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Showing papers on "Convective available potential energy published in 2000"


Journal ArticleDOI
TL;DR: In this article, a toy model of large-scale deep convection variations is constructed around a radiative-convective equilibrium climate, with an observed mean sounding as its thermodynamic basic state.
Abstract: A toy model of large-scale deep convection variations is constructed around a radiative–convective equilibrium climate, with an observed mean sounding as its thermodynamic basic state. Vertical structure is truncated at two modes, excited by convective (one-signed) and stratiform (two-signed) heating processes in tropical deep convection. Separate treatments of deep and shallow convection are justified by observations that deep convection is more variable. Deep convection intensity is assumed to be modulated by convective available potential energy (CAPE), while occurrence frequency is modulated by the ratio of convective inhibition (CIN) to “triggering energy” K, a scalar representing the intensity of subgrid-scale fluctuations. Deep convective downdrafts cool and dry the boundary layer but also increase K. Variations of K make the relationship between convection and thermodynamic variables (CAPE, CIN, θe) nonunique and amplify the deep convective response to temperature waves of small (∼1°C) am...

373 citations


Journal ArticleDOI
TL;DR: In this article, a quasi-equilibrium tropical circulation model (QTCM) is proposed to simulate the tropical and subtropical climate variations, and the model is coupled with a one-layer land surface model with interactive soil moisture.
Abstract: A class of model for simulation and theory of the tropical atmospheric component of climate variations is introduced. These models are referred to as quasi-equilibrium tropical circulation models, or QTCMs, because they make use of approximations associated with quasi-equilibrium (QE) convective parameterizations. Quasiequilibrium convective closures tend to constrain the vertical temperature profile in convecting regions. This can be used to generate analytical solutions for the large-scale flow under certain approximations. A tropical atmospheric model of intermediate complexity is constructed by using the analytical solutions as the first basis function in a Galerkin representation of vertical structure. This retains much of the simplicity of the analytical solutions, while retaining full nonlinearity, vertical momentum transport, departures from QE, and a transition between convective and nonconvective zones based on convective available potential energy. The atmospheric model is coupled to a one-layer land surface model with interactive soil moisture and simulates its own tropical climatology. In the QTCM version presented here, the vertical structure of temperature variations is truncated to a single profile associated with deep convection. Though designed to be accurate in and near regions dominated by deep convection, the model simulates the tropical and subtropical climatology reasonably well, and even has a qualitative representation of midlatitude storm tracks. The model is computationally economical, since part of the solution has been carried out analytically, but the main advantage is relative simplicity of analysis under certain conditions. The formulation suggests a slightly different way of looking at the tropical atmosphere than has been traditional in tropical meteorology. While convective scales are unstable, the large-scale motions evolve with a positive effective stratification that takes into account the partial cancellation of adiabatic cooling by diabatic heating. A consistent treatment of the moist static energy budget aids the analysis of radiative and surface heat flux effects. This is particularly important over land regions where the zero net surface flux links land surface anomalies. The resulting simplification highlights the role of top-of-the-atmosphere fluxes including cloud feedbacks, and it illustrates the usefulness of this approach for analysis of convective regions. Reductions of the model for theoretical work or diagnostics are outlined.

328 citations


Journal ArticleDOI
TL;DR: The mean genesis environment was constructed for each of five mesoscale convective complex population centers around the world: Africa, Australia, China, South America, and the United States.
Abstract: The mean genesis environment was constructed for each of five mesoscale convective complex (MCC) population centers around the world: Africa, Australia, China, South America, and the United States. It is found that the environments are very similar and exhibit many of the same dynamic and thermodynamic structures that are present with systems in the United States. In particular, MCCs initiate within prominent baroclinic zones characterized by locally large values of lower-tropospheric vertical wind shear and convective available potential energy (CAPE). Typically, a low-level jet of air with low static stability, high equivalent potential temperature, oriented nearly perpendicular to the baroclinic zone, intrudes into the genesis region and is forced to ascend over a relatively shallow, surface-based layer of relatively cool air. Pronounced warm advection accompanied by strong lower-tropospheric veering overlays the surface-based cool layer. A local maximum in absolute humidity and a local minimu...

171 citations


Journal ArticleDOI
TL;DR: In this article, the influence of including higher-order moments in convective ensembles is investigated using a simple stochastic convective parameterization that includes a random contribution to the convective available potential energy (CAPE) in the deep convective scheme.
Abstract: Convective parameterizations used in general circulation models (GCMs) generally only simulate the mean or first-order moment of convective ensembles and do not explicitly include higher-order moments. The influence of including unresolved higher-order moments is investigated using a simple stochastic convective parameterization that includes a random contribution to the convective available potential energy (CAPE) in the deep convective scheme. Impacts are tested in an tropical atmospheric model of intermediate complexity. Adding convective noise noticeably affects tropical intraseasonal variability, suggesting inclusion of such noise in GCMs might be beneficial. Model response to the noise is sensitive not only to the noise amplitude, but also to such particulars of the stochastic parameterization as autocorrelation time.

