夜间大气边界层的高阶矩数值模拟

本文建立了一个二阶矩模式,用此摸式对Wangara资料进行了实例模拟.结果表明,该模式能模拟出夜间边界层发展以及湍流结构的一些主要特征.计算发现在夜间边界层发展过程中存在一种突变现象.     

Processes in the surface mixed layer of the ocean

Models for the evolution of the surface mixed layer need to be improved to include dominant processes such as Langmuir circulation. It is shown that the wave forcing in Langmuir circulation models is much stronger than that due to a surface buoyancy loss, and studies of the erosion by the cells of a pre-existing stratification are described. Mixed layer models will also need to allow for horizontal inhomogeneity. It is shown, for example, that the horizontal buoyancy gradient that may be left behind after a storm produces restratification that can be significant. The nonlinearity of the equation of state is another real-world factor; it gives rise to an annual average surface buoyancy that is misleading as it is compensated by interior cabbeling. Current work linking the mixed layer to water mass formation is also introduced.     

Applied model for the growth of the daytime mixed layer

A slab model is proposed for developing the height of the mixed layer capped by stable air aloft. The model equations are closed by relating the consumption of energy (potential and kinetic) at the top of the mixed layer to the production of convective and mechanical turbulent kinetic energy within the mixed layer. By assuming that the temperature difference at the top of the mixed layer instantaneously adjusts to the actual meteorological conditions without regard to the initial temperature difference that prevailed, the model is reduced to a single differential equation which easily can be solved numerically. When the mixed layer is shallow or the atmosphere nearly neutrally stratified, the growth is controlled mainly by mechanical turbulence. When the layer is deep, its growth is controlled mainly by convective turbulence. The model is applied on a data set of the evolution of the height of the mixed layer in the morning hours, when both mechanical and convective turbulence contribute to the growth process. Realistic mixed-layer developments are obtained.     

A lidar study of the atmospheric entrainment zone and mixed layer over Hong Kong

In this paper we present the lidar study of the atmospheric boundary layer using the City University of Hong Kong lidar system. Three cases of determination of the entrainment zone thickness and the mixed layer depth in the atmospheric boundary layer over Hong Kong were selected for detailed study. The data collected have been analysed using the visual inspection method and the Steyn et al. [J. Atmos. Ocean. Technol. 16 (1999) 953] detection method. During the cold front passage, the mixed layer depth stayed at the level almost constant over the transition period, and the entrainment zone was thickening at a steady entrainment rate.     

Numerical simulation of the boundary layer above waves

A new approach to investigations of the structure of the boundary layer above waves is discussed. The approach is based on direct numerical simulation of wave motions in the boundary layer produced by a moving curved surface. Model equations are derived, which are the Reynolds equations in a curvilinear nonstationary system of co-ordinates, evolution equations for turbulent kinetic energy, and Kolmogorov's approximate similarity formulae relating the coefficient of turbulent viscosity to the dissipation of turbulent energy; the length scale is assumed to grow linearly with increasing distance from the surface. Principles of constructing the model numerical scheme are described. Results are given of modelling the structure of the boundary layer above a nonsteady surface, which, in a general case, is a superposition of progressive waves with assigned dispersion relations and amplitudes. Mechanisms of energy and momentum transfer to the surface, effects of density stratification and energy structure in the boundary layer are studied. Merits and demerits of the approach are discussed.     

Interannual variability of the Tropical Indian Ocean mixed layer depth

In the present study, interannual fluctuations of the mixed layer depth (MLD) in the tropical Indian Ocean are investigated from a long-term (1960–2007) eddy permitting numerical simulation and a new observational dataset built from hydrographic in situ data including Argo data (1969–2008). Both datasets show similar interannual variability patterns in relation with known climate modes and reasonable phase agreement in key regions. Due to the scarcity of the observational dataset, we then largely rely on the model to describe the interannual MLD variations in more detail. MLD interannual variability is two to four times smaller than the seasonal cycle. A large fraction of MLD interannual variations is linked to large-scale climate modes, with the exception of coastal and subtropical regions where interannual signature of small-scale structures dominates. The Indian Ocean Dipole is responsible for most variations in the 10°N–10°S band, with positive phases being associated with a shallow MLD in the equatorial and south-eastern Indian Ocean and a deepening in the south-central Indian Ocean. The El Niño signature is rather weak, with moderate MLD shoaling in autumn in the eastern Arabian Sea. Stronger than usual monsoon jets are only associated with a very modest MLD deepening in the southern Arabian Sea in summer. Finally, positive Indian Ocean Subtropical Dipoles are associated with a MLD deepening between 15 and 30°S. Buoyancy fluxes generally appear to dominate MLD interannual variations except for IOD-induced signals in the south-central Indian Ocean in autumn, where wind stirring and Ekman pumping dominate.     

