Showing papers on "Convective available potential energy published in 2001"

Influence of the spatial distribution of vegetation and soils on the prediction of cumulus Convective rainfall

Abstract: This paper uses published work to demonstrate the link between surface moisture and heat fluxes and cumulus convective rainfall. The Earth's surface role with respect to the surface energy and moisture budgets is examined. Changes in land-surface properties are shown to influence the heat and moisture fluxes within the planetary boundary layer, convective available potential energy, and other measures of the deep cumulus cloud activity. The spatial structure of the surface heating, as influenced by landscape patterning, produces focused regions for deep cumulonimbus convection. In the tropics, and during midlatitude summers, deep cumulus convection has apparently been significantly altered as a result of landscape changes. These alterations in cumulus convection teleconnect to higher latitudes, which significantly alters the weather in those regions. The effect of tropical deforestation is most clearly defined in the winter hemisphere. In the context of climate, landscape processes are shown to be as much a part of the climate system as are atmospheric processes.

A mass-flux convection scheme for regional and global models

Abstract: A bulk mass-flux convection parametrization for deep and shallow convection is presented that includes an efficient and straightforward treatment of numerics, moist thermodynamics and convective downdraughts. The scheme is evaluated in a single-column model context for a tropical deep-convective period and a trade-wind cumulus case. Preliminary applications in a global numerical weather-prediction model and a mesoscale model are also discussed. The results suggest that the present scheme provides reasonable solutions in terms of predicted rainfall, and tropical temperature and moisture structures. The application of the scheme to various scales is supported by the use of a convective available potential energy convective closure that assures a smooth interaction with the large-scale environment and efficiently suppresses conditional instability of the second kind-like spin-up processes on the grid-scale. Finally, the theoretical and practical limits of the present approach are discussed together with possible future developments.

Some Common Ingredients for Heavy Orographic Rainfall

Abstract: The purpose of this paper is to synthesize some common synoptic and mesoscale environments conducive to heavy orographic rainfall. Previous studies of U.S. and Alpine cases and new analyses of some Alpine and east Asian cases have shown the following common synoptic and mesoscale environments are conducive to heavy orographic rainfall: 1) a conditionally or potentially unstable airstream impinging on the mountains, 2) a very moist low-level jet (LLJ), 3) a steep mountain, and 4) a quasi-stationary synoptic system to slow the convective system over the threat area. A deep short-wave trough is found to approach the threat area in the U.S. and European cases, but is not found in the east Asian cases. On the other hand, a high convective available potential energy (CAPE) value is observed in east Asian cases, but is not consistently observed in the U.S. and European cases. The enhancement of low-level upward motion and the increase of instability below the trough by the approaching deep short-wave tr...

Numerical Modeling of Gravity Wave Generation by Deep Tropical Convection

Abstract: Although convective clouds are known to generate internal gravity waves, the mechanisms responsible are not well understood. The present study seeks to clarify the dynamics of wave generation using a high-resolution numerical model of deep convection over the Tiwi Islands, Australia. The numerical calculations presented explicitly resolve both the mesoscale convective cloud cluster and the gravity waves generated. As the convective clouds evolve, they excite gravity waves, which are prominent features of the model solutions in both the troposphere and stratosphere. The source location is variable in time and space but is related to the development of individual convective cells. The largest amplitude gravity waves are generated when the cloud tops reach the upper troposphere. A new analysis technique is introduced in which the nonlinear terms in the governing equations are taken as the forcing for linear gravity waves. The analysis shows that in the present calculation, neither the shear nor the diabatic heating are the dominant forcing terms. Instead, the wave source is most easily understood when viewed in a frame of reference moving with the wind at the level of neutral buoyancy, whereupon the source may be described as a vertically oriented, oscillating convective updraft. This description is consistent with the properties of the modeled stratospheric waves.

The Electrification of Severe Storms

Abstract: This review is concerned with electrification and lightning in severe weather. Based on substantial evidence that the electrical processes active in ordinary (nonsevere) thunderstorms are also present in severe storms, the initial discussion here is focused on ordinary thunderstorms. This material forms a physical basis for understanding the often marked departures in electrical behavior in severe storms, which are defined by exceedence criteria for surface wind speed [50 knots (26 m s−1)], hailstone diameter [3/4″ (19 mm)], and/or the occurrence of a tornado.

