Mesoscale Organization Research Articles

Witherite nanorods form mesocrystals: a direct experimental examination of a dipole-driven self-assembly model

Several theories or models have been proposed to account for the biofabrication of the architecture of nacre layers in mollusk shell; among them, a dipole-driven self-assembly mineralization model has been put forward to account for the oriented alignment of aragonite nanocrystals in an aragonite tablet and the formation of local co-oriented aragonite columns in nacre. In order to test the dipole-driven self-assembly model, we select witherite, which is isostructural with aragonite, as a model mineral to examine the concept. Herein, nanoscale witherite rods (nanorods) were first synthesized by use of dimethyl sulfoxide (DMSO) as a stabilizer, and then the completely washed up nanorods were redispersed in deionized water to obtain organized mesoscale witherite rods (mesocrystals or mesorods) at ambient temperature. The nano- or mesorods obtained under different conditions were characterized by a range of techniques involving XRD, FTIR, FESEM, TEM, SAED, and HRTEM. The SEM, TEM and SAED results demonstrate that the witherite nanorods can be spontaneously organized into mesorods in deionized water at ambient temperature via an oriented-attachment growth process, indicating that the intrinsic anisotropic dipole-dipole interactions between the assembled nanorods should be responsible for the self-assembly of the nanorods into the mesorods of witherite. Therefore, our results can provide a direct examination for dipole driven self-assembly, and show that dipole-dipole interactions can reasonably account for some aspects of the formation of hierarchical biominerals.

Relationships between the Raindrop Size Distribution and Properties of the Environment and Clouds Inferred from TRMM

Abstract Raindrop size distribution (DSD) retrievals from two years of data gathered by the Tropical Rainfall Measuring Mission (TRMM) satellite and processed with a combined radar–radiometer algorithm over the oceans equatorward of 35° are examined for relationships with variables describing properties of the vertical precipitation profile, mesoscale organization, and background environment. In general, higher freezing levels and relative humidities (tropical environments) are associated with smaller reflectivity-normalized median drop size (εDSD) than in the extratropics. Within the tropics, the smallest εDSD values are found in large, shallow convective systems where warm rain formation processes are thought to be predominant, whereas larger sizes are found in the stratiform regions of organized deep convection. In the extratropics, the largest εDSD values are found in the scattered convection that occurs when cold, dry continental air moves over the much warmer ocean after the passage of a cold front. These relationships are formally attributed to variables describing the large-scale environment, mesoscale organization, and profile characteristics via principal component (PC) analysis. The leading three PCs account for 23% of the variance in εDSD at the individual profile level and 45% of the variance in 1°-gridded mean values. The geographical distribution of εDSD is consistent with many of the observed regional reflectivity–rainfall (Z–R) relationships found in the literature as well as discrepancies between the TRMM radar-only and radiometer-only precipitation products. In particular, midlatitude and tropical regions near land tend to have larger drops for a given reflectivity, whereas the smallest drops are found in the eastern Pacific Ocean intertropical convergence zone.

Mesoscale Organization in a Physically Separated Vacuum Residue: Comparison to Asphaltenes in a Simple Solvent

Physical separation of heavy oils and bitumen is of particular interest because it improves the description of the chemical and structural organization in these industrial and challenging fluids (Zhao, B.; Shaw, J. M.Composition and size distribution of coherent nanostructures in Athabasca bitumen and Maya crude oil. Energy Fuels 2007, 21, 2795−2804). In this study, permeates and retentates, differing in aggregate concentrations and sizes, were obtained from nanofiltration of a vacuum residue at 200 °C with membranes of varying pore size. Elemental composition and density extrapolations show that aggregates are best represented as n-pentane asphaltenes, while the dispersing phase corresponds to n-pentane maltenes. Small-angle X-ray scattering (SAXS) measurements are processed, on this basis, to calculate the size and mass of the aggregates. Aggregates in the vacuum residue are similar in size and mass to asphaltenes in toluene, and temperature elevation decreases the size of the aggregates. Wide-angle X-r...

