Baroclinic Wave Life Cycles Research Articles

A climatological-dynamical analysis of tropopause folds over Southwest Asia in the period of 1989–2018

Tropopause folding is believed to be connected to the development of cyclonic systems in midlatitudes as well as the larger-scale, lower-frequency processes in the lower latitudes. To investigate such connections, a statistical and dynamical analysis of tropopause folds occurred in Southwest Asia is carried out for the period of 1989–2018. Tropopause folds are identified using the Era-Interim reanalysis dataset from the European Centre for Medium-Range Weather Forecasts. The spatial–temporal distributions of tropopause folding show that the frequency of folding is higher in the winter hemisphere than in the summer hemisphere and greater values of the frequency are found mostly within an elongated zonal band which is situated between 20 and 40 degrees latitude. Southwest Asia also has strong positive anomalous values of tropopause folding frequency and achieves the values more than five times the average of the Northern Hemisphere in the summer, especially in the northern latitudes of this region including Afghanistan, Iran and the eastern Mediterranean. This can be attributed mainly to the formation of summer monsoon in the subtropical latitudes of the Indian Ocean. In this regard, the examination of the effects of El Niño and La Niña on the spatial–temporal distributions of tropopause folding reveals that the values of 3-month running average of tropopause folding frequency coincident with La Niña tend to be larger than those with El Nino in both the hemispheres and Southwest Asia. This result is consistent with the previous findings on the tendency for higher occurrence of anticyclonic (cyclonic) baroclinic wave life cycles during La Niña (El Niño). Dynamically, the folds occurred in the study area are associated with the formation of waves at 500 hPa level and subsequently the creation of surface cyclones. In areas of high frequency of folding events, strong meridional shear of the zonal winds and jet stream amplification are observed at 200 hPa level. The analysis of wave activity flux and its divergence for the selected month of June 2015 shows two regions of wave propagation, one with a wave-packet like signature of flux divergence/convergence centers in the upper troposphere and one over the lower troposphere. The upper-tropospheric wave packet ends with an area of flux convergence related with the receipt and decay of Rossby waves, while in the lower troposphere, a flux divergence region as a source of stationary waves associated with the Indian monsoon activity is identified.

Structure, Energy, and Parameterization of Inertia–Gravity Waves in Dry and Moist Simulations of a Baroclinic Wave Life Cycle

Abstract The impact of moisture on inertia–gravity wave generation is assessed for an idealized unstable baroclinic wave using the Weather Research and Forecasting Model (WRF) in a channel on the f plane. The evolution of these waves in a moist simulation is compared with a dry simulation. The centers of action for inertia–gravity wave activity are identified as the equatorward-moving upper-level front and the poleward-progressing upper-level jet–surface front system. Four stratospheric wave packets are found, which are significantly more intense in the moist simulation and have slightly higher frequency. They are characterized by their structure and position during the baroclinic wave life cycle and are related to forcing terms in jet, front, and convection systems. By exploring the time series of mass and energy, it is shown that the release of latent heat leads to a change in enthalpy, an increase in the eddy kinetic energy, and an intensification of the inertia–gravity wave energy. The ratio of the inertia–gravity wave energy to the eddy kinetic energy is estimated to be about 1/200 for the moist simulation, which is 3 times larger than that for the dry simulation. An empirical parameterization scheme for the inertia–gravity wave energy is proposed, based on the fast large-scale ageostrophic flow associated with the jet, front, and convection. The diagnosed stratospheric inertia–gravity wave energy is well captured by this parameterization in six WRF simulations with different moisture and resolutions. The approach used to construct the parameterization may serve as a starting point for state-dependent nonorographic gravity wave drag schemes in general circulation models.

Spatiotemporal development of irreversible mixing in midlatitude baroclinic wave life cycles: Morphology, energetics, and nonisentropic mixing activity

