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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-Luc Redelsperger, Françoise Guichard, and Sylvain Mondon

Abstract

The paper investigates the enhancement of surface fluxes by atmospheric mesoscale motions. The authors show that horizontal wind variabilities induced by these motions (i.e., gustiness) need to be considered in the parameterization of surface fluxes used in general circulation models (GCMs), as they always occur at subgrid scale.

It is argued that there are two different sources of gustiness: deep convection and boundary layer free convection. The respective scales (time and length) and the convective patterns are very different for each of these sources. A general parameterization of the gustiness distinguishing these two effects is proposed.

For boundary layer free convection, the gustiness is related to the free convection velocity. To establish this relationship, both observations and numerical simulations are used. Revisiting the Coupled Ocean–Atmosphere Response Experiment data, the authors propose a new value of the proportionality coefficient that links the free convection velocity and the gustiness.

For deep convection, the dominant source of gustiness is the occurence of downdrafts and updrafts generated by convective cells. It is shown that these motions produce large enhancement of surface fluxes and should be parameterized in GCMs. Results indicate that the gustiness can be related either to the precipitation or to the updraft and downdraft mass fluxes.

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Sophie Belamari, Jean-Luc Redelsperger, and Marc Pontaud

Abstract

Dynamical impacts of a strong westerly wind burst (WWB) are studied using an ocean–atmosphere coupled simulation in which an intense westerly wind burst is introduced. The ocean response includes local and remote components with the development of a zonal surface jet and the appearance of positive sea surface temperature anomalies in the eastern Pacific. As observed during the triggering of the 1997/98 El Niño, remote warming results from the eastward propagation of a subsurface temperature anomaly generated in the western Pacific. A detailed heat budget helps to describe and understand the major mechanisms taking part to the formation of the subsurface temperature anomaly in the western Pacific, contributing to its eastward propagation and leading to the surface warming simulated in the eastern Pacific. Advective contributions are proved to be widely predominant in the heat budget. Further decomposition of advective terms into linear and nonlinear parts allows insights into the role of mean state and perturbation dynamics in the formation and evolution of the subsurface temperature anomaly.

Vertical advection is shown to have a major role from the formation of the subsurface temperature anomaly in the western Pacific to its rise in the eastern Pacific: (i) linear vertical advection of the mean temperature by the Kelvin wave perturbation is the main process explaining the formation of a temperature anomaly in the western Pacific and its eastward propagation; (ii) nonlinear vertical advection acts to modify the vertical structure of the subsurface temperature anomaly; (iii) linear vertical upward advection of the temperature anomaly by the mean equatorial upwelling is shown to be the major process leading to the development of simulated surface warming in the eastern Pacific.

Zonal advection is a second-order contribution but takes part in the propagation and evolution of the thermal pattern through linear and nonlinear processes. Meridional advection is a third-order contribution, only significant during the onset phase through a nonlinear contribution due to a WWB-induced Gill-type oceanic recirculation characterized by surface convergence and subsurface divergence.

It is suggested that the previously mentioned mechanisms could be implied in the evolution of “subsurface El Niño events” in contrast with “surface El Niño events” on which numerous conceptual models, such as the delayed oscillator or the advective–reflective model, as well as theoretical unstable coupled modes have mainly focused.

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Jean-Luc Redelsperger and Terry L. Clark

Abstract

Two- and three-dimensional numerical simulations were performed to investigate the scale selection and initiation of both moist and dry convection over gentle western and gentle eastern slopes where the latter represents an idealization of the eastern Colorado region of the Great Plains of North America. This work extends earlier studies of thermally forced convection by considering a model framework that is large enough to resolve both the convective scale dynamics as well as the larger scale dynamics within which the convection is embedded. As a result, the scale interaction problem leading to the selection of the dominant deep modes of the troposphere and consequent convection initiation is more realistically treated. The main physical mechanisms involved in the initiation of convection in these studies are the usual boundary-layer instabilities leading to the development of eddies and/or shear-aligned rolls, the excitation of gravity waves by the boundary-layer motions interacting with the free atmosphere, and the eventual development of coherent vertical structures that link the boundary layer motions and the overlying gravity waves into larger horizontally spaced modes than typically obtained from an isolated boundary layer.