146 citations


Journal ArticleDOI
TL;DR: In this paper, the authors used TOGA COARE data to study the effect of dry air on the recovery of tropical tropical air from middle-latitude waves, and they found that the time it takes for the atmosphere to recover to moist conditions was ∼ 10-20 days.
Abstract: Recent studies using TOGA COARE data have found that extremely dry air from middle-latitude waves frequently intrudes into the equatorial troposphere over the western Pacific. Using sounding data taken during the COARE, the magnitude of the advection of water vapour for one event is calculated, and it is estimated that the lime for the atmosphere to recover to moist conditions was ∼ 10-20 days. From the magnitude of the drying and from the frequency of these events, it is proposed that dry intrusions must be a major contributor to the tropospheric moisture budget over the region during the COARE, making it difficult for the atmosphere to reach a radiative-convective equilibrium, intrusions, instead, can help to recharge the tropical atmosphere by decreasing convective activity and, thus, driving the atmosphere toward unusually large values of convective available potential energy. A variety of atmospheric and oceanic measurements are also used to study the recovery process in detail. A conceptual model is proposed based on this work and previous investigations. As in past studies, the recovery of the atmosphere to moist conditions is accomplished through detrainment from convective clouds that began to form soon after the arrival of the dry air mass and slowly deepen in height as the recovery progresses. Previous investigators concluded that the entrainment of dry air into convective ceils is generally the factor that tends to suppress convective activity and limits the height of any convection that does develop under these adverse conditions. The idea that entrainment limits convective activity is consistent with the commonly held perception that the western Pacific is a region where there is little inhibition to deep convection and, when inhibition does occur, it can be removed by surface fluxes within hours. In contrast, it is found that convective inhibition can be large enough to suppress convection following dry intrusions, and that the diurnal variation in rainfall is due partly to modulations in convective inhibition. The modulations in convective inhibition are, in turn, caused by diurnal variations in the vertical profiles of radiation, in surface fluxes, and perhaps in large-scale subsidence, leading to a minimum in convective inhibition during the late afternoon. In contrast, studies of this type of convection have generally emphasized diurnal variations in the surface fluxes, and often ignored convective inhibition and diurnal variations in atmospheric radiative heating.

127 citations


Journal ArticleDOI
TL;DR: In this article, the authors analyzed the causes of the large thermal biases in the simulation of the National Center for Atmospheric Research's Community Climate Model Version 3 (CCM3) single-column model (SCM) when it is forced with the Atmospheric Radiation Measurement Program's (ARM) summer 1995 Intensive Observing Period (IOP) data.
Abstract: We analyze the causes of the large thermal biases in the simulation of the National Center for Atmospheric Research's Community Climate Model Version 3 (CCM3) single-column model (SCM) when it is forced with the Atmospheric Radiation Measurement Program's (ARM) summer 1995 Intensive Observing Period (IOP) data. We have found that deficiencies of the convection triggering function used in the model can explain a significant part of the biases. In the model, convection is triggered whenever there is positive convective available potential energy (CAPE), which happens to occur during daytime due to solar heating of the land surface. In observations, however, convection takes place only when the large-scale dynamic condition is favorable to the release of CAPE. On the basis of observations we propose a new triggering function for the CCM3 deep convection scheme. We assume that model convection occurs only when the large-scale dynamic forcing makes positive contributions to the existing positive CAPE. Improved simulation results are obtained when the new triggering function is implemented in the model. We further test the new triggering function using the ARM Southern Great Plains summer 1997 IOP data and the Global Atmospheric Research Program's Atlantic Tropical Experiment phase III observations. These tests confirm the results obtained from the ARM 1995 experiments.

122 citations


Journal ArticleDOI
TL;DR: In this article, the surface conditions, vertical vorticity, and horizontal vortivity near the boundary were documented using conventional and special observations from the VORTEX field program.
Abstract: On 2 June 1995, the large-scale environment of eastern New Mexico and western Texas was generally favorable for the occurrence of supercells because of the presence of strong deep shear and storm-relative helicity, as well as sufficient convective available potential energy (CAPE). Indeed, many supercells occurred, but the only storms to produce tornadoes were those supercells that crossed, or developed and persisted on the immediate cool side of a particular outflow boundary generated by earlier convection. Surface conditions, vertical vorticity, and horizontal vorticity near this boundary are documented using conventional and special observations from the VORTEX field program. It is shown that the boundary was locally rich in horizontal vorticity, had somewhat enhanced vertical vorticity, and enhanced CAPE. Theoretical arguments indicate that the observed horizontal vorticity (around 1 × 10−2 s−1), largely parallel to the boundary, can be readily produced with the type of buoyancy contrast obse...

