Effects of Roll Vortices on the Evolution of Hurricane Harvey During Landfall

Horizontal boundary layer roll vortices are a series of large-scale turbulent eddies that prevail in a hurricane’s boundary layer. In this paper, a one-way nested sub-kilometer-scale large eddy simulation (LES) based on the Weather Research and Forecasting model (WRF) was used to examine the impact of roll vortices on the evolution of Hurricane Harvey around its landfall from 0000z on 25 to 1800z 27 August 2017. The simulation results imply that the turbulence in the LES can be attributed mainly to roll vortices. With the representation of roll vortices, the LES simulation provided a better simulation of hurricane wind vertical structure and precipitation. In contrast, the mesoscale simulation with the YSU PBL scheme overestimated the precipitation for the hurricane over the ocean.Further analysis indicates that the roll vortices introduced a positive vertical flux and thinner inflow layer, whereas a negative flux maintained the maximum tangential wind at around 400 m above ground. During hurricane landfall, the weak negative flux maintained the higher wind in the LES simulation. The overestimated low-level vertical flux in the mesoscale simulation with the YSU scheme led to overestimated hurricane intensity over the ocean and accelerated the decay of the hurricane during landfall. Rainfall analysis reveals that the roll vortices led to a weak updraft and insufficient water vapor supply in the LES. For the simulation with the YSU scheme, the strong updraft combined with surplus water vapor eventually led to unrealistic heavy rainfall for the hurricane over the ocean.

FFD Analysis for the Effect of Fouling on the Permeate Flux in High-Pressure Membranes

Porous high-pressure membranes have been widely used for both brackish water and seawater desalination. However, fouling (concentration polarization) extensively reduces permeate flux in high-pressure membranes such that reverse osmosis (RO) and/or nanofiltration (NF). In this study, we have attempted to understand the effect of membrane fouling on the permeate water flux by modeling the salt concentration profile within a membrane of interest. A parabolic (or diffusion) partial differential equation was used to describe the change in salt concentration inside the membrane. Subsequently, the PDE equation was solved numerically, under certain assumptions, by using forward finite difference (FFD) explicit method. It was found that salt accumulation occurs at the membrane feed-side surface and there was a noticeable decrease in water flux as fouling increased. For waters with an initial salt concentration of 10000 ppm (NaCl) and with an average diffusivity of , results showed that both RO/NF would have flux rates of 74.9, 67.4, 22.5, 0, –37.4, –74.9 LMH for the feed-side surface concentrations 0, 1000, 7000, 10000, 15000 and 20000 ppm, respectively, where negative flux indicates a back-flow scenario.

Continuous Estimate of Atlantic Oceanic Freshwater Flux at 26.5°N

The first continuous estimates of freshwater flux across 26.5°N are calculated using observations from the RAPID–MOCHA–Western Boundary Time Series (WBTS) and Argo floats every 10 days between April 2004 and October 2012. The mean plus or minus the standard deviation of the freshwater flux (FW) is −1.17 ± 0.20 Sv (1 Sv ≡ 106 m3 s−1; negative flux is southward), implying a freshwater divergence of −0.37 ± 0.20 Sv between the Bering Strait and 26.5°N. This is in the sense of an input of 0.37 Sv of freshwater into the ocean, consistent with a region where precipitation dominates over evaporation. The sign and the variability of the freshwater divergence are dominated by the overturning component (−0.78 ± 0.21 Sv). The horizontal component of the freshwater divergence is smaller, associated with little variability and positive (0.35 ± 0.04 Sv). A linear relationship, describing 91% of the variance, exists between the strength of the meridional overturning circulation (MOC) and the freshwater flux (−0.37 − 0.047 Sv of FW per Sverdrups of MOC). The time series of the residual to this relationship shows a small (0.02 Sv in 8.5 yr) but detectable decrease in the freshwater flux (i.e., an increase in the southward freshwater flux) for a given MOC strength. Historical analyses of observations at 24.5°N are consistent with a more negative freshwater divergence from −0.03 to −0.37 Sv since 1974. This change is associated with an increased southward freshwater flux at this latitude due to an increase in the Florida Straits salinity (and therefore the northward salinity flux).

Investigating uptake of N₂O in agricultural soils using a high-precision dynamic chamber method

Uptake (or negative flux) of nitrous oxide (N2O) in agricultural soils is a controversial issue which has proved difficult to investigate in the past due to constraints such as instrumental precision and methodological uncertainties. Using a recently developed high-precision quantum cascade laser gas analyser combined with a closed dynamic chamber, a well-defined detection limit of 4 μg N2O-N m−2 h−1 could be achieved for individual soil flux measurements. 1220 measurements of N2O flux were made from a variety of UK soils using this method, of which 115 indicated uptake by the soil (i.e. a negative flux in the micrometeorological sign convention). Only four of these apparently negative fluxes were greater than the detection limit of the method, which suggests that the vast majority of reported negative fluxes from such measurements are actually due to instrument noise. As such, we suggest that the bulk of negative N2O fluxes reported for agricultural fields are most likely due to limits in detection of a particular flux measurement methodology and not a result of microbiological activity consuming atmospheric N2O.

