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Jean-Pierre Chaboureau and Peter Bechtold

Abstract

A simple statistical parameterization of cloud water–related variables that has been originally developed for nonprecipitating boundary layer clouds is extended for all cloud types including deep precipitating convection. Based on three-dimensional cloud resolving model (CRM) simulations of observed tropical maritime and continental midlatitude convective periods, expressions for the partial cloudiness and the cloud water content are derived, which are a function of the normalized saturation deficit Q 1. It turns out that these relations are equivalent to boundary layer cloud relations described earlier, therefore allowing for a general description of subgrid-scale clouds.

The usefulness of the cloud relations is assessed by applying them diagnostically and prognostically in a mesoscale model for a midlatitude cyclone case and a subtropical case, and comparing the simulated cloud fields to satellite observations and to reference simulations with an explicit microphysical scheme. The comparison uses a model-to-satellite approach where synthetic radiances are computed from the meteorological fields and are compared to Meteosat satellite observations both in the visible and the thermal infrared spectral channels. The impact of the statistical cloud scheme is most pronounced for shallow and deep convective cloud fields (where Q 1 < 0), provided that the host models convection parameterization is able to correctly represent the ensemble average water vapor profile in the troposphere. The scheme significantly reduces the biases in the infrared and especially shortwave spectral range with respect to the explicit microphysical scheme. Furthermore, it produces more realistic (smooth) horizontal and vertical condensate distributions in both diagnostic or prognostic applications showing the potential use of this simple parameterization in larger-scale models.

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Irene Reinares Martínez and Jean-Pierre Chaboureau

Abstract

The radiative effect of dust on precipitation and mesoscale convective systems (MCSs) is examined during a case of dust emission and transport from 9 to 14 June 2006 over northern Africa. The same method to identify and track different cloud types is applied to satellite observations and two convection-permitting simulations (with grid mesh of 2.5 km), with and without the radiative effect of dust, performed with the MesoNH model. The MCSs produce most of the observed total precipitation (66%), and the long-lived systems (lasting 6 h or more) are responsible for 55% of the total. Both simulations reproduce the observed distribution of precipitation between the cloud categories but differ due to the radiative effects of dust. The overall impacts of dust are a warming of the midtroposphere; a cooling of the near surface, primarily in the western parts of northern Africa; and a decrease in precipitation due to a too-low number of long-lived MCSs. The drop in their number is due to the stabilization of the lower atmosphere, which inhibits the triggering of convection. The long-lived MCSs are a little longer lived, faster, and more efficient in rainfall production when accounting for the dust–radiation interaction. This higher degree of organization is due to the larger convective available potential energy and an intensified African easterly jet. The latter is, in turn, a response to the variation in the meridional gradient of the temperature induced by the dust.

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Irene Reinares Martínez and Jean-Pierre Chaboureau

Abstract

Precipitating systems are analyzed during a dust event from 9 to 14 June 2006 over northern Africa. A common analysis is applied to satellite observations and two Meso-NH simulations: one convection permitting (grid spacing x = 2.5 km) and the other with parameterized convection ( x = 20 km). The precipitating systems are identified as cloud objects and classified as deep convective clouds (DCCs) or other clouds according to their infrared signature. Large DCCs [hereafter named mesoscale convective systems (MCSs)] are tracked, characterized in terms of precipitation and thermodynamic profiles, and analyzed in southern West Africa (SWA), central Africa, and Ethiopia. Precipitation is mostly observed along 0°–15°N, with 71% of the total precipitation produced by all DCCs and 55% by long-lived MCSs. It shows a marked diurnal cycle with a peak in the evening, mainly due to long-lived MCSs, which are characterized by an increase in size, zonal speed, and duration from east to west, with the largest, fastest, and longest-lived ones found over SWA. This is due to an enhanced African easterly jet (AEJ) and monsoon flow leading to stronger shear and greater conditional instability. The simulation with parameterized convection fails to distribute precipitation correctly. The convection-permitting simulation captures most of the observed precipitation features, but lacks the increase in organization of the long-lived MCSs over SWA. Excess moisture in a too zonal AEJ flow suggests that the long-lived MCSs in SWA are poorly located with respect to African easterly waves. The convection-permitting model improves the representation of precipitation but without fully resolving the long-lived MCSs.