111 citations


Journal ArticleDOI
TL;DR: One hundred and thirty dropwindsondes deployed within 500 km radius of the eye of six North Atlantic hurricanes were used to determine the magnitudes and trends in convective available potential energy, and 10-1500m and 0-6-km shear of the horizontal wind as a function of radius, quadrant, and hurricane intensity as discussed by the authors.
Abstract: One hundred and thirty Omega dropwindsondes deployed within 500-km radius of the eye of six North Atlantic hurricanes are used to determine the magnitudes and trends in convective available potential energy, and 10–1500-m and 0–6-km shear of the horizontal wind as a function of radius, quadrant, and hurricane intensity. The moist convective instability found at large radii (400–500 km) decreases to near neutral stability by 75 km from the eyewall. Vertical shears increase as radius decreases, but maximum shear values are only one-half of those found over land. Scatter for both the conditional instability and the shear is influenced chiefly by hurricane intensity, but proximity to reflectivity features does modulate the pattern. The ratio of the conditional instability to the shear (bulk Richardson number) indicates that supercell formation is favored within 250 km of the circulation center, but helicity values are below the threshold to support strong waterspouts. The difference between these oce...

99 citations


Journal ArticleDOI
TL;DR: In this article, single-column model (SCM) simulations of a tropical squall-line case observed during the Coupled Ocean-Atmosphere Response Experiment of the Tropical Ocean/Global Atmosphere Programme are presented.
Abstract: This paper presents single-column model (SCM) simulations of a tropical squall-line case observed during the Coupled Ocean-Atmosphere Response Experiment of the Tropical Ocean/Global Atmosphere Programme. This case-study was part of an international model intercomparison project organized by Working Group 4 ‘Precipitating Convective Cloud Systems’ of the GEWEX (Global Energy and Water-cycle Experiment) Cloud System Study. Eight SCM groups using different deep-convection parametrizations participated in this project. The SCMs were forced by temperature and moisture tendencies that had been computed from a reference cloud-resolving model (CRM) simulation using open boundary conditions. The comparison of the SCM results with the reference CRM simulation provided insight into the ability of current convection and cloud schemes to represent organized convection. The CRM results enabled a detailed evaluation of the SCMs in terms of the thermodynamic structure and the convective mass flux of the system, the latter being closely related to the surface convective precipitation. It is shown that the SCMs could reproduce reasonably well the time evolution of the surface convective and stratiform precipitation, the convective mass flux, and the thermodynamic structure of the squall-line system. The thermodynamic structure simulated by the SCMs depended on how the models partitioned the precipitation between convective and stratiform. However, structural differences persisted in the thermodynamic profiles simulated by the SCMs and the CRM. These differences could be attributed to the fact that the total mass flux used to compute the SCM forcing differed from the convective mass flux. The SCMs could not adequately represent these organized mesoscale circulations and the microphysicallradiative forcing associated with the stratiform region. This issue is generally known as the ‘scale-interaction’ problem that can only be properly addressed in fully three-dimensional simulations. Sensitivity simulations run by several groups showed that the time evolution of the surface convective precipitation was considerably smoothed when the convective closure was based on convective available potential energy instead of moisture convergence. Finally, additional SCM simulations without using a convection parametrization indicated that the impact of a convection parametrization in forced SCM runs was more visible in the moisture profiles than in the temperature profiles because convective transport was particularly important in the moisture budget.

78 citations


Journal ArticleDOI
TL;DR: In this article, the authors discussed the meteorological implications of a bias discovered in the measurement of water vapor in widely deployed radiosonde systems and developed a correction scheme that intends to remove the bias.
Abstract: Accurate measurements of atmospheric water vapor are crucial to many aspects of climate research and atmospheric science. This paper discusses some of the meteorological implications of a bias discovered in the measurement of water vapor in widely deployed radiosonde systems. This problem apparently arose in the early 1990s and a correction scheme has been recently developed that intends to remove the bias. The correction scheme also includes improvements in the humidity measurements in the upper troposphere and near the surface. This scheme has been applied to data taken during the Tropical Ocean and Global Atmosphere Coupled Ocean­Atmosphere Response Experiment (TOGA COARE). The impact of the bias on the general stability of the tropical atmosphere to deep convection, as measured by the convective available potential energy (CAPE) and the convective inhibition (CIN), is quite large. On the basis of the uncorrected data set, one might erroneously conclude that it is difficult to trigger deep convection over the region. When the correction is taken into account, the atmosphere over the tropical western Pacific becomes typically unstable to deep convection, with convective instability similar to that measured from aircraft in the vicinity of active convective sytems. Radiative fluxes are also significantly modified. For clear sky conditions, it is found that, on average, the net surface radiative flux increases by 4 W m-2 due to the humidity correction. Under more realistic cloudy conditions, the differences are weaker, but still significant. Changes in radiative fluxes are explained at first order by the the precipitable water increase.

