Search Results

You are looking at 1 - 10 of 63 items for

  • Author or Editor: Ming Li x
  • Refine by Access: All Content x
Clear All Modify Search
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.

Full access
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.

Full access
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.

Full access
Ming Li and Chris Garrett

Abstract

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.

Full access
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.

Free access
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.

Full access
Ming-Dah Chou, Li Peng, and Albert Arking

Abstract

The sensitivity of climate to a doubling of the atmospheric CO2, content has been studied using the GLAS multi-layer energy balance model. In response to a doubled CO2 content, tropospheric temperature lapse rate decreases at low latitudes but increases at high latitudes. Averaged over the Northern Hemisphere, the change is +2.3°C in the surface temperature and +0.47°C in the earth's brightness temperature. The increase, in surface temperature is mainly caused by the intensified downward IR radiation in the water vapor bands at all latitudes and by the increased absorption of solar radiation at high latitudes due to the reduction of ice/snow cover.

The effects of some feedback mechanisms on the climate sensitivity to a doubled CO2 content have also been studied. It is found that the sensitivity of surface temperature is approximately doubled at all latitudes due to the change in water vapor content. There are two important, yet offsetting, effects of ice/snow cover on the climate sensitivity. As the ice/snow cover decreases due to an increased surface temperature, both the absorption of solar radiation and evaporation increase greatly. The effect of allowing the ice/snow cover to change with temperature is to enhance the global sensitivity by 40%. Other feedback mechanisms studied include meridional transport of latent heat and e-type absorption.

Full access
Xiaohui Xie, Ming Li, and William C. Boicourt

Abstract

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.

Full access
Chris Garrett, Ming Li, and David Farmer

Abstract

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.

Full access
Ming Li, Huidong Jin, and Jaclyn N. Brown

Abstract

Seasonal climate forecasts from raw climate models at coarse grids are often biased and statistically unreliable for credible crop prediction at the farm scale. We develop a copula-based postprocessing (CPP) method to overcome this mismatch problem. The CPP forecasts are ensemble based and are generated from the predictive distribution conditioned on raw climate forecasts. CPP performs univariate postprocessing procedures at each station, lead time, and variable separately and then applies the Schaake shuffle to reorder ensemble sequence for a more realistic spatial, temporal, and cross-variable dependence structure. The use of copulas makes CPP free of strong distributional assumptions and flexible enough to describe complex dependence structures. In a case study, we apply CPP to postprocess rainfall, minimum temperature, maximum temperature, and radiation forecasts at a monthly level from the Australian Community Climate and Earth-System Simulator Seasonal model (ACCESS-S) to three representative stations in Australia. We evaluate forecast skill at lead times of 0–5 months on a cross-validation theme in the context of both univariate and multivariate forecast verification. When compared with forecasts that use climatological values as the predictor, the CPP forecast has positive skills, although the skills diminish with increasing lead times and finally become comparable at long lead times. When compared with the bias-corrected forecasts and the quantile-mapped forecasts, the CPP forecast is the overall best, with the smallest bias and greatest univariate forecast skill. As a result of the skill gain from univariate forecasts and the effect of the Schaake shuffle, CPP leads to the most skillful multivariate forecast as well. Further results investigate whether using ensemble mean or additional predictors can enhance forecast skill for CPP.

Free access