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Nathalie Söhne, Jean-Pierre Chaboureau, and Françoise Guichard

Abstract

The 3-hourly brightness temperatures (BTs) at 10.8 μm from the Meteosat Second Generation (MSG) satellite were used to document the cloud system variability over West Africa in the summer of 2006 and to evaluate the quality of the Méso-NH model forecasts of cloud cover in the African Monsoon Multidisciplinary Analysis (AMMA) framework. Cloud systems were observed over the Guinean and Sahelian bands with more frequent occurrence and patchier structures in the afternoon. Some intraseasonal variations of the number of cloud systems were found, partly related to the intermittency of the African easterly wave (AEW) activity. Compared to the MSG observations, the Méso-NH model reproduces the overall variation of the BT at 10.8 μm well at D + 1 forecast. The model captures the BT diurnal cycle under conditions of clear-sky and high-cloud cover, but misses the lowest BT values associated with deep convection. Forecasted cloud systems are more numerous and smaller, hence patchier, than those observed. These results suggest some deficiencies in the model’s convection and cloud parameterization schemes. The use of meteorological scores further documents the skill of the model to predict cloud systems. Beyond some systematic differences between simulations and observations, analysis also suggests that the model high-cloud forecast is improved under specific synoptic forcing conditions related to AEW activity. This indicates that room exists for improving the skills of weather forecasting over West Africa.

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Luiz A. T. Machado and Jean-Pierre Chaboureau

Abstract

This study evaluates the cloud and rain cell organization in space and time as forecasted by a cloud-resolving model. The forecast fields, mainly describing mesoscale convective complexes and cold fronts, were utilized to generate synthetic satellite and radar images for comparison with Meteosat Second Generation and S-band radar observations. The comparison was made using a tracking technique that computed the size and lifetime of cloud and rain distributions and provided histograms of radiative quantities and cloud-top height. The tracking technique was innovatively applied to test the sensitivity of forecasts to the turbulence parameterization. The simulations with 1D turbulence produced too many small cloud systems and rain cells with a shorter lifetime than observed. The 3D turbulence simulations yielded size and lifetime distributions more consistent with the observations. As shown for a case study, 3D turbulence yielded longer mixing length, larger entrainment, and stronger turbulence kinetic energy inside clouds than 1D turbulence. The simulation with 3D turbulence had the best scores in high clouds. These features suggest that 1D turbulence did not produce enough entrainment, allowing the formation of more small cloud and rain cells than observed. Further tests were performed on the sensitivity to the mixing length with 3D turbulence. Cloud organization was very sensitive to in-cloud mixing length and the use of a very small value increased the number of small cells, much more than the simulations with 1D turbulence. With a larger in-cloud mixing length, the total number of cells, mainly the small ones, was strongly reduced.

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Daria Kuznetsova, Thibaut Dauhut, and Jean-Pierre Chaboureau

Abstract

The passage of the Madden–Julian oscillation (MJO) over the Indian Ocean and the Maritime Continent is investigated during the episode of 23–30 November 2011. A Meso-NH convection-permitting simulation with a horizontal grid spacing of 4 km is examined. The simulation reproduces the MJO signal correctly, showing the eastward propagation of the primary rain activity. The atmospheric overturning is analyzed using the isentropic method, which separates the ascending air with high equivalent potential temperature from the subsiding air with low equivalent potential temperature. Three key circulations are found. The first two circulations are a tropospheric deep circulation spanning from the surface to an altitude of 14 km and an overshoot circulation within the tropical tropopause layer. As expected for circulations associated with deep convection, their intensities, as well as their diabatic tendencies, increase during the active phase of the MJO, while their entrainment rates decrease. The third circulation is characterized by a rising of air with low equivalent potential temperature in the lower free troposphere. The intensity of the circulation, as well as its depth, varies with the MJO activity. During the suppressed phase, this circulation is associated with a dry air intrusion from the subtropical region into the tropical band and shows a strong drying of the lower to middle troposphere.

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Beatriz M. Funatsu, Chantal Claud, and Jean-Pierre Chaboureau

Abstract

A characterization of the large-scale environment associated with precipitating systems in the Mediterranean region, based mainly on NOAA-16 Advanced Microwave Sounding Unit (AMSU) observations from 2001 to 2007, is presented. Channels 5, 7, and 8 of AMSU-A are used to identify upper-level features, while a simple and tractable method, based on combinations of channels 3–5 of AMSU-B and insensitive to land–sea contrast, was used to identify precipitation. Rain occurrence is widespread over the Mediterranean in wintertime while reduced or short lived in the eastern part of the basin in summer. The location of convective precipitation shifts from mostly over land from April to August, to mostly over the sea from September to December. A composite analysis depicting large-scale conditions, for cases of either rain alone or extensive areas of deep convection, is performed for selected locations where the occurrence of intense rainfall was found to be important. In both cases, an upper-level trough is seen to the west of the target area, but for extreme rainfall the trough is narrower and has larger amplitude in all seasons. In general, these troughs are also deeper for extreme rainfall. Based on the European Centre for Medium-Range Weather Forecasts operational analyses, it was found that sea surface temperature anomalies composites for extreme rainfall are often about 1 K warmer, compared to nonconvective precipitation conditions, in the vicinity of the affected area, and the wind speed at 850 hPa is also stronger and usually coming from the sea.

