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Richard C. Easter

Abstract

Two modified versions of Bott's forward-in-time, positive-definite numerical advection scheme are described. One version is a conservative flux form; the other is a nonconservative advective form. Both versions demonstrate improved stability in tests involving a strongly deformational flow field. The modified versions are identical to Bott's original scheme for cases of uniform fluid density and purely rotational flow. Additional forms of the odd-order versions of the scheme are presented that exhibit lower phase-speed errors than the original odd-order versions.

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Mikhail Ovtchinnikov and Richard C. Easter

Abstract

Monotonicity constraints and gradient-preserving flux corrections employed by many advection algorithms used in atmospheric models make these algorithms nonlinear. Consequently, any relations among model variables transported separately are not necessarily preserved in such models. These errors cannot be revealed by traditional algorithm testing based on advection of a single tracer. New types of tests are developed and conducted to evaluate the monotonicity of a sum of several number mixing ratios advected independently of each other—as is the case, for example, in models using bin or sectional representations of aerosol or cloud particle size distributions. The tests show that when three tracers with an initially constant sum are advected separately in one-dimensional constant velocity flow, local errors in their sum can be on the order of 10%. When cloudlike interactions are allowed among the tracers in the idealized “cloud base” test, errors in the sum of three mixing ratios can reach 30%. Several approaches to eliminate the error are suggested, all based on advecting the sum as a separate variable and then using it to normalize the sum of the individual tracers’ mixing ratios or fluxes. A simple scalar normalization ensures the monotonicity of the total number mixing ratio and positive definiteness of the variables, but the monotonicity of individual tracers is no longer maintained. More involved flux normalization procedures are developed for the flux-based advection algorithms to maintain the monotonicity for individual scalars and their sum.

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Richard C. Easter and Peter V. Hobbs

Abstract

The production of ammonium sulfate by oxidation of dissolved sulfur dioxide in cloud droplets in a wave cloud situation, and the resulting enhancement of cloud condensation nuclei (CCN) released from the cloud on evaporation, have been calculated. The condensational growth of droplets formed on 75 initial CCN sizes is considered simultaneously with the production of sulfates via the Scott-Hobbs mechanism in the droplets in an air parcel moving through a wave cloud. The results show that significant increases in the concentrations of CCN active at 0.5% supersatutation can be produced by SO2 oxidation in wave clouds with short “flow-through” times (4 min) and with concentrations of SO2 and NH3 typical of unpolluted air (1 and 3 ppb, respectively). The sulfate production is found to decrease as the SO2 concentration rises above 10 ppb. This effect is due to the limited buffering capacity of the NH3.

The results of the calculations indicate that the in-cloud production of ammonium sulfate can probably explain previous observations of higher than ambinent CCN concentrations in air from evaporating clouds, and that the rate of production of ammonium sulfate in clouds is sufficiently fast that it is probably the major worldwide source of these particles.

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Steven J. Ghan and Richard C. Easter

Abstract

Bulk cloud microphysics parameterizations typically employ time steps of a few tens of seconds. Although the computational burden of these parameterizations is acceptable for the 1-day mesoscale cloud simulations for which they were designed, the time steps are unacceptably short for direct application of them parameterizations to global-climate simulation. To increase the computational efficiency of bulk cloud microphysics parameterizations, we introduce two approximations that are appropriate for stratiform clouds. By diagnosing rather than predicting rain and snow concentrations and by assuming instantaneous melting of snow, we have found that the permissible time step is increased tenfold (to 2–6 min) with little loss in accuracy for vertical motions and time scales characteristic of those resolved by general circulation models (GCMs). Such time steps are sufficiently long to permit application of bulk cloud microphysical parameterizations to GCMs for multiyear global simulations. However, we also find that the vertical resolution must be considerably finer (100–200 m) than that currently employed in GCMs.

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Richard C. Easter and Leonard K. Peters

Abstract

Binary homogeneous nucleation of sulfuric acid and water vapor is thought to be the primary source of new particles in the marine atmosphere. The rate of binary homogeneous nucleation depends strongly on temperature and the gas-phase concentrations of both sulfuric acid and water vapor. This paper investigates the effects of these nonlinear dependencies on the rate of formation of new particles. An increase of 2°-3°C can reduce the particle formation rate by an order of magnitude. Large-scale fluctuations such as those characteristic of a well-mixed boundary layer can alternately “turn on” and “shut off” the nucleation process, giving rise to regions of new particle formation that are quite localized. These “bursts” of nucleation correspond to higher altitudes in the boundary layer. Small-scale fluctuations, more typical of normal atmospheric turbulence, can increase the binary homogeneous nucleation rate severalfold above the rate calculated based on mean conditions.

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Peter V. Hobbs, Richard C. Easter, and Alistair B. Fraser

Abstract

Expressions are derived for the horizontal and vertical components of the wind, the temperature, and the mass of water vapor condensed when air flows over a long mountainous ridge. The growth of solid precipitation particles in the orographic clouds by deposition from the vapor phase, riming and aggregation are considered. The trajectories of these precipitation particles are then computed from their fallspeeds and the airflow model.

The model is used to investigate the effects of the microstructure of clouds on the growth and fallout of solid precipitation over the Cascade Mountains. It is shown that, under suitable conditions, increases in the concentration of ice particles in the clouds from about 1 to 100 liter−1 can cause the solid precipitation to be carried farther downwind and over the Cascade crest, so that snowfall is deposited on the eastern rather than the western slopes of the mountains.

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Alistair B. Fraser, Richard C. Easter, and Peter V. Hobbs

Abstract

A model for airflow over mountainous terrain is presented. The equations for steady, two-dimensional, laminar inviscid flow, including pseudo-adiabatic latent heat release, are derived. Approximate solutions to the linearized equations are obtained for stably stratified conditions, and a terrain consisting of broad ridges (width≳25 km), through an iterative transform technique which allows the nonlinear boundary conditions to be satisfied. The model indicates that the dynamical effects of latent heat are significant in some cases but are generally secondary to the barrier effect of the terrain.

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Jerome D. Fast, William I. Gustafson Jr., Elaine G. Chapman, Richard C. Easter, Jeremy P. Rishel, Rahul A. Zaveri, Georg A. Grell, and Mary C. Barth

Abstract

The current paradigm of developing and testing new aerosol process modules is haphazard and slow. Aerosol modules are often tested for short simulation periods using limited data so that their overall performance over a wide range of meteorological conditions is not thoroughly evaluated. Although several model intercomparison studies quantify the differences among aerosol modules, the range of answers provides little insight on how to best improve aerosol predictions. Understanding the true impact of an aerosol process module is also complicated by the fact that other processes—such as emissions, meteorology, and chemistry—are often treated differently. To address this issue, the authors have developed an Aerosol Modeling Testbed (AMT) with the objective of providing a new approach to test and evaluate new aerosol process modules. The AMT consists of a more modular version of the Weather Research and Forecasting model (WRF) and a suite of tools to evaluate the performance of aerosol process modules via comparison with a wide range of field measurements. Their approach systematically targets specific aerosol process modules, whereas all the other processes are treated the same. The suite of evaluation tools will streamline the process of quantifying model performance and eliminate redundant work performed among various scientists working on the same problem. Both the performance and computational expense will be quantified over time. The use of a test bed to foster collaborations among the aerosol scientific community is an important aspect of the AMT; consequently, the longterm development and use of the AMT needs to be guided by users.

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