A method for computing the three-dimensional radial distribution function of cloud particles from holographic images

Reliable measurements of the three-dimensional radial distribution function for cloud droplets are desired to help characterize microphysical processes that depend on local drop environment. Existing numerical techniques to estimate this three-dimensional radial distribution function are not well suited to in situ or laboratory data gathered from a finite experimental domain. This paper introduces and tests a new method designed to reliably estimate the three-dimensional radial distribution function in contexts in which (i) physical considerations prohibit the use of periodic boundary conditions and (ii) particle positions are measured inside a convex volume that may have a large aspect ratio. The method is then utilized to measure the three-dimensional radial distribution function from laboratory data taken in a cloud chamber from the Holographic Detector for Clouds (HOLODEC)

Sulfate Reduction in Sediments Produces High Levels of Chromophoric Dissolved Organic Matter

Sulfate reduction plays an important role in altering dissolved organic matter (DOM) in estuarine and coastal sediments, although its role in the production of optically active chromophoric DOM (CDOM) and a subset of fluorescent DOM (FDOM) has not been previously investigated in detail. Freshwater sediment slurries were incubated anaerobically with added sulfate and acetate to promote sulfate-reducing bacteria. Ultraviolet visible (UV-Vis) absorbance and 3-dimensional excitation emission matrix (EEM) fluorescence spectra were measured over a five weeks anaerobic dark incubation period. Parallel Factor Analysis (PARAFAC) of FDOM determined components that increased significantly during dark and anaerobic incubation matching three components previously considered of terrestrially-derived or humic-like origin published in the OpenFluor database. The observed FDOM increase was strongly correlated (R2 = 0.96) with the reduction of sulfate. These results show a direct experimental link between sulfate reduction and FDOM production, which impacts our understanding of coastal FDOM sources and early sediment diagenesis. As 3D fluorescence techniques are commonly applied to diverse systems, these results provide increasing support that FDOM can have many diverse sources not consistently captured by common classifications such as “humic-like” fluorescence

Refinements to data acquired by 2-dimensional video disdrometers

The 2-Dimensional Video Disdrometer (2DVD) is a commonly used tool for exploring rain microphysics and for validating remotely sensed rain retrievals. Recent work has revealed a persistent anomaly in 2DVD data. Early investigations of this anomaly concluded that the resulting errors in rain measurement were modest, but the methods used to flag anomalous data were not optimized, and related considerations associated with the sample sensing area were not fully investigated. Here, we (i) refine the anomaly-detecting algorithm for increased sensitivity and reliability and (ii) develop a related algorithm for refining the estimate of sample sensing area for all detected drops, including those not directly impacted by the anomaly. Using these algorithms, we explore the corrected data to measure any resulting changes to estimates of bulk rainfall statistics from two separate 2DVDs deployed in South Carolina combining for approximately 10 total years of instrumental uptime. Analysis of this data set consisting of over 200 million drops shows that the error induced in estimated total rain accumulations using the manufacturer-reported area is larger than the error due to considerations related to the anomaly. The algorithms presented here imply that approximately 4.2% of detected drops are spurious and the mean reported effective sample area for drops believed to be correctly detected is overestimated by ~8.5%. Simultaneously accounting for all of these effects suggests that the total accumulated rainfall in the data record is approximately 1.1% larger than the raw data record suggests

Observations and analysis of uncorrelated rain

Most microphysical models in precipitation physics and radar meteorology assume (at least implicitly) that raindrops are completely uncorrelated in space and time. Yet, several recent studies have indicated that raindrop arrivals are often temporally and spatially correlated. Resolution of this conflict must begin with observations of perfectly uncorrelated rainfall, should such “perfectly steady rain” exist at all. Indeed, it does. Using data with high temporal precision from a two-dimensional video disdrometer and the pair-correlation function, a scale-localized statistical tool, several ∼10–20-min rain episodes have been uncovered where no clustering among droplet arrival times is found. This implies that (i) rain events exist where current microphysical models can be tested in an optimal manner and (ii) not all rain can be properly described using fractals

