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C. S. Chen

Abstract

The osmotic coefficient of a solvent with a mixture of solutes is evaluated by taking the weighted average over the molality of each solute. The vapor pressure over a drop with a mixture of solutes is intermediate to the vapor pressures over the single-solute drops.

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C. S. Chen

Abstract

No abstract available.

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Shu-Lin Chong and C. S. Chen

Abstract

A numerical model has been developed to investigate water films or shells on ice pellets and hailstones with radii from 0.1 to 0.5 cm.

The model considers a hydrometeor consisting of a rigid, spherical ice core of density 0.9 gm cm−3 surrounded by a shell or film of water. The water volume of the hydrometeor is assumed to be conserved. A set of deformation equations for the water film is constructed from a series expansion of an equation which is based on the balance of pressures acting on the hydrometeor.

The results indicate that an ice pellet or hailstone with radius <0.45 cm can have a water film more or less evenly distributed over the ice core; for a hailstone >0.45 cm radius a rather unrealistic water distribution is obtained from the present model.

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R. D. Farley and C. S. Chen

Abstract

The Wisner one-dimensional time-dependent model has been modified to allow the condensed water forms to be represented by 52 logarithmically spaced size categories covering a range of just under 2 μm radius to slightly less than 5 mm radius. The size distribution of the water drops was allowed to evolve with time as a result of the physical processes of vertical advection, condensation/evaporation, stochastic coalescence, and drop breakup. Salt seeding was simulated by the introduction of a distribution of raindrop embryos at cloud base for a specified period of time. The raindrop embryo distribution was derived from calculations on the diffusional growth of a distribution of salt particles in the unsaturated air below cloud base. This model was applied to the 23 July 1970 salt seeding case reported by Biswas and Dennis. This “detailed microphysical” study has indicated that salt seeding can be effective in stimulating the warm rain process only if breakup-induced chain reactions result. In order for the chain reaction to develop, high vertical velocities (greater than 10 m s−1) are required. Salt seeding acts mainly as a catalyst in initiating this Langmuir-type chain reaction. Without breakup, salt seeding has little effect other than to allow a few of the big drops to fall out of the model clouds. Breakup acting alone may cause the model clouds to precipitate but much longer periods are required than when seeding and breakup are combined. The effects of salt seeding and breakup-induced chain reactions are also strongly dependent on the dynamics of the model cloud.

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Mark R. Hjelmfelt, Richard D. Farley, and Philip C. S. Chen

Abstract

Numerical simulations are conducted to determine the possible effects of air pollution from coal-fired power plants on cloud and precipitation processes in the northern Great Plains. This study can only be considered as preliminary in nature since a complete cloud simulation is not employed and the ice phase is not considered.

Natural and polluted particulate distributions are developed based on observations in the northern Great Plains and of coal-fired power plant effluent. Cloud droplet growth on these distributions is simulated in a condensation model. Results of this model indicate that the number concentration and dispersion coefficient (breadth) of the cloud droplet size distributions are increased by the addition of pollutant particles, especially if these are more hygroscopic than the background nuclei.

Coalescence calculations using the results of the condensation studies as input are also reported. These results indicate that the rate of production of large drops, while being slowed by an increase in the number concentration, is hastened by an increase in the dispersion coefficient. These two effects nearly cancel each other out so that the time required for precipitation development is very nearly the same for cloud droplet distributions initialized on background and polluted particulate distributions. If, however, both distributions have the same dispersion, the polluted case requires a considerably longer time to develop precipitation.

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J. O. Roads, S-C. Chen, and K. Ueyoshi

Abstract

The National Meteorological Center's (NMC's) twice-daily, global 2.5° pressure analyses of temperature, relative humidity, and wind speed are compared, over the coterminous United States, to the National Climatic Data Center's twice-daily, upper-air rawinsonde observations and hourly, first-order, surface observations for the period 1 January 1988 through 31 December 1992. NMCs analyses have clearly improved during this time period. Still, there are some noticeable differences especially near the surface and at 1200 UTC. During the early morning there is a warm bias, relative humidity is too low, and the surface wind speed is too strong. Weaker systematic errors occur during the late afternoon: there is a cold bias, relative humidity is too high, and the surface wind speed is still too strong. Aloft, the bias is noticeably reduced except for the wind speed, which is somewhat too weak. The analysis wind speed also has too strong temporal variations near the surface and too weak temporal variations aloft. The analysis climatology can be corrected at each station by removing the bias. Transient variations can be corrected simply by multiplying the analysis anomalies by the ratio of the station standard deviation to the analysis standard deviation. Correcting for the biases and spatially interpolating the analysis and station collections to a 0.5° grid provides a useful guess for local conditions, especially if there is not a surface or rawinsonde station within about 200 km.

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C-P. Chang, S. C. Hou, H. C. Kuo, and G. T. J. Chen

Abstract

The East Asian summer monsoon (Mei-yu) disturbance of 17–25 June 1992 was the most intense 850-hPa low center of such systems during a 7-yr period. Due to the moisture fluxes associated with the southwesterlies from the warm tropical oceans, diabatic heating has generally been considered the main energy source of these heavy-precipitation disturbances as they propagate eastward from the eastern flank of the Tibetan Plateau across southeastern China and move into the East China Sea. In this study piecewise potential vorticity inversion is used to analyze the physical mechanisms of this intense case, particularly the possible roles of midlatitude baroclinic processes in its development and evolution.

