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Frédéric Hourdin
and
Alexandre Armengaud

Abstract

In the context of advection of trace species by 3D atmospheric flows, a comparative test of a hierarchy of finite volume transport schemes initially derived by B. Van Leer is presented. Those schemes are conservative by construction and Van Leer proposed a simple way of ensuring monotonicity. One of the schemes, introduced independently in the atmospheric community by M. J. Prather, is now considered as a reference in the GCM community. An important aspect of the present work is to perform test simulations with various spatial resolutions in order to compare the various schemes at a comparable numerical cost. The result is that higher-order schemes are much more accurate than lower order at a given spatial resolution but much more comparable when the lower-order schemes are run on a finer grid to make the numerical costs equivalent. Moreover, the higher moments of the tracer distribution introduced in the more sophisticated schemes become an issue when other processes such as chemistry or turbulent mixing are accounted for. Finally, it is suggested that Van Leer scheme I is well suited for transport of trace species by 3D atmospheric winds. The results are shown of applications to the transport of radon in the GCM of Laboratoire de Météorologie Dynamique. The GCM implementation of Van Leer scheme I is conservative, positive, and monotonic, and it does not modify a uniform tracer distribution.

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Catherine Rio
and
Frédéric Hourdin

Abstract

The “thermal plume model,” a mass-flux scheme combined with a classical diffusive approach, originally developed to represent turbulent transport in the dry convective boundary layer, is extended here to the representation of cloud processes. The modified parameterization is validated in a 1D configuration against results of large eddy simulations (LES), as well as in a 3D configuration against in situ measurements, for a series of cases of dry and cloudy convective boundary layers. Accounting for coherent structures of the mixed layer with the mass-flux scheme improves the representation of the diurnal cycle of the boundary layer, particularly its progressive deepening during the day and the associated near-surface drying. Results also underline the role of the prescription of the mixing of air between the plume and its environment, and of submean-plume fluctuations.

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Jean-Louis Dufresne
,
Richard Fournier
,
Christophe Hourdin
, and
Frédéric Hourdin

Abstract

The net exchange formulation (NEF) is an alternative to the usual radiative transfer formulation. It was proposed by two authors in 1967, but until now, this formulation has been used only in a very few cases for atmospheric studies. The aim of this paper is to present the NEF and its main advantages and to illustrate them in the case of planet Mars.

In the NEF, the radiative fluxes are no longer considered. The basic variables are the net exchange rates between each pair of atmospheric layers i, j. NEF offers a meaningful matrix representation of radiative exchanges, allows qualification of the dominant contributions to the local heating rates, and provides a general framework to develop approximations satisfying reciprocity of radiative transfer as well as the first and second principles of thermodynamics. This may be very useful to develop fast radiative codes for GCMs.

A radiative code developed along those lines is presented for a GCM of Mars. It is shown that computing the most important optical exchange factors at each time step and the other exchange factors only a few times a day strongly reduces the computation time without any significant precision lost. With this solution, the computation time increases proportionally to the number N of the vertical layers and no longer proportionally to its square N  2. Some specific points, such as numerical instabilities that may appear in the high atmosphere and errors that may be introduced if inappropriate treatments are performed when reflection at the surface occurs, are also investigated.

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Frédéric Hourdin
,
Fleur Couvreux
, and
Laurent Menut

Abstract

Presented is a mass flux parameterization of vertical transport in the convective boundary layer. The formulation of the new parameterization is based on an idealization of thermal cells or rolls. The parameterization is validated by comparison to large eddy simulations (LES). It is also compared to classical boundary layer schemes on a documented case of a well-developed convective boundary layer observed in the Paris area during the Étude et Simulation de la Qualité de l'air en Ile de France (ESQUIF) campaign. For both LES and observations, the new scheme performs better at simulating entrainment fluxes at the top of the convective boundary layer and at near-surface conditions. The explicit representation of mass fluxes allows a direct comparison with campaign observations and opens interesting possibilities for coupling with clouds and deep convection schemes.

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Frédéric Hourdin
,
Phu Le Van
,
François Forget
, and
Olivier Talagrand

Abstract

It is commonly admitted that the seasonal surface pressure cycle, observed on Mars by the two Viking landers, is due to condensation and sublimation of the atmospheric carbon dioxide in the polar caps. A three Martian year numerical simulation has been performed with a Martian General Circulation Model developed from the terrestrial model of the Laboratoire de Météorologic Dynamique. The conditions of the simulation were those of a typical clear-sky situation. The results, validated by comparison to Viking pressure measurements and to temperature fields retrieved from Mariner-9 measurements, show that the pressure cycle depends on the location on the planet. They strongly suggest that, in addition to condensation and sublimation of the atmospheric carbon dioxide, two other effects significantly contribute to the pressure cycle: an orographic effect resulting from the difference in mean height between the two hemispheres, and a dynamical effect resulting from the geostrophic balance between the mass and wind field. In high latitudes, the pressure variation linked to the dynamical effect may have the same magnitude (about 25%) as the global mass variation due to the condensation-sublimation cycle. A shorter dust storm simulation is also in good agreement with observations, in particular as concerns the surface pressure variations and the low-level winds, independently estimated from observations of the bright streaks on the surface of the planet. These results show that the atmospheric mass budget cannot be correctly estimated from local measurements such as Viking measurements.

