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P. K. Wang and H. R. Pruppacher

Abstract

Experiments have been carried out to determine the efficiency with which aerosol particles of 0.25 μm radius are collected due to Brownian diffusion, and due to hydrodynamic, phoretic and electrical effects by water drops of 150 to 2500.μm equivalent radius falling in subsaturated air. In the absence of electrical effects it was found that with increasing drop size the collection efficiency decreases to a minimum and then rises again as the collection due to phoretic forces is overcompensated by the collection due to hydrodynamic forces. With further increase in drop size the collection efficiency was found to rise to a maximum, This rise was attributed to hydrodynamic effects in the rear of the drop which increase as the stagnant eddy at the downstream end of the falling drop increases in size, but progressively decrease as the drop assumes a size, and thus a Reynolds number, large enough for turbulent eddies to be shed from the rear of the drop. The present results are qualitatively consistent with the predictions of experiments reported in the literature and quantitatively agree with the theoretical predictions made by the model of Grover et al. (1977). Electrical charges on drop and aerosol particles were found to significantly raise the collection efficiency, in good agreement with the efficiency theoretically predicted by the model of Grover et al. (1977).

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P. K. Wang and H. R. Pruppacher

Abstract

A theoretical method is given which allows computing the acceleration to terminal velocity of cloud and raindrops at various levels in the atmosphere. For drops of equivalent radius 800 μm ≤ a 0 ≤ 3500 μm our theoretical predictions were found to agree well with the results of an experimental study carried out in the UCLA Rain-Shaft. For drops of 20 μm ≤ a 0 ≤ 80 μm our theoretical predictions were found to agree well with the experimental results of Sartor and Abbott (1975). Experiment and theory indicate that in air of 1000 mb and 20°C, drops of a 0 > 1000 μm need distances of at least 12 m to accelerate to terminal velocity.

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S. J. Martin, P. K. Wang, and H. R. Pruppacher

Abstract

Two theoretical models are presented which allow computing the efficiency with which aerosol particles of radius 0.001 ≤ r ≤ 10 μm are collected by simple ice crystal plates of radius 50 ≤ ac ≤ 640 μm, in air of various relative humidity, temperature and pressure. Particle capture due to Brownian diffusion, thermophoresis, diffusiophoresis and inertial impaction is considered. It is shown that, analogous to water drops, ice crystal plates exhibit a minimum collection efficiency within a specific size interval of aerosol particles. This minimum is strongly affected by the relative humidity of the ambient air. The collection efficiency of particles with r > 1 μm is controlled by the flow field around the ice crystal, while the collection efficiency of particles with r < 0.01 μm is controlled by convective Brownian diffusion. Trajectory analysis predicts that aerosol particles are preferentially captured by the ice crystal rim. Our theoretical results are found to agree satisfactorily with laboratory studies presently available.

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P. K. Wang, C. H. Chuang, and N. L. Miller

Abstract

Formulas suitable for calculating the electrostatic, temperature, and vapor density fields surrounding stationary columnar ice crystals are derived. Columnar ice crystals are approximated as circular cylinders of finite lengths. In this way the effects of sharp edges are taken into account. Results of electrostatic fields for some columnar ice crystals are shown. The potential distribution of a prolate spheroid is also determined and compared to that of a circular cylinder. The results show that the approximation of a columnar crystal by a prolate spheroid is inadequate. Formulas are also given to convert the electric fields into temperature and vapor density fields.

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P. K. Wang, S. N. Grover, and H. R. Pruppacher

Abstract

A theoretical model is described which determines the efficiency E with which aerosol particles of radius r are collected by water drops of radius a due to the combined action of Brownian diffusion, thermo- and diffusiophoresis and electric forces, in the absence of inertial impaction effects. The results of this model are combined with the results of our earlier model which determines the collection efficiency of drops for particles due to the combined action of inertial impaction, thermo- and diffusiophoresis and electric forces, in the absence of effects due to Brownian diffusion. Both models combined quantitatively determine the variation of E vs r for 0.001≤r≤10 μm, and 42≤a≤310 μm, for relative humidities up to and including 100%, and for electric charges on drops and aerosol particles ranging in magnitude up to that found under thunder-storm conditions. In particular, a combination of both models allows a quantitative description of the particle size range where the collection efficiency of drops is minimum.

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H. Wang, R. T. Pinker, P. Minnis, and M. M. Khaiyer

Abstract

Solar radiation reaching the earth’s surface provides the primary forcing of the climate system, and thus, information on this parameter is needed at a global scale. Several satellite-based estimates of surface radiative fluxes are available, but they differ from each other in many aspects. The focus of this study is to highlight one aspect of such differences, namely, the way satellite-observed radiances are used to derive information on cloud optical properties and the impact this has on derived parameters such as surface radiative fluxes. Frequently, satellite visible radiance in a single channel is used to infer cloud transmission; at times, several spectral channels are utilized to derive cloud optical properties and use these to infer cloud transmission. In this study, an evaluation of these two approaches will be performed in terms of impact on the accuracy in surface radiative fluxes. The University of Maryland Satellite Radiation Budget (UMD/SRB) model is used as a tool to perform such an evaluation over the central United States. The estimated shortwave fluxes are evaluated against ground observations at the Atmospheric Radiation Measurement Program (ARM) Central Facility and at four ARM extended sites. It is shown that the largest differences between these two approaches occur during the winter season when snow is on the ground.

