AROME‐France is a convective‐scale numerical weather prediction system which has been running operationally at Météo‐France since the end of 2008. In order to determine its initial conditions, it ...uses a 3D‐Var assimilation scheme at the same resolution as the model in a continuous data assimilation cycle. In addition to conventional and satellite observations used in global data assimilation systems, dedicated observations for the mesoscale such as surface observations and radar measurements (radial winds and reflectivities) are assimilated. A major update of this system occurred in April 2015 with, among several improvements, (i) an increase of both horizontal and vertical resolutions (1.3 km and 90 vertical levels versus 2.5 km and 60 levels), and (ii) the reduction of the period of the data assimilation cycle from 3 to 1 h (as a result of the model spin‐up reduction and the tuning of the background‐error covariances). This study presents the preparatory work to these modifications and explores the main impact expected on convective activity forecasts. (i) appears to result in more realistic convective cells and better rainfall and wind gust scores and (ii) allows assimilation of more observations with information at the mesoscale which provides more accurate initial conditions and hence better subsequent rainfall forecasts. The benefits of using both (i) and (ii) in a pre‐operational configuration are shown using objective precipitation scores and illustrated by a case‐study.
Major changes in the French high resolution operational model improve the simulation of convective activity. Precipitation rates (mm/h) simulated with the new configuration (middle) are closer to the radar observations (left) than previously (right). This article describes the preparatory work leading to the two main modifications: an increase of both horizontal and vertical resolutions and the reduction of the period of the data assimilation cycle from 3 to 1 h.
A challenge for cloud-resolving models is to make subgrid schemes suitable for deep convective clouds. A benchmark large-eddy simulation (LES) was conducted on a deep convective cloud with 50-m grid ...spacing. The reference turbulence fields for horizontal grid spacings of 500 m, 1 km, and 2 km were deduced by coarse graining the 50-m LES outputs, allowing subgrid fields to be characterized. The highest values of reference subgrid turbulent kinetic energy (TKE) were localized in the updraft core, and the production of subgrid TKE was dominated by thermal effects at coarser resolution (2 and 1 km) and by dynamical effects at finer resolution than 500 m. Countergradient areas due to nonlocal mixing appeared on the subgrid vertical thermodynamical fluxes in the updraft core and near the cloud top. The subgrid dynamical variances were anisotropic but the difference between vertical and horizontal variances diminished with increasing resolution. Then offline and online evaluations were conducted for this deep convective case with two different parameterization approaches at kilometer-scale resolution and gave the same results. A commonly used eddy-diffusivity turbulence scheme underestimated the thermal production of subgrid TKE and did not enable the countergradient structures to be reproduced. In contrast, the approach proposed by Moeng, parameterizing the subgrid vertical thermodynamical fluxes in terms of horizontal gradients of resolved variables, reproduced these characteristics and limited the overestimation of vertical velocity.
Abstract
A giga-large-eddy simulation of a cumulus congestus has been performed with a 5-m resolution to examine the fine-scale dynamics and mixing on its edges. At 5-m resolution, the dynamical ...production of subgrid turbulence clearly dominates over the thermal production, whereas the situation is reversed for resolved turbulence, the tipping point occurring near the 250-m scale. For cloud dynamics, the toroidal circulation already obtained in previous observational and numerical studies remains, with a strong signature on the resolved turbulent fluxes, the most important feature for the exchanges between the cloud and its environment even though numerous smaller eddies are also well resolved. The environment compensates for the upward mass flux through a large-scale compensating subsidence and the so-called subsiding shell composed of cloud-edge downdrafts, both having a significant contribution. A partition is used to characterize the dynamics, buoyancy, and turbulence of the inner and outer edges of the cloud, the cloud interior, and the far environment. On the edges of the cloud, downdrafts caused by the eddies and by evaporative cooling effects coexist with a buoyancy reversal while the cloud interior is mostly rising and positively buoyant. An alternative simulation in which evaporative cooling is suppressed indicates that this process reinforces the downdrafts near the edges of the cloud and causes a general decrease of the convective circulation. Evaporative cooling also has an impact on the buoyancy reversal and on the fate of the engulfed air inside the cloud.
This paper presents results of numerical experiments on five precipitating events: two cases of convective systems over the northern French plains that induced localized flooding, two cases of ...quasi-stationary systems over the southern French mountainous areas with recorded precipitation above 100 mm in less than 3-4 h, and the extreme flooding case of 12-13 November 1999 with daily precipitation having reached 550 mm, which led to more than 30 deaths. The ability of a high-resolution (2.5 km) model to forecast the quantitative precipitation associated with these five events is evaluated. Both qualitative evaluations that compare observed and modeled reflectivities or surface precipitation, and quantitative evaluations based on classical scores are performed to assess the quality of the numerical experiments. Starting from the same analysis as the operational models, the high-resolution model improves the results for one of the cases, the extreme flash flood case. For the four other cases, higher resolution and more advanced physics than those currently used in operational models do not improve the results. In order to verify that the failures of the high-resolution simulations are due to the initial conditions, the initial state of the simulations are modified for these four cases by using more mesoscale observations, such as mesonet surface observations, radar reflectivities, and Meteosat data. With these modified initial conditions, the high-resolution model produces more realistic precipitation fields. The description of the humidity field in the initial state is crucial for the forecast of the convective systems. Information, such as the presence of a mid- to upper-tropospheric saturated area associated with the developing convective clouds, or nearly saturated low levels, is essential.