78 citations


Journal ArticleDOI
TL;DR: In this article, nine mini-supercell storms were observed over the Kanto Plain of Japan in the northeast quadrant of Typhoon 9019 and three out of nine spawned a tornado.
Abstract: On the night of 19 September 1990, nine mini-supercell storms were observed over the Kanto Plain of Japan in the northeast quadrant of Typhoon 9019. Three storms out of nine spawned a tornado. The characteristics of the storms and tornadoes were studied by means of a single Doppler radar, conventional radars, surface meteorological observations, and damage surveys. The mini-supercell storms exhibited characteristics similar to typical supercell storms over the Great Plains of the United States, such as hook echo, bounded weak echo region, slower movement relative to the mean wind, long lives, and rightward deviation of the storm motion relative to the mean wind shear was observed. They also displayed several differences from typical supercell storms with respect to the following points: 1) horizontal scale of the mesocyclone was smaller ; and 2) vertical vorticity was confined to lower levels (less than 5 km above ground level). These similarities and differences correspond with those observed for miniature supercells in Hurricane Danny’s environment. The storm environment for these mini supercells was characterized by modest convective available potential energy (about 1600 J kg21) and strong low-level wind shear with veering. The storm updrafts likely tilted the ambient horizontal vorticity associated with the strong low-level wind shear to generate mesocyclonic vertical vorticity. Two of the tornadoes were spawned by mini-supercell storms moving along a preexisting surface boundary that was accompanied by significant convergence, vertical vorticity, and horizontal gradients of temperature. An estimation of vertical vorticity near the ground in the boundary suggests a possibility that preexisting vertical vorticity contributed to supercell evolution resulting in tornadogenesis.

Journal ArticleDOI
TL;DR: In this article, the authors simulated deep cumulonimbus clouds using a model that makes accurate diagnoses of entrainment and detrainment rates and of the properties of entrained and detrained air.
Abstract: Deep cumulonimbus clouds are simulated using a model that makes accurate diagnoses of entrainment and detrainment rates and of the properties of entrained and detrained air. Clouds generated by a variety of initial thermodynamic soundings are compared. In the simulations, updraft entrainment rates are large near and above cloud base, through the entire depth of the conditionally unstable layer. Stronger updrafts in a more unstable environment are better able to entrain relatively undisturbed environmental air, while weaker updrafts can entrain only air that has been modified by the clouds. When the maximum buoyancy is large, the updraft includes parcels with a wide range of buoyancies, while weaker clouds are more horizontally uniform. Strong downdrafts originate from levels at which updrafts detrain, and their mass flux depends on the mass flux of the updraft. The magnitude of mixing between cloud and environment, not the entrainment rate, varies inversely with the cloud radius. How much of the mixed air is entrained depends on the buoyancy.

Journal ArticleDOI
TL;DR: In this article, the authors investigated the mechanisms responsible for the development and maintenance of long-lived squall lines in dry environments through two-dimensional numerical experiments by using a nonhydrostatic cloud model.
Abstract: The mechanisms responsible for the development and maintenance of long-lived squall lines in dry environments are investigated through two-dimensional numerical experiments by using a nonhydrostatic cloud model. The squall line environments are characterized by a low convective available potential energy (CAPE), low moisture content, and a high level of free convection (LFC), which are based on observations of a squall line over an arid region in China. Although these environments seem to be unfavorable for the development of convective systems, a long-lived squall line is simulated in the environment of a well-mixed moisture profile within a deep, mixed boundary layer. During the mature stage of this simulated squall line, the air parcels originating in the upper part of the mixed layer ahead of a surface cold-air pool are lifted to the upper troposphere. On the other hand, the air parcels originating in the lower part of the mixed layer are forced to go rearward, never reaching the upper levels...

Journal ArticleDOI
TL;DR: In this article, the authors quantify the maximum amount of destabilization as a function of the amplitude and horizontal scale of the upper-level trough, represented by an elongated potential-temperature perturbation on the tropopause.
Abstract: The approach of an upper-level trough is accompanied by a local ascent of the isentropic surfaces in the troposphere. The associated cooling and moistening destabilizes the atmosphere to convection. In this paper we attempt to quantify the maximum amount of destabilization as a function of the amplitude and horizontal scale of the trough. The trough is represented by an elongated potential-temperature perturbation on the tropopause. Its structure, including the isentrope displacement it produces, is calculated from the balanced shear-line solutions of Juckes. These solutions are used to estimate the amount by which mid- and lower-tropospheric air masses are lifted when an upper-tropospheric trough passes overhead on the assumption that isentropic potential-vorticity anomalies are localized in the upper troposphere. The amount of destabilization is characterized by the maximum change in convective available potential energy (CAPE) and convective inhibition (CIN) brought about by the passage of the trough. The calculations seek to isolate one aspect of the interaction between an upper-level trough and a tropical cyclone. We show that the changes in CAPE are significant in the Tropics (up to 83% for the soundings examined) and even more so in the middle latitudes (up to 130%) and that the CIN may be completely removed.

