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R. Roehrig, F. Chauvin, and J.-P. Lafore


The understanding and forecasting of persistent dry or wet periods of the West African monsoon (WAM), especially those that occur at the intraseasonal time scale, are crucial to improve food management and disaster mitigation in the Sahel region. In the present study, the authors assess how the 10–25-day intraseasonal variability of convection over the Sahel is related to the recently documented intraseasonal variability of the Saharan heat low (SHL) and the associated extratropical circulation. Strongest and most frequent interactions occur when the SHL intraseasonal fluctuations lead those of convection over the Sahel with a 5-day lag. Using a nonlinear event-based approach, such a combination is shown to concern about one-third of Sahelian dry and wet spells and, in the case of dry spells, to yield convective anomalies that are stronger, last longer by at least 2 days, and reach a larger spatial scale. It is then argued that the 10–25-day intraseasonal variability of convection over the Sahel can be partly explained by the midlatitude intraseasonal variability, through a major role played by the SHL. The anomalous midlevel circulations observed during Sahelian wet and dry events can be shifted from the midlatitudes, which provide a complementary mechanism to that invoking equatorial Rossby wave dynamics. These two mechanisms are likely to interfere together in a constructive or destructive way, leading to high temporal and spatial variability of the Sahelian dry and wet spells.

As a particular intraseasonal event, the WAM onset is shown to be clearly favored by phases of the SHL intraseasonal variability, when the Mediterranean ventilation is weakened and the SHL is able to strengthen. Conversely, the formation of a strong cold air surge over Libya and Egypt and its propagation toward the Sahel lead to the collapse of the SHL, which inhibits the WAM onset. From these extratropical–tropical interactions, more skillful forecasts of the Sahelian wet and dry spells and of the WAM onset can be expected. In particular, the monitoring of both the SHL intraseasonal activity and that of the equatorial Rossby wave should provide relevant information to forecast at least two-thirds of such high-impact events.

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P. Bénard, J-L. Redelsperger, and J-P. Lafore


A series of experiments using a two-dimensional, nonhydrostatic, numerical cloud model with fine horizontal and vertical resolution is performed with the Hoskins-Bretherton solution to the Eady problem as initial condition. Dry and wet simulations are presented with 5-, 10-, and 40-km horizontal resolutions and vertical resolution from 160 m at the ground to 330 m at the domain top. Sensitivity experiments on the initial Brunt-Väisälä frequency and vertical shear are also discussed.

Two classes of narrow bands are identified: 1) A narrow cold-frontal rainband at the surface cold front, consisting of a line of shallow convection triggered by the frictionally induced instability in the boundary layer at the surface front. The associated precipitation is organized in a narrow line with a large rainfall rate. Latent heating due to condensation contributes in large part to the tilting of isentropes and to the increasing of the vertical jet strength. Sensitivity experiments show that both friction and condensation processes are important to simulate this jet. 2) Narrow free-atmosphere rainbands above the narrow cold-frontal band. A succession of updrafts and downdrafts are generated in the stable free atmosphere above the narrow cold-frontal rainband along the frontal surface. Weak precipitation is associated with these bands. The conditions for conditional convective instability and conditional symmetric instability are not met. Detailed analyses show that the linear theory of stationary and hydrostatic gravity waves gives a reasonable explanation of these bands. Simulations with different horizontal resolutions indicate that the horizontal wavelength is related to the width of the vertical jet.

Two classes of wide rainbands are also obtained in particular regions of the frontal system. 1) Wide cold-frontal rainbands consisting of bands periodic in the frontal zone, with a 75–100 km scale and a lifetime of 6–9 hours. 2) A single warm-sector wide rainband, located in the warm sector 300–400 km ahead of the surface cold front. The width of the vertical ascent varies during simulation time from 80 km up to 250 km. This band leads to widespread precipitation on an 80–120-km scale. These are discussed in detail in Part II.

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P. Jabouille, J. L. Redelsperger, and J. P. Lafore


The mesoscale variability of surface heat fluxes induced by atmospheric convection is studied by using 3D cloud explicit simulations and surface observations. Two convective cases observed during the Coupled Ocean–Atmosphere Response Experiment are simulated (26 November 1992 and 17 February 1993) corresponding to different ambient surface wind conditions, namely, light and moderate winds.