Severe Convective Storms—An Overview

Abstract: In general, convection, refers to the transport of some property by fluid movement, most often with reference to heat transport. As such, it is one of the three main processes by which heat is transported: radiation, conduction, and convection. Meteorologists typically use the term convection to refer to heat transport by the vertical component of the flow associated with buoyancy. Transport of heat (or any other property) by the nonbuoyant part of the atmospheric flow is usually called advection by meterologists; advection can be either horizontal or vertical.

The Sensitivity of Simulated Supercell Structure and Intensity to Variations in the Shapes of Environmental Buoyancy and Shear Profiles

Abstract: Convective storm simulations are conducted using varying thermal and wind profile shapes, subject to the constraints of strict conservation of convective available potential energy (CAPE) and hodograph trace. Small and large CAPE regimes and straight and curved hodographs are studied, each with a matrix of systematically varying thermal and wind profile shapes having identical levels of free convection and bulk Richardson numbers favorable to supercell development. Differences in storm intensity and morphology resulting from changes in the profile shapes can be profound, especially in the small CAPE regime, where, for the moderate shears studied here, storms are generally weak except when the buoyancy is concentrated at low levels. In stronger CAPE regimes, less dramatic relative enhancements of storm updraft intensity are found when both the buoyancy and shear are concentrated at low levels. Peak midlevel vertical vorticity correlates roughly with peak updraft speed in the small CAPE regime, but...

Models for Stratiform Instability and Convectively Coupled Waves

Abstract: A simplified intermediate model for analyzing and parameterizing convectively coupled tropical waves is introduced here. This model has two baroclinic modes of vertical structure: a direct heating mode and a stratiform mode. The key essential parameter in these models is the area fraction occupied by deep convection, sc. The unstable convectively coupled waves that emerge from perturbation of a radiative convective equilibrium are discussed in detail through linearized stability analysis. Without any mean flow, for an overall cooling rate of 1 K day21 as the area fraction parameter increases from sc 5 0.0010 to sc 5 0.0014 the waves pass from a regime with stable moist convective damping to a regime of ‘‘stratiform’’ instability with convectively coupled waves propagating at speeds of roughly 15 m s21; instabilities for a band of wavelengths in the supercluster regime, O(1000)‐O(2000) km; and a vertical structure with a ‘‘wave tilt’’ where the temperature structure in the upper troposphere lags behind that in the lower troposphere. Thus, these convectively coupled waves in the model reproduce several key features of convectively coupled waves in the troposphere processed from recent observational data by Wheeler and Kiladis. As the parameter sc is increased further to values such as sc 5 0.01, the band of unstable waves increases and spreads toward a mesoscale wavelength of O(100) km while the same wave structure and quantitative features mentioned above are retained for O(1000) km. A detailed analysis of the temporal development of instability of these convectively coupled waves is presented here. In the first stage of instability, a high convective available potential energy (CAPE) region generates deep convection and a front-to-rear ascending flow with enhanced vertical shear in a stratiform wake region. Thus, these intermediate models may be useful prototypes for studying the parameterization of upscale convective momentum transport due to organized convection. In the second stage of instability, detailed analysis of the CAPE budget establishes that the effects of the second baroclinic mode in the stratiform wake produce new CAPE, which regenerates the first half of the wave cycle. Finally, since these convectively coupled stratiform waves do not require a barotropic mean flow, a barotropic mean flow, which alters the surface fluxes, is added to study its effect on their stability. These effects of a barotropic mean flow are secondary; an easterly mean flow enhances instability of the eastward-propagating convectively coupled waves and diminishes the instability of the westward-propagating waves through a wind-induced surface heat exchange mechanism.

Convectively Driven High Wind Events

Abstract: On a day when the potential instability is sufficient, it is possible to initiate storms from rising air parcels. One estimate of the intensity of these buoyant plumes (also an indicator of the severity of the storm) is based on parcel theory (Bluestein et al. 1988, 1989; Holton 1992). These rising parcels of air rapidly cool until saturation occurs. Further lifting results in condensation and, in a short period of time, precipitation develops. It is often at this stage that another fundamental element of a storm commonly forms: the convective downdraft.