The development of the low-level vortex involved the coupling with two upper-level disturbances, one at 500 hPa that also originated from the eastern flank of the Tibetan Plateau, and another at 300 hPa. Both disturbances appeared later than and upstream of the low-level vortex. Faster eastward movements allowed them to catch up with the low-level vortex and led to a strong vertical coupling and deep tropopause folding. Initially, diabatic heating was the dominant mechanism for the low-level vortex while the tropopause process opposed it. Both mechanisms supported the 500-hPa disturbance, and tropopause folding was the dominant mechanism for the 300-hPa disturbance. As the vertical coupling developed, the tropopause process reversed its earlier role in the low-level disturbance and contributed to its development. Boundary layer and adiabatic effects also became contributive as the disturbance moved out of eastern China to the oceanic region.

The vertical coupling of the three disturbances was a major factor in the development. The timing and position of the middle-tropospheric disturbance was critical in bridging the upper- and lower-level disturbances and a deep tropopause folding. This midlatitude-originated process compounded the diabatic heating effect that was sustained by tropical moist air, leading to the strong intensification.

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X. M. Chen, S.-H. Chen, J. S. Haase, B. J. Murphy, K.-N. Wang, J. L. Garrison, S. Y. Chen, C. Y. Huang, L. Adhikari, and F. Xie

Abstract

This study evaluates, for the first time, the impact of airborne global positioning system radio occultation (ARO) observations on a hurricane forecast. A case study was conducted of Hurricane Karl during the Pre-Depression Investigation of Cloud-Systems in the Tropics (PREDICT) field campaign in 2010. The assimilation of ARO data was developed for the three-dimensional variational (3DVAR) analysis system of the Weather Research and Forecasting (WRF) Model version 3.2. The impact of ARO data on Karl forecasts was evaluated through data assimilation (DA) experiments of local refractivity and nonlocal excess phase (EPH), in which the latter accounts for the integrated horizontal sampling along the signal ray path. The tangent point positions (closest point of an RO ray path to Earth’s surface) drift horizontally, and the drifting distance of ARO data is about 2 to 3 times that of spaceborne RO, which was taken into account in these simulations.

Results indicate that in the absence of other satellite observations, the assimilation of ARO EPH resulted in a larger impact on the analysis than local refractivity did. In particular, the assimilation of ARO observations at the actual tangent point locations resulted in more accurate forecasts of the rapid intensification of the storm. Among all experiments, the best forecast was obtained by assimilating ARO data with the most accurate geometric representation, that is, the use of nonlocal EPH operators with tangent point drift, which reduced the error in the storm’s predicted minimum sea level pressure (SLP) by 43% beyond that of the control experiment.

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Z. Pan, M. Segal, R. W. Arritt, T-C. Chen, and S-P. Weng

Abstract

A combined observational and numerical modeling methodology was employed to examine the interaction of large-scale quasi-stationary waves with regional forcing in the drought of 1988 and flood of 1993 in the north-central United States. The study implements and tests a new lateral boundary condition closure approach enabling regional climate modeling sensitivity studies to account for anomalies external to the simulated domain.

Thirty-day regional simulations of the continental United States using the MM5 regional model successfully reproduced the anomalous drought and flood conditions. In an additional set of simulations the observed large-scale quasi-stationary wave anomalies in the dynamic and thermodynamic fields were filtered out through adjustment of the observationally based lateral boundary conditions. In these filtered simulations the simulated meteorological fields over the drought/flood areas for both years tended toward those for normal years, suggesting that the proposed filtering technique can be used as a tool to test the regional response to external forcing. Comparisons of the control and filtered simulations illustrated the significant influence of large-scale anomalies on the strength and geographical distribution of the low-level jet and related impacts on rainfall in the central United States.

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S. B. Trier, F. Chen, K. W. Manning, M. A. LeMone, and C. A. Davis

Abstract

A coupled land surface–atmospheric model that permits grid-resolved deep convection is used to examine linkages between land surface conditions, the planetary boundary layer (PBL), and precipitation during a 12-day warm-season period over the central United States. The period of study (9–21 June 2002) coincided with an extensive dry soil moisture anomaly over the western United States and adjacent high plains and wetter-than-normal soil conditions over parts of the Midwest. A range of possible atmospheric responses to soil wetness is diagnosed from a set of simulations that use land surface models (LSMs) of varying sophistication and initial land surface conditions of varying resolution and specificity to the period of study.

Results suggest that the choice of LSM [Noah or the less sophisticated simple slab soil model (SLAB)] significantly influences the diurnal cycle of near-surface potential temperature and water vapor mixing ratio. The initial soil wetness also has a major impact on these thermodynamic variables, particularly during and immediately following the most intense phase of daytime surface heating. The soil wetness influences the daytime PBL evolution through both local and upstream surface evaporation and sensible heat fluxes, and through differences in the mesoscale vertical circulation that develops in response to horizontal gradients of the latter. Resulting differences in late afternoon PBL moist static energy and stability near the PBL top are associated with differences in subsequent late afternoon and evening precipitation in locations where the initial soil wetness differs among simulations. In contrast to the initial soil wetness, soil moisture evolution has negligible effects on the mean regional-scale thermodynamic conditions and precipitation during the 12-day period.

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