Observations and analysis of uncorrelated rain

Most microphysical models in precipitation physics and radar meteorology assume (at least implicitly) that raindrops are completely uncorrelated in space and time. Yet, several recent studies have indicated that raindrop arrivals are often temporally and spatially correlated. Resolution of this conflict must begin with observations of perfectly uncorrelated rainfall, should such “perfectly steady rain” exist at all. Indeed, it does. Using data with high temporal precision from a two-dimensional video disdrometer and the pair-correlation function, a scale-localized statistical tool, several ∼10–20-min rain episodes have been uncovered where no clustering among droplet arrival times is found. This implies that (i) rain events exist where current microphysical models can be tested in an optimal manner and (ii) not all rain can be properly described using fractals

On the detection of statistical heterogeneity in rain measurements

The application of the Wiener–Khintchine theorem for translating a readily measured correlation function into the variance spectrum, important for scale analyses and for scaling transformations of data, requires that the data be wide-sense homogeneous (stationary), that is, that the first and second moments of the probability distribution of the variable are the same at all times (stationarity) or at all locations (homogeneity) over the entire observed domain. This work provides a heuristic method independent of statistical models for evaluating whether a set of data in rain is wide-sense stationary (WSS). The alternative, statistical heterogeneity, requires 1) that there be no single global mean value and/or 2) that the variance of the variable changes in the domain. Here, the number of global mean values is estimated using a Bayesian inversion approach, while changes in the variance are determined using record counting techniques. An index of statistical heterogeneity (IXH) is proposed for rain such that as its value approaches zero, the more likely the data are wide-sense stationary and the more acceptable is the use of the Wiener–Khintchine theorem. Numerical experiments as well as several examples in real rain demonstrate the potential of IXH to identify statistical homogeneity, heterogeneity, and statistical mixtures. In particular, the examples demonstrate that visual inspections of data alone are insufficient for determining whether they are wide-sense stationary. Furthermore, in this small data collection, statistical heterogeneity was associated with convective rain, while statistical homogeneity appeared in more stratiform or mixed rain events. These tentative associations, however, need further substantiation

Fine-scale droplet clustering in atmospheric clouds: 3D radial distribution function from airborne digital holography

The extent of droplet clustering in turbulent clouds has remained largely unquantified, and yet is of possible relevance to precipitation formation and radiative transfer. To that end, data gathered by an airborne holographic instrument are used to explore the three-dimensional spatial statistics of cloud droplet positions in homogeneous stratiform boundary-layer clouds. The three-dimensional radial distribution functions g(r) reveal unambiguous evidence of droplet clustering. Three key theoretical predictions are observed: the existence of positive correlations, onset of correlation in the turbulence dissipation range, and monotonic increase of g(r) with decreasing r. This implies that current theory captures the essential processes contributing to clustering, even at large Reynolds numbers typical of the atmosphere

Can gravitational infall energy lead to the observed velocity dispersion in DLAs?

The median observed velocity width v_90 of low-ionization species in damped Ly-alpha systems is close to 90 km/s, with approximately 10% of all systems showing v_90 > 210 km/s at z=3. We show that a relative shortage of such high-velocity neutral gas absorbers in state-of-the-art galaxy formation models is a fundamental problem, present both in grid-based and particle-based numerical simulations. Using a series of numerical simulations of varying resolution and box size to cover a wide range of halo masses, we demonstrate that energy from gravitational infall alone is insufficient to produce the velocity dispersion observed in damped Ly-alpha systems, nor does this dispersion arise from an implementation of star formation and feedback in our highest resolution (~ 45 pc) models, if we do not put any galactic winds into our models by hand. We argue that these numerical experiments highlight the need to separate dynamics of different components of the multiphase interstellar medium at z=3.Comment: 12 Pages, 9 Figures, accepted to ApJ, printing in colour recommende

Genome-wide profiling of human cap-independent translation-enhancing elements.

We report an in vitro selection strategy to identify RNA sequences that mediate cap-independent initiation of translation. This method entails mRNA display of trillions of genomic fragments, selection for initiation of translation and high-throughput deep sequencing. We identified >12,000 translation-enhancing elements (TEEs) in the human genome, generated a high-resolution map of human TEE-bearing regions (TBRs), and validated the function of a subset of sequences in vitro and in cultured cells