Journal ArticleDOI
TL;DR: In this paper, the authors document some of the unusual rotating updrafts (one of which produced a tornado) that developed over central Oklahoma on 28 October 1998 in an environment of strong (1.8 × 10−2 s−1) low-level (0-3 km) mean shear.
Abstract: The authors document some of the unusual rotating updrafts (one of which produced a tornado) that developed over central Oklahoma on 28 October 1998 in an environment of strong (1.8 × 10−2 s−1) low-level (0–3 km) mean shear. The maximum convective available potential energy (including virtual temperature effects) a “storm” could have realized was approximately 300 J kg−1; however, most of the storms probably realized less than 100 J kg−1. Average (maximum) parcel virtual temperature excesses were estimated to be 0.4–1.2 K (1.8–2.8 K). Echo tops were measured from less than 5 km to 11.2 km above ground level (AGL), although visual observations and radar data suggested echoes that extended above approximately 5–6 km AGL were not associated with significantly buoyant cloud elements. Radar characteristics of many of the storms were similar to supercell storms (e.g., weak echo regions, echo overhang, velocity couplets, hook echoes), as were some of the visual characteristics near cloud base (e.g., wal...

01 Jan 2000
TL;DR: In this paper, a prototypal probabilistic model was used to forecast the likelihood that a thunderstorm will produce a strong or violent tornado, given a certain value of convective available potential energy (CAPE) and helicity (or CAPE and wind shear).
Abstract: Several previous studies have established statistical relationships between the severity of convection and environmental conditions determined from rawinsonde observations. Here, the authors seek 1) to determine whether similar relationships are observed when severe weather reports are associated with gridded short-term numerical forecasts,and 2) to develop and demonstrate a prototypal probabilistic model to forecast the likelihood a thunderstorm will be tornadic. Severe weather reports and lightning network data from 1 January 1999 through 30 June 1999 were used to classify the weather at a set of Rapid Update Cycle (RUC-2) grid points into four weather categories. These were no thunderstorms, nonsupercellular thunderstorms, supercellular thunderstorms without significant tornadoes, and thunderstorms with significant tornadoes (F2 or greater). RUC-2 forecast convective available potential energy (CAPE), helicity, and 0‐4-km mean wind shear from the same period were associated with this gridded classification of the weather. In general similar relationships were found between environmental parameters and storm categorization as others have previously documented. The Bayesian probabilistic model used here forecasts the likelihood that a thunderstorm will produce a strong or violent tornado, given a certain value of CAPE and helicity (or CAPE and wind shear). For two selected cases when significant tornadoes occurred, this model reasonably located the high-threat areas many hours in advance of the severe weather. An enhanced version of this prototypal tool may be of use to operational severe weather forecasters.

Journal ArticleDOI
TL;DR: In this paper, the authors study the contribution of surface data to convection nowcasting over regions of modest orography and under weak synoptic forcing, and they find that the organization of convective systems into a reflectivity line is preceded by a similar organization in the MOCON field from one to three hours ahead.
Abstract: We study the contribution of surface data to convection nowcasting over regions of modest orography and under weak synoptic forcing. Hourly mesoscale analyses are performed using the CANARI optimal interpolation analysis scheme, which combines first-guess fields from the fine mesh (10 km) ALADIN model with hourly routine observations arising from a mesonet of automated ground stations. These analyses then allow the computation of diagnostic parameters that quantify convective instability, low-level lifting processes and moisture supply: these are the convective available potential energy (CAPE) and the moisture convergence (MOCON). A tuning of the analysis scheme is needed first for it to fit the meso-β-scale. Then, the skill of the computed diagnostics for convection nowcasting is evaluated by comparing their fields with radar reflectivities observed between one and four hours after the analysis time. This is done for four selected convective situations. With regard to thunderstorm triggering, results show that this usually happens over areas of persistently high values of CAPE which undergo convergence continuously from four to one hour before the event; on the other hand, areas of persistent divergence are never associated with convective developments. In addition, the proposed criteria allow a significant reduction in the areal extent of predicted thunderstorms (i.e. decreasing the false-alarm rate) compared with what can be currently done on an operational basis, while maintaining a low non-detection rate. As to convection monitoring, we find that the organization of convective systems into a reflectivity line is preceded by a similar organization in the MOCON field from one to three hours ahead. Copyright © 2000 Royal Meteorological Society

