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P. Bougeault
,
J. Noilhan
,
P. Lacarrère
, and
P. Mascart

Abstract

As a follow-up to the HAPEX-MOBILHY experiment in southwestern France, an advanced parameterization of land-surface processes has been designed. This paper and two companions report on the implementation of this new parameterization in the limited-area model of the French Weather Service and on results obtained in the numerical simulation of one of the most widely studied periods of the experiment, 16 June 1986. In Part I, we discuss the production of maps of parameters to be used in the three-dimensional model. The useful parameters are produced by tabulation of their most probable values, as function of some primary parameters, which are for the present time the soil texture, the soil depth, the dominant vegetation type, and the albedo. The methods giving the maps of these primary parameters are also briefly discussed. They rely on existing information, such as several soil maps and satellite products from AVHRR and Meteostat. Parts II and III discuss the results of the simulation of 16 June 1986.

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P. Bougeault
,
B. Bret
,
P. Lacarrère
, and
J. Noilhan

Abstract

In this second part, we report on a one-day simulation with the French Weather Service limited-area model PERIDOT, including the new parameterization of land-surface energy budget. The emphasis is put on the assessment of the general quality of the simulation at mesobeta scale, in order to establish the significance of the detailed comparisons with the HAPEX-MOBILHY dataset, presented in Part III. We examine the evolution of surface wind, temperature, and humidity, of the low-level clouds, of surface budget characteristics, of the planetary boundary layer structure and horizontal variability, and of induced mesoscale circulations. Whenever it is possible, we present both model and observed parameters and discuss remaining discrepancies. We conclude that the model has captured most of the phenomena occurring at mesoscale-scale on this particular day, and that the reference integration constitutes a good numerical laboratory to investigate the problems posed by the surface parameterization at scales ranging from 10 to 100 km.

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B. Bénech
,
J. Noilhan
,
A. Druilhet
,
J. M. Brustet
, and
C. Charpentier

Abstract

The work reported here describes the environmental impact of emitting about 1000 MW of dry heat from a concentrated source into the atmosphere. It is based on a large field program conducted jointly by the Centre de Recherches Atmosphériques and Electricité de France. This program provided an opportunity to evaluate the actual environmental impacts of large-scale heat release and to obtain data required to develop parameterization schemes for use in modeling heat releases by intense sources such as dry cooling towers.

The heat source is an array of 105 fuel-oil burners distributed over 15 000 m2. An aerial assemblage suspended at two levels (25 and 50 m) over the burner array has been used to collect data (temperature and velocity fields) for analyzing aspects of both the mean and the turbulent components of the flow near the heat source.

The flow field near the heat source comprises a cold downdraft upwind zone which supplies the burner area with ambient air, a convective zone containing a hot vertical air stream with rotation effects, and a cold updraft downwind zone where numerous vortices are initiated.

In the upwind zone, the horizontal flow is accelerated, steady state and divergent. In the convective zone, temperature and vertical velocity are closely correlates, as are temperature and horizontal velocity. The downstream flow shows strong convergence (∼0.3 s−1) and contains two counter-rotating vortices. Cross-correlation and spectral analysis of temperature and vertical velocity in the convective zone show that the major spectral energy contribution is located at wavelengths between 30 and 70 m. The slope of the temperature spectra tends to increase with the standard deviation of the temperature fluctuations. The turbulence in the core of the convective zone is characterized by large values of the dissipation rate ε (∼1 m2 s−3) and of the temperature structure parameter CT 2 (∼10 m−⅔ K). The comparison between the turbulent and advective heat fluxes suggests that the turbulence is not yet fully developed at the vicinity of the heat source. Finally, an estimation of the mean initial conditions as a function of the wind is given.

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E. M. Blyth
,
A. J. Dolman
, and
J. Noilhan

Abstract

A meso-β-scale model is used to model a frontal intrusion in southwest France during HAPEX-MOBILHY. The skill of the model to reproduce the observed variation in temperature, humidity, and wind speed over the domain is reasonable within the limitations of the model parameterizations and initialization procedure, although there were errors in the timing and positioning of the front. A stable boundary layer was both observed and modeled over the forested area. The associated negative sensible heat flux provided the energy to sustain evaporation from the wet forest canopy under conditions of low radiation. A large wind shear over the stably stratified boundary layer provided the required turbulent kinetic energy to maintain the downward transport of sensible heat. Sensitivity experiments showed that local rainfall with a full forest cover changed from 2.9 to 3.8 mm, which represents a 30% increase when compared with a bare-soil domain. Half of this increase is from positive feedback of the intercepted water that reevaporates. The high roughness length of the forest, with its associated physical and dynamical effects, accounts for the rest of the increase in rainfall and for the accompanying increase in soil moisture.