Representing the Sensitivity of Convective Cloud Systems to Tropospheric Humidity in General Circulation Models

Convective cloud variability on many times scales can be viewed as having three major components: a suppressed phase of shallow and congestus clouds, a disturbed phase of deep convective clouds, and a mature phase of transition to stratiform upper-level clouds. Cumulus parameterization development has focused primarily on the second phase until recently. Consequently, many parameterizations are not sufficiently sensitive to variations in tropospheric humidity. This shortcoming may affect global climate model simulations of climate sensitivity to external forcings, the continental diurnal cycle of clouds and precipitation, and intraseasonal precipitation variability. The lack of sensitivity can be traced in part to underestimated entrainment of environmental air into rising convective clouds and insufficient evaporation of rain into the environment. As a result, the parameterizations produce deep convection too easily while stabilizing the environment too quickly to allow the effects of convective mesoscale organization to occur. Recent versions of some models have increased their sensitivity to tropospheric humidity and improved some aspects of their variability, but a parameterization of mesoscale organization is still absent from most models. Evidence about the effect of these uncertainties on climate change projections suggests that climate modelers should make improved simulation of high and convective clouds as high a priority as better representations of low clouds.

Beyond Stereospecificity: Liquids and Mesoscale Organization of Cytoplasm

The cytoplasm is not a homogenous solution but instead consists of large dynamic assemblies that arise from transient molecular interactions. Some of these structures have been shown to represent liquid droplets of concentrated protein and RNA. Liquid phase separation of cytoplasm may be a fundamental principle of cytoplasmic organization.

Multiple‐scale simulations of stratocumulus clouds

This study introduces a flexible multiple nested modeling framework developed from the Weather Research and Forecasting (WRF) model for boundary layer cloud research. It features a nested large‐eddy simulation in a hindcasting mode that allows a direct comparison between simulations and observations. Using this approach, we simulated two stratocumulus cases observed at the southern Great Plains observing site. The simulation‐observation comparisons demonstrate that the multiple nested WRF well reproduces the cloud properties and vertical velocity fields detected by cloud radar and other remote sensing instruments. The simulations show that (1) the cloud fields can be strongly modulated by mesoscale organizations; (2) the vertical velocity variance and skewness in the boundary layer have well‐defined diurnal variations and are strongly dependent on external forcings, but the in‐cloud turbulent statistics appears to be more controlled by the internal turbulent processes; (3) the organized structures of stratocumulus are only slightly skewed, which is consistent with the cloud radar observations that the updraft fraction is close to 50%; and (4) the vertical distribution of cloud water and the associated high‐order moments of continental stratocumulus can be substantially different from its marine counterpart due to different ambient thermodynamic structures. This study suggests that the multiple‐scale WRF modeling system provides a great numerical framework that can be used to address various issues regarding the parameterization of boundary layer clouds.

A Complex Metacommunity Structure for Gastropods Along an Elevational Gradient

The metacommunity framework integrates species-specific responses to environmental gradients to detect emergent patterns of mesoscale organization. Abiotic characteristics (temperature, precipitation) and associated vegetation types change with elevation in a predictable fashion, providing opportunities to decouple effects of environmental gradients per se from those of biogeographical or historical origin. Moreover, expected structure is different if a metacommunity along an elevational gradient is molded by idiosyncratic responses to abiotic variables (expectation=Gleasonian structure) than if such a metacommunity is molded by strong habitat preferences or specializations (expectation=Clementsian structure). We evaluated metacommunity structure for 13 species of gastropod from 15 sites along an elevational transect in the Luquillo Experimental Forest of Puerto Rico. Analyses were conducted separately for the primary axis and for the secondary axis of correspondence extracted via reciprocal averaging. The metacommunity exhibited quasi-Clementsian structure along the primary axis, which represented a gradient of gastropod species specialization that was unassociated with elevation. The secondary axis represented environmental variation associated with elevation. Along this axis, the metacommunity exhibited Clementsian structure, with specialists characterizing each of three suites of sites that corresponded to three distinct forest types. These forest types are associated with low (tabonuco forest), mid- (palo colorado forest), or high (elfin forest) elevations. Thus, variation among sites in species composition reflected two independent processes: the first decoupled from elevational variation and its environmental correlates, and the second highly associated with environmental variation correlated with elevation.