Numerical results in the first case are successfully compared to surface observations. Local enhancements of two times for the latent heat flux and three times more for the sensible one are produced in the rainy area. Intense wind gusts generated by convective outflow are found mainly responsible for these increases. For the second case, the simulated surface fluxes are found to vary greatly, although they are structured in response to an organized convective system.

At the domain scale (90 km × 90 km) corresponding to a general circulation model (GCM) grid box, it is shown that convective activity significantly enhances the averaged surface heat fluxes. This effect is important since the preconvective wind is weak. To compute these surface fluxes with a bulk formula using fields defined on the domain scale, special attention must be given to the determination of the mean wind speed.

In GCMs, gusts generated by downdrafts are subgrid scale and are hence unresolved. This study suggests that flux enhancement due to clouds may be parameterized in GCMs by extending to deep convection the gustiness correction previously proposed for free convection by other authors. Analysis of both model simulations and observed time series suggest that once convection increases above a rather small threshold value, gustiness saturates at about 3 m s−1, whereas surface air humidity varies only slightly. These are the major elements of a proposed new parameterization of evaporation from the tropical ocean.

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J. P. Chalon, G. Jaubert, J. P. Lafore, and F. Roux


Durirg the night of 23/24 June 1981, new Korhogo, Ivory Coast, a squall line passed over the instrumented area of the COPT 81 experiment. Observations were obtained with a dual-Doppler radar system, a sounding station and 22 automatic meteorological surface stations. Data from these instruments and from satellite pictures were analyzed to depict the kinematic and thermodynamic structure of the squall line. Composite analysis techniques were used to obtain a vertical cross section of the reflectivity structure and of the wind field relative to the line. The redistributions of air, moisture and thermodynamic energy by the convection wet calculated through averaged two-dimensional wind fields from a dual-Doppler radar system. The method also allowed the evaluation of the exchanges that were occurring between the convective and the stratiform regions.

This squall line had many similarities with tropical squall lines previously described by others. The leading convective part, composed of intense updrafts and downdrafts, and the trailing part, containing weak mesoscale updraft and downdraft, were separated by a reflectivity trough. A notable feature of this line was the presence of a leading anvil induced by intense easterly environmental winds in the upper troposphere. Observations of the evolution of the system at different scales indicated that the mesoalpha-scale (following the classification of Orlanski) and the mosobeta-scale patterns combined to allow the system to have optimum conditions for maximum strength and a maximum lifetime.

A rear-to-front flow was found at midlevels in the stratiform region. The flow sloped downward to the surface and took on the characteristics of a density current in the forward half of the squall lice. Entering the convective region, this flow was supplied with cold air by the convective downdrafts and played an important role in forcing upward the less dense monsoon flow entering at the leading edge.

Calculations of mass, moisture and energy transports showed the importance of the transfers between the convective and the stratiform regions. Particularly large quantities of condensate and energy were transferred from the convective region toward the anvils and made important contributions to the precipitation budget in the stratiform region, while large quantities of water vapor and latent heat energy were transferred from the stratiform region toward the convective region through the rear-to-front flow. Diabatic heating resulting from condensation in the convective region was also evaluated.

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P. Pires, J-L. Redelsperger, and J-P. Lafore


Equatorial wave systems and their relationships with convective activity are analyzed in the western and central Pacific regions during the Coupled Ocean–Atmosphere Response Experiment (COARE) intensive observation periods. The study uses Geostationary Meteorological Satellite infrared temperature observations and the operational European Centre for Medium-Range Weather Forecasts analyses supplemented with COARE observations.

Spectral and complex principal component analysis are applied to the data. Using the linear theory of equatorially trapped waves as a guideline, the existence of three types of waves is detected. In the 7–10-day period range, n = 1 Rossby waves are found to the east of the date line, in a region of weak convective activity. Over the western equatorial Pacific, where intense convection occurs, the 7–10-day waves do not possess the general characteristics of linear Rossby waves, but they are strongly linked to the active phases of westerly wind bursts and of convection.

Analysis of the meridional wind reveals intense mixed Rossby–gravity waves with a mean 5-day period and westward phase and eastward group velocities. Over the western Pacific, the convection is found to be strongly correlated with the antisymmetric structure of the divergent field, as predicted by the linear theory.