AbstractThe significance of nonisentropic irreversible mixing processes is diagnosed for idealized simulations of synoptic‐scale baroclinic wave life cycles along the subtropical jet stream, using a nonhydrostatic, anelastic, mesoscale model subject to a free‐slip surface boundary condition. A variety of morphological features of mixing is identified such as a mesoscale columnar vortex associated with the onset of frontal fracture, episodic overturnings along the surface fronts, “wrinkling” of the tropopause, and injection of tropospheric air into the stratosphere. The evolution of the degree of nonisentropic irreversible mixing is first analyzed by computing the change of the “base” component of potential energy that cannot be converted into kinetic energy. The structure of the mixing activity is also diagnosed through inspection of spatiotemporal changes of entropy to demonstrate that the surface fronts are by far the most active regions of such activity. This activity is found to be primarily longitudinal, extending from the surface to the lower stratosphere, to form a three‐dimensional spiral in the synoptic‐scale cyclone and along the fronts. However, an exceptional region also exists along the warm front, where the structure becomes primarily transverse in the mature phase of frontal development in a model including an explicit representation of small‐scale turbulence. In all simulations, the net transfer of mass and heat across the tropopause is from troposphere to stratosphere. The maximum transfer occurs when the observed climatological level of stratification contrast is assumed between stratosphere and troposphere. The same climatological choice leads to a minimum net irreversible mixing, which occurs primarily at Earth's surface.

Mesoscale Gravity Waves in Moist Baroclinic Jet–Front Systems

Abstract A series of cloud-permitting simulations with the Weather Research and Forecast model (WRF) are performed to study the characteristics and source mechanisms of mesoscale gravity waves in moist baroclinic jet–front systems with varying degrees of convective instability. These idealized experiments are initialized with the same baroclinic jet but with different initial moisture content, which produce different life cycles of moist baroclinic waves, to investigate the relative roles of moist processes and baroclinicity in the generation and propagation of mesoscale gravity waves. The dry experiment with no moisture or convection simulates gravity waves that are consistent with past modeling studies. An experiment with a small amount of moisture produces similar baroclinic life cycles to the dry experiment but with the introduction of weak convective instability. Subsequent initiation of convection, although weak, may considerably amplify the gravity waves that are propagating away from the upper-level jet exit region crossing the ridge to the jet entrance region. The weak convection also generates a new wave mode of shorter-scale wave packets that are believed to interact with, strengthen, and modify the dry gravity wave modes. Further increase of the moisture content (up to 5 times) leads to strong convective instability and vigorous moist convection. Besides a faster-growing moist baroclinic wave, the convectively generated gravity waves emerge much earlier, are more prevalent, and are larger in amplitude; they are fully coupled with, and hardly separable from, the dry gravity wave modes under the complex background moist baroclinic waves.

Wave Activity for Large-Amplitude Disturbances Described by the Primitive Equations on the Sphere

Abstract Pseudomomentum and pseudoenergy are both measures of wave activity for disturbances in a fluid, relative to a notional background state. Together they give information on the propagation, growth, and decay of disturbances. Wave activity conservation laws are most readily derived for the primitive equations on the sphere by using isentropic coordinates. However, the intersection of isentropic surfaces with the ground (and associated potential temperature anomalies) is a crucial aspect of baroclinic wave evolution. A new expression is derived for pseudoenergy that is valid for large-amplitude disturbances spanning isentropic layers that may intersect the ground. The pseudoenergy of small-amplitude disturbances is also obtained by linearizing about a zonally symmetric background state. The new expression generalizes previous pseudoenergy results for quasigeostrophic disturbances on the β plane and complements existing large-amplitude results for pseudomomentum. The pseudomomentum and pseudoenergy diagnostics are applied to an extended winter from the European Centre for Medium-Range Weather Forecasts Interim Re-Analysis data. The time series identify distinct phenomena such as a baroclinic wave life cycle where the wave activity in boundary potential temperature saturates nonlinearly almost two days before the peak in wave activity near the tropopause. The coherent zonal propagation speed of disturbances at tropopause level, including distinct eastward, westward, and stationary phases, is shown to be dictated by the ratio of total hemispheric pseudoenergy to pseudomomentum. Variations in the lower-boundary contribution to pseudoenergy dominate changes in propagation speed; phases of westward progression are associated with stronger boundary potential temperature perturbations.