Convectively Generated Gravity Waves and Their Effect on the Cloud Environment

Abstract: This study uses a two-dimensional cloud-resolving model to examine how convectively generated gravity waves modify the environment of an isolated convective cloud. The model is initialized with an idealized sounding, and the cloud is initiated by adding a locally buoyant perturbation. The modeled convection generates a spectrum of gravity waves with vertical wavelengths that are harmonics of the depth of the troposphere. It is shown that the first three wave modes significantly modify the cloud environment. The modification of the cloud environment is quantified in terms of the convective available potential energy (CAPE) and convective inhibition (CIN). The first two wave modes travel fastest away from the cloud and are responsible for the changes in CAPE, whereas the third wave mode causes low-level lifting and hence a reduction in CIN. The maximum far-field perturbations in CAPE and CIN are approximately 15% and 33% of the initial background values, respectively. These results agree with previous studies of more organized convection, predicting the existence of a region surrounding the convective system that favors the development of new convection.

Understanding Hector: The Dynamics of Island Thunderstorms

Abstract: Linear and nonlinear models are used to examine the development of island thunderstorms, in particular the Hector convective system that forms over the Tiwi Islands just north of Australia. The linear model is used to examine the flow response to an isolated, elliptical, heat source. It is found that the low-level convergence is maximized when the flow is weak and along the major axis of the heat source. A dry version of the nonlinear model verifies the trends predicted by the linear model except at very low flow speeds where the convergence is bounded in the nonlinear model but increases indefinitely in the linear model. Deep convection develops over the heat source when a moisture profile with positive convective available potential energy (CAPE) is added to the nonlinear model. The sensitivity of the convective strength (defined by the accumulated rainfall and total condensate) to wind speed and direction, surface fluxes, and low-level moisture is then examined. It is shown that the strength i...

Preliminary Evaluation of a Revised Zhang-McFarlane Convection Scheme Using the NCAR CCM3 GCM

Abstract: This study investigates the interaction between convection, clouds, and the large-scale circulation. By examining the sensitivity of the large-scale fields to a modification of the convective parameterization scheme in the NCAR CCM3, we show that the convective parameterization has a strong impact on the temporal characteristics of the large-scale circulation and clouds. When Convective Available Potential Energy (CAPE) in the atmosphere is used to close the convective parameterization, the simulated convection is continuous, and lacks the observed intermittence. When the CAPE change due to the large-scale forcing in the free troposphere is used, the simulated temporal behavior of convection is in much better agreement with the observations. We attribute this improvement to the enhanced coupling between convection and the large-scale forcing in the convective parameterization.

Tropical Convective Variability as 1/f Noise

Abstract: Evidence is presented that the tropical convective variability behaves as 1/f noise for a 1–30-day period. This behavior is shown by analyzing the time series of convective available potential energy, which measures the degree of convective instability, as well as the boundary layer moisture and temperature for the 4-month period over the western Pacific during the Tropical Ocean and Global Atmosphere Coupled Ocean–Atmosphere Response Experiment. The long memory of 1/f noise associated with tropical convective variability has important implications for global climate modeling, particularly for ENSO predictions. A simple conceptual model is proposed to explain the 1/f-noise behavior.

Waves and Instabilities for Model Tropical Convective Parameterizations

Abstract: Models of the tropical atmosphere with crude vertical resolution are important as intermediate models for understanding convectively coupled wave hierarchies and also as simplified models for studying various strategies for parameterizing convection and convectively coupled waves. Simplified models are utilized in a detailed analytical study of the waves and instabilities for model convective parameterizations. Three convection schemes are analyzed: a strict quasi-equilibrium (QE) scheme and two schemes that attempt to model the departures from quasi equilibrium by including the shorter timescale effects of penetrative convection, the Lagrangian parcel adjustment (LPA) scheme and a new instantaneous convective available potential energy (CAPE) adjustment (ICAPE) scheme. Unlike the QE parameterization scheme, both the LPA and ICAPE schemes have scale-selective finite bands of unstable wavelengths centered around typical cluster and supercluster scales with virtually identical growth rates and wave structure. However, the LPA scheme has, in addition, two nonphysical superfast parasitic waves that are artifacts of this parameterization while such waves are completely absent in the new ICAPE parameterization. Another topic studied here is the fashion in which an imposed barotropic mean wind triggers a transition to instability in the Tropics through suitable convectively coupled waves; this is the simplest analytical problem for studying the influence of midlatitudes on convectively coupled waves. For an easterly barotropic mean flow with the effect of rotation included, both supercluster-scale moist Kelvin waves and cluster-scale moist mixed Rossby‐gravity waves participate in the transition to instability. The wave and stability properties of the ICAPE parameterization with rotation are studied through a novel procedure involving complete zonal resolution but low-order meridional truncation. Besides moist Kelvin, mixed Rossby‐gravity, and equatorial Rossby waves, this approximation retains other slowly propagating moist gravity waves in a consistent fashion.

