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William R. Cotton

Abstract

A review of convective cloud modeling spanning the period from the days of the NOAA Experimental Meteorology Laboratory (EML) in the late 1960s to 2000 is presented. The intent is to illustrate the evolution of cloud models from the one-dimensional parcel-type models to the current generation of three-dimensional convective storm models and cloud ensemble models. Moreover, it is shown that Dr. Joanne Simpson played a pivotal role in the evolution of cloud models from the very first models to current generation cloud ensemble models. It is also shown that the first concept of the Regional Atmospheric Modeling System (RAMS) began while Drs. Cotton and Pielke worked under Dr. Simpson's supervision at EML. It is then illustrated how far cloud modeling has come with recent applications of RAMS to atmospheric research and numerical weather prediction. The chapter concludes with an outline of the major limitations of current generation convective cloud models.

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William R. Cotton

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No abstract available.

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William R. Cotton

Abstract

A one-dimensional time-dependent cumulus model is developed and discussed. Data predicted by the model along with a bulk entrainment model are compared with a case study observation and Warner's mean profile of Q/QA. While a great deal of the discrepancy between observed and predicted data can he attributed to the transient nature of convection, the consistent pattern of overprediction of such cloud properties as Q/QA and vertical velocity is indeed disturbing. It is concluded that neither the entrainment model nor the scalar nonlinear eddy viscosity model can adequately treat the general problem of turbulent transport in convective clouds. There is, however, sufficient evidence suggesting the models can he of practical value if their use is limited to dynamically active clouds and, in the case of the entrainment model, to a restricted portion of the cloud cycle life. Furthermore, there is little doubt that the entrainment coefficient is not a universal constant while the universality of the mixing length coefficients in the eddy viscosity models is still in question.

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WILLIAM R. COTTON

Abstract

Based on numerical experiments in droplet collection with a stochastic model similar to Berry's, a new quantitative definition of autoconversion is discussed. The new formulation of autoconversion is compared with Kessler's and with Berry's. The new formulation has the decisive advantage over Berry's model of being directly compatible with Kessler's accretion model.

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William R. Cotton

This review is begun with a brief summary of the current status of our understanding of the physics of precipitation in warm clouds. The impact of warm-cloud precipitation processes on the evolution of the ice phase in supercooled clouds also is discussed.

This is followed by a review of experimental attempts to modify the microstructure of warm clouds. Modeling studies of warm cloud modification and observational studies of inadvertent warm cloud modification also are drawn upon to further elucidate the physics of warm cloud modification. The hypotheses, and evidence, for dynamic modification of warm clouds are then discussed. A few brief comments on modeling of warm cloud processes also are given. These comments are intended to serve as a warning to the non-modeler to be very cautious in taking the results of the modeling studies at face value. Finally, the review is concluded with specific recommendations regarding the current status of warm cloud modification, and future directions for the scientist and the weather modification practitioner.

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WILLIAM R. COTTON

Abstract

A numerical model of supercooled cumuli is developed and discussed. Water substance in the model is idealized to be partitioned into the five phase components; namely, water vapor, liquid cloud water, liquid rainwater, frozen rainwater, and ice crystals. Continuity equations are developed that predict the distribution of water substance among the five phase components. The cloud dynamic framework consists of a simple one-dimensional Lagrangian model that includes the effects of entrainment. The model is able to operate either as a steady-state model or as a spherical vortex model.

The results of two case study experiments illustrated that the principle action of ice particles nucleated on sublimation nuclei, or by the freezing of cloud droplets in cumulus clouds containing moderate to heavy amounts of supercooled rainwater, is to promote the freezing of supercooled rainwater. On the other hand, clouds containing small amounts of supercooled rainwater are dynamically insensitive to moderate concentrations of ice crystals. In such clouds, extensive riming and vapor deposition growth of crystals in concentrations of several thousand per liter are required before they make significant contributions to the dynamic structure of the cloud.

Finally, it was found that the warm-cloud precipitation process can either invigorate or retard the dynamic behavior of a supercooled cloud, depending upon the height and magnitude of the precipitation process.