Response of Idealized Baroclinic Wave Life Cycles to Stratospheric Flow Conditions

Abstract Dynamical stratosphere–troposphere coupling through a response of baroclinic waves to lower stratospheric flow conditions is investigated from an initial value approach. A series of adiabatic and frictionless nonlinear baroclinic wave life cycles in a midlatitude tropospheric jet with different initial zonal flow conditions in the stratosphere is simulated, using a dry primitive equation model with spherical geometry. When a stratospheric jet, located at various latitudes between 35° and 70°, is removed from the initial conditions, the wavenumber-6 life cycle behavior changes from the well-known LC1 to LC2 evolution, characterized by anticyclonic and cyclonic wave breaking, respectively. Linear theory, in terms of refractive index and the structure of the corresponding fastest-growing normal mode, is found to be unable to explain this stratosphere-induced LC1 to LC2 transition. This implies that altered nonlinear wave–mean flow interactions are important. The most significant stratosphere-induced change that extends into the nonlinear baroclinic growth stage is a region of downward wave propagation in the lower stratosphere associated with positive values of the squared refractive index near 20 km. Furthermore, it is demonstrated that the difference between the response of the tropospheric circulation to LC1 and LC2 life cycles closely resembles the meridional and vertical structure of the North Atlantic Oscillation (NAO), with positive (negative) NAO-like anomalies being driven by LC1 (LC2). Thus, a weakened stratospheric jet induces the generation of negative NAO-like anomalies in the troposphere, consistent with the observed stratosphere–NAO connection.

The Mesoscale Kinetic Energy Spectrum of a Baroclinic Life Cycle

Abstract The atmospheric mesoscale kinetic energy spectrum is investigated through numerical simulations of an idealized baroclinic wave life cycle, from linear instability to mature nonlinear evolution and with high horizontal and vertical resolution (Δx ≈ 10 km and Δz ≈ 60 m). The spontaneous excitation of inertia–gravity waves yields a shallowing of the mesoscale spectrum with respect to the large scales, in qualitative agreement with observations. However, this shallowing is restricted to the lower stratosphere and does not occur in the upper troposphere. At both levels, the mesoscale divergent kinetic energy spectrum—a proxy for the inertia–gravity wave energy spectrum—resembles a −5/3 power law in the mature stage. Divergent kinetic energy dominates the lower stratospheric mesoscale spectrum, accounting for its shallowing. Rotational kinetic energy, by contrast, dominates the upper tropospheric spectrum and no shallowing of the full spectrum is observed. By analyzing the tendency equation for the kinetic energy spectrum, it is shown that the lower stratospheric spectrum is not governed solely by a downscale energy cascade; rather, it is influenced by the vertical pressure flux divergence associated with vertically propagating inertia–gravity waves.

Self‐limiting feedback between baroclinic waves and a NAO‐like sheared zonal flow

The eddy‐mean flow interaction associated with the North Atlantic Oscillation (NAO) is examined by using the baroclinic wave life cycle experiments. When a sheared zonal flow perturbation akin to the NAO‐related dipole wind anomaly is added to the basic state, momentum fluxes due to baroclinic waves tend to reinforce the initial zonal flow dipole in the upper troposphere both for the anticyclonic and cyclonic shears. The eddy feedback is stronger for the anticyclonic shear because the node of the dipole flow is asymmetric about the basic jet, suggesting that the positive NAO is more favored by the eddy feedback. For the zonal wind anomaly with extremely large amplitude, the baroclinic wave breaking cannot efficiently intensify the zonal flow dipole, indicating a self‐limitation in the positive eddy feedback to the NAO‐like zonal flow anomaly.

Sensitivity of Mesoscale Gravity Waves to the Baroclinicity of Jet-Front Systems

Abstract This study investigates the sensitivity of mesoscale gravity waves to the baroclinicity of the background jet-front systems by simulating different life cycles of baroclinic waves with a high-resolution mesoscale model. Four simulations are made starting from two-dimensional baroclinic jets having different static stability and wind shear in order to obtain baroclinic waves with significantly different growth rates. In all experiments, vertically propagating mesoscale gravity waves are simulated in the exit region of upper-tropospheric jet streaks. A two-dimensional spectral analysis demonstrates that these gravity waves have multiple components with different wave characteristics. The short-scale wave components that are preserved by a high-pass filter with a cutoff wavelength of 200 km have horizontal wavelengths of 85–161 km and intrinsic frequencies of 3–11 times the Coriolis parameter. The medium-scale waves that are preserved by a bandpass filter (with 200- and 600-km cutoff wavelengths) have horizontal wavelengths of 250–350 km and intrinsic frequencies less than 3 times the Coriolis parameter. The intrinsic frequencies of these gravity waves tend to increase with the growth rate of the baroclinic waves; gravity waves with similar frequency are found in the experiments with similar average baroclinic wave growth rate but with significantly different initial tropospheric static stability and tropopause geometry. The residuals of the nonlinear balance equation are used to assess the flow imbalance. In all experiments, the developing background baroclinic waves evolve from an initially balanced state to the strongly unbalanced state especially near the exit region of upper-level jet fronts before mature mesoscale gravity waves are generated. It is found that the growth rate of flow imbalance also correlates well to the growth rate of baroclinic waves and thus correlates to the frequency of gravity waves.