Sensitivity of tropical storms simulated by a general circulation model to changes in cumulus parametrization

Abstract: A number of recent studies have examined the statistics of tropical storms simulated by general circulation models (GCMs) forced by observed sea surface temperatures. Many GCMs have demonstrated an ability to simulate some aspects of the observed interannual variability of tropical storms, in particular, variability in storm frequency. This has led to nascent attempts to use GCMs as part of programs to produce operational seasonal forecasts of tropical-storm numbers. In this study, the sensitivity of the statistics of GCM-simulated tropical storms to changes in the model's physical parametrizations is examined. After preliminary results indicated that these statistics were most sensitive to details of the convective parametrization, GCM simulations with identical dynamical cores but different convective parametrizations were created. The parametrizations examined included moist convective adjustment, two variants of the Arakawa-Schubert scheme, and several variants of the relaxed Arakawa-Schubert (RAS) scheme; the impact of including a shallow-convection parametrization was also examined. The simulated tropical-storm frequency, intensity, structure, and interannual variability were all found to exhibit significant sensitivities to changes in convective parametrization. A particularly large sensitivity was found when the RAS and Arakawa-Schubert parametrizations were modified to place restrictions on the production of deep convection. Climatologies of the GCM tropical atmosphere and composites of tropical storms were examined to address the question of whether the tropical-storm statistics were directly impacted on by changes in convection associated with tropical storms, or if they were indirectly affected by parametrization-induced changes in the tropical mean atmosphere. A number of results point to the latter being the primary cause. A regional hurricane model, initialized with mean states from the GCM simulation climatologies, is used to further investigate this point. Particularly compelling is the fact that versions of the RAS scheme that produce significantly less realistic simulations of tropical storms nevertheless produce a much more realistic interannual variability of storms, apparently due to an improved tropical mean climate. A careful analysis of the background convective available potential energy (CAPE) is used to suggest that this quantity is particularly relevant to the occurrence of tropical storms in the low-resolution GCMs, although this may not be the case with observations. If the tropical CAPE is too low, tropical storms in the low-resolution GCMs cannot form with realistic frequency.

The diurnal cycle and African easterly waves: A land surface perspective

Abstract: Fluxes from the land surface into the atmosphere exhibit strong variability on diurnal and daily time-scales during the Sahelian wet season. The influence of this variability on the atmosphere is examined here using a general circulation model. A land-surface scheme with an improved description of the sparse vegetation which characterizes the Sahel is introduced, and compared with simulations using existing parameters. With smaller evaporation rates, the sparse vegetation results in a warmer and deeper planetary boundary layer (PBL) in better agreement with observations. The diurnal cycle in simulated rainfall is much weaker, due to less frequent triggering of daytime convection. The damping of diurnal variability is accompanied by enhanced daily variability, with a deep moist PBL developing over several days. Above the sparsely vegetated surface, there is also greater African easterly wave (AEW) activity, and more long-lived rain events associated with it. A mechanism is proposed to explain the increased frequency of AEWs above sparser vegetation, based on wave composites in the model. Surface heating of the lower atmosphere is modulated strongly by AEWs. This occurs via a surface-convection-radiation feedback. Estimates of the boundary-layer heat budget indicate that variations in surface heating can, to a large extent, account for the low-level temperature signal characteristic of AEWs. By reinforcing the low-level temperature anomalies, the surface may influence the meridional wind signature of the wave above the PBL. Further evidence of the close coupling between the land surface and atmosphere is found by examining the modulation of precipitation by AEWs. Surface heat and moisture fluxes influence the build-up of convective available potential energy ahead of the wave trough. In this way, the properties of the surface can affect the timing and intensity of rainfall associated with, an AEW.