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Thibaut Dauhut, Jean-Pierre Chaboureau, Juan Escobar, and Patrick Mascart

Abstract

The dynamics of Hector the Convector, which overshot into the stratosphere on 30 November 2005 over the Tiwi Islands, Australia, is investigated using a giga-large-eddy simulation with a 100-m cubic mesh. Individual updrafts, defined as 3D objects with vertical velocity above 10 m s−1 are identified. Among the 20 000 updrafts formed during the most intense phase, only a dozen were more than 4 km tall. The two tallest updrafts accounted for more than 90% of the total vertical mass flux through the tropical tropopause layer. Their locations were determined by low-level convergence lines first created by the sea breeze in the morning, then enhanced by cold pools due to cumulus congestus. They finally reinforced each other as they moved inland and intersected. The two tallest updrafts that overshot the tropopause were contrasted with those occurring 1 h earlier and later. They presented larger widths (up to 8 km), greater buoyancy (up to 0.1 m s−2), stronger vertical velocities (up to 50 m s−1), and larger hydrometeor contents (more than 10 g kg−1). They kept their core weakly diluted on their way to the stratosphere with an entrainment rate as low as 0.08 km−1. Both the low-level convergence lines intensified by cold pools and the reduced mixing in the troposphere were found to be the determinant for the transition from deep to very deep convection.

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Thibaut Laffineur, Chantal Claud, Jean-Pierre Chaboureau, and Gunnar Noer

Abstract

Polar lows are intense high-latitude mesocyclones that form during the cold season over open sea. Their relatively small-scale and short life span lead to a rather poor representation in model outputs and meteorological reanalyses. In this paper, the ability of the Interim European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-Interim) to represent polar lows over the Norwegian and Barents Sea is assessed, and a comparison with the 40-yr ECMWF Re-Analysis (ERA-40) is provided for three cold seasons (1999–2000 until 2001–02). A better representation in ERA-Interim is found, with 13 systems captured out of the 29 observed, against 6 in the case of ERA-40. Reasons for the lack of representation are identified. Unexpectedly, the representation of different polar low sizes does not appear to be linked to the resolution. Rather, it is the representation of synoptic conditions that appears to be essential. In a second part, a downscaling is conducted using the mesoscale model Méso-NH. For each observed polar low, a pair of simulations is performed: one initialized by ERA-Interim and the other one by ERA-40. An improvement is noted with 22 polar lows represented when ERA-Interim is used. Through a model-to-satellite approach, it is shown that even if polar lows are simulated, convective processes remain insufficiently represented. Wind speeds, which were underestimated in reanalyses, are nevertheless more realistic in the Méso-NH simulations. These results are supported by a spectral analysis of reanalyses and Méso-NH fields.

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Thibaut Dauhut, Jean-Pierre Chaboureau, Patrick Mascart, and Olivier Pauluis

Abstract

The overturning of Hector the Convector, a tropical multicellular convective system of northern Australia that regularly overshoots into the stratosphere, is synthesized at the scale of a large-eddy simulation. The isentropic analysis offers the advantage of filtering out the reversible motions due to gravity waves and taking into account the turbulent fluxes that contribute to the vertical transport. Two key circulations are characterized: the troposphere deep overturning and the mass exchange due to the overshoots into the stratosphere. The transition from deep to very deep convection is associated with a change in the diabatic tendency inside the tallest updrafts: the latent heat release due to the formation of a large amount of icy hydrometeors exceeds the loss of energy due to mixing with the drier, colder air of the environment. In agreement with a previous study of Hector examining the properties of its two tallest updrafts, the entrainment rate exhibits a minimum during the very deep convection phase as low as 0.04 km−1. The overturning intensity corroborates the Eulerian computation of the vertical mass flux in the midtroposphere and in the lower stratosphere. It however gives a lower estimate of the flux in the upper troposphere, filtering out the reversible motions, and a larger estimate in the lower troposphere and at the tropopause, where slow vertical motions contribute significantly to the transport.

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