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J. Shen, M. Yu, and J. Lin

Abstract

For nearly 60 years, the lognormal distribution has been the most widely used function in the field of atmospheric science for characterizing atmospheric aerosol size distribution. We verify whether the three-parameter inverse Gaussian distribution (IGD) is a more suitable function than the lognormal distribution for characterizing aerosol size distribution. An attractive feature of IGD is that with it a new method of moments (MOM) can be established for resolving atmospheric aerosol dynamics which is described by a kinetic aerosol dynamics equation, i.e., inverse Gaussian distributed MOM (IGDMOM). The advantage of IGDMOM is that all of its moments can be analytically calculated using a closure moment function inherited from IGD. The precision and efficiency of IGDMOM are verified by comparing it with other recognizable methods in test cases of four representative atmospheric aerosol dynamics. Several key statistical quantities determining aerosol size distributions, including kth moments (k = 0, 1/3, 2/3, and 2), geometric standard deviation, skewness, and kurtosis, are evaluated. IGDMOM has higher precision than the lognormal MOM with nearly identical efficiency. The article provides a novel alternative to atmospheric scientists for solving kinetic aerosol dynamics equations.

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Richard P. Mied, Colin Y. Shen, and Matthew J. Kidd

Abstract

Acoustic Doppler profiler measurements of inertial waves embedded within the high shell region of a cold core ring in the Sargasso Sea are described. By repeatedly traversing the same point from different directions, a sampling pattern resembling an asterisk was obtained. The data reveal the presence of two different wave signals which are advected through the test region: a strong, monochromatic, downgoing wave, and a less well defined ensemble of mostly downgoing waves. Calculations of the phase of the vertical shear and coherence in the vertical and horizontal planes establishes the horizontal and vertical wavenumbers. These are 33 m and 11.8 km, respectively, and the wave propagates in a nearly cross-stream direction. The weaker ensemble of waves advected through the test region later in the experiment has similar dominant scales: ∼30 m in the vertical and a horizontal wavelength in the range 11.6–30.0 km. For all of these waves, the ratio of vertical to horizontal wavelength is small and the intrinsic frequency is 1.09f, so that the waves are of near-inertial frequency. By examining individual terms in the near-inertial ray equations, it is seen that the horizontal crosscurrent variations cause the horizontal wavenumber to be rotated so as to propagate perpendicular to the current. Consideration of the full ray equations for all internal wave frequencies in a barotropic current shows a filtering effect that also preserves waves propagating across the current. Both of these results are consistent with the observation that the waves are propagating nearly cross stream.

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J. H. Shen, K. Klier, and A. C. Zettlemoyer

Abstract

A fluorine mica, fluorophlogopite, has been found to produce higher bulk water freezing temperature than many other nucleating agents including the parent hydroxyphlogopite and even silver iodide. It is the most efficient catalyst yet found in this Laboratory. Fluorophlogopite has an inherently large mismatch with ice crystals but the water cluster embryo is apparently sustained by an F-H-O hydrogen bond assisted by neighboring potassium ions.

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K-M. Lau, G. J. Yang, and S. H. Shen

Abstract

This study attempts to bridge a gap in our knowledge and understanding of the East Asian monsoon by presenting a documentation of the rainfall and intraseasonal rainfall climatology of summer monsoon rainfall over East Asia. The rainfall climatology is viewed both in terms of its regional characteristics and in relation to the large-scale circulation with the aim of bringing to light the myriad phenomena associated with the East Asian monsoon and its relevance to current mainstream research in monsoon and climate dynamics.

Specially, climatology of 30-day and 10-day rainfall records from East Asian stations are studied and compared with satellite outgoing longwave radiation and the large-scale circulation field. Results show that climatologically two major monsoon rainfall onsets over East Asia are identified during the period from April to September. The first, known as the Mei-yu (or plum rain), occurs over central China around the beginning or the middle of June, and the second over northeast China during late July. The multiple onsets occur as the major rainbands make rapid transitions or sudden jumps between three somewhat stationary positions over southern China (pre-monsoon rain), central China (the Mei-yu front) and northeastern China (the polar front).

Also found are the presence of 40-day and the 20-day oscillations in the rainfall climatology. Both oscillations exhibit structure and propagation consistent with previous studies. Abrupt changes by the major rainbands appear to be related to phase-locking between intraseasonal oscillations, such as the 40-day mode and the 20-day mode, and the seasonal variation. A comparison of the rainfall climatology with that over India is also discussed.

