation. Convection over mountainous regions is not considered. The case-study days are grouped according to their synoptic type in order to evaluate the usefulness of the forecast diagnostics. In the evaluation it is found that by combining the CAPE (convective available potential energy) and low-level relative humidity diagnostics one can delineate quite large areas where convective triggering is possible and outside which convection will not develop. Organisation of the mesoscale convective systems is also considered. It is found that forecast strong low-level shears tend to be well correlated with observed linear convective systems. A set of stability indices are evaluated quantitatively and qualitatively by comparing the forecast stability indices with observations at a sounding location. CAPE and a simple index based on a moist temperature difference between two layers (Adedokun2) are identified as the most useful forecast stability indices for all of the cases. Copyright © 1998 Royal Meteorological Society

Journal ArticleDOI
TL;DR: In this paper, a three-dimensional cloud model is used to simulate the early development and propagation of an arc-shaped line element similar to that found in the GARP (Global Atmospheric Research Program) Tropical Atlantic Experiment (GATE) 4−5 September 1974 squall line system.
Abstract: A three-dimensional cloud model is used to simulate the early development and propagation of an arc-shaped line element similar to that found in the GARP (Global Atmospheric Research Program) Tropical Atlantic Experiment (GATE) 4‐5 September 1974 squall line system. The simulated squall line element forms in a relatively unexplored environment with moderate convective available potential energy and a strong low-level jet (bulk Richardson number 5 37) associated with an easterly wave. The simulated line element develops in a large-scale convergence region from an initial cell that splits and elongates in a manner reminiscent of some midlatitude lines. Simulation features compare favorably with observed characteristics of some of the line elements including line orientation (approximately perpendicular to the average wind shear below the low-level jet), propagation speed (11 m s21 to the southwest), length (75 km), and maximum precipitation rate (187 mm h21). In addition, the simulated line merges with cells that form ahead as observed. The simulated arc-shaped line element consists of four regions several hours after its initiation. The northern region or right flank of the line contains a long-lived cell exhibiting three-dimensional characteristics similar to midlatitude supercells including long-lasting and steady updrafts with midlevel cyclonic rotation and movement to the right of the mean winds. Although the simulated supercell characteristics cannot be confirmed because of insufficient data from the limited GATE 4‐5 September observations, updrafts with strong vertical vorticity have been observed to occur in other GATE rainbands where waterspouts have been seen. The region just to the south of this cell is the site where a cell that formed out ahead of the line segment merges with the line and also develops supercell-like characteristics. Farther to the south, convection develops with quasi twodimensional characteristics below 3‐4 km but breaking into several multicells above. A rear-inflow jet does not accompany these features and the winds in the downdraft are oblique too and nonuniform along the associated gust front. Finally, on the southernmost or left flank of the line element, cells with both long and short lifetimes develop. Sensitivity tests indicate that once the early line structure has developed, its evolution and structure are not seriously altered by the removal of large-scale forcing. Further, the formation of the cold pool and gust front between the initial separating (splitting) cells is crucial to the filling in of the line. Changes in the thermodynamic profile consistent with nonsquall observations one-half day earlier result in only modest differences. Changes in the wind profile (based on the same observations) led to significant differences, such as the lack of convection between the initial separating cells and the merger taking place to the north of the right flank creating a convective complex in this region.

Journal ArticleDOI
TL;DR: In this paper, the CANARI optimal interpolation mesoscale analysis scheme is used, which combines guess-fields from the fine-mesh (10 km) ALADIN model with hourly routine observations arising from a mesonet of automated ground stations.
Abstract: We study the contribution of surface data to convection nowcasting outside mountainous areas and under weak synoptic forcing. The CANARI optimal interpolation mesoscale analysis scheme is used, which combines guess-fields from the fine-mesh (10 km) ALADIN model with hourly routine observations arising from a mesonet of automated ground stations. A tuning of this scheme is realized in order to fit convective systems scales (meso-β scale) as well as the mesonet scale. Then, these grid point analyses allow the computation of diagnostic fields such as Convective Available Potential Energy (CAPE) and MOisture CONvergence (MOCON), which are relevant in convection nowcasting. These diagnostic fields are compared with radar reflectivities observed from 1 to 5 hours later, so as to estimate their skill in predicting convection triggering. First results on six case studies show that convection generally occurs over areas where high CAPE values and persisting convergence were analyzed 4 to 1 hour before.