Journal ArticleDOI
TL;DR: In this article, a prototypal probabilistic model was developed to forecast the likelihood a thunderstorm will be tornadic, based on weather reports and lightning network data from 1 January 1999 through 30 June 1999.
Abstract: Several previous studies have established statistical relationships between the severity of convection and environmental conditions determined from rawinsonde observations. Here, the authors seek 1) to determine whether similar relationships are observed when severe weather reports are associated with gridded short-term numerical forecasts, and 2) to develop and demonstrate a prototypal probabilistic model to forecast the likelihood a thunderstorm will be tornadic. Severe weather reports and lightning network data from 1 January 1999 through 30 June 1999 were used to classify the weather at a set of Rapid Update Cycle (RUC-2) grid points into four weather categories. These were no thunderstorms, nonsupercellular thunderstorms, supercellular thunderstorms without significant tornadoes, and thunderstorms with significant tornadoes (F2 or greater). RUC-2 forecast convective available potential energy (CAPE), helicity, and 0–4-km mean wind shear from the same period were associated with this gridded ...

Journal ArticleDOI
TL;DR: The most destructive tornadoes developed along a preexisting surface boundary where lower-tropospheric moisture convergence and frontogenesis were enhanced by unseasonably warm moist air at lower levels resulting in significant instability.
Abstract: On 1 March 1997 violent tornadoes caused numerous fatalities and widespread damage across portions of central and eastern Arkansas and western Tennessee. In addition, the associated thunderstorms produced very heavy rainfall and flash flooding, with a few locations receiving up to 150 mm (6 in.) of rainfall in 3 h. The initial environment appeared favorable for strong tornadoes with unseasonably warm moist air at lower levels resulting in significant instability (convective available potential energy values between 1400 and 1800 J kg21) where 0‐2-km storm-relative helicities exceeded 300 m2 s22 and the middle-tropospheric storm-relative flow was conducive for tornadic supercells. The most destructive tornadoes developed along a preexisting surface boundary where lower-tropospheric moisture convergence and frontogenesis were enhanced. Tornadoes and heaviest rainfall only ensue after upward motion associated with the direct circulation of an upper-tropospheric jet streak became collocated with lower-tropospheric upward forcing along the surface boundaries. From a flash flood perspective the event occurred in a hybrid mesohigh-synoptic heavy rain pattern as thunderstorms developed and moved along surface boundaries aligned nearly parallel to the mean wind. In addition, strong flow and associated moisture flux convergence in the lower troposphere favored the formation of cells to the southwest or upstream of the initial convection with thunderstorms, including a a tornadic supercell, traversing over the same area. The available moisture and ambient instability also supported both vigorous updrafts and high precipitation rates.

Journal ArticleDOI
TL;DR: In this paper, the effects of sub-grid-scale land use differences in modifying the stability of prestorm environments has been studied using data from the Department of Energy Cloud and Radiation Testbed (CART) in Kansas and Oklahoma and a mesoscale model.
Abstract: The effects of sub-grid-scale land use differences in modifying the stability of prestorm environments has been studied using data from the Department of Energy Cloud and Radiation Testbed (CART) in Kansas and Oklahoma and a mesoscale model. To quantify the atmospheric instability, three indices were used: the lifted index, the modified K index, and convective available potential energy. The spatial variations of these indices were calculated from simulations using spatially varying and spatially uniform surface fluxes as lower boundary conditions. The CART is approximately 105 km2 in area and is characterized by large areas of contrasting vegetation cover and surface sensible and latent heat fluxes. The spatially varying fluxes were calculated with the SiB2 model using data from the CART. Six days, during which isolated thunderstorms developed, were chosen for the study. The results suggest that sub-grid-scale variations in land use differences do not modify the spatial distribution of the stability indices in the southern Great Plains to any significant degree. Predictions of areas of preferred development of deep convection, based on changes in the indices, are not improved by accounting for sub-grid-scale variations in land use. The indices were found to be potentially useful but imperfect indicators of the occurrence of deep convective precipitation at scales smaller than that of the CART. At the model resolution used (grid spacing of 2.08 km) a close correlation was not found between regions of precipitation and regions with enhanced simulated vertical updrafts related to land-use differences, but a detailed study of local triggering has not been carried out.

