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J. L. Brenguier

Abstract

Meat of the procedures proposed in the past to correct the coincidence and dead time losses in droplet counters FSSP* or ASSP** are based on empirical formulae. A more rigorous analysis of the statistical behavior of these counters yields a better estimate of the high droplet concentration measurements and moreover, it becomes possible to explain the fluctuations of the operating characteristics of the probes (rejection criteria). A new method is then proposed to detect heterogeneities at scales smaller than the sampling scale and to estimate the concentration peak values in the sample, with the current parameters measured by the FSSP.

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J. L. Brenguier

Abstract

A theoretical formulation is given that allows separate consideration of the major factors governing cloud droplet spectra in nonprecipitating cumulus clouds: activation of nuclei, condensational growth, and turbulent mixing. Broad spectra are treated as combinations of elementary droplet populations, each of which is characterized by its spectrum of activated nuclei (f 0) and its degree of condensational growth (b 2). The broadening of the spectra is described in terms of density functions of f 0 and b 2.

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J. L. Brenguier and L. Amodei

Abstract

Counting loss due to coincidences limit the efficiency of the retriggerable particle counters. When the particle rate increases, the counted rate rapidly reaches a maximum so that it becomes impossible to precisely estimate the actual value. On the contrary, the activity time of the probe increases continuously and is a more efficient parameter for high concentration measurements. Exact relationships between these parameters and the actual particle rate are given here for particles randomly dispersed according to a Poisson distribution. Relations between activity and counted particle rate are also useful for the evaluation of the heterogeneities in the spatial distribution of the particles.

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H. Pawlowska, J. L. Brenguier, and G. Salut

Abstract

Particle concentration is generally derived from measurements by cumulating particle counts on a given sampling period and dividing this particle number by the corresponding sampled volume. Such a procedure is a poor estimation of the concentration when the number of counts per sample is too low. It is shown that counting particles in a cloud is a conditionally Poisson random process given its intensity, which is proportional to the local average concentration of particles. Because of turbulence and mixing processes, the particle concentration in clouds fluctuates and so does the intensity of the counting process, which is referred to as an inhomogeneous Poisson process. The series of counts during a cloud traverse is a unique realization of the process. The estimation of the expected number of particles is thus a Bayesian procedure that consists in the estimation of the intensity of an a priori random inhomogeneous Poisson process from a unique realization of the process. This implies, of course, an a priori model for the possible variations of this intensity. The general theory of optimal estimation for point processes addresses the above problem. It is briefly recalled here, and its application to particle measurements in the atmosphere is tested with simulated series of particle counts. Two examples of estimation from droplet measurements in clouds are also shown and compared to the current method. Nonlinear estimation removes the noise inherent to the counting process while preserving sharp discontinuities in the droplet concentration.

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J. L. Brenguier, D. Baumgardner, and B. Baker

Abstract

The forward-scattering spectrometer probe (FSSP) is an optical particle counter widely used for the measurement of cloud droplet size distributions and concentration. Previous studies have identified operational limitations of these probes and a number of techniques have been developed to minimize the impact of these limitations on the measurements. The majority of effort has been focused on accounting for droplets missed by the FSSP as a result of droplet coincidence and electronic dead time. This note reviews the algorithms that have been developed to account for these losses, describes how and when to apply them to previously acquired measurements, and recommends methods to improve the quality of future measurements.

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P. Personne, J. L. Brenguier, J. P. Pinty, and Y. Pointin

Abstract

Measurements of cloud liquid water content made with various probes reveal a disagreement between their results.

We show in this paper that comparisons between actual airborne measurements make it possible to define correction procedures which improve the accuracy of each probe.

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H. Gerber, G. Frick, S. P. Malinowski, J-L. Brenguier, and F. Burnet