The counter-propagating Rossby-wave perspective on baroclinic instability. Part IV: Nonlinear life cycles

Pairs of counter-propagating Rossby waves (CRWs) can be used to describe baroclinic instability in linearized primitive-equation dynamics, employing simple propagation and interaction mechanisms at only two locations in the meridional plane—the CRW ‘home-bases’. Here, it is shown how some CRW properties are remarkably robust as a growing baroclinic wave develops nonlinearly. For example, the phase difference between upper-level and lower-level waves in potential-vorticity contours, defined initially at the home-bases of the CRWs, remains almost constant throughout baroclinic wave life cycles, despite the occurrence of frontogenesis and Rossby-wave breaking. As the lower wave saturates nonlinearly the whole baroclinic wave changes phase speed from that of the normal mode to that of the self-induced phase speed of the upper CRW. On zonal jets without surface meridional shear, this must always act to slow the baroclinic wave. The direction of wave breaking when a basic state has surface meridional shear can be anticipated because the displacement structures of CRWs tend to be coherent along surfaces of constant basic-state angular velocity, U. This results in up-gradient horizontal momentum fluxes for baroclinically growing disturbances. The momentum flux acts to shift the jet meridionally in the direction of the increasing surface U, so that the upper CRW breaks in the same direction as occurred at low levels

A Potential Vorticity Diagnostic Approach to Upper-Level Frontogenesis within a Developing Baroclinic Wave

The process of tropopause folding is studied in the context of the life cycle of baroclinic waves. Previous studies of upper-level frontogenesis have emphasized the role of the vertical circulation in driving stratospheric air down into the midtroposphere. Here, a potential vorticity‐based approach is adopted that focuses on the generation of a folded tropopause. To facilitate comparison of the two approaches, the diagnosis is applied to the upper-level front previously simulated and studied by Rotunno et al. The potential vorticity approach clarifies the primary role played by the horizontal nondivergent wind in producing a fold and explains why folding should be a common aspect of baroclinic development. Between the trough and upstream ridge, prolonged subsidence within a region of weak system-relative flow generates a tropopause depression oriented at an angle to the large-scale flow. The large-scale vertical shear then locally increases the slope of the tropopause, eventually leading to a tropopause fold. In contrast, tropopause folding in the base of the trough is caused by the nondivergent cyclonic circulation associated with the surface thermal wave. The winds associated with the thermal wave amplify the potential vorticity wave aloft, and these winds, which decrease with height, rapidly generate a tropopause fold within the trough.

Wave Packets and Life Cycles of Troughs in the Upper Troposphere: Examples from the Southern Hemisphere Summer Season of 1984/85

Abstract In this paper, reanalysis data for the Southern Hemisphere summer season of 1984/85, produced by the European Centre for Medium-Range Weather Forecasts, have been analyzed to examine wave packets and life cycles of baroclinic waves. A semiobjective algorithm has been devised to track wave packets. It is found that the perturbations in the upper troposphere are dominated by wave packets. Some of these wave packets can be tracked for several weeks, during which they can make up to two trips around the latitude circle. The energy life cycles of waves that make up several of these wave packets were analyzed, and apart from the incipient waves that started the wave packets, most of the subsequent wave development was found to be dominated by convergence and divergence of energy fluxes rather than due to baroclinic or barotropic conversions, demonstrating that these wave packets are indeed coherent entities that propagate due to downstream development (or dispersion). The energy life cycle of the most ...

