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M. B. Baker

Abstract

The statistical properties of airborne tracer concentrations, and the time-lagged correlation between concentration and wind velocity, are calculated from a simple model for the fluctuations in wind velocity. The mean concentration distribution thus calculated is compared with that obtained using the usual diffusivity formulation for transport, and the limitations on the diffusivity approximation discussed for various sources and sinks. It is shown that the model provides a method for calculation of the concentration statistics in the presence of velocity-dependent sources.

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T. Ackerman and M. B. Baker

Abstract

Clouds in some polluted areas may contain high concentrations of anthropogenic aerosol particles. The possible role of these particles in perturbing the optical and dynamical properties of the clouds is an important question for climate studies. The direct radiative effects of unactivated aerosol particles in stable stratus clouds have been calculated at λ = 0.5 μm. Several simplifying assumptions have been made relating the behavior of such particles in the high humidity environment within the cloud to their physicochemical make-up. It is shown that the energy absorbed by particles within the clouds may be, for realistic concentrations, comparable to the latent heat released and thus may play a significant role in cloud dynamics in some areas. These results are shown to be relatively insensitive to the assumptions about the particle properties within the cloud.

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M. B. Baker and J. Latham

Abstract

Calculations have been made of the evolution of droplet spectra within small cumulus clouds which are entraining undersaturated environmental air. The mixing process is assumed to be highly inhomogeneous. In the extreme situation considered, environmental air is entrained in discrete blobs or parcels, causing some droplets of all sizes to be completely removed from the condensate spectrum, while others do not change in size. This model, which is based on laboratory experiments, corresponds to a situation in which the time constant for droplet evaporation is small relative to that for turbulent mixing; in the classical (homogeneous) model, which has been used by other workers, the reverse applies. The calculations produce spectral shapes which agree well with those observed in cumulus by Warner (1969), and they indicate that favored droplets may grow very much faster through the condensate spectrum than is predicted classically.

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J. M. Baker, D. C. Reicosky, and D. G. Baker

Abstract

Many models in a variety of disciplines require air temperature throughout the day as an input, yet often the only data available are daily extrema. Several methods for estimating the diurnal change in temperature from daily extrema have been reported. This paper compares the performance of three such algorithms (a sine wave, a sine-exponential, and a linear model) at all times of the year. Each was used to generate four years of hourly temperatures using as input the daily highs and lows recorded by the National Weather Service at the Minneapolis-St. Paul International Airport for the years 1970, 1971, 1973 and 1974. The output from the models was compared with the actual hourly values recorded at the same site. Residual sums and standard errors computed for all three models, both as functions of day and month of the year, showed that all three were more accurate in summer than in winter.

Differences in the overall standard errors of estimate among the three models were small relative to the standard errors themselves. The sinusoidal model was the most accurate of three in predicting midday summer temperatures, suggesting that it would be the best choice for photosynthesis or transpiration models. There was little difference in overall residual sums, indicating equal suitability for use in models driven by accumulated heat units.

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Keith Ronnholm, M. B. Baker, and Halstead Harrison

Abstract

While it is now possible to compute the optical transmission, reflection and absorption of a homogeneous horizontal layer of known parameters to great accuracy, the input parameters (optical depth, scattering phase function and single-scattering albedo) are in general neither precisely known nor exactly constant over a layer. In this paper several simple representations of the distributions of input parameters are used to compute the mean values and standard deviations of layer transmission, reflection and absorption. The effects of variability in input parameters depend on the mean-layer properties; under certain conditions this variability induces errors on the order of 5% in the derived optical properties. This magnitude is comparable to the differences between those obtained by the Eddington, delta-Eddington, and discrete ordinate four-steam approximations, and a more precise 20-stream doubling method.