Journal ArticleDOI
TL;DR: In this paper, the authors investigated the sensitivity of nonsupercell tornadogenesis to variations in convective available potential energy (CAPE), outflow boundary vortex sheet strength, and boundary layer vertical shear.
Abstract: Nonsupercell tornadogenesis has been investigated in a three-part numerical study. Building on the results of Parts I and II, Part III addresses the sensitivity of nonsupercell tornadogenesis to variations in convective available potential energy (CAPE), outflow boundary vortex sheet strength, and boundary layer vertical shear. A three-dimensional, nonhydrostatic, quasi-compressible convective cloud model has been employed to examine nonsupercell tornado (NST) development in an environment typical of the Colorado high plains. A strong relationship was shown to exist between the magnitude of the environmental CAPE and the structure and intensity of the misocyclones and nonsupercell tornadoes that developed. As CAPE was increased from 0 to 1700 J kg−1, the simulated vortices markedly contracted and intensified. Multiple CAPE thresholds were identified that yielded markedly different vortex intensity. The highest CAPE runs produced NST families with peak ground-relative surface winds of ∼47 m s−1. V...

Journal ArticleDOI
TL;DR: In this paper, an assimilation system that performs continuous assimilation of satellite imager data and intermittent assimilation on hourly surface observations is described, applied to a case study of the southeast United States that was heavily influenced by the shading effect of an area of morning stratiform clouds.
Abstract: An assimilation system that performs continuous assimilation of satellite imager data and intermittent assimilation of hourly surface observations is described. The system was applied to a case study of the southeast United States that was heavily influenced by the shading effect of an area of morning stratiform clouds. The results of analyses produced during the assimilation show improvement in the depiction of the modified surface heating effects beneath the cloudy region as well as in important convective precursors such as mass and moisture convergence and convective available potential energy in the cloudy and adjoining regions. Without assimilation of these data, the numerical model was less able to simulate these thermally forced circulations.

Journal ArticleDOI
TL;DR: In this article, two numerical models are used to investigate aspects of thunderstorm dynamics and thunderstorm initiation in the northern Alpine foreland, and the model is initialized with wind, temperature and moisture profiles from a radiosonde sounding ahead of the squall line.
Abstract: Two numerical models are used to investigate aspects of thunderstorm dynamics and thunderstorm initiation in the northern Alpine foreland. The first, an isentropic model of airflow over and around the Alps, is used to investigate flow patterns favourable for the initiation of deep convection in the region. It is found that a stably-stratified southerly flow towards the Alps leads to a southwesterly flow in the Alpine foreland, a situation most often found during thunderstorm periods, and to the formation of a gravity wave in the lee of the Alps. This wave is accompanied by raised isentropes which, in reality, would lead to a reduction in static stability and convective inhibition as well as an increase in convective available potential energy. The second model, a cloud model, is used to study the development of an observed squall line over southern Bavaria. The model is initialized with wind, temperature and moisture profiles from a radiosonde sounding ahead of the squall line and the squall line is initiated by an array of thermal bubbles. The model simulation is used to interpret the evolution of the squall line.

01 Jan 2000
TL;DR: In this paper, the authors document some of the unusual rotating updrafts (one of which produced a tornado) that developed over central Oklahoma on 28 October 1998 in an environment of strong (1.8 3 1022 s21) lowlevel (0.3 km) mean shear.
Abstract: The authors document some of the unusual rotating updrafts (one of which produced a tornado) that developed over central Oklahoma on 28 October 1998 in an environment of strong (1.8 3 1022 s21) lowlevel (0‐3 km) mean shear. The maximum convective available potential energy (including virtual temperature effects) a ‘‘storm’’ could have realized was approximately 300 J kg 21; however, most of the storms probably realized less than 100 J kg21. Average (maximum) parcel virtual temperature excesses were estimated to be 0.4‐1.2 K (1.8‐2.8 K). Echo tops were measured from less than 5 km to 11.2 km above ground level (AGL), although visual observations and radar data suggested echoes that extended above approximately 5‐6 km AGL were not associated with significantly buoyant cloud elements. Radar characteristics of many of the storms were similar to supercell storms (e.g., weak echo regions, echo overhang, velocity couplets, hook echoes), as were some of the visual characteristics near cloud base (e.g., wall clouds, rain-free bases, and striated low-level updrafts); however, visual characteristics in middle to upper portions of the storms were not characteristic of typical severe storms, supercells, or previously documented ‘‘minisupercells.’’ Furthermore, the buoyancy realized by the updrafts was estimated to be considerably less than environments associated with the aforementioned minisupercells.

