Hayder Abdul-Razzak

A parameterization of aerosol activation. 2. Multiple aerosol types

A parameterization of the activation of a lognormal size distribution of aerosols to form cloud droplets is extended to the case of multiple externally mixed lognormal modes, each composed of a uniform internal mixture of soluble and insoluble material. The Kohler theory is used to relate the aerosol size distribution and composition to the number activated as a function of maximum supersaturation. The supersaturation balance is used to determine the maximum supersaturation, accounting for particle growth both before and after the particles are activated. Comparison of the parameterized activation of two competing aerosol modes with detailed numerical simulations of the activation process yields agreement to within 10% under a wide variety of conditions, including diverse size distributions, number concentrations, compositions, and updraft velocities. The parametization error exceeds 10% only when the mode radius of the two size distributions differs by an order of magnitude. Errors for the mass fraction activated are always much less than errors for the number fraction activated.

Prediction of cloud droplet number in a general circulation model

A predictive treatment of droplet number is applied to both a single-column cloud model and a global circulation model. Droplet number is predicted from the droplet number balance, which accounts for droplet nucleation, mixing, and droplet loss due to autoconversion of droplets to rain and collection by rain, ice, and snow. Droplet nucleation is parameterized in terms of the parameters of a lognormal aerosol size distribution and in terms of a Gaussian probability distribution of vertical velocity within each grid cell. The predicted droplet number is found to be significantly less than observations unless vertical resolution provides at least 10 levels within the planetary boundary layer. When droplet number is simply diagnosed from the number nucleated, droplet concentrations are found to be consistently greater than with the predictive treatment. Predicted droplet number concentrations are found to be nonlinearly related to aerosol number concentration.

Kinetic limitations on cloud droplet formation and impact on cloud albedo

Under certain conditions mass transfer limitations on the growth of cloud condensation nuclei (CCN) may have a significant impact on the number of droplets that can form in a cloud. The assumption that particles remain in equilibrium until activated may therefore not always be appropriate for aerosol populations existing in the atmosphere. This work identifies three mechanisms that lead to kinetic limitations, the effect of which on activated cloud droplet number and cloud albedo is assessed using a one-dimensional cloud parcel model with detailed microphysics for a variety of aerosol size distributions and updraft velocities. In assessing the effect of kinetic limitations, we have assumed as cloud droplets not only those that are strictly activated (as dictated by classical Köhler theory), but also unactivated drops large enough to have an impact on cloud optical properties. Aerosol number concentration is found to be the key parameter that controls the significance of kinetic effects. Simulations indicate that the equilibrium assumption leads to an overprediction of droplet number by less than 10% for marine aerosol; this overprediction can exceed 40% for urban type aerosol. Overall, the effect of kinetic limitations on cloud albedo can be considered important when equilibrium activation theory consistently overpredicts droplet number by more than 10%. The maximum change in cloud albedo as a result of kinetic limitations is less than 0.005 for cases such as marine aerosol; however albedo differences can exceed 0.1 under more polluted conditions. Kinetic limitations are thus not expected to be climatically significant on a global scale, but can regionally have a large impact on cloud albedo.

A parameterization of aerosol activation: 1. Single aerosol type

This paper presents a parameterization of the fraction of aerosol particles activated to form cloud droplets in a parcel of air rising adiabatically. The study applies to aerosols of a single lognormal size distribution with uniform chemical composition and capable of serving as cloud condensation nuclei. The parameterization is based on analysis of the rate equations for the parcel supersaturation and aerosol activation process. The analysis leads to the identification of only four dimensionless parameters on which the fraction of activation strongly depends. Using results of detailed numerical simulations by a size-resolving Lagrangian parcel model, errors due to simplifying assumptions used in the analysis were largely eliminated by employing adjustment coefficients. For a wide range of governing parameters (e.g., particle radius, standard deviation, updraft velocity, etc.), differences between the parametric equations and the numerical model results are less than 10% for most conditions and less than 25% for some extreme but realistic conditions. This new parameterization is significantly more accurate and successful in representing the fraction of activation in terms of governing parameters than any known parameterization.

Competition between Sea Salt and Sulfate Particles as Cloud Condensation Nuclei

Abstract The influence of sea salt on the cloud droplet activation of sulfate particles is investigated using a size-resolving model of the aerosol activation process. The authors found that the total number concentration of activated cloud droplets increases with increasing sea salt concentration if the sulfate number concentrations are relatively low and updrafts are strong, but it decreases with higher sea salt if sulfate number concentrations are high and cloud updrafts are weak. The increase is due to the activation of accumulation mode sea salt particles, while the decrease is due to the reduction in maximum cloud supersaturation due to competition with coarse mode sea salt particles. These conclusions are insensitive to the sulfate size distribution and surface wind speed.

Correction to “Droplet nucleation: Physically‐based parameterizations and comparative evaluation”

6 1. Atmospheric and Global Change Division, Pacific Northwest National 7 Laboratory, PO Box 999, Richland, Washington, 99352 8 9 2. Department of Mechanical Engineering, Texas A&M University-Kingsville, MSC 10 191, 700 University Blvd, Kingsville, Texas, 78363 11 12 3. Schools of Earth & Atmospheric Sciences and Chemical & Biomolecular 13 Engineering, Georgia Institute of Technology, 311 Ferst Drive, Atlanta, Georgia, 14 30332-0340 15 16 4. Geophysical Fluid Dynamics Laboratory, P. O. Box 308, Princeton, New Jersey, 17 08542 18 19 5. United Kingdom Meteorology Office, Exeter, United Kingdom 20 21 6. Department of Marine, Earth, and Atmospheric Sciences, North Carolina State 22 University, 2800 Faucette Dr, Raleigh, North Carolina, 27695‐8208 23 24 7. Chinese Research Academy of Environment Sciences, No.8 Dayangfang, 25 Beiyuan, Chaoyang District, Beijing 100012, China 26 27 8. Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 10029, 28 China 29 30 31 Submitted to Journal of Advances in Modeling of Earth Systems, April 7, 2011 32

Analysis of Blood Flow in a Partially Blocked Bifurcated Blood Vessel

Coronary artery disease is a major cause of death in the United States. It is the narrowing of the lumens of the coronary blood vessel by a gradual build‐up of fatty material, atheroma, which leads to the heart muscle not receiving enough blood. This my ocardial ischemia can cause angina, a heart attack, heart failure as well as sudden cardiac death [9]. In this project a solid model of bifurcated blood vessel with an asymmetric stenosis is developed using GAMBIT and imported into FLUENT for analysis. In FLUENT, pressure and velocity distributions in the blood vessel are studied under different conditions, where the size and position of the blockage in the blood vessel are varied. The location and size of the blockage in the blood vessel are correlated with the pressures and velocities distributions. Results show that such correlation may be used to predict the size and location of the blockage.