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Michael N. Baker and Derek M. Cunnold

Abstract

A contour advection technique, contour advection with surgery (CAS), is applied to the Northern Hemisphere Arctic vortex during several dynamically active periods in midwinter and at several different levels in the stratosphere. The ability of the technique to accurately depict vortex evolution is assessed. Isentropic potential vorticity (PV) is used as the dynamic tracer, and observed winds on isentropic surfaces are used to advect the PV contours. Results of the current study show that while it may provide a continuous view of vortex evolution that is spatially and temporally consistent, quantitative information regarding vortex area change derived from CAS is of limited utility. The results are shown to be somewhat sensitive to subtle differences in the wind and PV fields for quantities (such as area) that develop slowly in time. Increasing the temporal resolution of the advecting wind field does not appear to improve the agreement between CAS results and model or observed data. The poor correspondence between the area changes from CAS and those derived from analyzed data suggests that diabatic effects during most of these events are important. Percentage differences between the CAS and the analyzed PV contours are more pronounced above the lower region of the stratosphere, and a large part of the differences are acquired in the first day of a CAS calculation. Contour length parameters such as e-folding time are relatively insensitive to uncertainties in the initial conditions, suggesting that quantities that change rapidly (as length does) are better determined by CAS; however, the filamentary structure contributes little to vortex area results.

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Peter W. Baker and M. C. Hodson

Abstract

In 1964, R.R. Rogers proposed a method for estimating vertical air velocity in rainfall which is based on the Marshall-Palmer raindrop size distribution. Many investigators have shown that there are deviations from the Marshall-Palmer distribution such that the intercept N 0, and the shape may vary. Essential to Rogers' method is the relationship between reflectivity Z and Doppler fall velocity . A numerical method has been used to obtain versus Z relationships for raindrop size distributions different from those of Marshall and Palmer and to compare these with results obtained using Rogers's method. Thus errors that are likely to occur in estimating vertical air motions have been evaluated and are shown to be a maximum of 1.4 m s−1.

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W. P. Giddings and M. B. Baker

Abstract

Haze particles coated by organic substances have been found in some maritime regions. It is shown that the most likely origin of surfactant material on atmospheric droplets is the ocean surface, which contains fatty acids and other organics previously identified in atmospheric aerosol particles as well as calcium soaps and proteinaceous substances which could act as surfactants. Persistent hazes, but not fogs, may result from stabilization of unactivated atmospheric droplets by organic surfactants in maritime regions. Calculations show that under time-varying atmospheric conditions the primary effect of surfactants is to decrease the steady-state portion of. the growth rate. A kinetic mechanism is proposed for the monolayer inhibitory action, implying limitations on the utility of the accommodation coefficient formulation.

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E. Kalnay, M. Kanamitsu, and W.E. Baker

In this paper we describe the global numerical weather prediction system of the National Meteorological Center, and review recent improvements, the evolution in skill, and current research projects and plans.

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W. E. Baker, R. Atlas, M. Halem, and J. Susskind

Abstract

In this study we examine the sensitivity of forecast to individual components of the First GARP (Global Atmospheric Research Programme) Global Experiment database as well as to some modifications in the data analysis techniques. Several short assimilation experiments (0000 GMT 18 January 1979 through 0000 21 January) are performed in order to test the effects of each database or analysis change. Forecasts are then generated from the initial conditions provided by these experiments. The 0000 21 January case is chosen for a detailed investigation because or the poor forecast skill obtained earlier over North America for that particular case. Specifically, we conduct experiments to test the sensitivity of forecast skill to: 1) the addition of individual satellite observing system components; 2) temperature data obtained with different satellite retrieval methods; and 3) the method of vertical interpolation between the mandatory pressure analysis levels and the model sigma levels.

For the single case examined, TIROS-N infrared land retrievals produced operationally are found to degrade the forecast, while the use of TIROS-N retrievals produced with a physical inversion method as part of an analysis/forecast cycle results in an improved forecast. The use of oceanic VTPR (Vertical Temperature Profile Radiometer) satellite retrievals also results in an improved forecast over North America. The forecast is also found to be sensitive to the method of vertical interpolation between the mandatory pressure analysis levels and the model sigma levels.

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