Book ChapterDOI
01 Jan 2000
TL;DR: The role of clouds in the climate system is very complex and is the subject of much interest and research as mentioned in this paper, and clouds interact nonlinearly with radiative, dynamical, chemical, and hydrological processes in the atmosphere on a wide range of temporal and spatial scales.
Abstract: The role of clouds in the climate system is very complex and is the subject of much interest and research. Clouds interact nonlinearly with radiative, dynamical, chemical, and hydrological processes in the atmosphere on a wide range of temporal and spatial scales. Clouds play a fundamental role in controlling the amount of solar and infrared radiation available to the climate system. The radiative properties of clouds make them a key component in the energy balance of the Earth. In particular, clouds are involved in both heating and cooling in the determination of the Earth’s temperature. On average, roughly 50% of the Earth is covered by clouds. They contribute to the planet’s albedo by reflecting some incident sunlight (shortwave radiation) back to space (they also absorb some). However, they also partially block the escape of infrared radiation from below; that is, they exert a greenhouse effect on Earth. (Clouds are the primary contributors to the greenhouse effect.) They also emit some longwave radiation. Clouds also play an essential role in controlling the amount of moisture available to the climate system. Through precipitation, clouds serve as a conduit for the transfer of heat from the oceans to the atmosphere. They are also important in many chemical processes such as the absorption of water-soluble chemicals and pollutants in cloud droplets and their elimination by precipitation. See [Tre92] for further discussion.

Journal ArticleDOI
TL;DR: In this article, the subgrid-scale vertical flux c ≥g −1 c p of equivalent temperature υ, referred to as convective flux (diagnosed from routine gridscale data), analyzed surface precipitation; the convective available potential energy CAPE; lightning records; and radar patterns from the NORDRAD network.
Abstract: Following the scientific objectives of BALTEX, this report contributes to a diagnostic understanding of higher latitude convection. For the PIDCAP period (Aug-Sep-Oct 1995) we have studied daily fields of: The subgridscale vertical flux c≡g -1 c p of equivalent temperature υ, referred to as convective flux (diagnosed from routine gridscale data); analyzed surface precipitation; the convective available potential energy CAPE; lightning records; and radar patterns from the NORDRAD network.- We have suggested earlier that the convective flux is the proper dynamic measure to quantify convection. Here we show in a first case-study part that in areas with significant upward values of c the other parameters also indicate convective activity. Yet there is nowhere a one-to-one relationship between c and rain, or c and CAPE. In some cases the diagnosed convective fluxes are caused by gridscale frontal processes while in others they are due to static instability.-In a second statistical part we compare the first three EOF profiles of c with the subjectively classified convective flux types deep convection (DC) and boundary layer convection (BL). EOF1 is highly correlated with DC while the rotated EOF2+EOF3 is well correlated with BL. The classical convection indicators (CAPE, lightning) are weakly correlated among each other and with precipitation but hardly at all with the convective flux.-These results corroborate our earlier conclusion that the convective flux c is a useful diagnostic parameter. Nevertheless, understanding convection requires a concerted effort in which more than one tool is to be employed.

01 Jan 2000
TL;DR: In this article, the authors used a two-dimensional cloud model with radiative-convective interaction process, found a QBO-like (quasibiennial oscillation) oscillation of mean zonal wind that affected the convective system.
Abstract: Cloud resolving model (CRM) has widely been used in recent years for simulations involving studies of radiative-convective systems and their role in determining the tropical regional climate. The growing popularity of CRMs usage can be credited for their inclusion of crucial and realistic features such like explicit cloud-scale dynamics, sophisticated microphysical processes, and explicit radiative-convective interaction. For example, by using a two-dimensional cloud model with radiative-convective interaction process, found a QBO-like (quasibiennial oscillation) oscillation of mean zonal wind that affected the convective system. Accordingly, the model-generated rain band corresponding to convective activity propagated in the direction of the low-level zonal mean winds; however, the precipitation became "localized" (limited within a small portion of the domain) as zonal mean winds were removed. Two other CRM simulations by S94 and Grabowski et al. (1996, hereafter G96), respectively that produced distinctive quasi-equilibrium ("climate") states on both tropical water and energy, i.e., a cold/dry state in S94 and a warm/wet state in G96, have later been investigated by T99. They found that the pattern of the imposed large-scale horizontal wind and the magnitude of the imposed surface fluxes were the two crucial mechanisms in determining the tropical climate states. The warm/wet climate was found associated with prescribed strong surface winds, or with maintained strong vertical wind shears that well-organized convective systems prevailed. On the other hand, the cold/dry climate was produced due to imposed weak surface winds and weak wind shears throughout a vertically mixing process by convection. In this study, considered as a sequel of T99, the model simulations to be presented are generally similar to those of T99 (where a detailed model setup can be found), except for a more detailed discussion along with few more simulated experiments. There are twelve major experiments chosen for presentations that are introduced in section two. Several significant feature analyses regarding the rainfall properties, CAPE (Convective Available Potential Energy), cloud-scale eddies, the stability issue, the convective system propagation, relative humidity, and the effect on the quasi-equilibrium state by the imposed constant. radiation or constant surface fluxes, and etc. will be presented in the meeting. However, only three of the subjects are discussed in section three. A brief summary is concluded in the end section.