Impressions of the turbulence variability in a weakly stratified,flat-bottom deep-sea ‘boundary layer’

The character of turbulent overturns in a weakly stratified deep-sea is investigated in some detail using 144 high-resolution temperature sensors at 0.7 m intervals, starting 5 m above the bottom. A 9-day, 1 Hz sampled record from the 912 m depth flat-bottom (<0.5% bottom-slope) mooring site in the central-north Alboran Sea (W-Mediterranean) demonstrates an overall conservative temperature range of only 0.05 °C, a typical mean buoyancy period as large as 3 h and a 1 Hz-profile-vertically-averaged turbulence dissipation rate maximum of only 10⁻⁸ m² s⁻³. Nonetheless, this ‘boundary layer’ varies in height between <6 and >104 m above the bottom and is thus not homogeneous throughout; the temperature variations are seldom quiescent and are generally turbulent in appearance, well exceeding noise levels. The turbulence character is associated with small-scale internal waves; examples are found of both shear- and convection-driven turbulence; particular association, although not phase-locked, is found between turbulence variations and tidal rather than with inertial motions; the mean buoyancy frequency of a few times the inertial frequency implies the importance of ‘slantwise convection’ in the direction of the earth rotational vector rather than in the direction of gravity. Such convection is observed both in near-homogeneous and weakly stratified form.     

An extended Miles' theory for wave generation by wind

Miles' inviscid theory of surface wave generation by wind is (a) modified by replacing the logarithmic shear velocity profile with one which applies right down to the wave surface and which exhibits an explicit dependence on the roughness of the surface, and (b) extended to include the effects of the interaction of wave with air flow turbulence by considering the wave-modified mean flow as the mean of the actual turbulent air flow over water waves and using this in a mixing-length model.The surface pressure is shown to depend significantly on the flow conditions being aerodynamically smooth or rough. Its component in phase with the surface elevation is practically unaffected by the wave-turbulence interaction. However, such interaction tends to increase the rate of energy input ß from wind to waves travelling in the same direction, e.g., the increase is 2gk ² for aerodynamically rough flow, where gk is the Von Karman constant. It also provides damping of waves in an adverse wind which can be about 10% of the growth rate in a favourable wind.     

Atmospheric Disturbances that Generate Intermittent Turbulence in Nocturnal Boundary Layers

Using the unprecedented observational facilities deployed duringthe 1999 Cooperative Atmosphere-Surface Exchange Study (CASES-99),we found three distinct turbulent events on the night of 18October 1999. These events resulted from a density current,solitary wave, and internal gravity wave, respectively. Our studyfocuses on the turbulence intermittency generated by the solitarywave and internal gravity wave, and intermittent turbulenceepisodes associated with pressure change and wind direction shiftsadjacent to the ground. Both the solitary and internal gravitywaves propagated horizontally and downward. During the passage ofboth the solitary and internal gravity waves, local thermal andshear instabilities were generated as cold air was pushed abovewarm air and wind gusts reached to the ground. These thermal andshear instabilities triggered turbulent mixing events. Inaddition, strong vertical acceleration associated with thesolitary wave led to large non-hydrostatic pressure perturbationsthat were positively correlated with temperature. The directionaldifference between the propagation of the internal gravity waveand the ambient flow led to lateral rolls. These episodic studiesdemonstrate that non-local disturbances are responsible for localthermal and shear instabilities, leading to intermittentturbulence in nocturnal boundary layers. The origin of thesenon-local disturbances needs to be understood to improve mesoscalenumerical model performance.     