Sensitivity analysis of the equations for a convective mixed layer

Jump or slab models are frequently used to calculate the depth of the convectively mixed layer and its potential temperature during the course of a clear day. Much attention has been paid theoretically to the parameterization of the budget for turbulent kinetic energy that is required in these models. However, for practical applications the sensitivity of the solutions of the model equations to variations in the entrainment formulation and in the initial and boundary conditions is also very important. We analyzed this sensitivity on the basis of an analytical solution for the model which uses the well-known constant heat flux ratio. The initial conditions for the mixed-layer height (h) and potential temperature ( _m ) quickly lose their influence. Only the initial temperature deficit is important. The mixed-layer temperature at noon on convective days is insensitive to the entrainment coefficient c. It is governed by the integral of the heat input and by the stable lapse rate. A change in c from 0.2 to 0.5 leads to a variation of 20% in h. This is not very much considering the accuracy in the determination of h from actual observations.     

Coupled oceanic and atmospheric mixed layer model

A coupled, one-dimensional atmospheric-oceanic boundary layer model based on a single station assessment has been formulated from independent oceanic and atmospheric bulk boundary layer models. Sensitivity analyses are conducted to determine major differences in the responses of the coupled model compared to those of the separate oceanic and atmospheric models. The general behavior of the coupled model atmosphere is not significantly different from that of the atmospheric model over short term simulations (12–24 h). However, large differences may occur under certain limited conditions when winds are light and the lifting condensation level is close to the height of the inversion. Major differences between the predicted evolution of the ocean boundary layer by the ocean model and coupled model are more common, and the short term predictive ability of the ocean model in coupled form is enhanced.     

坡地斜压稳定大气边界层结构的数值研究

本文用数值模式较为全面地探讨了斜坡地形斜压稳定大气边界层的结构,并与平坦地形的结果进行了比较,给出了两者之间很有意思的差别.     

The role of molecular diffusion in the deepening of the mixed layer

Turbulent mixing across heat-stratified density interfaces was studied in the laboratory using oscillating-grid generated turbulence. The aim was to study the transition between the entrainment regimes dominated by interfacial wave-breaking and molecular diffusion, and to study the characteristics of the latter. It was observed that, above a critical Richardson number Ri_c, which depends on the Peclet number Pe, the mixing due to wave breaking disappears and that Ri_c Pe⁻ⁿ, where the mean value of the exponent n is approximately . Above Ri_c, the entrainment is molecular-diffusion dominated and takes place through a sequence of events: the buoyancy gradient of the initially sharp density interface is weakened by molecular diffusion until the mixed-layer eddies can engulf a portion of the interfacial layer wherefore the interface sharpens again. Thus, the entrainment events are recurrent with a rate-controlling diffusion stage between them. An entrainment law of the form E Ri⁻²Pe⁻², where E is the entrainment coefficient and Ri is the Richardson number, is suggested for the diffusion-dominated entrainment regime.     

Thermodynamic Feedback between Clouds and the Ocean Surface Mixed Layer

A cloud-ocean planetary boundary layer (OPBL) feedback mechanism is presented and tested in this paper. Water vapor, evaporated from the ocean surface or transported by the large-scale air flow, often forms convective clouds under a conditionally unstable lapse rate. The variable cloud cover and rainfall may have positive and negative feedback with the ocean mixed layer temperature and salinity structure. The coupling of the simplified Kuo’s (1965) cumulus cloud model to the Kraus-Turner’s (1967) ocean mixed layer model shows the existence of this feedback mechanism. The theory also predicts the generation of low frequency oscillation in the atmosphere and oceans.     

Second order modeling of turbulent transport in the surface mixed layer

A model for the simultaneous transport of heat and water vapor is presented. In an effort to resolve the structure of the entrainment region at the inversion base, models are constructed so as to satisfy realizability as far as possible. Density anomaly and water vapor mixture fraction (specific humidity) are taken as the basic variables. Algebraic expressions for the third moments are derived from first principles, and contain no adjustable constants. Separate equations are carried for the dissipation of each variance, constructed to give rational behavior of all time scale ratios. New forms for relaxation and cross-dissipation terms are constructed in such a way as to guarantee realizability. We describe how realizability was used as a tool to construct these models. We present preliminary results without mean velocity gradients for a dry surface mixed layer leaving the land and starting over water, producing a stable internal humidity boundary layer, but with large fluxes of sensible heat and water vapor (local advection).Prepared for presentation at the 29th OHOLO Conference on Boundary Layer Structure — Modelling and Application to Air Pollution and Wind Energy, Zichron Ya'acov, Israel, March 25–28, 1984. Supported in part by the U.S. Office of Naval Research under the following programs: Physical Oceanography (Code 422PO), Power (Code 473); in part by the U.S. National Science Foundation under grant no. ATM 79-22006; and in part by the U.S. Air Force Geophysics Laboratory.     

