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Philippe Peyrillé and Jean-Philippe Lafore

Abstract

The idealized 2D model developed in Part I of this study is used to study the West African monsoon sensitivity to large-scale forcing. Using ECWMF reanalyses, a large-scale forcing is introduced in the 2D model in terms of temperature and humidity advection.

A coherent structure of cooling–moistening near the surface and drying–warming in the 2–4-km layer is found in the Saharan heat low region. The effect of the advective forcing is to block the monsoon propagation by strengthening the northerly flux and by an increase of convective inhibition. The heat low thus appears to play a key role in the monsoon northward penetration through its temperature and humidity budget. Ultimately, warmer low levels and/or more moist midlevels in the heat low favor a more northerly position of the ITCZ.

A detailed view of the continental diurnal cycle is also presented. Potential temperature and humidity budgets are performed in the deep convective and heat low area. The moistening process to sustain deep convection is made through nocturnal advection at low levels and daytime turbulence that redistributes humidity vertically. The same mechanism occurs in the heat low except that the vertical transfers by turbulence help maintain the dryness of the low levels. A possible mechanism of interaction between the deep convective zone and the Saharan heat low is also proposed that involves gravity waves in the upper troposphere.

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Gérald Desroziers and Jean-Philippe Lafore

Abstract

The analysis of a frontal discontinuity is difficult as the hypothesis of isotropy commonly assumed by inter- polation schemes is obviously erroneous in that case. Starting from the semigeostrophic theory, the authors propose a kind of flow-dependent analysis based on the use of geostrophic coordinates. The idea behind this approach is that the discontinuity should appear much more regular in geostrophic space, and then it should better fulfill the above-mentioned hypothesis of isotropy. The validity of such a scheme is first checked in the Hoskins and Bretherton dry and inviscid shear model of frontogenesis.

In order to treat more realistic cases, the authors introduce a filtered geostrophic advection coordinate (FGAC) using real wind instead of geostrophic wind. Applied to simulations of the wet and viscous Eady's problem, this procedure brings a clear gain with a maximum positive impact when data spacing is in the 100–200-km range. Finally, the authors apply this method to a high-resolution dropsonde dataset collected during the FRONTS 87 experiment. Again, the FGAC transformation is shown to greatly improve the analysis, producing more consistent wind and mass fields.

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Jean-Yves Grandpeix and Jean-Philippe Lafore
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Jean-Luc Redelsperger and Jean-Philippe Lafore

Abstract

Three-dimensional convective-scale simulations of an African squall line, observed during the French COPT 81 experiment, are presented. Three simulations with different representations of large-scale forcing are performed on a domain of 50 km (along the line) by 80 km (across the line). They exhibit a similar circulation pattern characteristic of a squall line, but differ in intensity. The first simulation supposes an unperturbed environment and produces a slow-moving squall line (7 m s−1) with weaker total precipitation rate then observed (25%). The second one includes a representation of observed thermodynamic and dynamic environment modifications, and produces a fast-moving squall line (10 m s−1) still weaker than observations (50% or the rain rate). The third simulation takes into account the forcing induced by the rear inflow jet as depicted by Smull and Houze and observed on that day. It allows the system to reach an intensity in agreement with observations.

The convective region (30 km wide) appears as the superposition of several convective cells at different stages of their life cycle. New elements are formed in front of the system and are fed by the forced convergence band along the squall-line front. Mature cells produce precipitation that feeds downdrafts by loading and evaporation. Old convective cells dissipate at the simulated system rear. Between the convective updrafts, intrusions of low equivalent potential temperature (θe) are found. These are unsaturated downdraft cells feeding the gravity current.

At low levels (up to 2 km), the simulated system has a two-dimensional structure, but it becomes progressively three-dimensional with height. This three-dimensional structure allows the crossing of two inflow layers of high and low θe, respectively between 2 and 6 km. This is the crossover zone whose existence was hypothesized by Zipser. A detailed description of the gravity current at small scale is given, showing an inner circulation whose intensity depends on the forcing imposed by the stratiform part.

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Jean-Yves Grandpeix and Jean-Philippe Lafore