A Lagrangian study of the three-dimensional transport of boundary-layer tracers in an idealised baroclinic-wave life-cycle

The role of baroclinic-wave driven chemical transport is examined using the framework of a Lagrangiantrajectory model.The Lagrangian motion of transported trace gases are closely monitored through labelledboundary-layer tracers binned accordingto their latitudinal locations.From a set of 14-day Lagrangian paths, the mechanistically liftedsubtropical boundary-layer tracers track along tilted poleward paths, whilethe subsiding high-latitude tracers track along tilted equator-ward paths.The most significantmovements of tracers occur between days 6 and8. The vertical and latitudinal displacements during this time interval are3 km and 15° latitude. During a baroclinic-wave life-cycle,boundary-layertracers can either ascend vertically from 1 km to 7 km or descend to thesurface, while they arelatitudinally transported from 37° to 73° and from 45° to near 15° during the poleward and equator-ward motions,respectively.Vertical mixing of tracers occurs vigorously at mid-latitudes, where more than50%, by day 7, and a maximum of 70%, between days 9 and 10, of the boundary layer tracers have beentransported intothe free troposphere during the baroclinic-wave life-cycle.A clear 3D picture emerges from a Lagrangian analysis.Each time the tracer travels equator-ward, it descends, whilewhen it travels poleward, it ascends.Almost all of the low latitude tracers show tilted upward and poleward paths,while high latitude tracersshows downward tilted and equator-ward path.The maximum vertical displacement between poleward andequator-ward tracers are shown in mid-latitudes.Two types of the tilted upward and poleward paths are generally seen in thelatitude-height projections:anti-clockwise and clockwise paths.Both types of path transport tracers upward, however, the anti-clockwise pathsdeliver tracersequator-ward, while the clockwise paths deliver tracers poleward.Hence, whenlow latitude warm air arrives at mid-latitude, it can pick up enhanced tracerconcentrationand carry them on either poleward or equator-ward.

A Simple Model for the Baroclinic Life Cycle of Meridionally Elongated Eddies in Uniform Shear

Abstract A highly simplified model for the wave–mean flow interaction in a baroclinic wave life cycle is derived from the quasigeostrophic two-layer system. The simplification is based on a sheared disturbance embedded in a zonal mean flow with uniform vertical and meridional shear. This system, referred to as the uniform shear model, can only be justified in the limit of large meridional eddy elongation. However, the model captures well-known features of baroclinic wave life cycles. These include the excitation of a baroclinic growth and a barotropic decay phase where the zonal mean flow is effectively stabilized by the barotropic governor effect. This effect is a consequence of a nonlinear tilting instability where enstrophy is driven to decreasing scales while the kinetic energy is transformed into the zonal mean barotropic shear flow. A detailed examination of the uniform shear model reveals that the wave evolution depends sensitively on the initial barotropic shear, wave amplitude, and wave tilt. The...

The Advection of High-Resolution Tracers by Low-Resolution Winds

Abstract The usefulness of any simulation of atmospheric tracers using low-resolution winds relies on both the dominance of large spatial scales in the strain and time dependence that results in a cascade in tracer scales. Here, a quantitative study on the accuracy of such tracer studies is made using the contour advection technique. It is shown that, although contour stretching rates are very insensitive to the spatial truncation of the wind field, the displacement errors in filament position are sensitive. A knowledge of displacement characteristics is essential if Lagrangian simulations are to be used for the inference of airmass origin. A quantitative lower estimate is obtained for the tracer scale factor (TSF): the ratio of the smallest resolved scale in the advecting wind field to the smallest “trustworthy” scale in the tracer field. For a baroclinic wave life cycle the TSF = 6.1 ± 0.3 while for the Northern Hemisphere wintertime lower stratosphere the TSF = 5.5 ± 0.5, when using the most stringent ...

Transport of Passive Tracers in Baroclinic Wave Life Cycles

The transport of passive tracers in idealized baroclinic wave life cycles is studied using output from the National Center for Atmospheric Research Community Climate Model (CCM2). Two life cycles, LCn and LCs, are simulated, starting with baroclinically unstable initial conditions similar to those used by Thorncroft et al. in their study of two life cycle paradigms. The two life cycles LCn and LCs have different initial horizontal wind shear structures that result in distinctive nonlinear development. In terms of potential vorticity-potential temperature (PV-theta) diagnostics, the LCn case is characterized by thinning troughs that are advected anti-cyclonically and equatorward, while the LCs case has broadening troughs that wrap up cyclonically and poleward. Four idealized passive tracers are included in the model to be advected by the semi-Lagrangian transport scheme of the CCM2, and their evolutions are investigated throughout the life cycles. Tracer budgets are analyzed in terms of the transformed Eulerian mean constituent transport formalism in pressure coordinates and also in isentropic coordinates. Results for both LCn and LCs show transport that is downgradient with respect to the background structure of the tracer field, but with a characteristic spatial structure that maximizes in the middle to high latitudes. For the idealized tropospheric tracers in this study, this represents a net upward and poleward transport that enhances concentrations at high latitudes. These results vary little with the initial distribution of the constituent field. The time tendency of the tracer is influenced most strongly by the eddy flux term. with the largest transport occurring during the nonlinear growth stage of the life cycle. The authors also study the transport of a lower-stratospheric tracer, to examine stratosphere-troposphere exchange for baroclinic waves.