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William R. Cotton

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In this paper, testing, implementation, and evolution of both static and dynamic seeding concepts are reviewed. A brief review of both waterspray and hygroscopic seeding is first presented. This is followed by reviews of static seeding of stable orographic clouds and supercooled cumuli. We conclude with a review of dynamic seeding concepts with particular focus on the Florida studies.

It is concluded that it is encouraging that our testing procedures have evolved from single-response-variable “blackbox” experiments to randomized experiments that attempt to test a number of components in the hypothesized chain of physical responses to seeding. It is cautioned, however, that changes in the seeding strategy to optimize detection of a physical response (in any of the intermediate links in the hypothesized chain of responses) can have an adverse effect upon rainfall on the ground.

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William R. Cotton and Albert Boulanger

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Using the one-dimensional cumulus model developed by Cotton, predictions of the effects of seeding cumulus clouds were performed during the month of July 1973 as part of the Experimental Meteorology Laboratory's Florida Area Cumulus Experiment 1973 experiment. In Part I we compared seedability predictions with the Miami 1200 GMT soundings and soundings taken over the center of the experimental area (Central Site) at 1400 GMT. It was found that substantial differences between the two predictions occurred on a number of days in spite of the fact that the soundings are separated in time by only 2 h and in space by only 110 km.

In this paper we compare seedability predictions with the MIA 1200 GMT soundings and the CS 1800 GMT soundings. The CS 1800 GMT soundings were assumed to be representative of conditions over the experimental area during the period of operation of the experiment. We found that the predictions with the MIA 1200 GMT soundings were, on the average, more representative of conditions over the center of the experimental area during the period of operation of the experiment than were the predictions with the CS 1400 GMT soundings. The results of this study indicate that the choice of a sounding site and sounding time to be used for prediction of seeding effects over an experimental area must be carefully considered in the design of the experiment.

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Shuowen Yang and William R. Cotton

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Theoretical analysis shows that when water activity is larger than its threshold value and the dry radius of a particle is larger than 0.005 µm, the deviation of curvature correction from unity can be accurately represented by the product of two terms, with one term strongly depending upon water activity and the other depending upon dry radius. Moreover, experimental data show that the water-activity-dependent term can be approximated by linear and one-third power functions of water activity. According to the approximation made to curvature correction, water activity is solved as analytical functions of relative humidity (RH). The analytically solved water activity is then used to compute particle equilibrium sizes using a known (observed) relationship between water activity and water uptake by unit mass of dry material. The accuracy of equilibrium sizes calculated with this method is checked with seven typical classes of aerosols. Results show that when RH ≤ 99.99%, the equilibrium radius computed with this method is accurate to within 3% (6%) if the dry radius of a particle is larger (smaller) than 0.02 µm and that when RH > 99.99%, equilibrium sizes can be estimated with an accuracy higher than 84%. Analytical approximation formulas are also derived for calculating critical equilibrium radii and critical supersaturation. The maximum relative errors of these formulas range from 3% to 15%.

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William R. Cotton and Albert Boulanger

Abstract

Using the one-dimensional cumulus model developed by Cotton, predictions of the effects of seeding cumulus clouds were performed during the month of July 1973 as a part of the Experimental Meteorology Laboratory FACE 1973 Experiment. The calculations were performed with the Miami 1200 GMT soundings and soundings taken in the interior of Florida at 1400 GMT at the so-called Central Site (CS) location.A comparison of “seedability” predictions using the Miami 1200 GMT and CS 1400 GMT soundings have shown that substantial differences between the two seedability predictions occur on a number of days in spite of the fact that the soundings are separated in time by only 2 h and in space by only 110 km. The differences can be attributed to the frequent intrusion of dry air masses of varying height and thickness. The intensity of the dry layers is generally greatest over the higher-latitude CS location. The greatest differences between the two soundings, and hence the greatest difference between the predicted seeding effects, occurs during periods of transition from a disturbed, westerly flow regime to a well-defined, deep, easterly flow regime.

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