Southeast Pacific stratocumulus clouds, precipitation and boundary layer structure sampled along 20° S during VOCALS-REx

Abstract. Multiplatform airborne, ship-based, and land-based observations from 16 October–15 November 2008 during the VOCALS Regional Experiment (REx) are used to document the typical structure of the Southeast Pacific stratocumulus-topped boundary layer and lower free troposphere on a~transect along 20° S between the coast of Northern Chile and a buoy 1500 km offshore. Strong systematic gradients in clouds, precipitation and vertical structure are modulated by synoptically and diurnally-driven variability. The boundary layer is generally capped by a strong (10–12 K), sharp inversion. In the coastal zone, the boundary layer is typically 1 km deep, fairly well mixed, and topped by thin, nondrizzling stratocumulus with accumulation-mode aerosol and cloud droplet concentrations exceeding 200 cm−3. Far offshore, the boundary layer depth is typically deeper (1600 m) and more variable, and the vertical structure is usually decoupled. The offshore stratocumulus typically have strong mesoscale organization, much higher peak liquid water paths, extensive drizzle, and cloud droplet concentrations below 100 cm−3, sometimes with embedded pockets of open cells with lower droplet concentrations. The lack of drizzle near the coast is not just a microphysical response to high droplet concentrations; smaller cloud depth and liquid water path than further offshore appear comparably important. Moist boundary layer air is heated and mixed up along the Andean slopes, then advected out over the top of the boundary layer above adjacent coastal ocean regions. Well offshore, the lower free troposphere is typically much drier. This promotes strong cloud-top radiative cooling and stronger turbulence in the clouds offshore. In conjunction with a slightly cooler free troposphere, this may promote stronger entrainment that maintains the deeper boundary layer seen offshore. Winds from ECMWF and NCEP operational analyses have an rms difference of only 1 m s−1 from collocated airborne leg-mean observations in the boundary layer and 2 m s−1 above the boundary layer. This supports the use of trajectory analysis for interpreting REx observations. Two-day back-trajectories from the 20° S transect suggest that eastward of 75° W, boundary layer (and often free-tropospheric) air has usually been exposed to South American coastal aerosol sources, while at 85° W, neither boundary-layer or free-tropospheric air has typically had such contact.

A Mixed Scheme for Subgrid-Scale Fluxes in Cloud-Resolving Models

Abstract A large-domain large-eddy simulation of a tropical deep convection system is used as a benchmark to derive and test a mixed subgrid-scale (SGS) scheme for scalar and momentum fluxes in cloud-resolving models (CRMs). The benchmark simulation resolves a broad range of scales ranging from mesoscale organizations, through gravity waves and individual clouds, down to energy-containing turbulent eddies. A spectral analysis shows that the vertical-velocity kinetic energy peaks at scales from hundreds of meters in the lower cloud layer to several kilometers higher up; these scales are typical grid sizes of today’s CRMs. The analysis also shows that a significant portion of the scalar and momentum fluxes in the benchmark simulation are carried by motions smaller than several kilometers (i.e., smaller than a typical grid resolution of CRMs). The broad range of scales of the benchmark simulation is split into two components: filter scale (mimicking CRM resolvable scale) and subfilter scale (mimicking CRM SGS), using filter widths characteristic of a typical CRM grid spacing. The local relationship of the subfilter-scale fluxes to the filter-scale variables is examined. This leads to a mixed SGS scheme to represent the SGS fluxes of scalars and momentum in CRMs. A priori tests show that the mixed SGS scheme yields spatial distributions of subfilter-scale fluxes that correlate much better with those retrieved from the benchmark when compared with an eddy viscosity/diffusivity scheme that is commonly used in today’s CRMs.