In the 200-hPa divergence field, n = 1 gravity waves are visible, having periods shorter than 2.5 days. They rapidly propagate (about 25 m s−1) both westward and eastward, and have strong correlations with convective clusters.

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P. Bénard, J-P. Lafore, and J-L. Redelsperger


The different processes responsible for the occurrence of wide rainbands, as obtained by high-resolution (5-km) nonhydrostatic two-dimensional simulations of frontogenesis induced by shear, with an explicit representation of the convection are discussed. The study is restricted to a case of strong friction at surface without any surface heat flux.

A budget of the moist potential vorticity (qe) has been implemented for a rigorous investigation of generating mechanisms of wide rainbands. The balance between sources, transport, and evolution of qe in the model is first successfully validated. The parameterized turbulent subgrid-scale processes represent the main qe source in these simulations, especially at the PBL top.

It is shown that friction acts as a source of intense qe vertical flux at the ground, maximum below the alongfront low-level jets in both warm and cold air masses. An intense positive qe anomaly is obtained in the warm sector, appears to be generated by frictional processes in the far prefrontal zone, and is then transported towards the frontal system. This anomaly induces an intensification of the alongfront low-level jet on its warm flank. In the present shear-driven case, this jet corresponds to a maximum of warm moist advection: the warm conveyor belt, resulting in the formation of an intense warm sector wide rainband located 300 km ahead the surface cold front, lies in a region of strong to weak moist symmetric stability.

Wide cold-frontal rainbands, on the other hand, occur in a region of moist symmetric instability, which thus seems to enhance the circulation forced by the geostrophic shearing deformation and frictional convergence in the frontal zone and favors the development of these bands. They efficiently transport the lowest qe values upward and are thus diffusive. It is suggested that these bands were initiated by the dissipation of convective cells generated during the previous convective stage.

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E-P. Zahiri, M. Gosset, J-P. Lafore, and V. Gouget


A full radar simulator, which works with the 3D output fields from a numerical mesoscale model, has been developed. This simulator uses a T-matrix code to calculate synthetic radar measurements, accounts for both backscattering and propagation effects, and includes polarimetric variables. The tool is modular to allow several options in the derivation of the synthetic radar variables. A measurement uncertainty can be taken into account on both the simulated reflectivities and the differential phase shift. A scheme can also be switched on to allow for the gate-to-gate variability of the rain drops size distribution or, also, their oblateness. This work was done in the framework of the installation in West Africa of a polarimetric X-band radar for the observation of tropical rain. Accordingly, the first objective pursued with this simulation setup is a detailed analysis of X-band polarimetric rain retrieval algorithms. Two retrieval schemes, a simple RK DP formula and a profiler that uses both reflectivity and ϕ DP, are tested. For that purpose the simulator is run on a model case study of an African squall line, then the two schemes are used to retrieve the rain rates from the synthetic radar variables and compare them to the original. The scores of the schemes are discussed and compared. The authors analyze the sensitivity of the results to the measurement uncertainty and also to several aspects of drop size distribution and drop shape variability.

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G. Caniaux, J-P. Lafore, and J-L. Redelsperger


In a companion paper, a two-dimensional simulation of a fast-moving tropical squall line was successfully compared to observations performed during the COPT81 experiment over West Africa. The full ice phase parameterization is shown to be crucial in the simulation of trailing anvil precipitation. Different diagnostic tools are applied to the simulated fields to further our understanding of the scale interactions within a squall line-type mesoscale convective system.

The pressure organization is characterized by two marked features important for explaining the inner circulation: first, a front-to-rear midlevel pressure gradient and, second. the surface pressure mesohigh extending from the gust front to the rear of the most active part of the trailing stratiform region. Based on the hydrostatic approximation, an original method of decomposition of the pressure field is proposed, whereby dynamical and buoyant contributions depend only on the horizontal and vertical, respectively. The mean pressure increase through the whole system is in part related to the horizontal momentum changes occurring in the system. Concerning the mass contribution, the midlevel system-scale pressure gradient is mainly due to the widespread rear anvil injecting a large amount of water vapor behind the system and to the adiabatic warming underneath the rear anvil.