It has previously been shown that the mean wind shear spanning the region between the top of the boundary layer and the overlying stable layer plays an important role in producing energetic deep modes in the presence of thermal forcing. In the present simulation this shear results from a combination of initial baroclinicity associated with the westerlies and production by the differential thermal gradients formed by heating gently sloping terrain. Westerly geostrophic shears of either 3 or 5 m s−1 km−1 over the first 5.5 km above sea level were used as initial conditions. A balance is maintained between shear production through large-scale forcing and shear destruction through boundary-layer mixing that results in significant shear. The experiments showed a broad range of responses as a consequence of the horizontal variability of the shear structures. The preferred region of both dry and moist convection was found to be the eastern slope where the terrain effects result in an enhancement of the low-level shear. In response to the directional structure of the shear spanning the boundary layer and free atmosphere both a banded and a less coherent scattered organization were obtained for the waves and clouds.

Dominant deep modes were found to organize and initiate moist convection. West–east horizontal scales of the deep modes in the dry experiments were found to range from about 11 to 28 km with either a banded or a cellular structure with scales between 4 to 6 km in the south–north direction. The timing of the onset of the moist convection appeared to affect the final horizontal-scale selection in the moist experiments. The moist convection appears to lock onto the scales of the dry modes that initiate the convection for these particular experiments. The largest horizontal scales of dominant modes in the dry experiments were about 28 km and developed rather slowly as compared with the 11 km scale dominant modes. These largest horizontal scales did not develop in the moist experiments where clouds appeared early but did develop in those moist experiments where moist convection took longer to develop.

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Jean-Philippe Lafore, Jean-Luc Redelsperger, and Geneviève Jaubert

Abstract

Results from a detailed three-dimensional cloud model are extensively compared with Doppler radar data in the case of a fast-moving tropical squall line, observed during the COPT81 experiment. The comparisons use a two-dimensional statistical analysis, in which we consider each four-dimensional field as the sum of a two-dimensional averaged vertical field and a four-dimensional residual field, representative of along-line and temporal fluctuations. These fluctuations are analyzed by computing the time average of spatial variances and the temporal variance of spatial averages. The results indicate good agreement in both quality and quantity, particularly for the vertical velocity and line-normal wind. The consequences of this agreement are two-fold. First, a physical and comprehensive model of the convective part of a tropical squall line emerges that is coherent with both observations and simulations. This conceptual model emphasizes the importance of along-line fluctuations. Second, it is possible to use the simulation results to compute thermodynamic budgets and vertical transports of mass and momentum.

Both observations and simulations confirm that, for the convective part, the vertical transport of line-normal momentum is predominant relative to the along-line one. The line-normal transport cannot be either neglected or simply explained in terms of diffusion. The mean scale lifting and rear-to-front mean circulation give the mean structure of the momentum flux vertical profile. However, the transport due to convective eddies is important and serves to extend vertically the total transport. Except in the first kilometer, the effect of total advection is a loss of momentum at midlevels and a gain aloft. The pressure gradient across the convective part acts in opposition to the advection but is insufficient to neutralize it and to reach, at this scale, a steady state in the squall line's moving frame. For both observations and simulation, that results in a weak evolution of the convective part's structure.

The latent heating provides a good approximation of the apparent heating source; nevertheless, computation of this latent heating should include rain evaporation. The computation of the apparent moisture sink necessitates that of its convective eddy transport. The transport due to convective eddies decouples the vertical distributions of heat and moisture. Normalization by the rainfall rate allowed us to compare successfully the simulated apparent heating source and moisture sink with previous diagnostic studies with larger time and space scales. We thus confirmed the double-peak structure of apparent humidity sink Q 2 that is often seen in tropical budget composite studies. However, we propose a different interpretation through the vertical transport of Q 2 by convective eddies.

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Thibaut Montmerle, Jean-Philippe Lafore, and Jean-Luc Redelsperger

Abstract

Results from a three-dimensional cloud model are extensively compared with airborne Doppler radar data in the case of a tropical oceanic squall line observed during the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment. The comparison is based on the precipitation patterns, the dynamical and thermodynamical distributions, and the vertical transport of horizontal momentum.

