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Ping Chen

Abstract

It is shown analytically and numerically that in certain shear flows the linearized nondivergent barotropic vorticity equation has a limited number of neutral normal modes. The latitudinal structures of these shear flows can be expressed as polynomials of the sine of latitude. The first few such shear flows resemble the gross features of the zonal winds in the atmosphere of the earth at different tines and altitudes. The spatial structures of the neutral normal modes in these shear flows are spherical harmonics, and, as a consequence, these modes are also the exact solutions of the fully nonlinear equation because the nonlinear interaction term vanishes identically.

The spatial structures of the observed 5-, 4-, 2-, and 16-day free traveling waves in the atmosphere are often identified with the spherical harmonics with indices of (m, n) = (1, 2), (2, 3), (3, 3), and (1, 4), which are known previously as the neutral normal equation in a motionless background state. Our results could explain why these free traveling waves can survive the shearing effects of zonal flows that are far different from rest because these spherical harmonies are also normal modes in certain shear flows that resemble the observations of the atmosphere.

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Ping Chen

Abstract

The influences of the zonal-mean flow on Rossby wave breaking and tropical-extratropical interaction in the lower stratosphere are investigated using a high-resolution barotropic model. It is found that the zonal-mean wind in the subtropics of the winter hemisphere, denoted as ū30°, is pivotal to the location and intensity of Rossby wave breaking. When ū30° is positive and relatively large, significant wave breaking occurs in two regions: one in the midlatitudes of the winter hemisphere and the other in the Tropics. When ū30° is small or negative, on the other hand, wave breaking occurs primarily in the middle-to-high latitudes of the winter hemisphere. It is also found that when ū30° is large, wave breaking is sensitive to the phase of the equatorial quasi-biennial oscillation (QBO) in that if the QBO is westerly, significant wave breaking takes place in the midlatitudes of the winter hemisphere and the subtropics of the summer hemisphere and no wave breaking occurs in the equatorial region, and if the QBO is easterly, significant wave breaking occurs only in the winter hemisphere. When ū30° is small or negative, wave breaking is insensitive to the phase of the QBO.

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Ping Chen

Abstract

Analytical solutions of thermally forced stationary waves in a linear quasigeostrophic model are obtained. It is found that the zonal flow has a profound impact on the structure of the responses. The inviscid solutions on a resting basic state are the Sverdrup solutions that are confined to the heating region. The solutions on a westerly zonal flow are composed of a local and a vertically propagating part. The local response exists only in the heating region. The vertically propagating response exists in the far field as well as in the heating region. The thermally forced vertically propagating response can be conceptualized as a response to an “equivalent topography,” the height of which is proportional to the intensity and zonal scale of the heating, and inversely proportional to the strength of the zonal flow. Particular solutions forced by realistic summer heating fields reveal that, for weak westerlies, the height of the equivalent topography is much larger than that of the real topography, suggesting that heating is more important than topography in forcing the summer stationary waves in the subtropics. It is also found that Newtonian cooling has a significant effect on the structure of the thermally forced stationary waves.

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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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Ping Chen and Matthew Newman

Abstract

The upper-tropospheric circulation is investigated for the three months of April, May, and June 1988 during which the Great Plains region of the United States experienced one of its most severe droughts in history. It is found that during this period the April–June (AMJ) seasonal-mean anomaly was not representative of the variability of 10-day low-pass anomalies. Rather, over North America large fluctuations on monthly and shorter timescales occurred, with the dominant streamfunction anomalies not strongly anticyclonic until June. In fact, the AMJ anomaly was dominated by two episodes of rapidly developing, intense anomalous anticyclones in early and late June.

Examination of the daily 10-day low-pass streamfunction anomalies at 300 mb suggests that propagating Rossby waves originating in the west Pacific played a dominant role in the initiation of these intense anomalous anticyclones. Numerical experiments with a linear, time-dependent, barotropic model also support this hypothesis. These results suggest that the AMJ anomaly, which has been characterized as a wave train seemingly forced in the east Pacific, may not provide a useful picture of the circulation associated with the drought. Instead, the drought may be better studied not as a single seasonal event, but rather as a succession of events that together produced a serious hydrological deficit.

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Ping Huang and Dong Chen

Abstract

El Niño–Southern Oscillation (ENSO) is one of the most important sources of climate interannual variability. A prominent characteristic of ENSO is the asymmetric, or so-called nonlinear, local rainfall response to El Niño (EN) and La Niña (LN), in which the maximum rainfall anomalies during EN are located farther east than those during LN. In this study, the changes in rainfall anomalies during EN and LN are examined based on the multimodel ensemble mean results of 32 CMIP5 models under the representative concentration pathway 8.5 (RCP8.5) scenario. It is found that robust EN–LN asymmetric changes in rainfall anomalies exist. The rainfall anomalies during EN and LN both shift eastward and intensify under global warming, but the eastward shift during EN is farther east than that during LN. A simplified moisture budget decomposition method is applied to study the mechanism of the asymmetric response. The results show that the robust increase in mean-state moisture can enlarge the EN–LN asymmetry of the rainfall anomalies, and the spatial relative changes in mean-state SST with an El Niño–like pattern can shift the rainfall anomalies farther east during EN than during LN, enlarging the difference in the zonal locations of the rainfall response to EN and LN. The role of the relative changes in mean-state SST can also be interpreted as follows: the decreased zonal gradient of mean-state SST due to El Niño–like warming leads to a larger EN–LN asymmetry of rainfall anomalies under a future warming climate.

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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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Ping Chen and Walter A. Robinson

Abstract

The propagation of planetary-scale Rossby waves in the stratosphere is investigated in a linear, time-dependent, primitive equation model. Two distinct maxima in the convergence of the Eliassen-Palm flux (EP flux) are found for steady and for transient waves in a variety of realistic Northern hemisphere, winter season, zonal flows; one below the stratospheric polar jet and the other north of the zero wind line. These maxima appear at higher altitudes for wave 1 than for wave 2, especially in low latitudes.

Different mechanisms cause the formation of these two maxima. The confinement of wave activity in high latitudes is mainly due to the dissipation of waves in that region under non-WKBJ conditions. The maximum in lower latitudes is related to the absorption of Rossby waves near their critical lines.

For wave 2, the intensity of the high latitude maximum is sensitive to the transience of the wave forcing at the tropopause, while for wave 1 this sensitivity is reduced. For both waves the low latitude maximum shifts northward with increasing transience.

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Lie-Yauw Oey and Ping Chen

Abstract

A time-dependent, three-dimensional primitive-equation model is used here to study meanders that develop upstream of a western boundary current along a continental slope blocked by a diabathic topographic feature. It is found that episodic, large-amplitude meanders and shelfward intrusions occur at upstream distances, which coincide approximately with the topographic standing wavelength. In addition to its potential applications to other problems of front-topography interaction, the phenomenon may be relevant to branching and eddy intrusions of the Kuroshio southwest of Kyushu, Japan, and to the initiation and amplification of Gulf Stream meanders upstream of the “Charleston Bump,” a topographic hump on the continental slope of the U.S. South Atlantic Bight.

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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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