Thermodynamics of Eastern Mediterranean Rainfall Variability

Abstract: This note focuses on thermodynamic changes caused by Eastern Mediterranean (EM) subsidence anomalies. Subsidence anomalies are shown to modulate EM-wide stability with respect to moist ascent. Additionally, convective available potential energy (CAPE) generation rates, as well as mean CAPE, change coherently during extreme EM rainfall anomalies. It is suggested that the resulting modulation of convective rain generation is the process directly responsible for the observed rainfall anomalies.

Cumulus Ensembles in Shear: Implications for Parameterization

Abstract: A systematic numerical investigation is conducted into the role of ambient shear on the macrophysical properties of tropical cumulus ensembles maintained by convective available potential energy generated by constant surface fluxes of temperature and moisture and large-scale advective cooling and moistening. The effects of five distinct idealized wind profiles on the organization of convection, and quantities relevant to the parameterization of convection and convectively generated clouds, are examined in a series of 6-day two-dimensional cloud-resolving simulations. Lower-tropospheric shear affects the mesoscale organization of convection through interaction with evaporatively driven downdraft outflows (convective triggering), while shear in mid-to-upper levels determines the amount of stratiform cloud and whether the convective transport of momentum is upgradient or downgradient. Shear significantly affects the convective heating and drying, momentum transport, mass fluxes, and cloud fraction. ...

Westward Generation of Eastward-Moving Tropical Convective Bands in TOGA COARE

Abstract: Mechanisms responsible for westward generation of eastward-moving tropical convective bands in the Tropical Ocean and Global Atmosphere Coupled Ocean‐Atmosphere Response Experiment (TOGA COARE) are investigated using a two-dimensional numerical cloud model. Sequential generation of new convective bands to the west of an old eastward-moving convective band is successfully simulated in an environment of a convectively active day during TOGA COARE, characterized by west winds at low levels and strong easterlies aloft. It is concluded that the westward generation of new convective bands is explained by a gravity wave mechanism. Two westward-propagating modes excited below the convective cells moving westward relative to the convective bands appear to play an important role. A slow-propagating mode (; 15 ms 21) excited by a shallow convective band is ducted in the troposphere under an unstable layer of small Richardson number containing its critical level. A fast-propagating mode ( ;25 m s21) excited by a deep convective band is ducted in the troposphere under the remaining region of the convective cell containing its critical level. These two modes propagate horizontally to the west and promote the growth of shallow convection into long-lived convective bands. A dry model with thermal forcing representing the convective cell showed that preferential excitation of westward-propagating waves below the convective cell is due to westward motion and ascension of the convective cell. A comparative simulation without the critical level confirms the proposed gravity wave mechanism.

Parcel Theory in Three Dimensions and the Calculation of SCAPE

Abstract: Slantwise convective available potential energy (SCAPE) is a measure of the degree to which the atmosphere is unstable to conditional symmetric instability (CSI). It has, until now, been defined by parcel theory in which the atmosphere is assumed to be nonevolving and balanced, that is, two-dimensional. When applying this two-dimensional theory to three-dimensional evolving flows, these assumptions can be interpreted as an implicit assumption that a timescale separation exists between a relatively rapid timescale for slantwise ascent and a slower timescale for the development of the system. An approximate extension of parcel theory to three dimensions is derived and it is shown that calculations of SCAPE based on the assumption of relatively rapid slantwise ascent can be qualitatively in error. For a case study example of a developing extratropical cyclone, SCAPE calculated along trajectories determined without assuming the existence of the timescale separation show large SCAPE values for parcels ascending from the warm sector and along the warm front. These parcels ascend into the cloud head within which there is some evidence consistent with the release of CSI from observational and model cross sections. This region of high SCAPE was not found for calculations along the relatively rapidly ascending trajectories determined by assuming the existence of the timescale separation.

Effects of land-use modification on potential increase of convection: A numerical mesoscale study over south Israel