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J. Noilhan
,
B. Bénech
,
G. Letrenne
,
A. Druilhet
, and
A. Saab

Abstract

Some aspects of the mean and turbulent structures of artificial thermal plumes in the boundary layer (BL) are presented. This analysis is based mainly on measurements with an instrumented aircraft. As initial conditions for plume rise, the characteristics of the BL during the 10 experiments are summarized. Under neutral conditions, plume rise in the BL follows approximately the classical prediction. Plume structure aspects are analyzed inside the upwind active zone corresponding to the region of strongest gradients within the convective column. In an attempt to compare the various experiments, scaling parameters are derived from energetic considerations for both dissipative and first-order parameters. Individual and normalized vertical profiles are given. The statistics presented discriminate between dry and condensed plumes. For both populations, vertical profile tendencies are rather similar in the BL. However, condensed plumes show higher values of the perturbed variables inside the whole column. Mean and turbulent structure within the plume is sensitive to the stratification of the environment. In the BL, the main structural aspects consist of quasi-constant profiles of vertical velocity and specific humidity excess, and of rapid decreases of temperature excess, dissipation rate and temperature structure parameter, such decreases are due to mixing with the surrounding atmosphere. Penetration into the drier stable layers results in a reversal of the sign of θ p , an increase in the specific humidity excess and a small decrease in the dissipative parameters. The intensity of the plume turbulence is found to be more dependent on wind speed and condensation processes than on atmospheric turbulence.

The oscillations inside the horizontal part of the plume in the upper stable layer are modulated by mean advection and by possible interaction between the buoyant cells and natural waves.

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B. Decharme
,
H. Douville
,
A. Boone
,
F. Habets
, and
J. Noilhan

Abstract

This study focuses on the influence of an exponential profile of saturated hydraulic conductivity, k sat, with soil depth on the water budget simulated by the Interaction Soil Biosphere Atmosphere (ISBA) land surface model over the French Rhône River basin. With this exponential profile, the saturated hydraulic conductivity at the surface increases by approximately a factor of 10, and its mean value increases in the root zone and decreases in the deeper region of the soil in comparison with the values given by Clapp and Hornberger. This new version of ISBA is compared to the original version in offline simulations using the Rhône-Aggregation high-resolution database. Low-resolution simulations, where all atmospheric data and surface parameters have been aggregated, are also performed to test the impact of the modified k sat profile at the typical scale of a climate model. The simulated discharges are compared to observations from a dense network consisting of 88 gauging stations.

Results of the high-resolution experiments show that the exponential profile of k sat globally improves the simulated discharges and that the assumption of an increase in saturated hydraulic conductivity from the soil surface to a depth close to the rooting depth in comparison with values given by Clapp and Hornberger is reasonable. Results of the scaling experiments indicate that this parameterization is also suitable for large-scale hydrological applications. Nevertheless, low-resolution simulations with both model versions overestimate evapotranspiration (especially from the plant transpiration and the wet fraction of the canopy) to the detriment of total runoff, which emphasizes the need for implementing subgrid distribution of precipitation and land surface properties in large-scale hydrological applications.

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J-F. Mahfouf
,
A. O. Manzi
,
J. Noilhan
,
H. Giordani
, and
M. DéQué

Abstract

This paper describes recent developments in climate modeling at Météo-France related to land surface processes. The implementation of a simple land surface parameterization, Interactions between Soil Biosphere Atmosphere (ISBA), has gained from previous validations and calibrations at local scale against field datasets and from aggregation procedures devised to define effective land surface properties. Specific improvements for climate purposes are introduced: spatial variability of convective rainfall in canopy drainage estimation and subsurface gravitational percolation. The methodology used to derive climatological maps of land surface parameters at the grid-scale resolution of the model from existing database for soil and vegetation types at global scale is described. A 3-yr integration for the present day climate with a T42L30 version of the climate model has been performed. Results obtained compare favorably with available observed climatologies related to the various components of the continental surface energy and water budgets. Differences are due mostly to a poor simulation of the precipitation field. However, some differences suggest specific improvements in the surface scheme concerning representation of the bare soil albedo, the surface runoff, and the soil moisture initialization. As a first step prior to tropical deforestation experiments presented in Part II, regional analyses over the Amazon forest indicate that the modeled evaporation and net radiation are in good agreement with data collected during the Amazon Region Micrometeorological Experiment campaign.

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L. Bouilloud
,
E. Martin
,
F. Habets
,
A. Boone
,
P. Le Moigne
,
J. Livet
,
M. Marchetti
,
A. Foidart
,
L. Franchistéguy
,
S. Morel
,
J. Noilhan
, and
P. Pettré

Abstract

A numerical model designed to simulate the evolution of a snow layer on a road surface was forced by meteorological forecasts so as to assess its potential for use within an operational suite for road management in winter. The suite is intended for use throughout France, even in areas where no observations of surface conditions are available. It relies on short-term meteorological forecasts and long-term simulations of surface conditions using spatialized meteorological data to provide the initial conditions. The prediction of road surface conditions (road surface temperature and presence of snow on the road) was tested at an experimental site using data from a comprehensive experimental field campaign. The results were satisfactory, with detection of the majority of snow and negative road surface temperature events. The model was then extended to all of France with an 8-km grid resolution, using forcing data from a real-time meteorological analysis system. Many events with snow on the roads were simulated for the 2004/05 winter. Results for road surface temperature were checked against road station data from several highways, and results for the presence of snow on the road were checked against measurements from the Météo-France weather station network.

