Search Results

You are looking at 1 - 10 of 13 items for

  • Author or Editor: J-L. Redelsperger x
  • Refine by Access: All Content x
Clear All Modify Search
J. L. Redelsperger
and
G. Sommeria

Abstract

This article presents the main features of a three-dimensional model for deep convection developed with special care given to the formulation of subgrid turbulent processes. It explicitly simulates the dynamics of turbulent eddies, including condensation and precipitation processes. Second-order moments are expressed as a function of the grid-averaged field of variables and of a prognostic turbulent kinetic energy. The formulation includes a simple statistical treatment of subgrid condensation and subgrid conversion of cloud water into rain water. The coherence and relative importance of the various closure hypotheses are tested in an idealized case of precipitating cloud.

Results indicate the extent that features of the computed field are dependent on hypotheses used in the turbulence closure, choice of the basic turbulent variables, and formulation of the second-order moments. Significant benefits are obtained from the use of variables that are conserved in the condensation process. The computation of grid-scale condensation and precipitation is mostly dependent on the hypotheses made respectively for subgrid condensation and precipitation. Finally, it is shown that an advanced subgrid turbulence parameterization can partially compensate for the effects of a low spatial resolution.

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

Abstract

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.

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

Abstract

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.

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

Abstract

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.

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

Abstract

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.

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

Abstract

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.

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

Abstract

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.

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

Abstract

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.

Full access
C. Lac
,
J-P. Lafore
, and
J-L. Redelsperger

Abstract

The role of gravity waves in the initiation of convection over oceanic regions during the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) experiment is investigated. First, an autocorrelation method is applied to infrared temperature observations of convective events from satellite images. It reveals that new deep convective cells often occur a few hours after a previous intense event at a typical distance of a few hundred kilometers. Such fast moving modes (faster than 15 m s−1) are interpreted as the trace of gravity waves excited by previous convection and contributing to trigger further convection.

Second, the specific case of 11–12 December 1992, during which an active squall line is generated after the collapse of a previous mesoscale convective system (MCS) nearby, is analyzed with a nonhydrostatic model. The triggering of the second MCS is well reproduced explicitly, owing to the use of the two-way interactive grid nesting. The convective source appears to emit pulses of gravity waves on a wide range of small scales. On the contrary, the troposphere response to the convective source exhibits a spectral simplicity. A slowly evolving mode, characterized by an ascent in the PBL and a compensating subsidence in the free troposphere, favors shallow convection and inhibits deep convection, respectively. Traveling modes propagating away from the convective source are characterized by a fast mode (∼50 m s−1) and a slower mode (∼25 m s−1) associated with the convective and stratiform development of the source, respectively, in agreement with previous studies. A budget analysis reveals the different factors leading to the deep convection triggering. First, an active PBL characterized by strong surface fluxes and mean ascent, initiates shallow convection, lasting about 2 h, inhibited above by the subsiding motions maintaining a dry layer. Second, horizontal advection of moist and cooler air at midlevels, and detrainment from cumuli, contribute to destroy the dry air layer capping the shallow convective layer. Finally, vertical advection induced by the gravity waves passage modulates the vapor and temperature evolution. Ascending phases favor moistening and cooling, whereas subsiding phases stop these effects, delaying the deep convection onset. The triggering occurs after a strong subsidence, when the ascent phases of deep and shallow modes are combined.

Full access
D. Paradis
,
J-P. Lafore
,
J-L. Redelsperger
, and
V. Balaji

Abstract

A linearized version of a nonhydrostatic model is used to study the normal-mode selection and the structure of the African easterly waves in dry and moist environments associated with an idealized African easterly jet structure. The dry mode reproduces the principal observed characteristics of the lower troposphere, in particular the low-level ascent and convergence in the trough. A simple CISK-type parameterization improves the upper-level circulation. A budget of kinetic energy shows barotropic and baroclinic contributions and adequately reproduces the analysis of Norquist et al. The scale selection and the structure of the most unstable mode appears sensitive to the jet structure, in particular to the meridional and vertical shear. Several discrepancies between observations and these linear modes emphasize the importance of an accurate description of convection.

Full access