Integrated modeling of the dynamic meteorological and sea surface conditions during the passage of Typhoon Morakot

In August 2009, Typhoon Morakot caused massive flooding and devastating mudslides in the southern Taiwan triggered by extremely heavy rainfall (2777 mm in 4 days) which occurred during its passage. It was one of the deadliest typhoons that have ever attacked Taiwan in recent years. In this study, numerical simulations are performed for the storm surge and ocean surface waves, together with dynamic meteorological fields such as wind, pressure and precipitation induced by Typhoon Morakot, using an atmosphere–waves–ocean integrated modelling system. The wave-induced dissipation stress from breaking waves, whitecapping and depth-induced wave breaking, is parameterized and included in the wave–current interaction process, in addition to its influence on the storm surge level in shallow water along the coast of Taiwan. The simulated wind and pressure field captures the characteristics of the observed meteorological field. The spatial distribution of the accumulated rainfall within 4 days, from 00:00 UTC 6 August to 00:00 UTC 10 August 2009, shows similar patterns as the observed values. The 4-day accumulated rainfall of 2777 mm at the A-Li Shan mountain weather station for the same period depicted a high correlation with the observed value of 2780 mm/4 days. The effects of wave-induced dissipation stress in the wave–current interaction resulted in increased surge heights on the relatively shallow western coast of Taiwan, where the bottom slope of the bathymetry ranges from mild to moderate. The results also show that wave-breaking has to be considered for accurate storm surge prediction along the east coast of Taiwan over the narrow bank of surf zone with a high horizontal resolution of the model domain.     

Interaction of internal waves and turbulence in the ocean

Within the framework of the semiempirical theory of turbulence for stratified fluids some aspects of the problem of internal wave-turbulence interaction in the upper layer of the ocean are discussed. The conditions of amplification and sustaining of turbulence by internal waves are investigated. Stationary distributions of turbulent energy are found for a stratified fluid with a shear flow produced, for example, by a low-frequency internal wave. The internal wave damping due to both turbulent viscosity and turbulent diffusion in the thermocline is studied. For a two-layer model damping constant is determined as a function of the wave number. The variation of surface turbulence by internal waves is estimated and the role of this process in slick formation is considered.     

Evaluation of turbulent Prandtl (Schmidt) number parameterizations for stably stratified environmental flows

In this study, we evaluate four different parameterizations of the turbulent Prandtl (Schmidt) number Pr_t = ν_t/Γ_t where ν_t is the eddy viscosity and Γ_t is the scalar eddy diffusivity, for stably stratified flows. All four formulations of Pr_t are strictly functions of the gradient Richardson number Ri, which provides a measure of the strength of the stratification. A zero-equation (i.e. no extra transport equations are required) turbulence model for ν_t in a one-dimensional, turbulent channel flow is considered to evaluate the behavior of the different formulations of Pr_t. Both uni-directional and oscillatory flows are considered to simulate conditions representative of practical flow problems such as atmospheric boundary layer flows and tidally driven estuarine flows, to quantify the behavior of each of the four formulations of Pr_t. We perform model-to-model comparisons to highlight which of the models of Pr_t allow for a higher rate of turbulent mixing and which models significantly inhibit turbulent mixing in the presence of buoyancy forces resulting from linear (continuous) stratification as well as two-layer stratification. The basis underlying the formulation of each model in conjunction with the simulation results are used to emphasize the considerable variability in the different formulations and the importance of choosing an appropriate parameterization of Pr_t given a model for ν_t in stably stratified flows.     

The effect of idealized water waves on the turbulence structure and kinetic energy budgets in the overlying airflow

The influence of an idealized moving wavy surface on the overlying airflow is investigated using direct numerical simulations (DNS). In the present simulations, the bulk Reynolds number is Re = 8000 (; where U₀ is the forcing velocity of the flow, h the height of the domain and v the kinematic viscosity) and the phase speed of the imposed waves relative to the friction velocity, i.e., the wave age varies from very slow to fast waves. The wave signal is clearly present in the airflow up to at least 0.15λ (where λ is the wave length) and is present up to higher levels for faster waves. In the kinetic energy budgets, pressure transport is mainly of importance for slow waves. For fast waves, viscous transport and turbulent transport dominate near the surface. Kinetic energy budgets for the wave and turbulent perturbations show a non-negligible transport of turbulent kinetic energy directed from turbulence to the wave perturbation in the airflow. The wave-turbulent energy transport depends on the size, tilt, and phase of the wave-induced part of the turbulent Reynolds stresses.According to the DNS data, slow waves are more efficient in generating isotropic turbulence than fast waves.Despite the differences in wave-shape as well as in Reynolds number between the idealized direct numerical simulations and the atmosphere, there are intriguing similarities in the turbulence structure. Important information about the turbulence above waves in the atmosphere can be obtained from DNS—the data must, however, be interpreted with care.     

