Jinbao Song

Impact of Surface Waves on the Steady Near-Surface Wind Profiles over the Ocean

The impacts of surface waves on the steady near-surface wind profiles in the marine atmospheric boundary layer (ABL) are studied based on the Ekman theory, modified by introducing a wave-induced component on the total stress. An analytic solution is presented for the wave-modified Ekman model for an eddy viscosity coefficient varying linearly with height. The solution can be determined by the two-dimensional wavenumber spectrum of ocean waves, the wave-growth or decay rate, the geostrophic wind velocity, the Coriolis parameter and the densities of air and water. Wind profiles are calculated as examples for two cases: one with a monochromatic wave and the other with a fully-developed wind-generated sea. The effects of the surface waves on the wind profiles in the marine ABL are illustrated, and solutions proposed are compared with those of the model where the wave-induced stress is neglected. The solutions are also compared with observations from a tower on Östergarnsholm Island in the Baltic Sea. Illustrative examples and the comparisons between observations and the theoretical predictions demonstrate that the surface waves have a considerable impact, not only on the near-surface mean wind profile, but also on the turbulence structure of the marine ABL, as they change qualitatively the structure of the ABL.

Air-sea carbon-dioxide flux estimated by eddy covariance method from a buoy observation

Precise measurements of the CO2 gas transfer across the air-sea interface provide a better understanding of the global carbon cycle. The air-sea CO2 fluxes are obtained by the eddy covariance method and the bulk method from a buoy observation in the northern Huanghai sea. The effects of buoy motion on flux calculated by the eddy covariance method are demonstrated. The research shows that a motion correction can improve the correlation coefficient between the CO2 fluxes estimated from two different levels. Without the CO2-H2O cross-correlation correction which is termed as PKT correction, the air-sea CO2 fluxes estimated by eddy covariance method using the motion corrected data are nearly an order of magnitude larger than those estimated by the bulk method. After the CO2-H2O cross-correlation correction, some eddy covariance CO2 fluxes indeed become closer to the bulk CO2 flux, whereas some are overcorrected which are in response to small water vapor flux.

Effect of Langmuir circulation on upper ocean mixing in the South China Sea

Effect of Langmuir circulation (LC) on upper ocean mixing is investigated by a two-way wave-current coupled-model. The model is coupled of the ocean circulation model ROMS (regional ocean modeling system) to the surface wave model SWAN (simulating waves near shore) via the model-coupling toolkit. The LC already certified its importance by many one-dimensional (1D) research and mechanism analysis work. This work focuses on inducing LC’s effect in a three-dimensional (3-D) model and applying it to real field modeling. In ROMS, the Mellor-Yamada turbulence closure mixing scheme is modified by including LC’s effect. The SWAN imports bathymetry, free surface and current information from the ROMS while exports significant wave parameters to the ROMS for Stokes wave computing every 6 s. This coupled model is applied to the South China Sea (SCS) during September 2008 cruise. The results show that LC increasing turbulence and deepening mixed layer depth (MLD) at order of O (10 m) in most of the areas, especially in the north part of SCS where most of our measurements operated. The coupled model further includes wave breaking which will brings more energy into water. When LC works together with wave breaking, more energy is transferred into deep layer and accelerates the MLD deepening. In the north part of the SCS, their effects are more obvious. This is consistent with big wind event in the area of the Zhujiang River Delta. The shallow water depth as another reason makes them easy to influence the ocean mixing as well.

A motion correction on direct estimations of air-sea fluxes from a buoy

A flux system deployed on amoored buoy has been described, which is capable of directly estimating the airsea fluxes after removing the contamination in the signal due to buoy motion. A triple loop fitting method has been demonstrated for determining the three angular offsets between measurement axes of the sonic anemometer and motion pack. The data collected in an experiment in the Northern Huanghai Sea is used to correct the three sonic anemometer measurements of turbulent wind for buoy motion. The effective removal of wave-scale motion from the spectra and cospectra are demonstrated. Estimates of along-wind momentum flux, sensible heat flux and latent heat flux calculated by the eddy correlationmethod based on data obtained by sonic anemometer 81000V are shown to be in the same trend and scale with those determined by the bulk aerodynamic method after motion correction. The motion correction not only greatly improve the estimation of the momentum flux but also has a great impact on the calculated sensible heat flux.

Observations and Modeling of Typhoon Waves in the South China Sea

AbstractBuoy-based observations of surface waves during three typhoons in the South China Sea were used to obtain the wave characteristics. With the local wind speeds kept below 35 m s−1, the surface waves over an area with a radius 5 times that of the area in which the maximum sustained wind was found were mainly dominated by wind-wave components, and the wave energy distribution was consistent with fetch-limited waves. Swells dominated the surface waves at the front of and outside the central typhoon region. Next, the dynamics of the typhoon waves were studied numerically using a state-of-the-art third-generation wave model. Wind forcing errors made a negligible contribution to the surface wave results obtained using hindcasting. Near-realistic wind fields were constructed by correcting the idealized wind vortex using in situ observational data. If the different sets of source terms were further considered for the forcing stage of the typhoon, which was defined as the half inertial period before and aft...