Observational Analysis of an Upper-Level Inverted Trough during the 2004 North American Monsoon Experiment

Abstract Upper-level inverted troughs (IVs) associated with midlatitude breaking Rossby waves or tropical upper-troposphere troughs (TUTTs) have been identified as important contributors to the variability of rainfall in the North American monsoon (NAM) region. However, little attention has been given to the dynamics of these systems owing to the sparse observational network over the NAM region. High temporal and spatial observations taken during the 2004 North American Monsoon Experiment (NAME) are utilized to analyze a significant IV that passed over northwestern Mexico from 10 to 13 July 2004. The Colorado State University gridded dataset, which is independent of model analysis over land, is the primary data source used in this study. Results show that the 10–13 July IV disturbance was characterized by a warm anomaly around 100 hPa and a cold anomaly that extended from 200 to 700 hPa. The strongest cyclonic circulation was in the upper levels around 200 hPa. Quasigeostrophic (QG) diagnostics indicate that the upper-level low forced weak subsidence (weak rising motion) to the west (east) of its center. Net downward motion to the west was a result of the Laplacian of thermal advection (forcing subsidence) outweighing differential vorticity advection (forcing weak upward motion). Despite the QG forcing of downward motion west of the upper-level IV, enhanced convection occurred west of the IV center along the western slopes of the Sierra Madre Occidental (SMO). This seemingly contradictory behavior can be explained by noting that the upper-level IV induced a midlevel cyclonic circulation, with northeasterly (southeasterly) midlevel flow to the west (east) of its center. Increased mesoscale organization of convection along the SMO foothills was found to be collocated with IV-enhanced northeasterly midlevel flow and anomalous northeasterly shear on the western (leading) flank of the system. It is proposed that the upper-level IV increased the SMO-perpendicular midlevel flow as well as the wind shear, thereby creating an environment favorable for convective storms to grow upscale as they moved off the high terrain.

Multiscale interaction with topography and extreme rainfall events in the northeast Indian region

Flash floods associated with extreme rain events are a major hydrological disaster in the northeast Indian (NEI) region because of the unique topographic features of the region as well as increased frequency of occurrence of such events. Knowledge of the spatiotemporal distribution of these events in the region and an understanding of the factors responsible for them, therefore, would be immensely useful for appropriate disaster preparedness. Using daily rainfall data from 15 stations over the region for 32 years (1975–2006), it is shown that the frequency of occurrence of these events is largest not during the premonsoon thunderstorm season but during the peak monsoon months (June–July–August). This fact together with the fact that most of these events occur during long rainy spells indicate that the extreme events in the NEI region largely occur in association with the monsoon synoptic events rather than isolated thunderstorms. We also find that the aggregate of extreme rain events over the region has a significant decreasing trend in contrast to a recent finding of an increasing trend of such events in central India (Goswami et al., 2006). This decreasing trend of extreme events is consistent with observed decreasing trend in convective available potential energy and increasing convective inhibition energy over the region for the mentioned period. Examination of the structure of convection associated with the extreme rain events in the region indicates that they occur through a multiscale interaction of circulation with the local topography. It is found that at all the stations, the events are associated with a mesoscale structure of convection that is embedded in a much larger scale convective organization. We identify that this large‐scale organization is a manifestation of certain phases of the tropical convergence zone associated with the northward propagating active‐break phases of the summer monsoon intraseasonal oscillation. Further, it is shown that the mesoscale circulation interacting with the local topography generates southward propagating gravity waves with diurnal period. The strong updrafts associated with the gravity waves within the mesoscale organization leads to very deep convective events and the extreme rainfall. The insights provided by our study would be useful when designing models to improve the prediction of extreme events.

