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Jean-Louis Brenguier

Abstract

Measurements of the droplet size distribution collected in a field of small nonprecipitating cumuli are analyzed. It appears that despite the high variability of the measured droplet concentration, the droplet spectral shape at given levels is almost invariable. An original method, based on the coincidence effects in the droplet counter, is used to show that most of the low concentration samples are in fact heterogeneous and that the actual droplet concentration in the cloudy parts of these samples is comparable to the concentration in regions of the cloud which are uniform.

Observed spectra are much broader than provided by adiabatic lifting. The frequency distribution of the spectral shapes is compared to the results of a one-dimensional cloud model for various values of the entrainment rate. The limitations of the previous models are discussed and the inhomogeneous mixing formulation of Baker et al. is then modified to allow nucleus activation in the entrained air before its fine scale mixing with the cloudy droplets. Finally the proposed scheme is simplified to derive a parameterization of the broad spectra. The range of spectral shapes reproduced at any level by the parameterization is in agreement with their observed frequency distribution.

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Jean-Louis Brenguier

Abstract

Current conceptual models of the processes that modify the droplet spectrum in convective clouds starts with entrainment of environmental air followed by turbulent mixing of these parcels into progressively finer filaments. Thus, one would expect, for scales larger than those at which molecular diffusion predominates, cloud structures should be observed with very sharp transitions between parcels with differing histories.

Recent advances in microphysical measurements allow exploration of these processes at centimeter scales. In the present work, observations from an aircraft-mounted forward-scattering spectrometer probe, modified for faster response than the conventional probes, are presented. Cloud structures with very sharp interfaces are reported to exist on the smallest observable scale. These observations are intended to provide input into current efforts to model the evolution of the droplet spectrum.

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Laure Chaumat
and
Jean-Louis Brenguier

Abstract

Cloud samples of narrow spectra observed in adiabatic cores of cumulus clouds are selected and the droplet spatial distribution is examined in order to document microscale heterogeneities of the droplet concentration. Counting statistics and the Fishing test are applied and compared to the properties of a random spatial distribution, that is, to Poisson statistics. These tests suggest that microscale heterogeneities of the concentration are not significant in adiabatic cores. A conceptual model is then developed to estimate the lifetime that would be required for the observed heterogeneities to generate the observed spectra broadening. The model implies that the lifetime of the heterogeneities should reach unrealistic values to significantly contribute to the observed broadening. The statistical properties of direct numerical simulations of the droplet inertial coupling with turbulence are then compared to the properties of the actual samples. It appears that the properties of the actual samples are closer to the Poisson reference than those of the fields generated by the models. It is concluded that the microscale observations of the droplet spatial distribution in adiabatic cores do not support the hypothesis that inertial coupling is a significant source of spectra broadening.

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Frédéric Burnet
and
Jean-Louis Brenguier

Abstract

Thermodynamical and microphysical measurements collected in convective clouds are examined within the frame of the homogeneous/inhomogeneous mixing concept, to determine how entrainment-mixing processes affect cloud droplets, their number concentration, and their mean size. The three selected case studies—one stratocumulus layer and two cumulus clouds—exhibit very different values of the cloud updraft intensity, of the adiabatic droplet mean volume diameter, and of the saturation deficit in the environment, all three parameters that are expected to govern the microphysical response to entrainmentmixing. The results confirm that the observed microphysical features are sensitive to the droplet response time to evaporation and to the turbulent homogenization time scale, as suggested by the inhomogeneous mixing concept. They also reveal that an instrumental artifact due to the heterogeneous spatial droplet distribution may be partly responsible for the observed heterogeneous mixing features. The challenge remains, however, to understand why spatially homogeneous cloud volumes larger than the instrument resolution scale (10 m) are so rarely observed. The analysis of the buoyancy of the cloud and clear air mixtures suggests that dynamical sorting could also be efficient for the selection, among all possible mixing scenarios, of those that minimize the local buoyancy production.

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Frédéric Burnet
and
Jean-Louis Brenguier

Abstract

Microphysical measurements performed during the Small Cumulus Microphysics Study (SCMS) experiment are analyzed in order to examine the instrumental limitations of forward scattering spectrometer probes (FSSPs). Complementary information collected with a modified version of the instrument, the Fast-FSSP, are used to address crucial issues such as the size calibration of the spectrometers and the effects on the measured spectra (distortion and broadening), of beam inhomogeneities, of variations of the sampling section, and of the coincidence of particles. Their impact on the calculation of liquid water content is evaluated by the comparison with measurements performed with a hot-wire probe and a particle volume monitor. In addition to the statistical approach that aims at evaluating the typical uncertainty of the measurements, special attention is given to the identification of circumstances under which some of the instrumental limitations combined are likely to affect significantly the accuracy of the measurements. The overall data quality is illustrated in the data summary of the 10 missions flown with the Météo-France Merlin-IV during SCMS. Droplet concentration measurements performed with the standard and the Fast-FSSP, and statistically processed for each flight separately, agree to within a bias lower than 10% and a standard deviation of ±20%. The derived liquid water content measurements, compared to the hot-wire probe, exhibit a larger standard deviation of ±30%, with a substantial degradation at high droplet concentration due to droplet spectra distortion by droplet coincidences in the detection beam.