Investigating uptake of N₂O in agricultural soils using a high-precision dynamic chamber method

Uptake (or negative flux) of nitrous oxide (N2O) in agricultural soils is a controversial issue which has proven difficult to investigate in the past due to constraints such as instrumental precision and unknown methodological uncertainties. Using a recently developed high-precision quantum cascade laser (QCL) gas analyser combined with a closed dynamic chamber, a well defined detection limit of 4 μg N2O-N m−2 h−1 could be achieved for individual soil flux measurements. 1220 measurements of N2O flux were made from a variety of UK soils using this method, of which 115 indicated uptake by the soil (i.e. a negative flux in the micrometeorological sign convention). Only 4 of these apparently negative fluxes were greater than the detection limit of the method, which suggests that the vast majority of reported negative fluxes from such measurements are actually due to instrument noise. As such, we suggest that the bulk of negative N2O fluxes reported for agricultural fields are most likely due to limits in detection of a particular flux measurement methodology and not as a result of microbiological activity consuming atmospheric N2O.

<i>Review article</i> "Air-sea exchanges of CO₂ in world's coastal seas"

The air-sea exchanges of CO2 in the world's 165 estuaries and 87 continental shelves are evaluated. Generally and in all seasons, upper estuaries with salinities of less than two are strong sources of CO2 (39 &amp;pm; 56 mol C m−2 yr−1, negative flux indicates that the water is losing CO2 to the atmosphere); mid-estuaries with salinities of between 2 and 25 are moderate sources (17.5 ± 34 mol C m−2 yr−1) and lower estuaries with salinities of more than 25 are weak sources (8.4 ± 14 mol C m−2 yr−1). With respect to latitude, estuaries between 23.5 and 50° N have the largest flux per unit area (63 ± 101 mmol C m−2 d−1); these are followed by mid-latitude estuaries (23.5–0° S: 44 ± 29 mmol C m−2 d−1; 0–23.5° N: 39 ± 55 mmol C m−2 d−1), and then regions north of 50° N (36 ± 91 mmol C m−2 d−1). Estuaries south of 50° S have the smallest flux per unit area (9.5 ± 12 molC m−2 d−1). Mixing with low-pCO2 shelf waters, water temperature, residence time and the complexity of the biogeochemistry are major factors that govern the pCO2 in estuaries but wind speed, seldom discussed, is critical to controlling the air-water exchanges of CO2. The total annual release of CO2 from the world's estuaries is now estimated to be 0.10 PgC yr−1, which is much lower than published values mainly because of the contribution of a considerable amount of heretofore unpublished or new data from Asia and the Arctic. The Asian data, although indicating high in pCO2, are low in sea-to-air fluxes because the wind speeds are lower than previously determined values, which rely heavily on data from Europe and North America, where pCO2 is lower but wind speeds are much higher, such that the CO2 fluxes are higher than in Asia. Newly emerged CO2 flux data in the Arctic reveal that estuaries there mostly absorb, rather than release CO2. Most continental shelves, and especially those at high latitude, are under-saturated in terms of CO2 and absorb CO2 from the atmosphere in all seasons. Shelves between 0° and 23.5° S are on average a weak source and have a small flux per unit area of CO2 to the atmosphere. Water temperature, the spreading of river plumes, upwelling, and biological production seem to be the main factors in determining pCO2 in the shelves. Wind speed, again, is critical because at high latitudes, the winds tend to be strong. Since the surface water pCO2 values are low, the air-to-sea fluxes are high in regions above 50° N and below 50° S. At low latitudes, the winds tend to be weak, so the sea-to-air CO2 flux is small. Overall, the world's continental shelves absorb 0.4 PgC yr−1 from the atmosphere.

FRACTIONAL QUANTUM HALL STATES WITH NEGATIVE FLUX: EDGE MODES IN SOME ABELIAN AND NON-ABELIAN CASES

We investigate the structure of gapless edge modes propagating at the boundary of some fractional quantum Hall states. We show how to deduce explicit trial wavefunctions from the knowledge of the effective theory governing the edge modes. In general, quantum Hall states have many edge states. Here, we discuss the case of fractions having only two such modes. The case of spin-polarized and spin-singlet states at filling fraction ν = 2/5 is considered. We give an explicit description of the decoupled charged and neutral modes. Then we discuss the situation involving negative flux acting on the composite fermions. This happens notably for the filling factor ν = 2/3 which supports two counterpropagating modes. Microscopic wavefunctions for spin-polarized and spin-singlet states at this filling factor are given. Finally, we present an analysis of the edge structure of a non-Abelian state involving also negative flux. Counterpropagating modes involve, in all cases, explicit derivative operators diminishing the angular momentum of the system.

Cone exchange transformations and boundedness of orbits

We introduce a class of two-dimensional piecewise isometries on the plane that we refer to as cone exchange transformations (CETs). These are generalizations of interval exchange transformations (IETs) to 2D unbounded domains. We show for a typical CET that boundedness of orbits is determined by ergodic properties of an associated IET and a quantity we refer to as the ‘flux at infinity’. In particular we show, under an assumption of unique ergodicity of the associated IET, that a positive flux at infinity implies unboundedness of almost all orbits outside some bounded region, while a negative flux at infinity implies boundedness of all orbits. We also discuss some examples of CETs for which the flux is zero and/or we do not have unique ergodicity of the associated IET; in these cases (which are of great interest from the point of view of applications such as dual billiards) it remains an outstanding problem to find computable necessary and sufficient conditions for boundedness of orbits.