Genesis of Pre–Hurricane Felix (2007). Part II: Warm Core Formation, Precipitation Evolution, and Predictability

Abstract This is the second of a two-part study examining the simulated formation of Atlantic Hurricane Felix (2007) in a cloud-representing framework. Here several open issues are addressed concerning the formation of the storm’s warm core, the evolution and respective contribution of stratiform versus convective precipitation within the parent wave’s pouch, and the sensitivity of the development pathway reported in Part I to different model physics options and initial conditions. All but one of the experiments include ice microphysics as represented by one of several parameterizations, and the partition of convective versus stratiform precipitation is accomplished using a standard numerical technique based on the high-resolution control experiment. The transition to a warm-core tropical cyclone from an initially cold-core, lower tropospheric wave disturbance is analyzed first. As part of this transformation process, it is shown that deep moist convection is sustained near the pouch center. Both convective and stratiform precipitation rates increase with time. While stratiform precipitation occupies a larger area even at the tropical storm stage, deep moist convection makes a comparable contribution to the total rain rate at the pregenesis stage, and a larger contribution than stratiform processes at the storm stage. The convergence profile averaged near the pouch center is found to become dominantly convective with increasing deep moist convective activity there. Low-level convergence forced by interior diabatic heating plays a key role in forming and intensifying the near-surface closed circulation, while the midlevel convergence associated with stratiform precipitation helps to increase the midlevel circulation and thereby contributes to the formation and upward extension of a tropospheric-deep cyclonic vortex. Sensitivity tests with different model physics options and initial conditions demonstrate a similar pregenesis evolution. These tests suggest that the genesis location of a tropical storm is largely controlled by the parent wave’s critical layer, whereas the genesis time and intensity of the protovortex depend on the details of the mesoscale organization, which is less predictable. Some implications of the findings are discussed.

Interaction of Tropical Deep Convection with the Large-Scale Circulation in the MJO

Abstract To better understand the interaction between tropical deep convection and the Madden–Julian oscillation (MJO), tropical cloud regimes are defined by cluster analysis of International Satellite Cloud Climatology Project (ISCCP) cloud-top pressure—optical thickness joint distributions from the D1 dataset covering 21.5 yr. An MJO index based solely on upper-level wind anomalies is used to study variations of the tropical cloud regimes. The MJO index shows that MJO events are present almost all the time; instead of the MJO event being associated with “on or off” deep convection, it is associated with weaker or stronger mesoscale organization of deep convection. Atmospheric winds and humidity from NCEP–NCAR reanalysis 1 are used to characterize the large-scale dynamics of the MJO; the results show that the large-scale motions initiate an MJO event by moistening the lower troposphere by horizontal advection. Increasingly strong convection transports moisture into the upper troposphere, suggesting a reinforcement of the convection itself. The change of convection organization shown by the cloud regimes indicates a strong interaction between the large-scale circulation and deep convection. The analysis is extended to the complete atmospheric diabatic heating by precipitation, radiation, and surface fluxes. The wave organizes stronger convective heating of the tropical atmosphere, which results in stronger winds, while there is only a passive response of the surface, directly linked to cloud radiative effects. Overall, the results suggest that an MJO event is an amplification of large-scale wave motions by stronger convective heating, which results from a dynamic reorganization of scattered deep convection into more intense mesoscale systems.

Multidimensional Architectures for Functional Optical Devices

Materials exhibiting multidimensional structure with characteristic lengths ranging from the nanometer to the micrometer scale have extraordinary potential for emerging optical applications based on the regulation of light-matter interactions via the mesoscale organization of matter. As the structural dimensionality increases, the opportunities for controlling light-matter interactions become increasingly diverse and powerful. Recent advances in multidimensional structures have been demonstrated that serve as the basis for three-dimensional photonic-bandgap materials, metamaterials, optical cloaks, highly efficient low-cost solar cells, and chemical and biological sensors. In this Review, the state-of-the-art design and fabrication of multidimensional architectures for functional optical devices are covered and the next steps for this important field are described.

