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  • Author or Editor: Jean-Pierre Chaboureau x
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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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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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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 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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Peter Bechtold
,
Noureddine Semane
,
Philippe Lopez
,
Jean-Pierre Chaboureau
,
Anton Beljaars
, and
Niels Bormann

Abstract

A new diagnostic convective closure, which is dependent on convective available potential energy (CAPE), is derived under the quasi-equilibrium assumption for the free troposphere subject to boundary layer forcing. The closure involves a convective adjustment time scale for the free troposphere and a coupling coefficient between the free troposphere and the boundary layer based on different time scales over land and ocean. Earlier studies with the ECMWF Integrated Forecasting System (IFS) have already demonstrated the model’s ability to realistically represent tropical convectively coupled waves and synoptic variability with use of the “standard” CAPE closure, given realistic entrainment rates.

A comparison of low-resolution seasonal integrations and high-resolution short-range forecasts against complementary satellite and radar data shows that with the extended CAPE closure it is also possible, independent of model resolution and time step, to realistically represent nonequilibrium convection such as the diurnal cycle of convection and the convection tied to advective boundary layers, although representing the late night convection over land remains a challenge. A more in-depth regional analysis of the diurnal cycle and the closure is provided for the continental United States and particularly Africa, including comparison with data from satellites and a cloud-resolving model (CRM). Consequences for global numerical weather prediction (NWP) are not only a better phase representation of convection, but also better forecasts of its spatial distribution and local intensity.

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Thibaut Dauhut
,
Jean-Pierre Chaboureau
,
Peter H. Haynes
, and
Todd P. Lane

Abstract

Overshoots are convective air parcels that rise beyond their level of neutral buoyancy. A giga-large-eddy simulation (100-m cubic resolution) of “Hector the Convector,” a deep convective system that regularly forms in northern Australia, is analyzed to identify overshoots and quantify the effect of hydration of the stratosphere. In the simulation, 1507 individual overshoots were identified, and 46 of them were tracked over more than 10 min. Hydration of the stratosphere occurs through a sequence of mechanisms: overshoot penetration into the stratosphere, followed by entrainment of stratospheric air and then by efficient turbulent mixing between the air in the overshoot and the entrained warmer air, leaving the subsequent mixed air at about the maximum overshooting altitude. The time scale of these mechanisms is about 1 min. Two categories of overshoots are distinguished: those that significantly hydrate the stratosphere and those that have little direct hydration effect. The former reach higher altitudes and hence entrain and mix with air that has higher potential temperatures. The resulting mixed air has higher temperatures and higher saturation mixing ratios. Therefore, a greater amount of the hydrometeors carried by the original overshoot sublimates to form a persistent vapor-enriched layer. This makes the maximum overshooting altitude the key prognostic for the parameterization of deep convection to represent the correct overshoot transport. One common convection parameterization is tested, and the results suggest that the overshoot downward acceleration due to negative buoyancy is too large relative to that predicted by the numerical simulations and needs to be reduced.

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Ingo Meirold-Mautner
,
Catherine Prigent
,
Eric Defer
,
Juan R. Pardo
,
Jean-Pierre Chaboureau
,
Jean-Pierre Pinty
,
Mario Mech
, and
Susanne Crewell

Abstract

Real midlatitude meteorological cases are simulated over western Europe with the cloud mesoscale model Méso-NH, and the outputs are used to calculate brightness temperatures at microwave frequencies with the Atmospheric Transmission at Microwave (ATM) radiative transfer model. Satellite-observed brightness temperatures (TBs) from the Advanced Microwave Scanning Unit B (AMSU-B) and the Special Sensor Microwave Imager (SSM/I) are compared to the simulated ones. In this paper, one specific situation is examined in detail. The infrared responses have also been calculated and compared to the Meteosat coincident observations. Overall agreement is obtained between the simulated and the observed brightness temperatures in the microwave and in the infrared. The large-scale dynamical structure of the cloud system is well captured by Méso-NH. However, in regions with large quantities of frozen hydrometeors, the comparison shows that the simulated microwave TBs are higher than the measured ones in the window channels at high frequencies, indicating that the calculation does not predict enough scattering. The factors responsible for the scattering (frozen particle distribution, calculation of particle dielectric properties, and nonsphericity of the particles) are analyzed. To assess the quality of the cloud and precipitation simulations by Méso-NH, the microphysical fields predicted by the German Lokal-Modell are also considered. Results show that in these midlatitude situations, the treatment of the snow category has a high impact on the simulated brightness temperatures. The snow scattering parameters are tuned to match the discrete dipole approximation calculations and to obtain a good agreement between simulations and observations even in the areas with significant frozen particles. Analysis of the other meteorological simulations confirms these results. Comparing simulations and observations in the microwave provides a powerful evaluation of resolved clouds in mesoscale models, especially the precipitating ice phase.

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