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Jen-Ping Chen

Abstract

The conventional Köhler theory, which describes the equilibrium sizes of hygroscopic aerosol particles in humid air, is modified by considering the solubility limitation so that the deliquescence and hysteresis processes can also be explained. A set of modified Köhler curves was constructed based on the modified theory. The deliquescence, dehydration, and spontaneous crystallization points (relative humidities) that are the characteristics of the aerosol particles can be identified on the modified Köhler curve diagram. These characteristic relative humidities were derived analytically. The relationships between the equilibrium (wet) size, dry size, and the corresponding solute concentration of ammonium sulfate particles were also established for various ambient relative humidities.

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Jen-Ping Chen

Abstract

The saturation development equation is solved analytically to give a solution that is more general than the existing analytical solution. This analytical solution provides accurate predictions of the saturation ratio and allows the use of relatively large time steps for the simulation of condensation processes. A statistical method that is nonanalytical in nature is also introduced for the prediction of saturation ratio. The performances of these prediction methods are compared for the simulation of drop growth in clouds under idealized situations. It is shown that the more general analytical solution provides improved predictions of saturation ratio under subsaturated conditions. Furthermore, the statistical method is shown to be more efficient and accurate than the analytical methods.

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Jen-Ping Chen
and
Dennis Lamb

Abstract

A detailed microphysical and chemical cloud model has been developed to investigate the redistribution of atmospheric trace substances through cloud processes. A multicomponent categorization scheme is used to group cloud particles into different bins according to their various properties. Cloud drops are categorized simultaneously and independently in both their water mass and solute mass components. Ice phase particles are additionally categorized according to their “shapes,” special effort having been paid to the parameterization of their growth and habit changes. The hybrid bin method used conserves the mass and number of particles while at the same time performing fast and accurate calculations for transferring various properties between categories within the multicomponent framework. With a minimum of parameterization, this model is capable of simulating detailed microphysical and chemical processes that occur during cloud and precipitation formation.

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Jen-Ping Chen
and
Dennis Lamb

Abstract

A theoretical analysis of surface kinetic and gas-phase diffusional effects permits the growth rates and habits of ice crystals to be specified in a self-consistent way. The analysis makes use of the fact that the difference between the condensation coefficients of the prism and basal faces determines the primary crystal habits, whereas the spatial variations of the vapor density contribute to the secondary habits. The parameterization scheme that results from the theoretical analysis yields a power law relationship between the a and c axial lengths that matches earlier empirical formulas derived from observational data for the temperature range of −30° to 0°C. Through application of this adaptive parameterization in a microphysical model that categorizes ice particles according to both their masses and shapes, it is shown that deviations from the power law relationship may develop if the crystals experience significant variations in the air temperature and in their inherent growth habits. A mass-dimension relationship is also derived through the theoretical analysis that can be used as a less detailed parameterization scheme for the growth of ice crystals by vapor deposition.

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Jen-Ping Chen
and
Dennis Lamb

Abstract

A detailed microphysical model is used to simulate the formation of wintertime orographic clouds in a two-dimensional domain under steady-state conditions. Mass contents and number concentrations of both liquid- and ice-phase cloud particles are calculated to be in reasonable agreement with observations. The ice particles in the cloud, as well as those precipitated to the surface, are classified into small cloud ice, planar crystals, columnar crystals, heavily rimed crystals, and crystal aggregates. Detailed examination of the results reveals that contact nucleation and rime splintering are the major ice-production mechanisms functioning in the warmer part of the cloud, whereas deposition/condensation-freezing nucleation is dominant at the upper levels. Surface precipitation, either in the form of rain or snow, develops mainly through riming and aggregation, removing over 17% of the total water vapor that entered the cloud.

The spectral distributions of cloud particles in a multicomponent framework provide information not only on particle sizes but also on their solute contents and, for ice particles, their shapes. Examination of these multicomponent distributions reveals the mechanisms of particle formation and interaction, as well as the adaptation of crystal habits to the ambient conditions. Additional simulations were done to test the sensitivity of cloud and precipitation formation to the size distribution of aerosol particles. It is found that the size distribution of aerosol particles has significant influence on not only the warm-cloud processes, but also the cold-cloud processes. A reduction in aerosol particle concentration not only causes an earlier precipitation development but also an increase in the amount of total precipitation from the orographic clouds.

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Jen-Ping Chen
and
Tzu-Chin Tsai

Abstract

A three-moment modal parameterization scheme was developed for describing variations in the shape of cloud ice crystals during growth by vapor deposition. The shape of ice crystals is represented using the volume-weighted aspect ratio, while the size spectrum of the crystal population is described using a three-parameter gamma function. Verified with binned spectral calculations, the proposed modal scheme performed quite accurately in the evolution of the mass and shape of cloud ice crystals growing under idealized conditions. The associated error is within 1% in mass after 1000 s of growth under water saturation. When the ventilation effect is taken into account, the error remains within 5%. Error with regard to the bulk aspect ratio is generally about 3%. A failure to take into account the ice crystal shape led to a 45% underestimation in mass growth. Using only two moments to describe the gamma distribution led to a 37% underestimation in mass and 28% underestimation in the bulk aspect ratio of the ice crystals. The proposed scheme is able to capture the shape memory effect and the gradual adaptation of ice crystal aspect ratios to a new growth habit regime.

