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  • Author or Editor: J. Noilhan x
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J. Noilhan
and
B. Bénech

Abstract

An experimental study of the dynamics within artificial thermal plumes rising in the boundary layer is presented.

In this third part, measurements just above the heat source and aircraft investigations in the plume aloft are used to reveal the internal structure of the airflow within the buoyant column. Analysis of the pressure perturbation obtained both by direct measurements and as a residual in the mean vertical motion equation for a plume, shows that the vertical pressure gradient accelerates the airflow near the heat source and then reduces the buoyancy in the upper levels. The pressure deficits, attaining maximum values of 1 mb in the core of the lower portion of the plume, are well correlated with large vertical velocities. During light ambient wind conditions, the reduced pressure near the heat source produces a large converging inflow sufficient to cause the lower portion of the plume to go into rotation as a whole. An analysis of the components of the velocity field and momentum fluxes within the column underscores the convergent and divergent characters of the flow, respectively, at the lower and upper portions of the plume. Strong vorticity concentration (∼4 10−2 s−1) is associated with a reduction of entrainment into the column.

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J. F. Mahfouf
and
J. Noilhan

Abstract

Vajious formulations of surface evaporation are tested against in situ data collected over a plot of loamy bare ground. Numerical simulations lasting seven days are compared with observations of near-surface water content and cumulative evaporation.

A comparison of classical bulk aerodynamic formulations shows similar predictions of daytime evaporation while significant differences are exhibited during the night. The so-called “surface moisture availability method” seems to overestimate the nocturnal evaporation flux.

In the context of this dataset, threshold methods strongly underestimate surface evaporation during the whole period of observations. A sensitivity analysis reveals that threshold evaporation (maximum sustainable water flux) is highly sensitive upon the depth of the top soil layer.

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J-F. Mahfouf
and
J. Noilhan

Abstract

This paper presents a simple parameterization of gravitational drainage for land surface schemes describing soil water transfers according to the force-restore method of Deardorff. A one-year time series of observed soil moisture period from HAPEX-MOBILHY (Hydrological Atmospheric Pilot Experiment-Mobilisation du Bilan Hydrique) 1986 revealed the importance of subsurface drainage during the wintertime period. This physical process is accounted for through a Newtonian restore to field capacity when soil moisture is above it. Simulation of the annual cycle of soil moisture by the land surface scheme ISBA (interactions soil biosphere atmosphere) is in this way greatly improved.

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J-C. Calvet
,
J. Noilhan
, and
P. Bessemoulin

Abstract

The bulk soil water content must be estimated accurately for short- and medium-term meteorological modeling. A method is proposed to retrieve the total soil moisture content as well as the field capacity from observed surface parameters such as surface soil moisture or surface temperature. A continuous series of micrometeorological and soil water content measurements was obtained in southwestern France over a fallow site in 1995. In addition, the database includes measurements of the surface temperature and soil moisture profiles within the top 5-cm soil layer. The surface soil moisture measurements are available twice a day during two 30-day intensive observing periods in spring and autumn 1995. Once calibrated, the ISBA (Interactions between Soil, Biosphere, and Atmosphere) surface scheme is able to properly simulate the measured surface variables and the bulk soil moisture. Then an assimilation technique is applied to analyze the field capacity and the total soil water content from the surface data. In particular, it is shown that knowing the atmospheric forcing and the precipitation, four or five estimations of the surface soil moisture spread out over a 15-day period are enough to retrieve the total soil water content by inverting ISBA. The use of the surface temperature seems more problematic because its sensitivity to the value of the total water content is meaningful in relatively dry conditions only.

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F. Bouttier
,
J-F. Mahfouf
, and
J. Noilhan

Abstract

This paper and its companion report on the development of a sequential assimilation technique based upon optimum interpolation in order to initialize soil moisture in atmospheric models. A previous study by Mahfouf has demonstrated that it is possible to estimate soil moisture from the evolution of atmospheric temperature and relative humidity near the surface. The main purpose of this paper is to examine more precisely the dependence of atmospheric low-level parameters upon soil moisture and how this dependence is affected by various factors (soil characteristics, vegetation type, low-level wind). The sensitivity of atmospheric parameters to soil moisture is expressed as the statistical quantities of the optimal interpolation. The importance of observation errors, which define the relevance of the atmospheric parameters for the assimilation procedure, is also investigated. An analytical formulation of the optimal interpolation coefficients is proposed. Finally, the usefulness and limitations of this work for soil moisture analysis in three-dimensional models are discussed.

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F. Bouttier
,
J-F. Mahfouf
, and
J. Noilhan

Abstract

A sequential assimilation technique based upon optimum interpolation is developed to initialize soil moisture in atmospheric models. Soil moisture increments are linearly related to forecast errors of near-surface atmospheric temperature and relative humidity. Part I has shown that soil moisture can be estimated from surface characteristics (vegetation coverage, soil texture). In this part, the behavior of the method is examined within a three-dimensional mesoscale model. The model includes a realistic land surface parameterization that relates soil moisture to atmospheric variables. Results reveal that after 48-h assimilations soil moisture has converged near reference values by blending atmospheric quantities in the algorithm. The convergence rate is almost independent of the first guess. Sensitivity studies show that the observational errors modulate the efficiency of the process and that results with an analytic formulation of the optimum coefficients are close to those obtained with a Monte Carlo method. These conclusions are of practical interest for an implementation in operational models.

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A. Boone
,
V. Masson
,
T. Meyers
, and
J. Noilhan

Abstract

The interactions between the soil, biosphere, and atmosphere (ISBA) land surface parameterization scheme has been modified to include soil ice. The liquid water equivalent volumetric ice content is modeled using two reservoirs within the soil: a thin surface layer that directly affects the surface energy balance, and a deep soil layer. The freezing/drying, wetting/thawing analogy is used, and a description of the modifications to the ISBA force–restore scheme, in particular to the hydrological and thermal transfer coefficients, is presented. In addition, the ISBA surface/vegetation scheme is coupled to a multilayer explicit diffusion soil heat and mass transfer model in order to investigate the accuracy of the force–restore formalism soil freezing parameterization as compared with a higher-order scheme.

An example of the influence of the inclusion of soil freezing in ISBA on predicted surface and soil temperatures and surface fluxes is examined using prescribed atmospheric forcing from a micrometeorological case study that includes freeze–thaw cycles. Surface temperature prediction is improved in comparison with the observed values, especially at night, primarily from the release of latent heat as the soil freezes. There is an improvement in the overall surface flux prediction, although for some specific periods there is increased error in the prediction of various components of the surface energy budget. Last, the simplified force–restore approach is found to produce surface flux and temperature predictions consistent with the higher-resolution model on typical numerical weather prediction model timescales (on the order of several days to two weeks) for this particular site.

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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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