Measurements of internal waves and turbulence in two-dimensional stratified shear flows

A turbulent stratified shear flow is generated in a towing tank by towing a grid or a circular cylinder through a tank of stratified salt water. The internal waves and turbulence generated in these flows are visualized with shadowgraphs and measured with quartz-coated hot-film probes (up to four probes for velocity fluctuations) and single-electrode conductivity probes (up to four probes for salinity fluctuations) which are towed at the same speed as the obstacle. The velocity and salinity signals are recorded on magnetic tapes. A portion of these signals is processed directly-on-line with a digital computer. From these shadowgraphs and probe measurements, we observe that

1. Far downstream of the obstacle where the turbulence has already subsided, the stratified fluid always has a layered structure. This layered structure persists for a long time, and is a result of the convection of turbulently mixed layers by the mean flow. These results indicate that in the regions of a stably stratified atmosphere and ocean where the turbulence has subsided, one could often find layered structure.
2. There are spectral peaks and valleys in the measured velocity and salinity autospectra when the stratifications are sufficiently strong. Under certain conditions, these spectral peaks tend to lift up the spectral curves to show substantialf ^?5/3 subranges, although the turbulence Reynolds numbers are too low for the flows to have recognizable inertial subranges. This anomalousf ^?5/3 subrange demonstrates the pitfalls of using spectral measurements in thef ^?5/3 subrange to predict the turbulent energy dissipation rate through the Kolmogorov hypothesis.
3. A diagnostic method is developed for distinguishing internal waves from turbulence, utilizing their phase characteristics. The phase characteristics can be conveniently examined from the cospectra and quadrature spectra measurements of: (a), two vertically separated velocity probes; (b), two vertically separated density probes; and (c), a velocity probe and a density probe. This method is demonstrated to be useful in the laboratory and can be applied directly to atmospheric and oceanic measurements to distinguish internal waves from turbulence.
4. From the coherency measurements, it is found that the entire turbulent stratified wake is actually whipping up and down at a frequency corresponding to the Brunt-Väisälä frequency. This indicates that similar stratified shear flows in the atmosphere and in the ocean, such as the jet streams in the atmosphere and the Cromwell current in the ocean, may oscillate vertically, which in turn can induce horizontal oscillation and meandering.

     

A method for determining thermophysical properties of organic material in aqueous solutions: Succinic acid

A method for determining evaporation rates and thermodynamic properties of aqueous solution droplets is introduced. The method combines evaporation rate measurements using modified TDMA technique with data evaluation using an accurate evaporation model. The first set of data has been collected and evaluated for succinic acid aqueous solution droplets.Evaporation rates of succinic acid solution droplets have been measured using a TDMA system at controlled relative humidity (65%) and temperature (298 K). A temperature-dependent expression for the saturation vapour pressure of pure liquid phase succinic acid at atmospheric temperatures has been derived by analysing the evaporation rate data with a numerical model. The obtained saturation vapour pressure of liquid phase succinic acid is ln(p) = 118.41 − 16204.8/T − 12.452ln(T). The vapour pressure is in unit of Pascal and the temperature in Kelvin. A linear expression for the enthalpy of vaporization for liquid state succinic acid is also presented.According to the results presented in the following, a literature expression for the vapour pressure of liquid phase succinic acid defined for temperatures higher than 461 K [Yaws, C.L., 2003. Yaws' Handbook of Thermodynamic and Physical Properties of Chemical Compounds, Knovel] can be extrapolated to atmospheric temperatures with very good accuracy. The results also suggest that at 298 K the mass accommodation coefficient of succinic acid is unity or very close to unity.     