The model simulates the evolution of the mesoscale convective system (MCS) frontal convective line from a quasi-linear to a broken pattern. The area located south of the “break,” which designates the region where the MCS leading edge reorientates from the N–S to the E–W direction, is composed of a pronounced bow-shaped structure with two vortices located on both sides of a strong rear inflow.

The vertical circulation is characterized by a jump updraft and an overturning downdraft. Both structures exhibit a vertical, intense updraft in the break zone, whereas the jump updraft is more sloped and less intense in the bow region. Front-to-rear momentum is injected mainly by the jump updraft. Both observations and simulation indicate the major role played by convective eddies in the vertical transport of cross-line and parallel-line horizontal momentum.

A synthesis summarizes the complex three-dimensional structure of the simulated system, based on three salient features and their relative locations: the deep convection region, the leading edge of the cold pool, and the melting area. The relative positions between the two last mentioned explains the observed asymmetric structure and the existence of more upright and narrow updrafts in the northern part of the system. Numerical experiments suggest that the wind profile at midlevel is mainly responsible for the location of the melting area relative to the cold pool. The system tends to generate new convective elements organized along the direction that reduces the angle between the convective line and the midlevel shear vector.

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

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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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Jean-Marcel Piriou, Jean-Luc Redelsperger, Jean-François Geleyn, Jean-Philippe Lafore, and Françoise Guichard

Abstract

An approach for convective parameterization is presented here, in which grid-scale budget equations of parameterization use separate microphysics and transport terms. This separation is used both as a way to introduce into the parameterization a more explicit causal link between all involved processes and as a vehicle for an easier representation of the memory of convective cells. The equations of parameterization become closer to those of convection-resolving models [cloud-system-resolving models (CSRMs) and large-eddy simulations (LESs)], facilitating parameterization development and validation processes versus a detailed budget of these high-resolution models.

The new Microphysics and Transport Convective Scheme (MTCS) equations are presented and discussed. A first version of a convective scheme based on these equations is tested within a single-column framework. The results obtained with the new scheme are close to those of traditional ones in very moist convective cases [like the Global Atmospheric Research Programme (GARP) Atlantic Tropical Experiment (GATE) Phase III, 1974]. The simulation of more difficult drier situations [European Cloud Systems Study/Global Energy and Water Cycle Experiment (GEWEX) Cloud System Studies (EUROCS/GCSS)] is improved through more memory due to higher sensitivity of simulated convection to dry midtropospheric layers; a prognostic relation between cloudy entrainment and precipitation evaporation dramatically improves the prediction of the phase lag of the convective diurnal cycle over land with respect to surface heat forcing.

The present proposal contains both a relatively general equation set, which can deal continuously with dry, moist, and deep precipitating convection, and separate—and still crude—explicit moist microphysics. In the future, when increasing the complexity of microphysical computations, such an approach may help to unify dry, moist, and deep precipitating convection inside a single parameterization, as well as facilitate global climate model (GCM) and limited-area model (LAM) parameterizations in sharing the same formulation of explicit microphysics with CSRMs.

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Jun-Ichi Yano, Peter Bechtold, and Jean-Luc Redelsperger

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Physical processes in numerical modeling are currently handled by a dichotomy of either an explicit or a parameterization approach. Herein, an alternative approach is proposed, in which degrading explicit physics with decreasing resolutions are compensated by a “renormalization.” More specifically, a “renormalization” factor depending on the model resolution is multiplied on explicit evaluations so that the subgrid-scale contributions to a given grid scale are approximately recovered without a parameterization. The approach is analogous to the renormalization approach in statistical physics, but without rigorously relying on its mathematical basis. For this reason, this name is evoked with a quotation.

In order to demonstrate this idea, the domain-mean vertical fluxes of heat, moisture, and momentum from cloud-resolving model experiments, corresponding to the grid-box averages in the large-scale modeling, are examined. In order to mimic the effects of degrading horizontal resolution, data are filtered in wavelet space. The “renormalization” factors that recover the full vertical fluxes are found to be relatively stable with time, and the associated errors by “renormalization” are overall less than the order of the vertical variance of the fluxes, indicating a potential usefulness of this approach. An analogous approach is found to work more effectively using data compression by wavelets.

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