Abstract

The aim of the present series of papers is to develop a density current parameterization for global circulation models. This first paper is devoted to the presentation of this new wake parameterization coupled with Emanuel’s convective scheme. The model represents a population of identical circular cold pools (the wakes) with vertical frontiers. The wakes are cooled by the precipitating downdrafts while the outside area is warmed by the subsidence induced by the saturated drafts. The budget equations for mass, energy, and water yield evolution equations for the prognostic variables (the vertical profiles of the temperature and humidity differences between the wakes and their exterior). They also provide additional terms for the equations of the mean variables. The driving terms of the wake equations are the differential heating and drying due to convective drafts. The action of the convection on the wakes is implemented by splitting the convective tendency and attributing the effect of the precipitating downdrafts to the wake region and the effect of the saturated drafts to their exterior. Conversely, the action of the wakes on convection is implemented by introducing two new variables representing the convergence at the leading edge of the wakes. The available lifting energy (ALE) determines the triggers of deep convection: convection occurs when ALE exceeds the convective inhibition. The available lifting power (ALP) determines the intensity of convection; it is equal to the power input into the system by the collapse of the wakes. The ALE/ALP closure, together with the splitting of the convective heating and drying, implements the full coupling between wake and convection. The coupled wake–convection scheme thus created makes it possible to represent the moist convective processes more realistically, to prepare the coupling of convection with boundary layer and orographic processes, and to consider simulating the propagation of convective systems.

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Philippe Peyrillé, Jean-Philippe Lafore, and Jean-Luc Redelsperger

Abstract

An idealized vertical–meridional zonally symmetric model is developed in order to recover a July typical monsoon regime over West Africa in response to surface conditions. The model includes a parameterization to account for heat and momentum fluxes associated with eddies. The sensitivity of the simulated West African monsoon equilibrium regime to some major processes is explored. It allows confirmation of the important role played by the sun’s latitudinal position, the aerosols, the albedo, and the SST’s magnitude in the Gulf of Guinea and in the Mediterranean Sea.

The important role of aerosols in warming the Saharan lower layers and their effect on the whole monsoon is underlined. Model results also stress the importance of the Mediterranean Sea, which is needed to obtain the extreme dryness of the Sahara. The use of this idealized model is finally discussed for studying the scale interactions and coupling involved in the West African monsoon as explored in a companion paper.

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Katia Chancibault, Véronique Ducrocq, and Jean-Philippe Lafore

Abstract

A case of a nontornadic supercell over France is simulated with a three-dimensional nonhydrostatic cloud model. The simulation starts from an operational data analysis without any superimposed perturbation. The initial convective cell is triggered by a mesoscale convergence line associated with a preexisting thermal boundary. The model succeeds in simulating right- and left-moving storms arising from a splitting process of the initial convective cell. The right-moving storm exhibits the characteristics of a supercell with a hooklike structure in the precipitation field, a midlevel rotating updraft, and a low-level cyclonic vortex.

A vorticity budget analysis is performed along backward parcel trajectories for the initial storm and for the supercell phases, with emphasis on the impacts of the preexisting thermal boundary and the associated low-level variations of shear. Similar mechanisms as those found for the cases over homogeneous environment operate globally. Indeed, the simulation exhibits a couplet of cyclonic and anticyclonic vortex on the flanks of the updraft leading to the split of the initial storm. The supercell derives its low-level cyclonic vorticity from tilting of baroclinically produced horizontal vorticity within the storm's forward flank region. Nevertheless, some differences in the rotational characteristics and in the formation of the initial cell arise from the heterogeneous environment compared to those of homogeneous environment. The vorticity analysis shows that the veering with height of the cyclonic and anticyclonic vertical vorticity cores at midlevels is not symmetric due to the heterogeneous field. It is also found that an additional mechanism operates in the low-level cyclonic vertical vorticity generation.

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Fabrice Chauvin, Romain Roehrig, and Jean-Philippe Lafore

Abstract

The Saharan heat low (SHL) is thought to be a key feature of the West African monsoon, and its variations during the summer season have not yet been systematically assessed. To characterize the intraseasonal variations of the SHL, real and complex empirical orthogonal function analyses were applied to the 850-hPa potential temperature field over northern Africa and the Mediterranean, using NCEP–Department of Energy (DOE) Atmospheric Model Intercomparison Project (AMIP-II) reanalysis results. A robust quasi-propagative mode was highlighted over North Africa and the Mediterranean. This mode consists of two phases. The west phase corresponds to a maximum temperature over the coast of Morocco–Mauritania, propagating southwestward, and a minimum between Libya and Sicily, propagating southeastward. The east phase corresponds to the opposite temperature structure, which propagates as in the west phase. A lag-composite analysis revealed that this SHL mode was preceded by large-scale, midlatitude, intraseasonal fluctuations of the atmosphere. The southward penetration of a Rossby wave disturbance over Europe and North Africa generates modulations of the three-dimensional atmospheric structure. The low-level ventilations and harmattan-like circulation are particularly impacted, as are the subtropical westerlies and the polar jets in the upper troposphere. The west phase is concomitant with an enhanced convective signal over the Darfur region, which propagates westward, as far as the middle of the Atlantic, at a speed similar to that of the well-known African easterly waves.

The SHL appears to be a bridge between the midlatitudes and the West African monsoon, which may offer promising sources of predictability over the Sahel on an intraseasonal time scale.