Abstract: The current research was designed to examine the potential of modified surface conditions in semiarid and arid central-southern Israel to enhance convective development and rainfall. The fifth-generation mesoscale model (MM5), coupled with a sophisticated land-surface submodel, is applied for the three-dimensional high resolution simulations of two convective rain case studies, on October 27, 1990 and October 18, 1987. Three surface conditions are modeled to examine the relative influence of land-use changes, which are present-time land-use (1990s), preirrigation time (1930s), and the hypothetical case of extended irrigated agricultural lands. The main conclusion of the study is that there exists a positive influence of the anthropogenic land-use changes on the enhancement of thermal convection and associated rainfall. Modification of surface parameters from semiarid land conditions to cultivated lands tends to consistently increase the potential for moist convection during the daytime heating hours, as expressed in the evolution of the PBL structure and the growth of convective available potential energy (CAPE), as well as the area-averaged rainfall. The model seems to accurately reproduce the observed atmospheric situations and rainfall, lending confidence to the assertion that these land-atmosphere effects can be quantified with an advanced mesoscale modeling system. This work is the first numerical mesobeta scale 3-D study over the south Israel area with its relatively sharp spatial change in land use as well as in climatic zone. It is believed to be a region, possibly the only in the world, where mesoscale surface and planetary boundary layer processes were suggested as causes for observed anti-desertification gains.

Near surface atmospheric characteristics over the North Bay of Bengal during the Indian Summer Monsoon

Abstract: Observations were made from a ship at 17.5 °N & 89 °E in the North Bay of Bengal during July-August 1999. The Bay was convectively active during this period with almost one convective event every week. The surface wind speed varied from 3 to 15 m s−1. SST decreased by 0.5° to 1 °C following convection. The latent heat flux was about 25–35% lower over the Bay compared to that over the West Pacific at comparable wind speeds. High resolution radiosonde observations have been made for the first time in the North Bay. The values of atmospheric mixed layer height, equivalent potential temperature, convective available potential energy and convection inhibition energy over the North Bay are comparable to those observed over the western Pacific warm pool region. The recovery timescale of the atmospheric variables such as mixed layer height, equivalent potential temperature and convective available potential energy is one to two days. © 2001 American Geophysical Union

The Sydney Hailstorm of April 14, 1999: Synoptic descriptionand numerical simulation

Abstract: During the evening of April 14, 1999 an intense hailstorm struck the most densely populated region of Australia, the eastern suburbs of Sydney. This thunderstorm, which transformed into a high precipitation supercell when it moved into a region of enhanced surface moisture convergence and increased helicity on the coast, maintained its identity for 5.5 hours. It produced the largest verified hail in Australia’s history with the biggest stones being 11 cm in diameter. A microburst was recorded at Sydney Airport. The damage inflicted by this hailstorm was immense with three deaths, numerous injuries and insured losses exceeding $1.7 billion Australian dollars. This storm is the most expensive Australian natural disaster since severe weather records commenced in 1975. The thunderstorm initially formed from surface heating of relatively dry air in a low shear environment but was advected by middle level west to south westerly winds into a region where the surface to 500 hPa wind shear had increased to 17 ms−1 west south westerly. The storm relative helicity in this region was , in the layer between the surface and 700 hPa. Diagnostics from the 1500 Australian Eastern Standard Time (AEST) radiosonde released from Sydney Airport, 150 km north of where the thunderstorm initially formed, are thought to be representative of the pre-storm environment. The Convective Available Potential Energy (CAPE) was moderately high at 1713 J/kg with a relatively low Convective Inhibition (CIN) value of 50 J/kg. The Total Totals Index (TT) was 55 and the Surface Lifted Index (SLI) was −5.5, capable of supporting severe convection. The freezing level was at 2900 m, near average for the time of the year. Convective cloud tops would be expected to reach the tropopause at 250 hPa. The coastal environment was assessed as being able to support a supercell thunderstorm. A preliminary high resolution numerical simulation of the severe thunderstorm has been conducted. The model was triply nested, with its highest resolution grid spacing being 1 km. It incorporates a multi (six) water – ice phase microphysics, enabling it to simulate hail growth associated with supercell developmet. The initial generation and subsequent northward propagation of a hail-producing thunderstorm are captured in this simulation.

Evolution of maritime continent thunderstorms under varying meteorological conditions over the Tiwi Islands