A numerical study on seasonal variations of the thermocline in the South China Sea based on the ROMS

On the basis of the regional ocean modeling system (ROMS), the seasonal variations of the thermocline in the South China Sea (SCS) were numerically investigated. The simulated hydrodynamics are in accordance with previous studies: the circulation pattern in the SCS is cyclonic in winter and anticyclonic in summer, and such a change is mostly driven by the monsoon winds. The errors between the modeled temperature profiles and the observations obtained by cruises are quite small in the upper layers of the ocean, indicating that the ocean status is reasonably simulated. On the basis of the shapes of the vertical temperature profiles, five thermocline types (shallow thermocline, deep thermocline, hybrid thermocline, double thermocline, and multiple thermocline) are defined herein. In winter, when the northeasterly monsoon prevails, most shallow shelf seas in the northwest of the SCS are well mixed, and there is no obvious thermocline. The deep region generally has a deep thermocline, and the hybrid or double thermocline often occurs in the areas near the cold eddy in the south of the SCS. In summer, when the southwesterly monsoon prevails, the shelf sea area with a shallow thermocline greatly expands. The distribution of different thermocline types shows a relationship with ocean bathymetry: from shallow to deep waters, the thermocline types generally change from shallow or hybrid to deep thermocline, and the double or multiple thermocline usually occurs in the steep regions. The seasonal variations of the three major thermocline characteristics (the upper bound depth, thickness, and intensity) are also discussed. Since the SCS is also an area where tropical cyclones frequently occur, the response of thermocline to a typhoon process in a short time scale is also analyzed.

On the parameterization of drag coefficient over sea surface

Six parameterization schemes of roughness or drag coefficient are evaluated on the basis of the data from six experiments. They present great consistency with measurement when friction velocity u*<0.5 m/s (approximately corresponding to 10 m wind speed U10<12 m/s) and large deviation from measurement when u*⩾0.5m/s (approximatelyU10⩾12m/s). In order to improve the deviation, a newparameterization of drag coefficient is derived on the basis of the similarity theory, Charnock relationship and Toba 3/2 power law. Wave steepness andwind-sea Reynolds number are considered in the new parameterization. Then it is tested on the basis of the measurements and shows significant improvement when u* ⩾0.5 m/s. Its standard errors are much smaller than the ones of the other six parameterizations. However, the new parameterization still needs more tests especially for high winds.

The spacing of Langmuir circulation under modest wind

Spacing characteristics of Langmuir circulation (LC) are computed by large eddy simulation (LES) model under modest wind. LC is an organized vertical motion, evidenced as buoyant materials forming lines nearly parallel to the wind direction. The horizontal distribution of velocity computed by LES shows clear lines formed by LC. These lines grow and parallel to each other for a while, which we call the stable state, before they finally form Y-junctions. We computed spacing between every two parallel lines by averaging them under the stable state. Statistically, spacing results of 154 tests (seven wind speed cases of 22 test runs each) show high correlations between spacing and wind speed, as well as mixed layer depth. The relationship of spacing and wind is important for future LC parameterization of upper-ocean mixing.

Uncertainty in air–sea CO2 flux due to transfer velocity

From TOPEX/Poseidon data, the significant uncertainty in global air–sea carbon dioxide (CO2) flux in 2000 is calculated from 21 wind-speed-dependent and 5 sea-state-dependent CO2 transfer velocities and the sea-to-air partial pressure difference proposed by Takahashi, Sutherland, and Kozyr (2010). The sea-state-dependent parameterizations are calculated based on the significant wave height (SWH) and the radar backscatter coefficient measured using the Ku-band altimeter. Uncertainty in air‒sea CO2 flux is compared using 26 various transfer velocity formulas. The maximum differences in global monthly mean and 4° zonal mean values for gas transfer velocity among these formulas are 33.20 and 108.20 cm hour‒1, respectively. The corresponding differences for sea-to-air CO2 flux are 6.41 and 0.58 Pg C year‒1, respectively. Monthly mean global maps of gas transfer velocity and flux are also presented. The average value of the global mean, transfer velocity obtained using the 26 formula is 27.33 ± 9.75 cm hour‒1, and the averaged total global net air-to-sea CO2 flux is 2.77 ± 1.02 Pg C year‒1 after area weighting and Schmidt number correction. The sea-state-dependent parameterizations are near these values, providing a successful method to estimate the air‒sea CO2 transfer velocity and flux.

Global air-sea surface carbon-dioxide transfer velocity and flux estimated using ERS-2 data and a new parametric formula

Using data from the European remote sensing scatterometer (ERS-2) from July 1997 to August 1998, global distributions of the air-sea CO2 transfer velocity and flux are retrieved. A new model of the air-sea CO2 transfer velocity with surface wind speed and wave steepness is proposed. The wave steepness (δ) is retrieved using a neural network (NN) model from ERS-2 scatterometer data, while the wind speed is directly derived by the ERS-2 scatterometer. The new model agrees well with the formulations based on the wind speed and the variation in the wind speed dependent relationships presented in many previous studies can be explained by this proposed relation with variation in wave steepness effect. Seasonally global maps of gas transfer velocity and flux are shown on the basis of the new model and the seasonal variations of the transfer velocity and flux during the 1 a period. The global mean gas transfer velocity is 30 cm/h after area-weighting and Schmidt number correction and its accuracy remains calculation with in situ data. The highest transfer velocity occurs around 60°N and 60°S, while the lowest on the equator. The total air to sea CO2 flux (calculated by carbon) in that year is 1.77 Pg. The strongest source of CO2 is in the equatorial east Pacific Ocean, while the strongest sink is in the 68°N. Full exploration of the uncertainty of this estimate awaits further data. An effectual method is provided to calculate the effect of waves on the determination of air-sea CO2 transfer velocity and fluxes with ERS-2 scatterometer data.