Abstract

Aircraft flights through stratocumulus clouds (Sc) during the Dynamics and Chemistry of Marine Stratocumulus II (DYCOMS-II) study off the California coast found narrow in-cloud regions with less liquid water content (LWC) and cooler temperatures than average background values. The regions are named cloud holes and are assumed to be a result of water evaporated by the entrainment of dryer air from above the Sc. While such features have been noted previously, this study provided a unique opportunity to investigate in much greater detail the nature of the holes, as well as their relationship to the entrainment rate, because high-speed temperature and LWC probes with maximum spatial resolution of 10 cm were flown together for the first time. Nine long-duration flights were made through mostly unbroken Sc for which conditional sampling was used to identify the location and size of the holes. The holes are concentrated near cloud top, their average width near cloud top is about 5 m, their relative length distribution is nearly constant for all flights, and they can penetrate hundreds of meters deep into the Sc before being lost by mixing. Entrainment velocities at cloud top are estimated from measurements of fluxes of reduced LWC and vapor mixing ratios in holes, the fraction of cloud area covered by holes, and the total water jump between cloud top and the free atmosphere. Rates as large as 10 mm s−1 are found for nocturnal flights, and these rates are about 3 times larger than for daytime flight segments. The rates correlate best with the size of the buoyancy jump above the Sc; the present conditional-sampling approach for measuring the rates gives larger rates than the “flux jump” rates determined by others for the same flights by a factor of about 2. The stability criterion for all Sc predicts thinning and breakup of the Sc, which does not occur. The minimal amount of cloud-top evaporative cooling caused by entrainment contributes little to the top-down convection dominated by radiative cooling during nocturnal flights; however, evaporative cooling caused by the mixing of holes as they subduct with the large-scale eddy circulation in the Sc may contribute, but with an as-of-yet unknown amount.

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M. Wendisch, H. Coe, D. Baumgardner, J.-L. Brenguier, V. Dreiling, M. Fiebig, P. Formenti, M. Hermann, M. Krämer, Z. Levin, R. Maser, E. Mathieu, P. Nacass, K. Noone, S. Osborne, J. Schneider, L. Schütz, A. Schwarzenböck, F. Stratmann, and J. C. Wilson

Aircraft inlets connect airborne instruments for particle microphysical and chemical measurements with the ambient atmosphere. These inlets may bias the measurements due to their potential to enhance or remove certain particle size fractions in the sample. The aircraft body itself may disturb the ambient air streamlines and, hence, the particle sampling. Also, anisokinetic sampling and transmission losses within the sampling lines may cause the sampled aerosol to differ from the ambient aerosol. In addition, inlets may change the particle composition and size through the evaporation of water and other volatile materials due to compressibility effects or heat transfer. These problems have been discussed at an international workshop that was held at the Leibniz-Institute for Tropospheric Research (IfT) in Leipzig, Germany, on 12–13 April 2002. The discussions, conclusions, and recommendations from this workshop are summarized here.

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Bjorn Stevens, Donald H. Lenschow, Gabor Vali, Hermann Gerber, A. Bandy, B. Blomquist, J. -L. Brenguier, C. S. Bretherton, F. Burnet, T. Campos, S. Chai, I. Faloona, D. Friesen, S. Haimov, K. Laursen, D. K. Lilly, S. M. Loehrer, Szymon P. Malinowski, B. Morley, M. D. Petters, D. C. Rogers, L. Russell, V. Savic-Jovcic, J. R. Snider, D. Straub, Marcin J. Szumowski, H. Takagi, D. C. Thornton, M. Tschudi, C. Twohy, M. Wetzel, and M. C. van Zanten

The second Dynamics and Chemistry of Marine Stratocumulus (DYCOMS-II) field study is described. The field program consisted of nine flights in marine stratocumulus west-southwest of San Diego, California. The objective of the program was to better understand the physics a n d dynamics of marine stratocumulus. Toward this end special flight strategies, including predominantly nocturnal flights, were employed to optimize estimates of entrainment velocities at cloud-top, large-scale divergence within the boundary layer, drizzle processes in the cloud, cloud microstructure, and aerosol–cloud interactions. Cloud conditions during DYCOMS-II were excellent with almost every flight having uniformly overcast clouds topping a well-mixed boundary layer. Although the emphasis of the manuscript is on the goals and methodologies of DYCOMS-II, some preliminary findings are also presented—the most significant being that the cloud layers appear to entrain less and drizzle more than previous theoretical work led investigators to expect.

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Robert M. Rauber, Harry T. Ochs III, L. Di Girolamo, S. Göke, E. Snodgrass, Bjorn Stevens, Charles Knight, J. B. Jensen, D. H. Lenschow, R. A. Rilling, D. C. Rogers, J. L. Stith, B. A. Albrecht, P. Zuidema, A. M. Blyth, C. W. Fairall, W. A. Brewer, S. Tucker, S. G. Lasher-Trapp, O. L. Mayol-Bracero, G. Vali, B. Geerts, J. R. Anderson, B. A. Baker, R. P. Lawson, A. R. Bandy, D. C. Thornton, E. Burnet, J-L. Brenguier, L. Gomes, P. R. A. Brown, P. Chuang, W. R. Cotton, H. Gerber, B. G. Heikes, J. G. Hudson, P. Kollias, S. K. Krueger, L. Nuijens, D. W. O'Sullivan, A. P. Siebesma, and C. H. Twohy
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