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Yixuan Shen, Yuan Sun, Suzana J. Camargo, and Zhong Zhong

Abstract

The ability to simulate tropical cyclones (TCs) realistically is an important factor in the performance evaluation of climate models. In previous studies, indirect evaluation methods have been proposed that are based on the comparison of TC-related background circulation between model results and observations. Direct model evaluation methods, in most cases, are limited to the model skill in simulating the TC frequency, intensity, and track density. Here we propose a new method to quantitatively and directly evaluate the ability of climate models in simulating TC tracks. The method consists of two indicators that account for the model performance in simulating TC track density and the geographic properties of TC tracks, respectively. This method is applied to evaluate the skill of climate models in simulating TC tracks over the western North Pacific Ocean. The explicit models include seven from phase 5 of the Coupled Model Intercomparison Project and eight from the U.S. CLIVAR Hurricane Working Group (HWG), as well as four downscaled HWG models. Our results indicate the order of these 15 explicit models according to their ability to simulate TC tracks. In addition, we show that, for one of the models, the TC track simulation is greatly improved by using downscaling.

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J. Shen, M. Yu, A. J. Koivisto, H. Jiang, Y. Liu, L. Wang, and T. Hussein

Abstract

The inverse Gaussian distributed method of moments (IGDMOM) was developed to analytically solve the kinetic collection equation (KCE) for the first time. Using the IGDMOM, we obtained both new analytical and asymptotic solutions to the KCE. This is shown for both the free-molecular and continuum regime collision frequency functions. The new analytical solutions are highly suitable for demonstrating the self-preserving size distribution (SPSD) theory. The SPSD theory is considered one of the most elegant research works in atmospheric science for aerosols or small cloud droplets. It was initially discovered by Friedlander and then developed by Lee with an assumption of the time-dependent lognormal size distribution function. In this study, we demonstrate that the SPSD theory of coagulating atmospheric aerosols can be presented in a simpler and more rigorous theoretical way, which is realized through the introduction of the IGDMOM for describing aerosol size distributions. Using the IGDMOM, the new formulas for the SPSD, as well as the time required for aerosols to reach the SPSD, are analytically provided and verified. Furthermore, we discover that the SPSD of atmospheric aerosols undergoing coagulation is only determined using a shape factor variable Ω, which is composed of the first three moments at an initial stage. This study has critical implications for developing tropospheric atmospheric aerosol or small cloud droplet dynamics models and further verifies the SPSD theory from the viewpoint of theoretical analysis.

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M. Segal, M. Leuthold, R. W. Arritt, C. Anderson, and J. Shen

The diversity of small lakes' (size < 50 km) configurations, sizes, surrounding terrain, and land use combined with relative sparsity of observations complicates the observational evaluation of the lake breezes (LB) that are induced by these lakes. In the present article observational data obtained from available documents, data archives, and special projects were surveyed to suggest characterization of the LB features. The observational survey was complemented by conceptual evaluations. A preliminary generalization of the LB intensity and inland penetration in relation to the surrounding land use was inferred. The conceptual evaluation suggested that for a given lake width the prime factor affecting the LB intensity is the magnitude of the surface sensible heat flux over the surrounding land. Cooling related to the lake water temperature was indicated to have usually a secondary effect on the LB intensity for small lakes. Surface observations implied that the onshore penetration of the LB by the early afternoon hours is typically less than the characteristic width of the lake. Lower atmosphere observations indicated that the vertical extent of the LB may reach several hundred meters. Implications of the observed LB features in support of characterization of the real-world vegetation breeze are discussed.

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Richard P. Mied, Colin Y. Shen, Clifford L. Trump, and Gloria J. Lindemann

Abstract

This work examines the presence of internal-inertial waves in a front in the North Atlantic subtropical convergence zone. Results of Doppler shear profiler and towed thermistor chain surveys are displayed to document the position and magnitude of the front. Objective maps of the total measured velocity are computed and subtracted from the observed velocity fields. The remaining wave signal is processed to yield horizontal (towed) and vertical (dropped) kinetic energy spectra across the front. From these, rotary spectra are also computed along the line of tow and in the vertical to determine the horizontal and vertical anisotropy. It is found that several nearly monochromatic waves are propagating northward and southward from the front with horizontal length scales of ∼32–50 km. It was also discovered that the region of anticyclonic frontal vorticity exhibits an excess of downgoing energy at the longest vertical wavelength thus sampled (∼50 m), while the region of cyclonic vorticity possesses more upgoing than downgoing energy at the same wavelengths. Vertical and horizontal spectra variances of the total kinetic energy within the region of the front am each enhanced by a factor of about five over the variances outside the front. These results are discussed in the light of recent work by Kunze.