Convection‐permitting limited‐area models based on the same spectral semi‐implicit semi‐Lagrangian (SL) techniques which are used in the ECMWF global model, are run operationally in several countries ...of the ALADIN/HIRLAM consortium. Forecasters have reported a general tendency for these models to produce overestimated precipitation and unrealistic divergent winds at the edges of the cold outflows generated by the precipitation evaporation in the vicinity of convective clouds. These grid‐point storms have been associated with a spurious behaviour of the pointwise interpolation used in the SL scheme, where grid‐scale buoyant updraughts create strong small‐scale convergence near the surface. A modification of the interpolation weights in the SL transport scheme introduces the concept of cell‐averaging into the traditional pointwise SL scheme which improves the conservation property of the scheme and eliminates the spurious mode. The COMAD (COntinuous Mapping about Departure points) correction applied to the standard interpolation weights takes into account the deformation of the air parcels along each direction of interpolation in order to improve the continuity and the conservative property of the re‐mapping between the model grid points and the origin points of the backward trajectories. The method has been validated with the small planet configuration of the Integrated Forecast System at ECMWF and with the limited‐area version of the same dynamics used for the AROME (Météo‐France) and HARMONIE (HIRLAM) models. The pathological behaviour of grid‐scale buoyant flows permitted by these dynamical cores is corrected by the COMAD interpolations. The precipitation forecasts in the convection‐permitting models AROME/HARMONIE which show an overestimation of intense convective precipitation are systematically improved by the new weights.
A climatological approach is developed to characterize the mesoscale environment in which heavily precipitating events (HPEs) grow over a mountainous Mediterranean area. This climatology that is ...based on three-dimensional variational data assimilation (3D-Var) mesoscale analyses is performed for a 5-yr period, considering cases with daily precipitation of >150 mm occurring over southern France during autumn. Different diagnostics are used to document the time evolution of mesoscale features associated with the HPEs for initiation, mature, and dissipation stages. To underline differences according to the location of precipitation, four subdomains are also considered: Languedoc-Roussillon, Cévennes-Vivarais, South Alps, and Corsica. Composite analyses show that these events are driven by some common features (slowly evolving trough–ridge pattern and diffluent midlevel flow). Instability and moisture are transported by the low-level jet (LLJ) toward the target area from their sources, which are located upstream over the Mediterranean Sea. Strong moisture convergence is located within the left exit of the LLJ. These parameters reach a maximum during the mature stage. During the life cycle of the HPEs, the low-level winds rotate clockwise. Composite analyses also show that the synoptic and mesoscale patterns can differ greatly as a function of the location of the precipitation. Indeed, the LLJ varies from southeasterly to southwesterly. The midlevel flow varies from southerly to southwesterly. The areas of high moisture and instability are stretched in different orientations. Long-lasting events are associated with a more pronounced quasi-stationary trough–ridge pattern, higher values of CAPE, a wetter troposphere, and faster LLJ. The most-heavily precipitating events are found to be in general associated with higher values of these parameters or with a low-level inflow that is closer to perpendicular to the relief.
The representation of deep convective clouds by convection‐permitting models could be improved by parametrizing the subgrid turbulent fluxes better. Following the work of Verrelle et al., a ...large‐eddy simulation (LES) of a population of convective clouds at 50‐m grid spacing was explored during the cloud life cycle to characterize the second‐order moment turbulent fluxes. The reference turbulence fields were deduced by coarse‐graining the LES outputs at horizontal grid resolutions of 500 m and 1 and 2 km. The ability of three parametrizations—the traditional K‐gradient model of Cuxart et al. (CBR), the Smagorinsky formulation, and the Moeng approach (MOENG) based on the horizontal gradients of the resolved variables—to reproduce the thermodynamical and dynamical fluxes was assessed in an offline configuration via a comparison with the reference fields. MOENG was the most appropriate scheme to represent the vertical and horizontal heat fluxes at all grid spacings in the clouds and their environment over the entire cloud life cycle, including the appropriate representation of countergradient areas. It also represented the vertical and horizontal moisture fluxes at 1‐km and 500‐m grid spacings best, while its advantage was reduced at a grid spacing of 2 km, as CBR performed slightly better for the statistical scores. MOENG was also able to represent the dynamical covariances well at grid spacings of 1 km and 500 m, even though the statistical scores were not as good as those obtained for the thermodynamical fluxes. The dynamical variances were well represented by CBR; however, for this offline evaluation, the subgrid turbulent kinetic energy, present in CBR formulations, is computed directly from the LES outputs, giving CBR an advantage over the two other diagnosed parametrizations. Also, the reference fluxes revealed an anisotropic deformation of turbulence throughout the troposphere, which was only captured by MOENG.