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Samo Diatta
,
Frédéric Hourdin
,
Amadou Thierno Gaye
, and
Nicolas Viltard

Abstract

Vertical rainfall profiles obtained with TRMM-PR 2A25 standard products are compared with rain profiles deduced from the Laboratoire de Météorologie Dynamique second generation global climate model (LMDZ, the Z stands for zoom capability) with two parameterization schemes: Emanuel’s and Tiedke’s. This paper focuses on the low layers of the atmosphere over West Africa during the monsoon [June–September (JJAS)]. The precipitation decrease above 4 km is systematically not represented in rainfall profiles generated by Emanuel’s parameterization scheme. However, Emanuel’s scheme shows a decrease similar to the observation from 4 km down to the surface, especially in the Sahel (proper depth of the layer dominated by reevaporation). As for Tiedtke’s scheme, it best describes the downward increase in the upper levels of the atmosphere, whereas the downward decrease in the lower levels begins too low when compared to the observations.

Tiedtke’s parameterization shows an overestimation of liquid water production over the ocean and over the Guinean region and a slightly too strong reevaporation in the Sahara and Sahel. The zonal distribution of vertical rain profiles is then biased with this model scheme compared to the 2A25-PR product. On the other hand, although Emanuel’s scheme detects too much reevaporation over the Sahara and underestimates liquid water production over the ocean compared to PR observation, it shows a good meridional distribution of these parameters. This is especially true in the Sahel where Emanuel’s scheme gives the best representation of reevaporation.

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Romain Roehrig
,
Dominique Bouniol
,
Francoise Guichard
,
Frédéric Hourdin
, and
Jean-Luc Redelsperger

Abstract

The present assessment of the West African monsoon in the models of the Coupled Model Intercomparison Project (CMIP) phase 5 (CMIP5) indicates little evolution since the third phase of CMIP (CMIP3) in terms of both biases in present-day climate and climate projections.

The outlook for precipitation in twenty-first-century coupled simulations exhibits opposite responses between the westernmost and eastern Sahel. The spread in the trend amplitude, however, remains large in both regions. Besides, although all models predict a spring and summer warming of the Sahel that is 10%–50% larger than the global warming, their temperature response ranges from 0 to 7 K.

CMIP5 coupled models underestimate the monsoon decadal variability, but SST-imposed simulations succeed in capturing the recent partial recovery of monsoon rainfall. Coupled models still display major SST biases in the equatorial Atlantic, inducing a systematic southward shift of the monsoon. Because of these strong biases, the monsoon is further evaluated in SST-imposed simulations along the 10°W–10°E African Monsoon Multidisciplinary Analysis (AMMA) transect, across a range of time scales ranging from seasonal to intraseasonal and diurnal fluctuations.

The comprehensive set of observational data now available allows an in-depth evaluation of the monsoon across those scales, especially through the use of high-frequency outputs provided by some CMIP5 models at selected sites along the AMMA transect. Most models capture many features of the African monsoon with varying degrees of accuracy. In particular, the simulation of the top-of-atmosphere and surface energy balances, in relation with the cloud cover, and the intermittence and diurnal cycle of precipitation demand further work to achieve a reasonable realism.

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Jason Edward Williams
,
Rinus Scheele
,
Peter van Velthoven
,
Idir Bouarar
,
Kathy Law
,
Béatrice Josse
,
Vincent-Henri Peuch
,
Xin Yang
,
John Pyle
,
Valérie Thouret
,
Brice Barret
,
Cathy Liousse
,
Frédéric Hourdin
,
Sophie Szopa
, and
Anne Cozic

The authors present results obtained during the chemistry-transport modeling (CTM) component of the African Monsoon Multidisciplinary Analysis Multimodel Intercomparison Project (AMMAMIP) using the recently developed L3JRCv2 emission dataset for Africa, where emphasis is placed on the summer of 2006. With the use of passive tracers, the authors show that the application of different parameterizations to describe advection, vertical diffusion, and convective mixing in a suite of state-of-the-art global CTMs results in significantly different transport mechanisms westward of the African continent. Moreover, the authors identify that the atmospheric composition over the southern Atlantic is governed by air masses originating from southern Africa for this period, resulting in maximal concentrations around 5°S. Comparisons with ozonesonde measurements at Cotonou (6.2°N, 2.2°E) indicate that the models generally overpredict surface ozone and underpredict ozone in the upper troposphere. Moreover, using recent aircraft measurements, the authors show that the high ozone concentrations that occur around 700 hPa around 5°N are not captured by any of the models, indicating shortcomings in the description of transport, the magnitude and/or location of emissions, or the in situ chemical ozone production by the various chemical mechanisms employed.

