A note on a consistent quasigeostrophic model in a multiply connected domain

The familiar quasigeostrophic equations are reconsidered to consistently include forcing and diffusive processes, as well as auxiliary conditions for making the model a fully determined one in both simply and multiply connected domains. In particular, Newtonian diffusions of momentum in the vertical direction and buoyancy in the horizontal must be excluded. The auxiliary conditions are such as to prevent spurious energy and mass fluxes through the boundaries and to assure the correct circulation magnitudes about interior obstacles (i.e., “islands”). Several examples of consistent model equations are presented where the vertical discretization is explicit.     

Coherent Structures and the Dissimilarity of Turbulent Transport of Momentum and Scalars in the Unstable Atmospheric Surface Layer

Atmospheric stability effects on the dissimilarity between the turbulent transport of momentum and scalars (water vapour and temperature) are investigated in the neutral and unstable atmospheric surface layers over a lake and a vineyard. A decorrelation of the momentum and scalar fluxes is observed with increasing instability. Moreover, different measures of transport efficiency (correlation coefficients, efficiencies based on quadrant analysis and bulk transfer coefficients) indicate that, under close to neutral conditions, momentum and scalars are transported similarly whereas, as the instability of the atmosphere increases, scalars are transported increasingly more efficiently than momentum. This dissimilarity between the turbulent transport of momentum and scalars under unstable conditions concurs with, and is likely caused by, a change in the topology of turbulent coherent structures. Previous laboratory and field studies report that under neutral conditions hairpin vortices and hairpin packets are present and dominate the vertical fluxes, while under free-convection conditions thermal plumes are expected. Our results (cross-stream vorticity variation, quadrant analysis and time series analysis) are in very good agreement with this picture and confirm a change in the structure of the coherent turbulent motions under increasing instability, although the exact structure of these motions and how they are modified by stability requires further investigation based on three-dimensional flow data.     

A further investigation of the decadal variation of ENSO characteristics with instability analysis

Based on instability theory and some former studies, the Simple Ocean Data Assimilation （SODA） data are analyzed to further study the difference between the propagation of the ENSO-related oceanic anomaly in the off-equatorial North Pacific Ocean before and after 1976. The investigation shows that after 1976 in the off-equatorial North Pacific Ocean, there is a larger area where the necessary conditions for baroclinic and/or barotropic instability are satisfied, which may help oceanic anomaly signals propagating in the form of Rossby waves to absorb energy from the mean currents so that they can grow and intensify. The baroclinic energy conversion rate in the North Pacific after 1976 is much higher than before 1976, which indicates that the baroclinic instability has intensified since 1976. Prom another perspective, the instability analysis gives an explanation of the phenomena that the ENSO-related oceanic anomaly signal in the North Pacific has intensified since 1976.     

The influence of the coast on the dynamics of upwelling fronts: Part I. Laboratory experiments

We describe laboratory experiments on the instability and later evolution of a front in a two-layer rotating fluid. In particular, we focus on the influence of a nearby boundary on instability growth and eddy formation. The front is produced through the adjustment of a buoyant fluid initially confined within a bottomless cylinder. Typically a front in quasi-cyclostrophic balance establishes after two rotation periods, after which it becomes unstable. Measurements of the velocity and vorticity fields at the surface are made which provide detailed information on the evolution of the front as the instability grows to finite amplitude. We focus on the time evolution of the vorticity and distinguish between the cyclonic and anticyclonic components. The spatial averages of the cyclonic and anticyclonic vorticity first grow exponentially. This growth saturates when eddies form and are advected across the front. The growth rate depends upon two nondimensional parameters: the width W of the upwelling region in units of the internal radius of deformation and the depth ratio δ between the two layers. Measurements of the growth rates for the average of the cyclonic and anticyclonic vorticity are compared to the values inferred from a simplified model for baroclinic instability. A good agreement is obtained when the front develops far from the boundary (i.e. W1). However, the agreement is only qualitative when the front is near the boundary (i.e. W1). We find that, as W decreases, the growth of cyclonic eddies consisting of dense—“coastal”—water is enhanced compared to that of anticyclonic vorticity consisting of buoyant—“off-shore”—water. This crucial effect of the boundary with respect to the instability of the front has significant impact on exchanges across the front.     