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K. P. Sooraj, H. Annamalai, Arun Kumar, and Hui Wang

Abstract

The 15-member ensemble hindcasts performed with the National Centers for Environmental Prediction Climate Forecast System (CFS) for the period 1981–2005, as well as real-time forecasts for the period 2006–09, are assessed for seasonal prediction skills over the tropics, from deterministic (anomaly correlation), categorical (Heidke skill score), and probabilistic (rank probability skill score) perspectives. Further, persistence, signal-to-noise ratio, and root-mean-square error analyses are also performed. The CFS demonstrates high skill in forecasting El Niño–Southern Oscillation (ENSO) related sea surface temperature (SST) anomalies during developing and mature phases, including that of different types of El Niño. During ENSO, the space–time evolution of anomalous SST, 850-hPa wind, and rainfall along the equatorial Pacific, as well as the mechanisms involved in the teleconnection to the tropical Indian Ocean, are also well represented. During ENSO phase transition and in the summer, the skill of forecasting Pacific SST anomalies is modest. An examination of CFS ability in forecasting seasonal rainfall anomalies over the U.S. Affiliated Pacific Islands (USAPI) indicates that forecasting the persistence of dryness from El Niño winter into the following spring/summer is skillful at leads > 3 months. During strong El Niño years the persistence is predicted by all members with a 6-month lead time. Also, the model is skillful in predicting regional rainfall responses during different types of El Niño. Since both deterministic and probabilistic skill scores converge, the suggestion is that the forecast is useful. The model’s skill in the real-time forecasts for the period 2006–09 is also discussed. The results suggest the feasibility that a dynamical-system-based seasonal prediction of precipitation over the USAPI can be considered.

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G. S. Kent, E. R. Williams, P-H. Wang, M. P. McCormick, and K. M. Skeens

Abstract

Data from the Stratospheric Aerosol and Gas Experiment II (SAGE II) solar occultation satellite instrument have been used to study the properties of tropical cloud over the altitude range 10.5–18.5 km. By virtue of its limb viewing measurement geometry, SAGE II has good vertical resolution and sensitivity to subvisual cloud not detectable by most other satellite instruments. The geographical distribution and temporal variation of the cloud occurrence have been examined over all longitudes on timescales from less than 1 day to that of the El Niño-Southern Oscillation (ENSO) cycle. Significant variations in cloud occurrence are found on each of these scales and have been compared with the underlying surface temperature changes. Maximum cloud occurs over the warm pool region of the Pacific Ocean, with secondary maxima over the South American and Central African landmasses, where the percentage of cloud occurrence in the upper troposphere can exceed 75%. Cloud occurrence at all altitudes within the Tropics, over both land and ocean, increases with the underlying surface temperature at a rate of approximately 13%°C−1. Extrapolated threshold temperatures for the formation of cloud are about 5°C lower than those found from nadir viewing observations. This difference is believed to be a consequence of the averaging process and the inclusion of outliers in the dataset. ENSO cycle changes in cloud occurrence are observed, not only over the Tropics but also over the subtropics, indicating a difference in the meridional Hadley circulation between ENSO warm and cold years. Sunrise–sunset cloud differences indicate that large-scale variations, whose form resembles that of the Hadley and Walker circulations, are present, with a timescale of 1 day or less. The global distribution of upper-tropospheric ice and its positive correlation with surface temperature on all timescales are generally consistent with the behavior of lightning and the global electrical circuit.

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C. Walcek, P. K. Wang, J. H. Topalian, S. K. Mitra, and H. R. Pruppacher

Abstract

An experimental method involving the UCLA Rain Shaft is described. This method allows determining the rate at which SO2 is scavenged from air by freely falling water drops. In the present experiment water drops of radii near 300 μm were allowed to pass through a chamber filled with SO2 whose partial pressure was determined by an infrared spectrometer. By varying the length of the gas compartment, the drops could be exposed to SO2 for different intervals of time. An electrochemical method verified by three quantitative chemical methods was used to determine the total amount of sulfur taken up by the drops falling through the gas compartment. The present experimental results were compared with the results from our theoretical model (Baboolal et at., 1981), which was evaluated for the present experimental conditions. Satisfactory agreement between experiment and theory was found.

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J. L. Zhang, Y. P. Li, G. H. Huang, C. X. Wang, and G. H. Cheng

Abstract

In this study, a Bayesian framework is proposed for investigating uncertainties in input data (i.e., temperature and precipitation) and parameters in a distributed hydrological model as well as their effects on the runoff response in the Kaidu watershed (a snowmelt–precipitation-driven watershed). In the Bayesian framework, the Soil and Water Assessment Tool (SWAT) is used for providing the basic hydrologic protocols. The Delayed Rejection Adaptive Metropolis (DRAM) algorithm is employed for the inference of uncertainties in input data and model parameters with global and local adaptive strategies. The advanced Bayesian framework can help facilitate the exploration of variation of model parameters due to input data errors, as well as propagation from uncertainties in data and parameters to model outputs in both snow-melting and nonmelting periods. A series of calibration cases corresponding to data errors under different periods are examined. Results show that 1) input data errors can affect the distributions of model parameters as well as parameters’ correlation, implying that data errors could influence the related hydrologic processes as well as their relations; 2) considering input data errors could improve the hydrologic simulation ability for peak streamflows; 3) considering errors of temperature and precipitation data as well as uncertainties of model parameters can provide the best modeling simulation performance in the snow-melting period; and 4) accounting for uncertainties in precipitation data and model parameters can provide the best modeling performance during the nonmelting period. The findings will help enhance hydrological model’s capability for simulating/predicting water resources during different seasons for snowmelt–precipitation-driven watersheds.

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