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David Farmer and Ming Li

Abstract

A commonly observed property of near-surface bubble distributions is their collective organization into long rows aligned with the wind under the influence of Langmuir circulation. Time series observations with sonars having fixed orientation reveal the temporal evolution of bubble distributions as they drift through the sonar measurement path, Here this concept is extended to provide a time sequence, at 37-s intervals, of two-dimensional images generated by horizontally rotating sonars. Observations obtained during a storm in the Strait of Georgia show individual Langmuir convergence zones as they evolve above the freely drifting sonar. The resulting images are processed to generate a binary representation of the convergence zone patterns from which their orientation, length, spacing, and other properties can be extracted. Although there is some angular spreading, most convergence lines are aligned within 20° of the wind. The spacing between convergence lines reveals a wide range of scales, but the mean spacing increases slightly with wind speed. Measurement of downwind length reveals the presence of numerous short bubble clouds, possibly associated directly with wave breaking; however, there is a general trend toward a length that increases with wind speed.

A dominant characteristic at higher wind speeds is the formation of Y junctions in which three linear bubble clouds are joined together. Each branch of a Y junction was observed to be approximately 50 m. The junctions preferentially point downwind with the angle between the two side branches being approximately 30°. Although the junctions deform with time, they can be readily tracked through successive images The existence of convergence zone junctions suggests the reconnection of counterrotating longitudinal vortices and the formation of U-shaped vortex tubes.

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Ming Li and Chris Garrett

Abstract

The ratio of the buoyancy force driving thermal convection to the surface wave vortex-force driving Langmuir circulation in the Craik–Leibovich mechanism involves the Hoenikker number Ho. The critical value Hoc, at which wave forcing and thermal convection contribute equally to the circulation, is found to increase with decreasing Langmuir number La and approaches 3 in the small La limit. For a typical wind speed and surface cooling, Ho is of order O(10−2) to O(10−1). Thus, wave forcing dominates over thermal convection in driving Langmuir circulation.

Stratification induced by strong surface heating suppresses the circulation generated by wave forcing and could completely inhibit the CL instability. In the physically plausible range of −0.1 < Ho < 0, however, this does not happen for small La and the dynamical effect of heating is very small.

For a given heat flux, the temperature difference between the regions of surface divergence and convergence in Langmuir circulation depends on Ho, Pr, and La and on the depth distribution of the heating, but is typically 0(10−2) K.

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Ming Li and Chris Garrett

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The interaction between wind-driven Langmuir circulation and preexisting stratification is examined in order to elucidate its role in the deepening of the ocean surface mixed layer. For linear stratification, a numerical model suggests that Langmuir cells initially engulf water and create a homogeneous surface layer. The depth of this layer can be understood in terms of a Froude number Fr = dn/(Nh̃), where dn is the maximum downwelling velocity generated by Langmuir circulation in homogeneous water and N is the buoyancy frequency. Numerical results show that Fr is a constant ≈ 0.6. Using computed values of dn, this implies that the rapid mixed layer deepening stops at = cu */N in which u * is the water friction velocity and the coefficient c is about 10 for fully developed seas. Alternatively, the deepening is arrested when the buoyancy jump Δb at the mixed layer base reaches about 50u2*/. The above formula, compared with the Price, Weller, and Pinkel value of 0.65 for the bulk Richardson number R b associated with shear mixing, suggests that engulfment by Langmuir circulation dominates mixed layer deepening if the velocity difference |Δũ| across the base of the mixed layer is less than about 0.01U w, where U w is the wind speed. The buoyancy jump criterion is tested for two-layer stratification profiles and found to be a robust formula suitable for incorporation into one-dimensional mixed layer models.

The possibility of further mixed layer deepening through shear instability is studied by examining the distribution of the gradient Richardson number Rig, particularly in a transition region beneath the mixed layer. It has great variability across wind, reaching minimum values beneath downwelling jets, but can fall below 0.25, indicating the onset of shear instability. Thus, Langmuir cells may facilitate shear instability in a horizontally confined region beneath downwelling jets, although further study will require allowance for a different background shear.