Turbulence Structure of Stable Boundary Layers with a Near-Linear Temperature Profile

By using a thermally stratified wind tunnel, we have successfullysimulated stably stratified boundary layers (SBL), in which the meantemperature increases upward almost linearly. We have investigated the flow structure and the effects of near-linearstable stratification on the transfer of momentum and heat. Thevertical profiles of turbulence quantities exhibit different behaviour in two distinct stability regimes of the SBLflows with weak and strong stability. For weak stability cases, theturbulent transfer of momentum and heat is basically similar to that for neutral turbulent boundary layers, although it is weakenedwith increasing stability. For strong stability cases, on the other hand,the time-mean transfer is almost zero over the whole boundary-layer depth.However, the instantaneous turbulent transfer frequently occurs in bothgradient and counter-gradient directions in the lower part of the boundary layer. This is due to the Kelvin–Helmholtz (K–H) shear instability and therolling up and breaking of K–H waves. Moreover, the internal gravity wavesare observed in the middle and upper parts of all stable boundary layers.     

Mesoscale convection from a large-scale perspective

This essay concerns precipitating convective cloud systems and convectively-driven mesoscale circulations (“mesoscale convection”) and their role in the large-scale structure of the atmosphere. Mesoscale convection is an important and ubiquitous process on scales of motion spanning a few kilometers to many hundreds of kilometers. It plays a role in the input of energy to the climate system through the radiative effect of upper-tropospheric cloud and water vapor, and enhanced surface fluxes. This is in addition to its important effect on energy, heat and momentum transport within the atmosphere. However, mesoscale convection is neither parameterized nor adequately resolved in atmospheric general circulation models. Its representation in mean-flow terms raises issues that are quite distinct from classical approaches to sub-grid scale convection parameterization.Cloud-resolving modeling and theoretical concepts pertinent to the transport properties and mean-flow effects of organized convection are summarized, as are the main convective parameterization techniques used in global models. Two principal themes that are relevant to the representation of organized mesoscale systems are discussed. First, mesoscale transports and their sub-grid scale approximation with emphasis on dynamical approaches. Second, long time-scale modeling of mesoscale cloud systems that involves the collective effect of convection, boundary and surface layers, radiation, microphysics acting under the influence of large-scale forcing.Finally, major research programs that address the role of precipitating convection and mesoscale processes in global models are summarized.     

Climatic inferences from the ventilated thermocline

Several computed cases of a model of the subtropical gyre with a partially ventilated thermocline (Luytenet al., 1982) are presented to illustrate the sensitivity of the field of density stratification, mean flow and location of unventilated regions to slight changes in surface boundary conditions. The structure of the low latitude thermocline is less sensitive to climatic change in amount of water forced down by convergence of wind-driven surface layers at higher latitudes than might be expected, even allowing for the well-known localness of vertically integrated meridional transport. On the other hand changes in structure at low latitudes do arise from changes in boundary conditions on the surface density at higher latitudes. The main climatic inference for transient tracers injected into a thermocline in steady state is that there are two time-scales in the subtropical thermocline: an advective time scale associated with distance from regions of direct ventilation of a density layer at the surface, and a subsurface diffusive time scale from ventilated to unventilated region.     

Wind–wave stabilization by a foam layer between the atmosphere and the ocean

The study is motivated by recent findings of the decrease in the momentum transfer from strong winds to sea. The Kelvin–Helmholtz instability (KHI) of a three-fluid system of air, foam and water is examined within the range of intermediately short surface waves. The foam-layer thickness necessary for effective separation of the atmosphere and the ocean is estimated. Due to high density contrasts in the three-fluid system, even a relatively thin foam layer between the atmosphere and the ocean can provide a significant stabilization of the water surface by the wavelength shift of the instability towards smaller scales. It is conjectured that such stabilization qualitatively explains the observed reduction of roughness and drag.     

The 6 h climatology of thunderstorms and rainfalls in the Friuli Venezia Giulia Plain

Friuli Venezia Giulia is a region located in the North-Eastern part of Italy. It has the Adriatic Sea (Gulf of Trieste) on the South and the Julian and Carnic Alps surrounding it on the North. For these geographical properties thunderstorms and precipitations are common events in the plain of this region.The climatology of thunderstorms and rainfalls, considering 6 h interval periods, is studied in this work. It is shown how the thunderstorm frequency, based on the recording of at least three lightning strikes during the 6 h period, is 16%. The occurrence frequency of at least 1 mm of rain accumulated in 6 h is 24%, while that of at least 5 mm in 6 h is 14%.The daily and monthly distributions of these events are then stratified in three classes, based on their “intensity” (weak, medium and strong), and the different behaviors are analysed. Finally, an explanation for the main monthly rain frequency is sought by looking at only two sounding-derived indices and in particular at their annual cycles. The two indices (related to the potential instability and to the water vapour flux) attempt to summarize the “convective” and “flux” mechanisms for producing rain. It is found that in some particular periods of the year the rain-originating process seems well identifiable, while in many others the two processes seem to be concomitant.     