The ability of three parametrizations to reproduce the second‐order moment turbulent fluxes is assessed via a comparison with the reference fields of a LES of convective clouds at 50‐m grid spacing. The PDFs of heat flux (K m s−1) at the 500‐m coarse‐grained grid spacing show a better agreement with the reference fluxes (a) for the Moeng approach (b) based on the horizontal gradients of the resolved variables than the traditional K‐gradient formulations of Cuxart et al. (c) and Smagorinsky (d).
Processes leading to the formation of strong surface wind gusts within an idealized sting‐jet extratropical cyclone are investigated using a high‐resolution mesoscale model. It is motivated by real ...case studies that have shown that damaging surface winds ahead of the bent‐back warm front of an extratropical cyclone are often due to the presence of a sting jet, which is a low‐level mesoscale jet, the air masses of which descend from the cloud head to the top of the boundary layer. Different numerical simulations show that surface winds below the leading edge of the sting jet increase with increased horizontal resolution and surface roughness. For typical land‐surface roughness, the intensity of near‐surface wind gusts increases rapidly with horizontal resolution, while the sting‐jet intensity above the boundary layer does not vary with resolution. A focus on the 1‐km grid‐spacing simulation with land‐surface roughness is then made. It shows stronger surface winds ahead of the bent‐back warm front than near the cold conveyor‐belt jet. It also exhibits multiple bands of strong surface wind speed, similar to real sting‐jet cyclones. These multiple bands are closely linked with multiple resolved convective rolls in the boundary layer, the descending branches of which are responsible for the downward transfer of momentum. Sensitivity experiments and a stability analysis show that cooling due to sublimation and melting of precipitating ice hydrometeors below the leading edge of the sting jet triggers and invigorates boundary‐layer convective rolls by reducing the buoyancy of air masses near the precipitation base and below. Closer to the surface, the transfer of momentum is predominantly taken over by subgrid‐scale turbulent fluxes.
Idealized simulations of a high‐resolution mesoscale model are performed to investigate boundary‐layer processes that transfer momentum from the sting jet down to the surface. Boundary‐layer convective rolls below the leading edge of the sting jet appear in the 1‐km grid‐spacing simulation with typical land‐surface roughness and lead to multiple bands of wind speed maxima near the surface. Convective rolls are formed in regions of reduced static stability which is largely due to the loss of buoyancy via evaporative cooling of precipitation.
A turbulence event arising in a jet exit region above the Belgium–Luxembourg area, determined from airliner in‐situ measurements, is reproduced using the meteorological models Application de la ...Recherche à l'Opérationnel à Méso‐Echelle (AROME) and Meso‐NH at horizontal resolutions of 1.3 km and 260 m. The behaviour of the subgrid turbulence scheme at 1.3 km and its sensitivity to various parameters are analysed, with results being evaluated using measurements. An increase of the vertical resolution around the tropopause levels with Δz≤300$$ \Delta z\le 300 $$ m is shown to greatly enhance the turbulence representation. The use of a non‐local formulation of the mixing length in the current parametrization at 1.3 km allows reproduction of a turbulence signal in agreement with the observations. On the contrary, the use of a fully three‐dimensional formulation has no impact on the simulation at this resolution (1.3 km). Using the 260 m runs, this turbulence event is linked to hydrodynamical wind shear instabilities characterized by horizontal wavelength of 4.5 km, sub‐resolved at the operational resolution. At these small grid‐size scales, turbulence evolution and equation budgets reflect an equilibrium between dynamical production and turbulence dissipation, and highlight the importance of horizontal gradients. Subgrid turbulence intensities are assessed to be underestimated by the current parametrization at 1.3 km when compared with this high‐resolution reference simulation. Finally, different tests on the turbulence parametrization illustrate a transfer between resolved and subgrid kinetic energy in the model. This transfer stresses the importance of a trade‐off between mixing intensity and the representation of wind at resolved scales for the upper troposphere.
A turbulence event arising in a jet diffluence zone above the Belgium–Luxembourg region, determined from airliner in‐situ measurements, is reproduced using the meteorological models Application de la Recherche à l'Opérationnel à Méso‐Echelle (AROME) and Meso‐NH at horizontal resolutions of 1.3 km and 260 m. The behaviour of the subgrid turbulence scheme at 1.3 km and its sensitivity to various parameters is analysed, with results being evaluated against measurements. Using the 260 m runs, this event is linked to hydrodynamical wind shear instabilities with horizontal wavelength of 4.5 km, sub‐resolved at the operational resolution.
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