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Xiaohui Xie and Ming Li

Abstract

Recent mooring observations at a cross-channel section in Chesapeake Bay showed that internal solitary waves regularly appeared during certain phases of a tidal cycle and propagated from the deep channel to the shallow shoal. It was hypothesized that these waves resulted from the nonlinear steepening of internal lee waves generated by lateral currents over channel-shoal topography. In this study numerical modeling is conducted to investigate the interaction between lateral circulation and cross-channel topography and discern the generation mechanism of the internal lee waves. During ebb tides, lateral bottom Ekman forcing drives a counterclockwise (looking into estuary) lateral circulation, with strong currents advecting stratified water over the western flank of the deep channel and producing large isopycnal displacements. When the lateral flow becomes supercritical with respect to mode-2 internal waves, a mode-2 internal lee wave is generated on the flank of the deep channel and subsequently propagates onto the western shoal. When the bottom lateral flow becomes near-critical or supercritical with respect to mode-1 internal waves, the lee wave evolves into an internal hydraulic jump. On the shallow shoal, the lee waves or jumps evolve into internal bores of elevation.

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Xiao-Ming Hu, Ming Xue, and Xiaolan Li

Abstract

Since the 1950s, a countergradient flux term has been added to some K-profile-based first-order PBL schemes, allowing them to simulate the slightly statically stable upper part of the convective boundary layer (CBL) observed in a limited number of aircraft soundings. There is, however, substantial uncertainty in inferring detailed CBL structure, particularly the level of neutral stability (z n), from such a limited number of soundings. In this study, composite profiles of potential temperature are derived from multiyear early afternoon radiosonde data over Beijing, China. The CBLs become slightly stable above z n ~ 0.31–0.33z i, where z i is the CBL depth. These composite profiles are used to evaluate two K-profile PBL schemes, the Yonsei University (YSU) and Shin–Hong (SH) schemes, and to optimize the latter through parameter calibration. In one-dimensional simulations using the WRF Model, YSU simulates a stable CBL above z n ~ 0.24z i, while default SH simulates a thick superadiabatic lower CBL with z n ~ 0.45z i. Experiments with the analytic solution of a K-profile PBL model show that adjusting the countergradient flux profile leads to significant changes in the thermal structure of CBL, informing the calibration of SH. The SH scheme replaces the countergradient heat flux term in its predecessor YSU scheme with a three-layer nonlocal heating profile, with f nl specifying the peak value and z*SL specifying the height of this peak value. Increasing f nl to 1.1 lowers z n, but to too low a value, while simultaneously increasing z*SL to 0.4 leads to a more appropriate z n ~ 0.36z i. The calibrated SH scheme performs better than YSU and default SH for real CBLs.

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Ming-Dah Chou and Li Peng

Abstract

A parameterization of the absorption in the 15 μm CO2 spectral region has been developed based upon the wing scaling approximation of Chou and Arking (1980, 1981). The spectrum is divided into a band-wing region and a band-center region, and the CO2 amount in an inhomogeneous atmosphere is scaled separately for the two regions. The spectrally averaged transmittance over each region is then expressed as a simple function of the scaled amount of CO2. Compared to fine-by-line calculations, the error of the parameterization is <0.025 in the transmittance and <0.04°C day−1 in the tropospheric and lower stratospheric cooling rates. The cooling rate error in the upper stratosphere is generally ten than a few tenths of a degree per day except for the region above the 3 mb level where the error is too large to be acceptable for some studies on the phenomena in that region.

The effect of the parameterization of absorption due to CO2 on climate studies has been investigated with the Multi-Layer Energy Balance Model (MLEBM) developed at GLAS (Peng et al, 1982). It is found that, compared to the accurate perturbation method, the parameterization introduces very small differences in the model temperatures and radiation budgets for both the normal and doubled CO2 concentrations. In addition, we have investigated the effect of including the CO2 absorption in the margins of the 15 μm spectral band on the CO2 climate sensitivity. It is found that the surface temperature sensitivity is enhanced by 20% for a doubled CO2 concentration and by 30% for a quadrupled CO2 concentration when the spectral range of CO2 absorption is extended from 580–760 to 540–800 cm−1.