A comparative study of observed and GCM-simulated turbulent surface fluxes at the positions of Atlantic weatherships

If oceanic models are to be driven with transient atmospheric input, data from standard daily analyses or from an atmospheric GCM simulation can be used. The question arises whether these “FF-data” (FF = field forcing) are appropriate to be used as a realistic oceanic forcing in the form of wind stress and turbulent heat fluxes.A series of different FF-data sets is compared with respective “LF-data” (LF = local forcing) derived from in situ weathership observations. We believe these LF data to be the most accurate and reliable long year maritime time series. The study is restricted to 8 Atlantic weatherships and January conditions and to fields obtained with the Hamburg University GCM or derived from analyses of the German Weather Service (DWD).It turns out that DWD based FF data sets are suitable only if long year mean values are required. In general, the interannual and synoptic scale variability is too small for all FF data sets. With respect to the windstress, the empirical formulae to obtain the surface wind (from the sea level pressure field) together with the usage of a windspread dependent drag coefficient yield the best though still unsatisfying results. The approach using generalized similarity theory gives worse results with respect to the synoptic scale and interannual variability.The GCM simulated data set is systematically biased over wide regions which is partly due to a shift in the model's quasistationary Icelandic Low and an increased temperature at the model's lowest level. The transients are simulated at some positions even poorer than those analysed by the DWD, but at other positions superior though still weaker than the LF data's.     

Growing interfacially coupled oscillations of the ocean–atmosphere

A stability analysis of the coupled ocean–atmosphere is presented which shows that the potential energy (PE) of the upper layer of the ocean is available to generate coupled growing planetary waves. An independent analysis suggests that the growth of these waves would be maintained in the presence of oceanic friction. The growing waves are a consequence of relaxing the rigid lid approximation on the ocean, thus allowing an upward transfer of energy across the sea surface. Using a two and a half layer model consisting of an atmospheric planetary boundary layer, coupled with a two layer ocean comprising an active upper layer and a lower layer in which the velocity perturbation is vanishingly small, it is shown that coupled unstable waves are generated, which extract PE from the main thermocline. The instability analysis is an extension of earlier work [Tellus 44A (1992) 67], which considered the coupled instability of an atmospheric planetary boundary layer coupled with an oceanic mixed layer, in which unstable waves were generated which extract PE from the seasonal thermocline. The unstable wave is an atmospheric divergent barotropic Rossby wave, which is steered by the zonal wind velocity, and has a wavelength of about 6000 km, and propagates eastward at the speed of the deep ocean current. It is argued that this instability, which has a multidecadal growth time constant, may be generated in the Southern Ocean, and that its properties are similar to observations of the Antarctic Circumpolar Wave (ACW).     

Turbulent transport processes of momentum and sensible heat in the surface layer over a paddy field

A sonic anemometer-thermometer was used to measure turbulent fluxes of momentum and sensible heat and related turbulence statistics just above plant canopies in unstable conditions. The stability dependence of transport processes is presented, using observational data in a wide range of instability. The analysis of joint probability distributions of w – u, w – T, w – wu, and w – wT confirmed that just above plant canopies, downdrafts were remarkably efficient for vertical transport of momentum and scalar quantities in near neutral conditions. Furthermore, it was shown that updrafts became more efficient than downdrafts for vertical transport, especially of scalar quantities, in very unstable conditions.     

On evaluation of the oceanic surface drift current speed and direction

The surface drift current speed and direction are defined using the resistance laws for turbulent Ekman boundary layers. Stratification conditions that coincide in the atmospheric and oceanic boundary layers yield approximate formulas whereby the drift current and geostrophic wind directions coincide and the geostrophic wind factor k is equal to the square root of the air and water density ratio. The theoretical estimates of k are compared with available experimental data.     

Nonlinear evolution of a layered stratified shear flow

We investigate numerically and theoretically the nonlinear evolution of a parallel shear flow at moderate Reynolds number which has embedded within it a mixed layer of intermediate fluid. The two relatively thin strongly stratified density interfaces are centered on the edges of the shear layer. We are particularly interested in the development of primary and secondary instabilities. We present the results of a stability analysis which predicts that such flows may be unstable to stationary vortical disturbances which are a generalization of an inviscid instability first considered by G.I. Taylor. We investigate the behavior of these “Taylor billows” at finite amplitude through two-dimensional numerical simulations. We observe that the braid regions connecting adjacent primary Taylor billows are susceptible to secondary, inherently two-dimensional instabilities. We verify that these secondary instabilities, which take the form of small elliptical vortices, arise due to a local intensification of the spanwise vorticity in the braid region.     