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Colin Y. Shen, James C. Mcwilliams, Bruce A. Taft, Curtis C. Ebbesmeyer, and Eric J. Lindstrom

Abstract

Dynamical properties examined in this paper are the dynamic height and the pressure on an isopycnal. Scalars are the salinity and the oxygen concentration on an isopycnal. The sea surface temperature and salinity are also examined. These properties are obtained from the spatially uniform and densely sampled hydrographic surveys conducted during a two-month period of the POLYMODE Local Dynamics Experiment in 1978 near 31°N, 69.5°W. The spatial maps of the dynamic height and the pressure of an isopycnal show that the baroclinic current in this area sometimes intensifies to a jetlike flow and at other times has the shape of elongated eddies. The current flows primarily in the NE–SW direction. Westward propagation occurs but varies in time and with depth. Eddies that transport water properties are also observed in this area. The salinity and the oxygen on an isopycnal are found to be correlated with itself and with each other from the surface to the 18°C water layer, within a 200-db range in the thermocline and an ∼400-db range centered near 1500 db. There are periods during which the salinity and the oxygen as well as the sea surface properties are correlated with the near-surface current structure; the correlation is not found at deeper depths. These scalar properties, which are interpreted as tracers, are more vigorously “stirred” in and above the thermocline than below, where the motion is more waveline. The net movement of water mass indicated by these tracers is due west through most of the water column during the period of the experiment.

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Bruce A. Taft, Eric J. Lindstrom, Curtis C. Ebbesmeyer, Colin Y. Shen, and James C. Mcwilliams

Abstract

The seven intensive hydrographic surveys of temperature, salinity and oxygen of the POLYMODE local Dynamics Experiment (LDE) (15 May–15 July 1978) were carded out in a 200 km-wide octagonal region centered at 31°05′N, 69°30′W; this location was within the southern portion of the Gulf Stream Recirculation. The vertical structure of the mean and standard deviations of water properties and physical parameters for the experiment are presented. Comparisons of LDE and climatological statistics at 31°N, 70°W showed that the profiles of mean quantities were not markedly different; however, LDE eddy potential energy in the water column above 2000 m was roughly one-half to one-third the long-term climatological values. There were peaks in variability of salinity on potential density surfaces in the 18°C-water, the midthermocline (850 db) and the lower thermocline (1400 db). The 18°C-peak was associated with a marked bimodal salinity distribution, the midthermocline peak with extreme outliers (positive and negative) on the frequency distribution and the lower thermocline with a positively skewed frequency distribution indicating Mediterranean Sea Water influence. Peaks in oxygen variability were observed at the same levels as for salinity. Examples of small anomalous water masses (outliers) are shown in vertical and horizontal sections. The dynamical fields (pressure of density surfaces, dynamic height) were highly anisotropic with the preferred orientation being in the NE/SW direction. There was evidence of northwest translation of dynamical features at a speed of 4 km d−1; the long horizontal scale of the dynamical features (ridges and troughs or elongated eddies) were not resolved by the experiment. Measurements adjacent to the LDE region indicate that the scale in the NE/SW direction was in excess of 400 km. The last four surveys showed the development of a very strong baroclinic feature (velocity shear > 50 cm s−1 between 700 and 1500 db). This jetlike structure, which extended across the survey region, had a gradient (NW/SE direction) with a horizontal able of 100 km. Because of the development of the Strong flow during the period of hydrographic surveys, the results are not typical of the whole period of the experiment; mooring measurements showed that two such events occurred in a 15-month period. There are three major differences between MODE and LDE results: the eddy potential and eddy kinetic energy was 3 to 4 times higher in the LDE; the LDE dynamic height fields were more anisotropic; and them were a number of intense small-scale (horizontal and vertical) property anomalies observed in the LDE that did not have a counterpart in MODE.

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