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Jean-Philippe Lafore and Mitchell W. Moncrieff

Abstract

A set of 13 two-dimensional numerical simulations based on the 22 and 23 June Soundings from the ConvectionProfonde Tropicale in 1981 (COPT81) experiment in West Africa is used to study the organization and interactionof the convective and stratiform regions of squall-line-type convective systems. The initial wind profiles arecharacterized by the African easterly jet (AEJ) and the tropical easterly jet (TEJ) located at about 3.5 km and14 km, respectively.

The physical processes that generate and maintain the mesoscale inflow at the rear of squall-line-type mesoscaleconvective systems are thereby examined. Horizontal potential temperature gradients generated by a combinationof latent heat release in the convective region and unsaturated mesoscale descent, both modulated by evaporation,cause a horizontal pressure gradient and generate horizontal, line-parallel vorticity. The rear inflow is a consequenceof these processes. The convective activity induces a significant upscale influence; ahead of the system the AEJstrength is reduced and the TEJ is enhanced while in the rear the TU is reduced. The velocity perturbationbelow 4 km, associated with the rear inflow, is the most marked signature in the horizontal momentum change.The effects of ice physics are examined by using a simple parameterization and the intensity of the AEJ is variedto test its effect on the rear inflow and the longevity of the convective system.

Generally, there is an extensive rotor circulation in the cold pool and the convective region consists of a seriesof transient convective cells traveling backwards relative to the cold pool at about 10 m s1. In many of thesimulations, the inflow to the convective-scale downdraft originates ahead of the line, crosses between thetransient cells and contributes to the maintenance of the cold pool and rotor. However, a significant proportionof the cold-pool mass can originate from the midlevel Stratiform region, demonstrating that the longevity ofthe convective system is influenced by a judicious combination of convective and mesoscale processes.

The density current mechanism for maintaining the convective region of the squall line is dominant onlyafter 3-4 h of simulation, while in the initial few hours the low-level inflow advects through the cooling region.With certain wind profiles this behavior persists throughout the lifetime of the system and a wavelike, low-levelconvergence (instead of a density current) organizes the development of new cells. Later stages are typified bya transition to a system having a lowered rear inflow, decreased convective depth, intensity and slope. Thisbehavior is most pronounced for a strong AEJ.

The system-scale organization is examined by using Lagrangian conservation properties. First, passive traceranalyses quantify the relative importance of individual transports. Second, the vorticity field is analyzed byusing a nonlinear steady state conservation theorem that, despite being applied to a system containing transientconvective cells, adequately represents the persistent nature of the vorticity dynamics and demomtes thestrong interaction between the convective and stratiform regions.

It is demonstrated that the vorticity structure in the COPT81 lines is much more complex than a balancebetween the inflow shear and the vorticity generated in the vicinity of the cold pool mainly because the system-scale (convective and stratiform) baroclinic vorticity generation cannot be neglected.

Due to the form of the initial wind profiles, the simulations are directed at tropical squall lines. Nevertheless,important characteristics of midlatitude lines are also evident, such as the transient cellular convective activityand the strong rear inflow. The latter is shown to have an important effect on the convective region by enhancingthe low-level convergence and the mass in the cold pool, thereby promoting a direct scale interaction. In addition,the ambient shear modulates the structure and the vorticity dynamics organizes the entire structure. Generalphysical properties of squall lines are therefore demonstrated.

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Rémy Roca, Jean-Philippe Lafore, Catherine Piriou, and Jean-Luc Redelsperger

Abstract

This paper investigates the relationship between large-scale dynamics, water vapor, and organized convection over West Africa. Making use of a simplified condensation hypothesis, a back-trajectory model fed by NCEP-analyzed winds is used to reconstruct the midtropospheric humidity field over Africa during July to August 1992. The approach documents both the moisture content and the origin of the air masses. Meteosat satellite infrared imagery is used to characterize the convective systems.

A case study analysis reveals that very dry air patches (RH < 5%) are located in the immediate midtropospheric environment of a typical squall line. Such dry-air structures are shown to originate in the upper levels (200–250 hPa) on the anticyclonic side of the polar jet stream at 50°N. Focusing on the Sahel region, dry events are isolated using the time series of the 500-hPa relative humidity distribution during the monsoon period. These dry events are shown to be composed of extratropical air. Composite analysis of the convective activity indicator exhibits a strong positive association between dry intrusions and convection on the eastern side of the Sahelian region. Organized convective systems that are fast moving and long lasting are more likely over this region when a dry intrusion is present. This coincides with the well-established theory that midtropospheric dry air, when combined with sufficient wind shear, can maintain and intensify previously triggered deep convection through rain evaporation that feeds the cold pools, especially within squall lines. This paper suggests that the extratropical dry-air intrusions modulate the occurrence and duration of convective systems and, therefore, the mode of variability of rainfall over West Africa during the monsoon.

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