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Jean-Louis Brenguier
and
Laure Chaumat

Abstract

Measurements of cloud droplet spectra performed with the Fast-Forward Scattering Spectrometer Probe during the Small Cumulus Microphysics Study (1995) are analyzed. Fifty cloud samples with narrow droplet spectra are selected. They are characterized by values of liquid water content slightly below the adiabatic value. Each observed spectrum is then compared to a narrow adiabatic spectrum predicted at the same level with the current theory of condensational growth in an adiabatic cloud cell, initialized with a reference spectrum measured right above the activation level, at cloud base. Broadening is characterized for each observed spectrum by the probability density function of condensational growth expressed as the Lagrangian integral of the ratio of supersaturation to vertical velocity, along the droplet trajectories. In particular it appears that the derived density functions show high probabilities of very low and very large values of condensational growth. The large values are related to a high relative density of big droplets in the measured spectra, higher than predicted by the adiabatic model. The contribution of the instrument to this feature is examined with a model of probe functioning. The simulations suggest that most of those big droplets are instrumental artifacts. The remaining broadening is parameterized by a linear relationship between the mean value and the standard deviation of the density function of condensational growth. This result will be used to examine the respective contributions to spectra broadening of microscale heterogeneities of the droplet concentration, in Part II, and of the mixing processes, in Part III of this series.

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Jean-Louis Brenguier
and
Wojciech W. Grabowski

Abstract

A simple numerical model designed to predict the evolution of cloud droplet spectra with special emphasis on the role of entrainment is developed for a case of nonprecipitating cloud. The model assumes that the cloud water mixing ratio at any grid location is equal to that predicted by a cloud model using bulk microphysics; that is, supersaturation/undersaturation inside cloud is neglected. Locally, only undiluted droplet concentrations are assumed to exist; any average dilution of cloud droplet concentration observed over a grid volume is interpreted as an effect of internal structure within the grid, with undiluted cloudy patches coexisting with those that are cloud free. Activation of cloud condensation nuclei is assumed to always produce the same initial spectrum of cloud droplets. Further condensational growth of this initial spectrum produces a set of base functions that are used to represent droplet spectral evolution.

The microphysical model, combined with the two-dimensional cloud model using bulk microphysics, is applied to an idealized case of small cumulus. It is shown that, within the current framework, entrainment usually leads to fresh activation of cloud droplets and results in multimodal spectra in actively growing cells. Both broad and narrow cloud droplet spectra are predicted in old, highly diluted cells at cloud periphery. These results are discussed in the context of both observational and theoretical studies of droplet spectral evolution.

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Frédérick Chosson
,
Jean-Louis Brenguier
, and
Lothar Schüller

Abstract

In general circulation models, clouds are parameterized and radiative transfer calculations are performed using the plane-parallel approximation over the cloudy fraction of each model grid. The albedo bias resulting from the plane-parallel representation of spatially heterogeneous clouds has been extensively studied, but the impact of entrainment-mixing processes on cloud microphysics has been neglected up to now. In this paper, this issue is examined by using large-eddy simulations of stratocumulus clouds and tridimensional calculations of radiative transfer in the visible and near-infrared ranges. Two extreme scenarios of mixing are tested: the homogeneous mixing scheme with constant concentration and reduced droplet sizes, against the inhomogeneous mixing scheme, with reduced concentration and constant droplet sizes. The tests reveal that entrainment-mixing effects at cloud top may substantially bias the simulated albedo. In the worse case, which corresponds to a fragmented and thin stratocumulus cloud, the albedo bias changes from −3% to −31% when using both mixing schemes alternatively.

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Jean-Louis Brenguier
,
A. R. Rodi
,
G. Gordon
, and
P. Wechsler

Abstract

A method for detecting malfunctions during collection of data with the forward-scattering spectrometer probe (FSSP) is discussed. Droplet spectra measured with the probe are not sufficient to alert operators of probe failures, such as those caused by fogging or icing of the optics. Exact solutions of the coincidence process and application of these formulas, using additional parameters provided by the FSSP, yield estimates of the transit time of the droplets through the laser beam. Comparison of these estimates with expected values derived from aircraft speed can be used to detect a variety of FSSP malfunctions. An example of such an analysis using data from a flight through supercooled cloud is presented.

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Lothar Schüller
,
Ralf Bennartz
,
Jürgen Fischer
, and
Jean-Louis Brenguier

Abstract

Algorithms are now currently used for the retrieval of cloud optical thickness and droplet effective radius from multispectral radiance measurements. This paper extends their application to the retrieval of cloud droplet number concentration, cloud geometrical thickness, and liquid water path in shallow convective clouds, using an algorithm that was previously tested with airborne measurements of cloud radiances and validated against in situ measurements of the same clouds. The retrieval is based on a stratified cloud model of liquid water content and droplet spectrum. Radiance measurements in visible and near-infrared channels of the Moderate Resolution Imaging Spectroradiometer (MODIS), which is operated from the NASA platforms Terra and Aqua, are analyzed. Because of uncertainties in the simulation of the continental surface reflectance, the algorithm is presently limited to the monitoring of the microphysical structure of boundary layer clouds over the ocean. Two MODIS scenes of extended cloud fields over the North Atlantic Ocean trade wind region are processed. A transport and dispersion model (the Hybrid Single-Particle Lagrangian Integrated Trajectory Model, HYSPLIT4) is also used to characterize the origin of the air masses and hence their aerosol regimes. One cloud field formed in an air mass that was advected from southern Europe and North Africa. It shows high values of the droplet concentration when compared with the second cloud system, which developed in a more pristine environment. The more pristine case also exhibits a higher geometrical thickness and, thus, liquid water path, which counterbalances the expected cloud albedo increase of the polluted case. Estimates of cloud liquid water path are then compared with retrievals from the Special Sensor Microwave Imager (SSM/I). SSM/I-derived liquid water paths are in good agreement with the MODIS-derived values.

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