Effects of land-surface heterogeneity on numerical simulations of mesoscale atmospheric boundary layer processes

Landscape heterogeneity that causes surface flux variability plays a very important role in triggering mesoscale atmospheric circulations and convective weather processes. In most mesoscale numerical models, however, subgrid-scale heterogeneity is somewhat smoothed or not adequately accounted for, leading to artificial changes in heterogeneity patterns (e.g., patterns of land cover, land use, terrain, and soil types and soil moisture). At the domain-wide scale, the combination of losses in subgrid-scale heterogeneity from many adjacent grids may artificially produce larger-scale, more homogeneous landscapes. Therefore, increased grid spacing in models may result in increased losses in landscape heterogeneity. Using the Weather Research and Forecasting model in this paper, we design a number of experiments to examine the effects of such artificial changes in heterogeneity patterns on numerical simulations of surface flux exchanges, near-surface meteorological fields, atmospheric planetary boundary layer (PBL) processes, mesoscale circulations, and mesoscale fluxes. Our results indicate that the increased heterogeneity losses in the model lead to substantial, nonlinear changes in temporal evaluations and spatial patterns of PBL dynamic and thermodynamic processes. The decreased heterogeneity favor developments of more organized mesoscale circulations, leading to enhanced mesoscale fluxes and, in turn, the vertical transport of heat and moisture. This effect is more pronounced in the areas with greater surface heterogeneity. Since more homogeneous land-surface characteristics are created in regional models with greater surface grid scales, these artificial mesoscale fluxes may have significant impacts on simulations of larger-scale atmospheric processes.

Impact of mesoscale vegetation heterogeneities on the dynamical and thermodynamic properties of the planetary boundary layer

This study uses aircraft observations over the Republic of Benin from the African Monsoon Multidisciplinary Analyses (AMMA) campaign to investigate the impact of vegetation heterogeneities on the dynamics within the planetary boundary layer, such as convection, transport, and mixing. Isoprene, a biogenic volatile organic compound emitted primarily by woody vegetation, was used as a tracer for transport to link the land surface to the boundary layer properties. Associated to planetary boundary layer (PBL) temperature gradients at vegetation boundaries, a persistent mesoscale organization of the winds which controlled patterns in the formation of cumulus congestus clouds was observed. A strong relationship among PBL temperatures, meridional wind velocity, isoprene concentrations, and fraction of forest or shrub cover was found, corroborating the land surface forcing of the observed dynamics. The observations show that the convergence zones tended to occur on the southern edge of warm surface and atmospheric anomalies. The northerly synoptic wind appears to have increased the coherency of the southerly part of the mesoscale flow and displaced the convergence zones southward. The relationships between the PBL potential temperatures and the meridional wind and isoprene concentrations were spatially coherent down to wavelengths of 10 and 8 km, respectively. A seasonal climatology of visible satellite data shows enhanced cloud cover in the afternoon over cropland, suggesting that the results presented are not limited to this case study but are of climatological significance in the region.

Interaction of Tropical Deep Convection with the Large-Scale Circulation in the MJO

To better understand the interaction between tropical deep convection and the Madden–Julian oscillation (MJO), tropical cloud regimes are defined by cluster analysis of International Satellite Cloud Climatology Project (ISCCP) cloud-top pressure—optical thickness joint distributions from the D1 dataset covering 21.5 yr. An MJO index based solely on upper-level wind anomalies is used to study variations of the tropical cloud regimes. The MJO index shows that MJO events are present almost all the time; instead of the MJO event being associated with “on or off” deep convection, it is associated with weaker or stronger mesoscale organization of deep convection. Atmospheric winds and humidity from NCEP–NCAR reanalysis 1 are used to characterize the large-scale dynamics of the MJO; the results show that the large-scale motions initiate an MJO event by moistening the lower troposphere by horizontal advection. Increasingly strong convection transports moisture into the upper troposphere, suggesting a reinforcement of the convection itself. The change of convection organization shown by the cloud regimes indicates a strong interaction between the large-scale circulation and deep convection. The analysis is extended to the complete atmospheric diabatic heating by precipitation, radiation, and surface fluxes. The wave organizes stronger convective heating of the tropical atmosphere, which results in stronger winds, while there is only a passive response of the surface, directly linked to cloud radiative effects. Overall, the results suggest that an MJO event is an amplification of large-scale wave motions by stronger convective heating, which results from a dynamic reorganization of scattered deep convection into more intense mesoscale systems.

