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Chih-Yue Jim Kao and Tetsuji Yamada

Abstract

A “four-dimensional data assimilation” technique is employed in a time-dependent, three-dimensional mesoscale model to simulate long-range pollutant transport and diffusion in the eastern United States using the 1983 Cross-Appalachian Tracer Experiment (CAPTEX) data. CAPTEX deployed 19 rawinsonde stations to measure upper-air meteorological conditions four times daily and 86 automatic sequential air samplers to measure tracer concentrations from a point source. The total area coverage of the data network is approximately 1000 km (east-west) × 800 km (north-south).

The assimilated wind fields and model-produced turbulence fields during a period of 2¼ days are used to simulate plume trajectories and surface concentrations through a random-particle statistical method. Two tracer releases in the CAPTEX are investigated: one was in a light-wind fair weather condition and produced a widely spread puff distribution; the other was associated with a surface cold front resulting in a rather narrow Puff distribution. The observed winds are successfully assimilated in both cases except in the period of the cold front passage, suggesting that a finer temporal resolution of the rawinsonde observations is desirable in dealing with special weather conditions. The general patterns of the puff distributions are also well simulated. Quantitatively, 57% of the modeled concentrations are within a factor of 4 in comparison with the observed concentrations in the light-wind case.

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Chih-Yue Jim Kao and Tetsuji Yamada

Abstract

Detailed observations of both mean and turbulence fields of an anticyclonic, quasi-steady state, stratocumulus-capped boundary layer obtained with ground-based and balloonborne equipment during the night of 19/20 November 1976 at Cardington, Bedford, UK, are simulated in relation to large-scale subsidence, longwave radiative model cooling, and large-scale moisture supply from sea to land, using a simplified second-order turbulence-closure radiative model.

Using a one-dimensional version of the model, most of the observed features are well simulated, including the large temperature “jump” in a thin layer at cloud top, thermodynamic profiles within the boundary layer, cloud depth and cloud liquid water content, turbulence in the cloud layer, and radiative fluxes and their associated cooling (heating) rates. The results also show that in order to reproduce the observed features, the large-scale subsidence rate and horizontal moisture input should be properly incorporated.

In addition to the one-dimensional simulations for the observed balloon profiles, we used a three-dimensional version of the model to investigate the mechanisms which resulted in a cloudless band embedded in this large sheet of stratocumulus, observed during the same night around the north shore of the English Channel. The physics derived from the one-dimensional simulations applies well in the three-dimensional model. The sensitivity tests show that the terrain effects, which induce larger downward vertical motion, are primarily responsible for this clear band.

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Chih-Yue Jim Kao and Lawrence H. Auer

Abstract

A straightforward iterative method is developed for solving the pressure field in three-dimensional, anelastic, nonhydrostatic, mesoscale models with uneven-spacing grid systems for which direct FFT (fan Fourier transform) schemes cannot be applied. This method utilizes the ADI (alternating direct implicit) integration scheme and a convergence-acceleration technique (Orthmin) to reach the asymptotic solution for a Poisson equation. It also permits the use of various types of boundary conditions in three dimensions. The efficiency of this method in terms of computing time and accuracy is presented.

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Chih-Yue Jim Kao and Yoshi Ogura

Abstract

A cumulus parameterization scheme developed by Arakawa and Schubert was tested through a semiprognostic approach using two different datasets: one for a tropical cloud band, the other for tropical composite easterly wave disturbances. Both were observed in GARP (Global Atmospheric Research Program) Atlantic Tropical Experiment using a computational algorithm different from that of Lord. Also, an efficient software package from the International Mathematics and Statistics Library was used in determining the cloud mass flux at the cloud base level.

The semiprognostic results indicate that the cloud heating and drying effects predicted by the Arakawa-Schubert scheme in both cases agree rather well with the observations. Also, the predicted cloud population in terms of cloud-base mass flux shows the prominent features as already revealed by other previous diagnostic studies in the tropical area. The Arakawa-Schubert scheme underestimates both condensation and evaporation rates substantially when compared with the cumulus ensemble model results for the cloud band case by Soong and Tao and another subsequent case by Tao. An inclusion of the downdraft effects associated with the evaporation of rainfall appears to alleviate this deficiency.

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Yoshi Ogura and Chih-Yue Jim Kao

Abstract

As an extension of the work presented in an accompanying paper of Kao and Ogura, the Arakawa-Schubert cumulus parameterization scheme is examined prognostically in a modeling study of the evolution of a convectively driven tropical mesoscale rainband that developed on 12 August 1974 over the eastern Atlantic. A two-dimensional hydrostatic model is used with a mixed layer parameterization. Observed soundings were used as initial conditions and 24-h integrations of the model are made.

In the control experiment, a prescribed time-independent large scale forcing is imposed in a limited area at low levels. Many aspects of the observed evolution of the rainband are well simulated by the model, including the shift of height of the area-averaged maximum upward motion from low levels to upper levels and the development of downward motion at low levels while upward motion of a significant magnitude is still present at upper levels. The predicted rainfall rate, in the mature stage, is also found to agree with the observation. A secondary development of deep convection away from the primary convective area is generated at later times due to the adiabatic subsidence warming at upper levels. The predicted cloud population is bimodal at the developing stage of the mesoscale circulation, whereas it is dominated by deep convection at the mature stage, consistent with earlier diagnostic studies. Quantitatively, however, some discrepancies are observed between the predicted and observed evolution of the mesoscale circulation.

Several sensitivity tests of the model are made. Removing the large-scale forcing at the time when the convective system reaches its peak intensity immediately results in the decay of the system. Including the vertical wind shear does not have much effect on the life cycle of the system except that the convective activities shift slightly to the downwind area. Permitting cumulus downdraft effects in the Arakawa-Schubert scheme reduces the intensity of circulation due to the cooling of subcloud layer introduced by the detrainment of the downdrafts.

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Scott Elliott, Xuepeng Zhao, Richard P. Turco, Chih-Yue Jim Kao, and Mei Shen

Abstract

Atmospheric photochemistry lies at the heart of global-scale pollution problems, but it is a nonlinear system embedded in nonlinear transport and so must be modeled in three dimensions. Total earth grids are massive and kinetics require dozens of interacting tracers, taxing supercomputers to their limits in global calculations. A matrix-free and noniterative family scheme is described that permits chemical step sizes an order of magnitude or more larger than time constants for molecular groupings, in the 1-h range used for transport. Families are partitioned through linearized implicit integrations that produce stabilizing species concentrations for a mass-conserving forward solver. The kinetics are also parallelized by moving geographic loops innermost and changes in the continuity equations are automated through list reading. The combination of speed, parallelization, and automation renders the programs naturally modular. Accuracy lies within 1% for all species in week-long fidelity tests. A 50-species, 150-reaction stratospheric module tested in a spectral GCM benchmarks at 10 min CPU time per day and agrees with lower-dimensionality simulations. Tropospheric nonmethane hydrocarbon chemistry will soon be added, and inherently three-dimensional phenomena will be investigated both decoupled from dynamics and in a complete chemical GCM.

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