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Li Peng, Ming-Dah Chou, and Albert Arking

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A nine-layer, zonally averaged, steady-state model has been developed for use in climate sensitivity studies. The model is based upon thermal energy balance and includes recently developed accurate treatment of radiative transfer, parameterized meridional and vertical energy transport, and thermodynamic interaction between the surface and the atmosphere. Cloud cover and relative humidity are prescribed parameters. Using present day boundary conditions for the Northern Hemisphere, the simulated temperature field, heat fluxes and radiation quantities are in good agreement with observations. In a study of sensitivity to changes in the solar constant, the model exhibits a high degree of nonlinearity. The change in the hemispheric mean surface temperature is +3.1°C in response to a 2% increase in the solar constant and −4.3°C in response to a 2% decrease in the solar constant. The sensitivity varies with latitude. In the polar region it is about three times larger than in the tropics, due mostly to the effect of ice-albedo feedback. There is also a variation of the response of atmospheric temperature with height. The response increases with height in the tropics but decreases with height in the polar regions. The results are in general agreement with those of Wetherald and Manabe (1975) using a GCM.

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Xiaohui Xie, Ming Li, and William C. Boicourt

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The 2-month-long mooring data were collected in a straight midsection of Chesapeake Bay to document the lateral circulation driven by along-channel winds. Under upestuary winds, the lateral circulation featured a clockwise (looking into estuary) circulation in the surface layer, with lateral Ekman forcing as the dominant generation mechanism. Under downestuary winds, however, the lateral circulation displayed a structure dependent on the Wedderburn number W: a counterclockwise circulation at small W and two counterrotating vortices at large W. The surface lateral velocity was phase locked to the along-channel wind speed. Analysis of the streamwise vorticity equation showed that the strength and structure of the lateral circulation in this stratified estuary were largely determined by the competition between the tilting of planetary vorticity by along-channel currents and lateral baroclinic forcing due to sloping isopycnals. Under strong, downestuary winds, the lateral baroclinic forcing offset or reversed the tilting of planetary vorticity on the western half of the estuarine channel, resulting in two counterrotating lateral circulation cells. A bottom lateral flow was observed in the deep channel and appeared to be generated by lateral Ekman forcing on the along-channel currents.

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Ming-Dah Chou, Li Peng, and Albert Arking

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The radiative and climatic effects of stratospheric volcanic aerosols are studied with a multilayer energy balance model. The results show that the latitudinal distribution of aerosols has a significant effect on climate sensitivity. When aerosols are assumed to be distributed uniformly in the 30–90°N region and decay exponentially with an e-folding time constant of 1 year, the maximum response is in the 60–70°N zone where the ice-albedo feedback is most active. The maximum occurs shortly (<0.5 years) after the eruption due to the large extent of land and, therefore, the small thermal inertia in that latitude zone. When the same amount of aerosols is assumed to be in the 0–30°N region, the response is much weakened due to smaller radiative forcing and lack of ice-albedo feedback in the tropics, but is prolonged due to the larger extent of the oceans. The maximum response is reduced to ⅕ of that of the former case and occurs at a much later time (∼1.5 years). A secondary maximum appears in the polar region as a result of ice-albedo feedback.

The climate sensitivity to some of the aerosol properties has also been studied. Model simulations indicate that the absorption of solar energy in the ultraviolet and visible spectral regions by the aerosols enhances the sensitivity of surface temperature due to the reduced solar radiation incident at the surface. With the same amount of forcing at the top of the atmosphere, the solar radiation is more important than the thermal IR radiation in affecting the surface temperature.

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Chris Garrett, Ming Li, and David Farmer

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A formula for the maximum size of a bubble for which surface tension forces can prevent bubble breakup by inertial forces, combined with the observed sizes of air bubbles in breaking waves, implies an energy dissipation rate. One dataset from the surf zone gives a dissipation rate of the order of 0.1 W kg−1, but the large number of small bubbles, and the bubble size spectrum generally, are puzzling. A simple dimensional cascade argument suggests that injected air beneath a breaking wave is rapidly broken up by turbulence, producing an initial size spectrum proportional to (radius)−10/3 before modification by dissolution and rising under buoyancy. This spectral slope is comparable with data from the surf zone. The cascade argument does, however, predict that for a constant dissipation rate there is a rapid accumulation of a large number of bubbles at the scale at which surface tension prevents further breakup; it is possible that the observed size spectrum reflects the range of turbulent energy dissipation rates rather than the result of a cascade. If so, an estimate of about 40 W kg−1 is obtained for the dissipation rate implied by the surf zone dataset. Once an initial size spectrum is formed by the rapid action of differential pressure forces, it will evolve subject to dissolution and buoyancy. It is shown that the former will tend to flatten the size spectrum at small scales, whereas the latter will tend to steepen the time-averaged spectrum observed at large scales. The slope change and transition radius predicted by a very simple model are in reasonable agreement with observations.

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