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Frédéric Hourdin
,
Thorsten Mauritsen
,
Andrew Gettelman
,
Jean-Christophe Golaz
,
Venkatramani Balaji
,
Qingyun Duan
,
Doris Folini
,
Duoying Ji
,
Daniel Klocke
,
Yun Qian
,
Florian Rauser
,
Catherine Rio
,
Lorenzo Tomassini
,
Masahiro Watanabe
, and
Daniel Williamson

Abstract

The process of parameter estimation targeting a chosen set of observations is an essential aspect of numerical modeling. This process is usually named tuning in the climate modeling community. In climate models, the variety and complexity of physical processes involved, and their interplay through a wide range of spatial and temporal scales, must be summarized in a series of approximate submodels. Most submodels depend on uncertain parameters. Tuning consists of adjusting the values of these parameters to bring the solution as a whole into line with aspects of the observed climate. Tuning is an essential aspect of climate modeling with its own scientific issues, which is probably not advertised enough outside the community of model developers. Optimization of climate models raises important questions about whether tuning methods a priori constrain the model results in unintended ways that would affect our confidence in climate projections. Here, we present the definition and rationale behind model tuning, review specific methodological aspects, and survey the diversity of tuning approaches used in current climate models. We also discuss the challenges and opportunities in applying so-called objective methods in climate model tuning. We discuss how tuning methodologies may affect fundamental results of climate models, such as climate sensitivity. The article concludes with a series of recommendations to make the process of climate model tuning more transparent.

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Jia-Lin Lin
,
George N. Kiladis
,
Brian E. Mapes
,
Klaus M. Weickmann
,
Kenneth R. Sperber
,
Wuyin Lin
,
Matthew C. Wheeler
,
Siegfried D. Schubert
,
Anthony Del Genio
,
Leo J. Donner
,
Seita Emori
,
Jean-Francois Gueremy
,
Frederic Hourdin
,
Philip J. Rasch
,
Erich Roeckner
, and
John F. Scinocca

Abstract

This study evaluates the tropical intraseasonal variability, especially the fidelity of Madden–Julian oscillation (MJO) simulations, in 14 coupled general circulation models (GCMs) participating in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). Eight years of daily precipitation from each model’s twentieth-century climate simulation are analyzed and compared with daily satellite-retrieved precipitation. Space–time spectral analysis is used to obtain the variance and phase speed of dominant convectively coupled equatorial waves, including the MJO, Kelvin, equatorial Rossby (ER), mixed Rossby–gravity (MRG), and eastward inertio–gravity (EIG) and westward inertio–gravity (WIG) waves. The variance and propagation of the MJO, defined as the eastward wavenumbers 1–6, 30–70-day mode, are examined in detail.

The results show that current state-of-the-art GCMs still have significant problems and display a wide range of skill in simulating the tropical intraseasonal variability. The total intraseasonal (2–128 day) variance of precipitation is too weak in most of the models. About half of the models have signals of convectively coupled equatorial waves, with Kelvin and MRG–EIG waves especially prominent. However, the variances are generally too weak for all wave modes except the EIG wave, and the phase speeds are generally too fast, being scaled to excessively deep equivalent depths. An interesting result is that this scaling is consistent within a given model across modes, in that both the symmetric and antisymmetric modes scale similarly to a certain equivalent depth. Excessively deep equivalent depths suggest that these models may not have a large enough reduction in their “effective static stability” by diabatic heating.

The MJO variance approaches the observed value in only 2 of the 14 models, but is less than half of the observed value in the other 12 models. The ratio between the eastward MJO variance and the variance of its westward counterpart is too small in most of the models, which is consistent with the lack of highly coherent eastward propagation of the MJO in many models. Moreover, the MJO variance in 13 of the 14 models does not come from a pronounced spectral peak, but usually comes from part of an overreddened spectrum, which in turn is associated with too strong persistence of equatorial precipitation. The two models that arguably do best at simulating the MJO are the only ones having convective closures/triggers linked in some way to moisture convergence.

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