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Weiqing Qu
,
A. Henderson-Sellers
,
A. J. Pitman
,
T. H. Chen
,
F. Abramopoulos
,
A. Boone
,
S. Chang
,
F. Chen
,
Y. Dai
,
R. E. Dickinson
,
L. Dümenil
,
M. Ek
,
N. Gedney
,
Y. M. Gusev
,
J. Kim
,
R. Koster
,
E. A. Kowalczyk
,
J. Lean
,
D. Lettenmaier
,
X. Liang
,
J.-F. Mahfouf
,
H.-T. Mengelkamp
,
K. Mitchell
,
O. N. Nasonova
,
J. Noilhan
,
A. Robock
,
C. Rosenzweig
,
J. Schaake
,
C. A. Schlosser
,
J.-P. Schulz
,
A. B. Shmakin
,
D. L. Verseghy
,
P. Wetzel
,
E. F. Wood
,
Z.-L. Yang
, and
Q. Zeng

Abstract

In the PILPS Phase 2a experiment, 23 land-surface schemes were compared in an off-line control experiment using observed meteorological data from Cabauw, the Netherlands. Two simple sensitivity experiments were also undertaken in which the observed surface air temperature was artificially increased or decreased by 2 K while all other factors remained as observed. On the annual timescale, all schemes show similar responses to these perturbations in latent, sensible heat flux, and other key variables. For the 2-K increase in temperature, surface temperatures and latent heat fluxes all increase while net radiation, sensible heat fluxes, and soil moistures all decrease. The results are reversed for a 2-K temperature decrease. The changes in sensible heat fluxes and, especially, the changes in the latent heat fluxes are not linearly related to the change of temperature. Theoretically, the nonlinear relationship between air temperature and the latent heat flux is evident and due to the convex relationship between air temperature and saturation vapor pressure. A simple test shows that, the effect of the change of air temperature on the atmospheric stratification aside, this nonlinear relationship is shown in the form that the increase of the latent heat flux for a 2-K temperature increase is larger than its decrease for a 2-K temperature decrease. However, the results from the Cabauw sensitivity experiments show that the increase of the latent heat flux in the +2-K experiment is smaller than the decrease of the latent heat flux in the −2-K experiment (we refer to this as the asymmetry). The analysis in this paper shows that this inconsistency between the theoretical relationship and the Cabauw sensitivity experiments results (or the asymmetry) is due to (i) the involvement of the β g formulation, which is a function of a series stress factors that limited the evaporation and whose values change in the ±2-K experiments, leading to strong modifications of the latent heat flux; (ii) the change of the drag coefficient induced by the changes in stratification due to the imposed air temperature changes (±2 K) in parameterizations of latent heat flux common in current land-surface schemes. Among all stress factors involved in the β g formulation, the soil moisture stress in the +2-K experiment induced by the increased evaporation is the main factor that contributes to the asymmetry.

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A. Boone
,
F. Habets
,
J. Noilhan
,
D. Clark
,
P. Dirmeyer
,
S. Fox
,
Y. Gusev
,
I. Haddeland
,
R. Koster
,
D. Lohmann
,
S. Mahanama
,
K. Mitchell
,
O. Nasonova
,
G.-Y. Niu
,
A. Pitman
,
J. Polcher
,
A. B. Shmakin
,
K. Tanaka
,
B. van den Hurk
,
S. Vérant
,
D. Verseghy
,
P. Viterbo
, and
Z.-L. Yang

Abstract

The Rhône-Aggregation (Rhône-AGG) Land Surface Scheme (LSS) intercomparison project is an initiative within the Global Energy and Water Cycle Experiment (GEWEX)/Global Land–Atmosphere System Study (GLASS) panel of the World Climate Research Programme (WCRP). It is a intermediate step leading up to the next phase of the Global Soil Wetness Project (GSWP) (Phase 2), for which there will be a broader investigation of the aggregation between global scales (GSWP-1) and the river scale. This project makes use of the Rhône modeling system, which was developed in recent years by the French research community in order to study the continental water cycle on a regional scale.

The main goals of this study are to investigate how 15 LSSs simulate the water balance for several annual cycles compared to data from a dense observation network consisting of daily discharge from over 145 gauges and daily snow depth from 24 sites, and to examine the impact of changing the spatial scale on the simulations. The overall evapotranspiration, runoff, and monthly change in water storage are similarly simulated by the LSSs, however, the differing partitioning among the fluxes results in very different river discharges and soil moisture equilibrium states. Subgrid runoff is especially important for discharge at the daily timescale and for smaller-scale basins. Also, models using an explicit treatment of the snowpack compared better with the observations than simpler composite schemes.

Results from a series of scaling experiments are examined for which the spatial resolution of the computational grid is decreased to be consistent with large-scale atmospheric models. The impact of upscaling on the domain-averaged hydrological components is similar among most LSSs, with increased evaporation of water intercepted by the canopy and a decrease in surface runoff representing the most consistent inter-LSS responses. A significant finding is that the snow water equivalent is greatly reduced by upscaling in all LSSs but one that explicitly accounts for subgrid-scale orography effects on the atmospheric forcing.

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