Note on the Growth Rate of Water Waves Propagating at an Arbitrary Angle to the Wind

The dependence of the turbulent airflow over water waves on the angle,, between mean wind and wavedirections is investigated. To this end,an existing semi-analytical model is extended. In this model, the main simplification of the problem is obtained by using the well-established divisionof the wave boundary layer into inner and outer regions for modelling turbulence. The effect of waves on turbulence is restricted to the thin inner region. Simulations show that the influence of the wind speed component transverse to the wave direction on the air flow, and hence on the growth rate of the waves, is small. This is confirmed by calculations with a numerical model that solves the full Reynolds equations using a second-order turbulence closure scheme. The growth rate of slowly moving waves (as compared to the wind speed) is then proportional to cos², whereas, for faster waves, it has a narrower angular distribution.     

Influence of regional pollution and sandstorms on the chemical composition of cloud/fog at the summit of Mt. Taishan in northern China

Cloud/fog samples were collected during spring of 2007 in the highly polluted North China Plain in order to examine the impact of pollution and dust particles on cloud water chemistry. The volume weighted mean pH of cloud water was 3.68. The cloud acidity was shown to be associated with air mass origins. Cloud water with its air mass trajectories originating from the southern part of China was more acidic than those from northern China. Anthropogenic source and dust had obvious impact on cloud water composition as indicated by the very high mean concentrations of SO₄^2? (1331.65 μeq L^? 1), NO₃^? (772.44 μeq L^? 1), NH₄⁺ (1375.92 μeq L^? 1) and Ca²⁺ (625.81 μeq L^? 1) in the observation periods. During sandstorm days, cloud pH values were relatively high, and the concentrations of all the ions in cloud water reached unusual high levels. Significant decreases in the mass concentrations of PM_2.5 and PM₁₀ were observed during cloud events. The average scavenging ratio for PM_2.5 and PM₁₀ was 52.0% and 55.7%, respectively. Among the soluble ions in fine particles, NO₃^?, K⁺ and NH₄⁺ tend to be more easily scavenged than Ca²⁺ and Na⁺.     

Collapse and recovery in a remote small island—A tale of adaptive cycles or downward spirals?

Few studies consider how social-ecological systems recover from disturbance. We consider the small semi-autonomous island of Rodrigues (Indian Ocean). Based on semi-structured interviews (n = 70), a fisher survey (n = 73), weather data and official records we build a timeline of key events. We tabulate local perceptions (5+ mentions) of changes (social, economic and natural capital) and look for signs of adaptive cycles in the island's social-ecological past. Rising human pressure and extreme weather event impacts are reported since first settlement. We propose a recent “collapse” phase catalysed in the 1970s by severe drought, based on respondents’ perceptions of still-ongoing changes in farming and fishing, water, external dependence, migration and inter-island political change. Connectivity (flows of people, goods, information, money, power) appear to have strengthed local island recovery, but degradation continued, not least due to water scarcity and a lack of shared political vision as Rodrigues became more tied into the wider world.Overall, our findings suggest social-ecological systems may get stuck in a post-collapse recovery without any new structure emerging, presuming adaptive cycles can even be detected. Data gaps and global change redefining spatial and temporal scales could mean the adaptive cycle's usefulness is limited in development policy-making contexts.     