Implication of a Theorem for a Class of Quasigeostrophic Flows

Large-scale atmospheric flows are often analyzed in the context of a quasigeostrophic (QG) model, which is characterized by conservation of quasigeostrophic potential vorticity (PV). Some of the subtle properties and limitations of a system may be best appreciated in terms of its intrinsic symmetry properties. For example, Nakamura (1993) points out that if the zonal mean flow and potential vorticity gradient in a QG system are vertically antisymmetric at one time, they remain antisymmetric for all time. It follows that the important barotropic governor mechanism (James 1987) cannot take place in such a situation, albeit a rather unrealistic one. Other symmetry properties have been noted and will be commented on shortly. Here we report yet another more general symmetry property of a QG system that has an implication for the constraint to baroclinic wave life cycle.

The semi-geostrophic diagnosis of vertical motion. II: Results for an idealized baroclinic wave life cycle

The semi-geostrophic diagnosis of vertical motion. II: Results for an idealized baroclinic wave life cycle

The semi‐geostrophic diagnosis of vertical motion. II: Results for an idealized baroclinic wave life cycle

AbstractIn this paper we apply the method of solution of the semi‐geostrophic (SG) omega equation developed in Part I to data generated from a numerical simulation of an idealized dry adiabatic frictionless baroclinic wave life cycle. The aim is to compare quantitatively the quasi‐geostrophic (QG) and SG solutions for the vertical motion, and to relate these to the model vertical motion field. This comparison allows an evaluation of how practical, and accurate, it would be to use such balance theories to estimate vertical motion from atmospheric data. It is found that the QG solution tends to overestimate both ascent and descent in the cyclone. This overestimation is due both to an overestimation of the forcing and to a poor treatment of the large horizontal gradients of static stability produced as the tropopause descends near the cyclone centre. As the SG theory more faithfully represents the Ertel‐Rossby potential vorticity (PV), the vertical motion which is implied by this distribution of PV is also quantitatively more similar to the model vertical motion than that produced by the QG theory. The PV distribution affects the vertical motion via the forcing and via explicit account of horizontal gradients of the local static stability distribution which appear in the differential operator in the omega equation. The residual between the vertical motion predicted by the SG, or any other balance, theory and that given by the model is produced partly by any inertia‐gravity wave activity and partly by inaccuracies in the assumed balance condition.

The importance of moisture distribution for the growth and energetics of mid-latitude systems

Abstract. A primitive equation model is used to study the sensitivity of baroclinic wave life cycles to the initial latitude-height distribution of humidity. Diabatic heating is parametrized only as a consequence of condensation in regions of large-scale ascent. Experiments are performed in which the initial relative humidity is a simple function of model level, and in some cases latitude bands are specified which are initially relatively dry. It is found that the presence of moisture can either increase or decrease the peak eddy kinetic energy of the developing wave, depending on the initial moisture distribution. A relative abundance of moisture at mid-latitudes tends to weaken the wave, while a relative abundance at low latitudes tends to strengthen it. This sensitivity exists because competing processes are at work. These processes are described in terms of energy box diagnostics. The most realistic case lies on the cusp of this sensitivity. Further physical parametrizations are then added, including surface fluxes and upright moist convection. These have the effect of increasing wave amplitude, but the sensitivity to initial conditions of relative humidity remains. Finally, 'control' and 'doubled CO2' life cycles are performed, with initial conditions taken from the time-mean zonal-mean output of equilibrium GCM experiments. The attenuation of the wave resulting from reduced baroclinicity is more pronounced than any effect due to changes in initial moisture.Key words. Meteorology and atmospheric dynamics (climatology; convective processes; synoptic-scale meteorology)