On a class of stable, slightly geostrophic mean gyres

An integral theorem is proved which establishes the sufficient conditions for the stability with respect to small perturbations of inviscid and unforced, non-geostrophic, inertial, mean currents in a fluid comprised of homogeneous layers of different densities within an enclosed ocean basin. The derivation is by a generalization of a technique employed by Drazin and Howard (1966) and Blumen (1973). The relevance of such solutions to the mean sub-tropical ocean gyres may be that they represent stable reservoirs of mean energy and, as such, might serve as “building blocks” for the general circulation. Several examples are discussed which support this hypothesis.     

大气静力稳定度的铅直分布和不同风速铅直廓线对天气系统发展的影响

采用一个准地转三层模式,对于高低空不同层结及不同风速铅直廓线下的斜压不稳定性问题进行了分析和讨论。指出:(1)大气层结特征对斜压不稳定的影响不仅表现在对不稳定临界波长和临界风速切变的制约上,而且其铅直分布的不均一性决定了大气的各个层次对斜压不稳定的贡献大小;扰动的斜压发展主要取决于静力稳定度较小的那些层次内的热力、动力学条件,在这些层次内,扰动也最为明显。(2)高低空风速比中空为大的“高低空强风型”风速铅直分布最有利于扰动的不稳定发展,在这种风速铅直廓线下,不稳定波波谱较宽,不稳定波临界波长和最不稳定波波长也较短。     

On momentum flux and velocity spectra over waves

Data from a research tower in Lake Ontario are used to study the validity of Monin--Obukhov scaling in the marine atmospheric boundary layer under various wave conditions. It is found that over pure wind seas, the velocity spectra and cospectra follow established universal scaling laws. However, in the presence of swells outrunning weak winds, velocity spectra and cospectra no longer satisfy universal spectral shapes. Here, Monin–Obukhov similarity theory, and the classical logarithmic boundary layers, are no longer valid. It is further shown that, in the presence of such swells, the momentum flux can be significantly modified in comparison to pure wind sea values. The implications of these findings for bulk flux estimations and on the inertial dissipation method for calculating fluxes are discussed.     

珠江三角洲春季龙卷发生的环境条件

采用日本气象研究所提供的ＴＢＢ资料分析了亚澳季风区的季节转换及亚洲夏季风建立的特征。发现４月份季节转换就已开始，５－６月是季节突变时期，６月份亚洲地区复季风形势完全建立。     

Climate drift in an ocean model coupled to a simple,perfectly matched atmosphere

A very simple, diffusive energy balance atmosphere is coupled to the GFDL ocean circulation model. This provides a useful tool for analyzing climate drift in the ocean model after coupling, and may be used to assess various schemes for minimizing such drift. In the experiment reported here, the atmosphere is constructed in such a way that it provides the ocean model at the moment of coupling with the same fluxes as during spinup. The experiment is therefore equivalent to coupling a perfectly flux-corrected atmosphere model, and is used to investigate the response of the ocean model under these conditions. In spite of the steady, passive, flux-corrected atmosphere, the ocean model drifts to a new equilibrium state after coupling. The transition takes about 2000 years; the new state is characterized by different sites of deep convection and resulting changes in high-latitude SST and global deep temperatures. The mechanism for the transition is an instability of the oceanic convection patterns under the new feedback, felt after coupling. A similar state transition of the ocean model may be triggered by the coupling shock in fully coupled GCMs. If this is so, the transition would contaminate the results of climate scenario experiments, and it would explain part of the residual drift observed in coupled models in spite of the use of flux corrections.     

A new model of tropical waves incorporating momentum mixing by cumulus convection

A comparison is made between the magnitudes of observed large-scale weather waves over the tropical Pacific and the magnitudes of the corresponding waves, predicted by wave-CISK theories, which are driven by the observed amount of latent heating (i.e., precipitation). The theoretical wave fields of meridional velocity, vorticity, and temperature rate are shown to exceed the observed quantities by an order of magnitude. An attempt is made to simulate the observed balance between the diabatic heating and adiabatic cooling within the context of the inviscid theories. For a broad class of heating profiles, geometries and basic states, it is found that this compensation without temperature change cannot be satisfactorily modelled, regardless of the vertical shape of the heating, when the vertical wavelength of the disturbance exceeds about 6 km.Scale analysis demonstrates that an important dynamical term has been neglected in the inviscid models, viz., the vertical transport of horizontal momentum by cumulus clouds. When this process is included in the wave model, velocities, relative vorticity, and the time rate of temperature change are all comparable to the observed values. The general behavior of a system where both forcing and cumulus “friction” are proportional to each other has not been previously examined. We find the behavior of such a system to have several novel features. For example, we find that for a wide range of precipitation amplitudes (from 1/4 to 4 times the precipitation amplitudes of waves in the western Pacific) we get essentially constant amplitudes for wind. The implications of this and other features for various aspects of tropical wave modelling are discussed.