Abstract: The development of tropical island thunderstorms in the maritime continent were investigated over the Tiwi Islands to the north of the Australian continent. A satellite cloud climatology was developed to examine these island thunderstorms, locally known as Hectors. During the pre-monsoon period of 1990 and 1991 Hectors were observed on 70% of days. These Hectors were observed to occur under two distinct environmental flow regimes. Under easterly flow, development occurred on the western end of the island and conversely under westerly flow, development occurred to the east. Hectors were also observed to have three distinct modes of development (Hector, suppressed and late developing) and one mode of non-development (non-Hector). Satellite cloud climatologies were developed and mean atmospheric characteristics were determined for each of these development modes. Hectors developed under conditions of moderate convective available potential energy, moderate shear and high moisture availability. Sub-cloud layer winds were also thought to contribute to the structure of the storm. Non-Hector and suppressed Hector days usually occurred after a previous convective disturbance such as a monsoon low or squall line and had typically low convective available potential energy and high values of convective inhibition. Conditions for late developing Hectors appeared to be conducive for thunderstorm development with high convective available potential energy and low values of convective inhibition. However, early initiation was suppressed by high shear conditions. Development in the late developing mode could be more intense than in other modes. Copyright © 2001 Royal Meteorological Society

Joint variations of temperature and water vapor over the midlatitude continents

Abstract: We have used warm-season data from the Atmospheric Radiation Measurement (ARM) Program's Southern Great Plains site to investigate the joint variability of the temperature and moisture soundings, and their relationship to the variability of the generalized convective available potential energy (GCAPE). The actual temperature and humidity soundings vary together in such a way as to produce variations of the GCAPE which are far smaller than those which would occur if the relative humidity varied while the temperature sounding was fixed, or vice versa.

Measure of CINE - A relevant parameter for forecasting pre-monsoon thunderstorms over GWB

Abstract: A method of testing the significance of Z- Statistic is introduced in this paper to discern the role of Convective Available Potential Energy (CAPE) and Convective Inhibition Energy (CINE) in forecasting the occurrence of pre-monsoon thunderstorms over Gangetic West Bengal (GWB). The result reveals that a negative correlation exists between CAPE and CINE. It further indicates that a range for the lower values of CINE can be fixed where the frequency of occurrence of such storms will be maximum, but such range, either for lower or for higher values of CAPE, is not possible. The paper, thus, ends with a very interesting finding that a measure of CINE is the only relevant parameter whereas CAPE has no significant role in forecasting the occurrence of pre-monsoon thunderstorms over GWB, which is in contrast to the concept of severe thunderstorms of Great Plains of America.

Scale Interactions Involved in the Initiation, Structure, and Evolution of the 15 December 1992 MCS Observed during TOGA COARE. Part I: Synoptic-Scale Processes

Abstract: This paper, the first of a series, examines the synoptic-scale mechanisms involved in the initiation, structure, and evolution of a mesoscale convective system observed during TOGA COARE. This study relies upon the use of the Japanese Geosynchronous Meteorological Satellite-4 imagery and ECMWF model outputs, from which diagnostic parameters are derived and interpreted. This mesoscale convective system consists initially of two groups of convective entities that progressively move toward each other and merge. It is shown that the synoptic-scale flow creates a favorable environment for the formation of this large convective system through the production of convective available potential energy (CAPE) by horizontal advection and the enhancement of low-level convergence in the region where the convective system formed. Moreover, the general evolution of the system is found to be governed by the synoptic-scale circulation and, more precisely, by the temporal evolution of CAPE and low-level convergence. The mechanisms leading to the initiation and general evolution of the system are examined. The easterly equatorial jet at 500 hPa triggered positive potential vorticity areas that propagated westward and generated an anticyclonic circulation. This anticyclonic circulation was enhanced during the development phase of the convective system through vortex stretching and tilting, which accelerated the low-level westerlies (corresponding to the southern branch of this circulation) and enhanced the low-level convergence associated with the studied convective system. In a companion paper, the mesoscale and convective-scale processes involved in the internal organization of this mesoscale convective system are examined using the airborne Doppler radar dataset collected within the system from 1700 to 2100 UTC. The downscale interactions (i.e., from synoptic scale to mesoscale and convective scale) are scrutinized using both the synoptic-scale context described in this paper and the mesoscale and convective-scale characteristics derived from the airborne radar observations.

NOTES AND CORRESPONDENCE Thermodynamics of Eastern Mediterranean Rainfall Variability

Abstract: This note focuses on thermodynamic changes caused by Eastern Mediterranean (EM) subsidence anomalies. Subsidence anomalies are shown to modulate EM-wide stability with respect to moist ascent. Additionally, convective available potential energy (CAPE) generation rates, as well as mean CAPE, change coherently during extreme EM rainfall anomalies. It is suggested that the resulting modulation of convective rain generation is the process directly responsible for the observed rainfall anomalies.