The line-normal momentum budget in the stratiform region shows that the nidlevel pressure mesohigh, induced by the system at its rear, can prevent the progression by advection of the nidlevel front-to-rear flow coming from the convective part and can force the mesoscale ascent in the anvil and the unsaturated, warm mesoscale descent underneath. The mesoscale ascent in the stratiform part transports front-to-rear momentum to the upper troposphere, whereas the mesoscale subsidence leads to a rear-to-front momentum vertical flux underneath. Its impact at the system scale is important due to its widespread extension.

The effects of the convection on the cross-line momentum field at large scale is quantified by computing the apparent source of line-normal momentum Qu. It is not negligible and the stratiform contribution can be significant.

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J-P. Lafore, J-L. Redelsperger, C. Cailly, and E. Arbogast


The dynamical mechanisms contributing to the cross-front ageostrophic circulation are identified in high-resolution (40 to 5 km) nonhydrostatic simulations of moist frontogenesis.

In a first step, the importance of the alongfront ageostrophic circulation is assessed. The structure of the intense thermal wind imbalance (TWI) occurring in the vicinity of the surface cold front is diagnosed and explained using a budget of the alongfront vorticity η. It allows one to propose a new balance in terms of the steadiness of the η field in the system moving framework. The TWI is thus found nearly equal to the total cross-front η transport by resolved and subgrid scales. It is shown that, first, the deviation of the prefrontal air toward the front, enhanced by the surface friction and cloud diabatic processes, allows generation of a layer of positive η near at the top of the PBL. Second, the frontal lifting of this η structure is responsible for the basic structure of the TWI.

In a second step, a general form of the Sawyer–Eliassen (SE) diagnostic equation is used, including diabatic effects as well as effects of thermal wind imbalances (or “ageostrophic residue”). This latter effect is evaluated using the steadiness balance, which is confirmed by a budget diagnosis. The solution of this SE equation provides an accurate diagnostic of the causes of the secondary circulation, both qualitatively and quantitatively, down to small scales.

Finally, the SE equation is used to explain the formation and localization of rainbands in regions of effective symmetric stability. In particular, it is shown that the “ageostrophic residue” plays a crucial role on the behavior of the bands. It explains about 25% and 60% of the intensity of the warm sector-wide rainband and of the narrow cold-frontal rainband, respectively, for a case with intense surface friction.

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G. Caniaux, J-L. Redelsperger, and J-P. Lafore


A two-dimensional nonhydrostatic cloud model is applied to the simulation of a tropical squall line that occurred on 23 June during the COPT 81 experiment. Owing to the use of an ice parameterization scheme, the simulation reproduces many interesting features of the stratiform part observed with Doppler radars. In particular, this includes the dynamical, thermodynamical, and microphysical structures of the stratiform part. Different parts are clearly identified from the simulation and observations: a leading convective zone 40-km wide with large precipitation; a developed stratiform zone stretching over 150 km with moderate precipitation; between these two regions, a transition zone 20-km wide giving only light precipitation; and a forward anvil near 12 km.

The mean horizontal circulation is characterized by two mean flows: the front-to-rear flow that represents upward and rearward injection of boundary-layer air and the underlying rear-to-front flow. The simulated vertical velocity, except in the convective part, is in good agreement with observations and is characterized by a mesoscale updraft in the midtroposphere just behind the transition zone and a mesoscale downdraft under the anvil. The level of zero vertical motion, separating the mesoscale updraft from the mesoscale downdraft, has a weak slope in the horizontal as observed, and stays under the 0°C isotherm everywhere. One consequence is that the bright band is embedded in the mesoscale ascent.

Detailed thermodynamical and microphysical budgets of the stratiform region are performed and lead to the following conclusions for the present simulation: 1) the warming in the poststratiform part is a consequence of the history of the storm when the subsiding low levels were dry; 2) the cooling in the mesoscale downdraft is mainly due to rainwater evaporation; 3) this net cooling is a temporal process that occurs when the stratiform anvil produced enough precipitation to counter the adiabatic warming; 4) the cooling at the base of the anvil is not due to melting but is the consequence of upward transport of low θ.

The apparent heat source and moisture sink of the whole system, as well as those of the convective and stratiform parts, are also presented at different times and compared with previous numerical results and observations.

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