Atmospheric and tidal forcing of the exchange between Prince William Sound and the Gulf of Alaska

Current meter data from a series of oceanographic moorings spanning a total of five years was analyzed to quantify the tidal and subtidal exchange of water between Prince William Sound and the adjacent continental shelf in the northern Gulf of Alaska. Velocity profiles were used to quantify the exchange in terms of a transport through each of the two largest passages: Montague Strait and Hinchinbrook Entrance. Buoy wind and atmospheric pressure observations, as well as bottom pressure records, are then used to elucidate the role of atmospheric forcing on the exchange.An EOF analysis shows that the barotropic component accounts for 62% or more of the variance in the velocity profiles even after tides are removed by low-pass filtering, and thus the analysis is concerned primarily with depth-integrated transport. The estimated depth-integrated transport can reach ±0.6 Sv in Montague Strait, and ±1.5 Sv in Hinchinbrook Entrance. The largest fluctuations occur in response to the semidiurnal tides. Transport variations on subtidal time scales, which can reach −0.2 Sv in Montague Strait, and +0.6 Sv in Hinchinbrook Entrance, are shown by a frequency domain analysis to be dominated by easterly wind stress events which occur at periods of 2–5 days in both summer and winter. Atmospheric pressure has much less impact on transport, but there is some evidence that it might play a small role on time scales of a few weeks.Bottom pressure records suggest that easterly wind events set up a sea level height gradient in Hinchinbrook Entrance such that it tilts up to the east, which under geostrophy drives a barotropic flow into Prince William Sound. The same winds also raise the sea level in Hinchinbrook Entrance relative to Montague Strait, encouraging an outflow there in agreement with the ADCP observations. There is no evidence that the wind drives a vertically sheared bi-directional flow in either entrance, as has been observed in some estuaries. It is hypothesized that the lack of such a flow is possible because Prince William Sound has two major connections to the shelf, which alters the mass conservation requirement for each passage when compared to a system with just one entrance.     

Single scattering from frozen hydrometeors at microwave frequencies

Scattering of electromagnetic waves from homogeneous or coated spheres can be computed in a mathematically exact way using the Mie theory. Therefore, for many approaches in remote sensing, frozen hydrometeors are parameterized as ice spheres. However, many frozen hydrometeors have non-spherical overall shapes and lack a spherically symmetric internal structure. They exist in a huge variety of shapes and exhibit different mixtures of ice, water and air. Therefore it is desirable to accurately compute scattering from non-spherical particles in order to clearly understand the effect the shape of a hydrometeor has on its scattering pattern.In this study, single scattering parameters like scattering cross section, absorption cross section, and asymmetry factor were calculated for frozen hydrometeors using the Discrete Dipole Approximation (DDA). The particles were modeled as hexagonal plates, columns, needles and dendrites by applying known dimensional relationships. The calculations were carried out over a wide range of centimeter and millimeter-wavelengths (1 GHz to 300 GHz), since millimeter-wave radiometers are highly sensitive to scattering by frozen hydrometeors in the atmosphere.The study results show that for size parameters < 1 (a ratio between wavelength and particle size) the scattering cross section of randomly orientated ice crystals is close to that of an equal volume ice sphere. Absorption cross section and asymmetry factor of non-spherical particles however are up to twice as high as that of equal volume ice spheres. Further the influence of the assumed model for the refractive index of ice at microwave wavelengths on the scattering parameters is investigated.     

A Mediterranean nocturnal heavy rainfall and tornadic event. Part I: Overview,damage survey and radar analysis

This study presents an analysis of a severe weather case that took place during the early morning of the 2nd of November 2008, when intense convective activity associated with a rapidly evolving low pressure system affected the southern coast of Catalonia (NE Spain). The synoptic framework was dominated by an upper level trough and an associated cold front extending from Gibraltar along the Mediterranean coast of the Iberian Peninsula to SE France, which moved north-eastward. South easterly winds in the north of the Balearic Islands and the coast of Catalonia favoured high values of 0–3 km storm relative helicity which combined with moderate MLCAPE values and high shear favoured the conditions for organized convection. A number of multicell storms and others exhibiting supercell features, as indicated by Doppler radar observations, clustered later in a mesoscale convective system, and moved north-eastwards across Catalonia. They produced ground-level strong damaging wind gusts, an F2 tornado, hail and heavy rainfall. Total lightning activity (intra-cloud and cloud to ground flashes) was also relevant, exhibiting several classical features such as a sudden increased rate before ground level severe damage, as discussed in a companion study. Remarkable surface observations of this event include 24 h precipitation accumulations exceeding 100 mm in four different observatories and 30 minute rainfall amounts up to 40 mm which caused local flash floods. As the convective system evolved northward later that day it also affected SE France causing large hail, ground level damaging wind gusts and heavy rainfall.     