On generation of internal waves by turbulence in the mixed layer

Turbulent fluctuations in active mixed layers can excite internal waves in stably stratified fluid regions adjoining them. Expressions are derived for the energy and momentum fluxes radiated away by internal waves from an oceanic mixed layer, in terms of the spectrum of the static pressure fluctuations imposed at the base of the mixed layer by the turbulent eddies. The role of these internal wave fluxes in questions such as the determination of the rate of deepening of the layer due to an applied surface stress and the origin of internal waves in the deep ocean is discussed.     

Parameterization for the depth of the entrainment zone above the convectively mixed layer

It has been noted that when the convective Richardson number Ri* is used to characterize the depth of the entrainment zone, various parameterization schemes can be obtained. This situation is often attributed to the invalidity of parcel theory. However, evidence shows that the convective Richardson number Ri^* might be an improper characteristic scaling parameter for the entrainment process. An attempt to use an innovative parameter to parameterize the entrainment-zone thickness has been made in this paper.Based on the examination of the data of water-tank experiments and atmospheric measurements, it is found that the total lapse rate of potential temperature across the entrainment zone is proportional to that of the capping inversion layer. Inserting this relationship into the so-called parcel theory, it thus gives a new parameterization scheme for the depth of the entrainment zone. This scheme includes the lapse rate of the capping inversion layer that plays an important role in the entrainment process. Its physical representation is reasonable. The new scheme gives a better ordering of the data measured in both watertank and atmosphere as compared with the traditional method using Ri^*. These indicate that the parcel theory can describe the entrainment process suitably and that the new parameter is better than Ri^*.     

论大气边界层的数值模拟方法

本文采用分层模式和连续模式模拟了一维、稳定的大气边界层中一些主要的气象变量随时间的变化规律,讨论了几种气象条件及下垫面条件下计算结果之间的差异,并且还讨论了常值湍流热通量层存在的条件及其高度随时间的变化,从而给出了两种模式适用的范围.     

Aspects of mixed layer modelling applied to JASIN data

Starting from the heat balance equation two different mixed layer models are developed: a very simple vertically integrated, so-called “thermal slab model” and a one-dimensional temperature profile model. Both models have been applied to the JASIN data, representing the averaged conditions for an ocean area of 150 × 150 km².Results for both model runs are presented. It is demonstrated that the temperature profile model in its presented form is capable of successfully reproducing the developments of the sea surface temperature, the heat budget, the mixed layer depth and the temperature profiles over a 2 month period.     

A new one-dimensional simple energy balance and carbon cycle coupled model for global warming simulation

Global warming and accompanying climate change may be caused by an increase in atmospheric greenhouse gasses generated by anthropogenic activities. In order to supply such a mechanism of global warming with a quantitative underpinning, we need to understand the multifaceted roles of the Earth's energy balance and material cycles. In this study, we propose a new one-dimensional simple Earth system model. The model consists of carbon and energy balance submodels with a north–south zonal structure. The two submodels are coupled by interactive feedback processes such as CO₂ fertilization of net primary production (NPP) and temperature dependencies of NPP, soil respiration, and ocean surface chemistry. The most important characteristics of the model are not only that the model requires a relatively short calculation time for carbon and energy simulation compared with a General Circulation Model (GCM) and an Earth system Model of Intermediate Complexity (EMIC), but also that the model can simulate average latitudinal variations. In order to analyze the response of the Earth system due to increasing greenhouse gasses, several simulations were conducted in one dimension from the years 1750 to 2000. Evaluating terrestrial and oceanic carbon uptake output of the model in the meridional direction through comparison with observations and satellite data, we analyzed the time variation patterns of air temperature in low- and middle-latitude belts. The model successfully reproduced the temporal variation in each latitude belt and the latitudinal distribution pattern of carbon uptake. Therefore, this model could more accurately demonstrate a difference in the latitudinal response of air temperature than existing models. As a result of the model evaluations, we concluded that this new one-dimensional simple Earth system model is a good tool for conducting global warming simulations. From future projections using various emission scenarios, we showed that the spatial distribution of terrestrial carbon uptake may vary greatly, not only among models used for climate change simulations, but also amongst emission scenarios.