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Tzu-Chin Tsai
and
Jen-Ping Chen

Abstract

To improve the parameterization of ice-phase microphysics in regional meteorological models, this study developed a triple-moment bulk scheme, which also tracks the variations in the shape and density of several hydrometeors. Solid-phase hydrometeors are classified into pristine ice, snow aggregates, rimed ice, and hailstones based on their physical mechanisms. The new scheme has been incorporated into the Weather Research and Forecasting Model and tested with an idealized two-dimensional simulation of a squall-line system. The simulation successfully revealed the smooth transition from the convective core to the stratiform anvil as well as the alternating pattern in the hydrometeor vertical distributions, as was similarly demonstrated in other similar studies. A few sensitivity tests were performed to reveal the importance of including shape and density variations, which strongly affect the mean particle size by up to 50% and fall speed by as much as 100% for individual hydrometeor categories. Furthermore, the inclusion of a third moment could enhance the diffusional growth rate of small crystals and reduce the ventilation effect of large particles compared with the conventional double-moment approach. These factors have a significant influence on cloud structure and precipitation amounts.

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Jen-Ping Chen
,
I-Jen Chen
, and
I-Chun Tsai

Abstract

The influence of present-day anthropogenic aerosols on the summer monsoon over the East Asia region was simulated using the Community Earth System Model coupled with a slab ocean model. The simulations revealed significant radiative forcing from anthropogenic aerosols and associated changes in clouds over East Asia and the northwestern Pacific; however, their spatial patterns differed from the exhibited surface temperature and precipitation responses. Two major dynamic feedback mechanisms were identified to explain such discrepancies. The wind–evaporation–sea surface temperature (WES) feedback, triggered by an initial cooling over the midlatitude sea surface, induced an equatorward expansion of ocean cooling through strengthened trade winds. The sea surface cooling excited a meridional wave pattern similar to the Pacific–Japan teleconnection pattern. Although the aerosol effect generally caused weakening in summer monsoon strength and regional precipitation over East Asia, precipitation increases were seen over the locations of the midlatitude mei-yu front and around the tropics. These precipitation increases are primarily associated with the WES feedback and teleconnection patterns. The aerosol effect also reached the upper troposphere, causing an equatorward shift of the jet stream over East Asia and the northwestern Pacific, indicating a much broader scale of teleconnection.

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Chein-Jung Shiu
,
Shaw Chen Liu
, and
Jen-Ping Chen

Abstract

In this work, 45 years (1961–2005) of hourly meteorological data in Taiwan, including temperature, humidity, and precipitation, have been analyzed with emphasis on their diurnal asymmetries. A long-term decreasing trend for relative humidity (RH) is found, and the trend is significantly greater in the nighttime than in the daytime, apparently resulting from a greater warming at night. The warming at night in three large urban centers is large enough to impact the average temperature trend in Taiwan significantly between 1910 and 2005. There is a decrease in the diurnal temperature range (DTR) that is largest in major urban areas, and it becomes smaller but does not disappear in smaller cities and offshore islands. The nighttime reduction in RH is likely the main cause of a significant reduction of fog events over Taiwan. The smaller but consistent reductions in DTR and RH in the three off-coast islands suggests that, in addition to local land use changes, a regional-scale process such as the indirect effect of anthropogenic aerosols may also contribute to these trends. A reduction in light precipitation (<4 mm h−1) and an increase in heavy precipitation (>10 mm h−1) are found over Taiwan and the offshore islands. The changes in precipitation are similar to the changes of other areas in Asia, but they are different from those of the United States, Europe, and the tropical oceans. The latter do not show any reduction in light precipitation.

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Guoxing Chen
,
Wei-Chyung Wang
, and
Jen-Ping Chen

Abstract

Atmosphere–ocean general circulation models tend to underestimate the solar radiative forcing by stratocumulus over the southeast Pacific, contributing to a warm sea surface temperature (SST) bias. The underestimation may be caused by biases in either macro- or micro- (or both) physical properties of clouds. This study used the WRF Model (incorporated with a physics-based two-moment cloud microphysical scheme) together with the 2008 Variability of the American Monsoon Systems Ocean–Cloud–Atmosphere–Land Study (VOCALS) field observations to investigate the effects of anthropogenic aerosols on the stratocumulus properties and their subsequent effects on the surface radiation balance. The effects were studied by comparing two cases: a control case with the anthropogenic aerosols and a sensitivity case without the anthropogenic aerosols. Results show that the control case produced cloud properties comparable with the measurements by aircraft and that aerosol–cloud microphysical interactions play an important role in regulating solar cloud radiative forcing. As expected, the anthropogenic aerosols increase the cloud droplet number and decrease the cloud droplet size, resulting in an enhancement of solar cloud radiative forcing and a reduction in solar radiation reaching the sea surface, up to a maximum of about 30 W m−2 near the coast. Results also show that aerosol–cloud microphysics–radiation interactions are sensitive to cloud fraction, thus highlighting the role of cloud diurnal variation in studying the cloud–radiation interactions. Analysis of the high-resolution (3 km) model simulations reveals that there exists an inherent scale dependence of aerosol–cloud–radiation interactions, with coarser horizontal resolution yielding a weaker variability.

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