Influence of drizzle on Z–M relationships in warm clouds

This paper addresses the sensitivity of the relationships between radar reflectivity (Z) and liquid water content (M) for liquid water clouds to microphysical drizzle parameters by means of simulated radar observation at a frequency of 3 GHz of modeled cumulus clouds. A power law relationship for non drizzling clouds with water content as high as 3 gm^− 3: Z_c = 0.026 M_c^1.61 is numerically derived and agreed with previous empirical relationships relative to cumulus and stratocumulus. This relationship is then used to explore the influence of drizzle on the correlation between radar reflectively and water content. Due to their large diameters with respect to cloud droplets, drizzle sized drops dominate radar reflectivity but do not carry the cloud water content so that reflectivity and liquid water content are expected to be not correlated in clouds containing drizzle. It is shown that for congestus or extreme congestus cumuli, microphysical conditions for which the Z_c–M_c relationship can be used with a tolerance of 5 and 10% are provided whereas for humilis or mediocris cumuli, the presence of drizzle breaks down the Z_c–M_c relationship whatever the situations.     

Investigations on meteorological conditions for elevated PM2.5 in Fairbanks,Alaska

The relationships between meteorological conditions (temperature, wind-speed and direction, relative humidity, surface-inversion depth and strength, and stability) and PM_2.5 concentrations in Fairbanks, Alaska were investigated using ten years of observational data. The results show that during wintertime (November through February) PM_2.5 concentrations exceeding the 24 h National Air Quality Standard (35 μg/m³) occurred under calm wind, extremely low temperature (≤20 °C) and moisture (water-vapor pressure < 2 hPa) multiday surface-inversion conditions that trap the pollutants in the breathing level and inhibit transport of polluted air out of Fairbanks. PM_2.5 concentrations tend to be higher under stable than other conditions, but are not sensitive to the degree of stability. The presence of a surface inversion and calm wind are necessary, but in combination with low temperatures and humidity, the conditions are sufficient for high PM_2.5 concentrations. The low temperatures are required because they lead to increased emission rates from domestic heating and power production. During multiday inversions with temperatures above ? 20 °C, high relative humidity (> 75%) partly caused by water-vapor emission reduces PM_2.5 concentrations.     

The observed generation and breaking of atmospheric internal gravity waves over the ocean

Observations are presented for internal gravity waves and their breaking at a height of 23.5 m over the ocean in surface-based inversions which are formed because of the advection of warm air over cold water. The spectral and cospectral analyses of velocity and temperature fluctuations were made to establish the characteristic features of the waves. Flow visualization photographs of smoke released during the breaking of a wave are also presented. Comparison between the turbulent energies present during and after breaking of a wave indicates enormous mixing and dispersion occurring during breaking.This research was performed under Contract No. EY-76-C-02-0016 with the United States Energy Research and Development Administration.     

Water on an urban planet: Urbanization and the reach of urban water infrastructure

Urban growth is increasing the demand for freshwater resources, yet surprisingly the water sources of the world's large cities have never been globally assessed, hampering efforts to assess the distribution and causes of urban water stress. We conducted the first global survey of the large cities’ water sources, and show that previous global hydrologic models that ignored urban water infrastructure significantly overestimated urban water stress. Large cities obtain 78 ± 3% of their water from surface sources, some of which are far away: cumulatively, large cities moved 504 billion liters a day (184 km³ yr⁻¹) a distance of 27,000 ± 3800 km, and the upstream contributing area of urban water sources is 41% of the global land surface. Despite this infrastructure, one in four cities, containing $4.8 ± 0.7 trillion in economic activity, remain water stressed due to geographical and financial limitations. The strategic management of these cities’ water sources is therefore important for the future of the global economy.