Diurnal variation of precipitation over southeastern China: 2. Impact of the diurnal monsoon variability

The monsoon flow influence on the diurnal variation of precipitation over southeastern China (SEC) is examined using satellite rainfall data, reanalysis data, and data sets from field experiments. The low‐level wind averaged over southern China is strongest at late night or in early morning, which is inferred as diurnal monsoon variability (DMV). Such nocturnal acceleration occurs frequently from presummer, when monsoon flow becomes active. It usually develops in the warm region south of frontal zone; it is related closely to the ageostrophic component. In response to the DMV, rainfall over SEC increases remarkably, especially during early morning when the wind maximum of southwesterlies occurs. Prolonged organized mesoscale convection is the main cause of this rainfall enhancement. On a diurnal timescale, the DMV may regulate the warm, moist inflow that feeds the frontal zone, thereby enhancing large‐scale frontogenesis from late night to morning. On a regional scale, the DMV, when coupling with local circulation diurnally, enhances the southerlies on the east flanks of plateaus and over the western SEC, generating convective instability successively there through the early morning. Consequently, the organized mesoscale convection is reinforced to produce more morning rainfall during presummer over SEC area, especially over its western region.

Modeling Mesoscale Cellular Structures and Drizzle in Marine Stratocumulus. Part I: Impact of Drizzle on the Formation and Evolution of Open Cells

Abstract A new modeling framework is used to investigate aerosol–cloud–precipitation interactions and dynamical feedbacks at the mesoscale. The focus is on simulation of the formation and evolution of cellular structures that are commonly seen in satellite images of marine stratocumulus clouds. Simulations are performed at moderate resolution in a 60 × 60 km2 domain for 16 h to adequately represent the mesoscale organization associated with open cells and precipitation. Results support the emerging understanding that precipitation plays a critical role in the formation and evolution of open cells. Evaporation of raindrops generates a dynamic response that manifests itself in cellular organization of updrafts and downdrafts and promotes and sustains the formation of an open cellular structure in cloud fields. Vertical motion in open-cell centers with thin clouds is minimal. It is shown that a mean surface rain rate as low as 0.02 mm day−1 is, for the case considered, sufficient to promote the formation of open cells. The maximum dimension of individual open cells ranges between 5 and 30 km. Individual cells grow at a mean rate of between 5 and 10 km h−1. Irregularity in the shape of open cells is caused by formation of new precipitating regions at the cell walls and interference with neighboring cells, which erode, and eventually eliminate, the old cells. The typical lifetime of large individual open cells is about 2 h, close to that observed by radar, although a collection of open cells as a whole may last for tens of hours.

Chiral amplification in a cyanobiphenyl nematic liquid crystal doped with helicene-like derivatives

The addition of a chiral non-racemic dopant to a nematic liquid crystal (LC) has the effect of transferring the molecular chirality to the phase organization and a chiral nematic phase is formed. This molecular chirality amplification in the LC provides a unique possibility for investigating the relationship between molecular structure, intermolecular interactions, and mesoscale organization. It is known that axially chiral or helical-shaped molecules with reduced conformational disorder are good candidates for high helical twisting power derivatives. In particular, biaryl derivatives are known to be efficient chiral inducers in biaryl nematic mesophases. In this paper, we focus on a new series of helicene-like molecules of known absolute configuration. We have integrated cholesteric pitch measurements with geometry optimization by DFT calculations and analysis of the twisting ability by the Surface Chirality model to shed light on the structural features responsible for the analogies and differences exhibited by these derivatives. The investigation of these dopants with well-defined geometry, by virtue of the low conformational freedom, and the substituents variously distributed around the core, allows us to extend our knowledge of the molecular origin of the chirality amplification in liquid crystals and to confirm the simple relationship “molecular P-helicity” → “cholesteric P-handedness” for helical-shaped helicene-like derivatives.