External Thermal Forcing Research Articles

A GPU-based 2D viscous flow model with variable density and heat exchange

Numerical simulation of unsteady viscous flow over variable topography under the influence of temperature changes is a challenge. In order to apply the model to large domains with complex topography, it becomes mandatory to reduce the model complexity from 3D to 2D by depth averaging the equations and to apply massive parallelization techniques for an efficient simulation. The depth averaged mass continuity, momentum and internal energy equations, combined with suitable friction laws, can be used for this type of flows. Variations in temperature can be accounted for from internal energy changes with density changing accordingly. The resulting system is solved using a finite volume technique on unstructured triangular grids well suited for problems over variable topography. A generic model applicable to a wide range of viscous fluids and validated with synthetic cases is presented to evaluate the performance of the numerical solution in presence of both external thermal forcing functions and discontinuous initial conditions. Finally, the model is applied to a realistic application and calibration to a particular case of lava flow taking into consideration variable density, viscosity and yield stress with temperature . A heat transfer with the air is included to consider the lava cooling. The numerical results of the lava front advance are compared to the Copernicus satellite observations at different dates. The efficiency of the GPU implementation allows to simulate a 11 day event in less than 1.7 h of simulation.

In situ measurements of thermal diffusivity in sediments of the methane-rich zone of Cascadia Margin, NE Pacific Ocean

Abstract Thermal diffusivity (TD) is a measure of the temperature response of a material to external thermal forcing. In this study, TD values for marine sediments were determined in situ at two locations on the Cascadia Margin using an instrumented sediment probe deployed by a remotely operated vehicle. TD measurements in this area of the NE Pacific Ocean are important for characterizing the upslope edge of the methane hydrate stability zone, which is the climate-sensitive boundary of a global-scale carbon reservoir. The probe was deployed on the Cascadia Margin at water depths of 552 and 1049 m for a total of 6 days at each site. The instrumented probe consisted of four thermistors aligned vertically, one sensor exposed to the bottom water and one each at 5, 10, and 15 cm within the sediment. Results from each deployment were analyzed using a thermal conduction model applying a range of TD values to obtain the best fit with the experimental data. TD values corresponding to the lowest standard deviations from the numerical model runs were selected as the best approximations. Overall TDs of Cascadia Margin sediments of 4.33 and 1.15 × 10–7 m2 s–1 were calculated for the two deployments. These values, the first of their kind to be determined from in situ measurements on a methane hydrate-rich continental margin, are expected to be useful in the development of models of bottom-water temperature increases and their implications on a global scale.

Dependence of Climate Response on Meridional Structure of External Thermal Forcing

Abstract This study shows that the magnitude of global surface warming greatly depends on the meridional distribution of surface thermal forcing. An atmospheric model coupled to an aquaplanet slab mixed layer ocean is perturbed by prescribing heating to the ocean mixed layer. The heating is distributed uniformly globally or confined to narrow tropical or polar bands, and the amplitude is adjusted to ensure that the global mean remains the same for all cases. Since the tropical temperature is close to a moist adiabat, the prescribed heating leads to a maximized warming near the tropopause, whereas the polar warming is trapped near the surface because of strong atmospheric stability. Hence, the surface warming is more effectively damped by radiation in the tropics than in the polar region. As a result, the global surface temperature increase is weak (strong) when the given amount of heating is confined to the tropical (polar) band. The degree of this contrast is shown to depend on water vapor– and cloud–radiative feedbacks that alter the effective strength of prescribed thermal forcing.

On the thermal instability induced by viscous dissipation

The effect of viscous dissipation may be the sole cause of a thermoconvective instability either in a fluid clear of solid material or in a fluid-saturated porous medium. Several recent investigations have contributed to illustrate this result under different flow and thermal conditions. The elementary physical nature of the dissipation induced instability is just the same as that of the Rayleigh–Bénard instability, namely the onset of a secondary buoyant flow taking place when the basic temperature gradient becomes sufficiently intense. The essential difference is that the dissipation instability is not induced by an external thermal forcing due to the temperature boundary conditions, as it happens for the Rayleigh–Bénard instability. On the other hand, the cause of the dissipation instability is the basic flow rate itself, acting thermally as a heat generation mechanism. Thus, the governing parameter determining the transition from stability to instability is not the Rayleigh number, as in the classical Rayleigh–Bénard problem, but the product between the Gebhart number and the square Péclet number.

Summer Rainfall Variability in Low-Latitude Highlands of China and Subtropical Indian Ocean Dipole

Abstract Based on reanalysis and observational datasets, this study proposes a reasonable mechanism for summer rainfall variations over the low-latitude highlands (LLH) of China, in which a subtropical Indian Ocean dipole (SIOD)-like pattern is the key external thermal forcing. In summers with a positive SIOD-like pattern, sea surface temperature (SST) anomalies may lead to lower-tropospheric divergence over the tropical Indian Ocean and convergence over the subtropical southwestern Indian Ocean and Arabian Sea. The convergence over the Arabian Sea can induce easterly anomalies of the divergent wind component off the eastern coast of the Bay of Bengal (BOB), while the divergence over the tropical Indian Ocean can change the interhemispheric vertical circulation and produce a descending motion over the same area and cyclonic anomalies in the rotational wind component over the Indian peninsula. The combined effect of the divergent and rotational wind anomalies and enhanced interhemispheric vertical circulation facilitates easterly anomalies and weakens climatological water vapor flux to the northern BOB. Therefore, anomalous water vapor divergence and less precipitation are observed over the LLH. In summers with a negative SIOD-like pattern, the situation is approximately the same but with opposite polarity and a weaker role of the divergent wind component. Further analyses indicate that the summertime SIOD-like pattern can be traced to preceding seasons, especially in positive SIOD-like years. The SST–wind–evaporation feedback mechanism could account for maintenance of the SIOD-like pattern. These results provide efficient prediction potential for summer rainfall variations over the LLH.

Dynamics and predictability of a low-order wind-driven ocean–atmosphere coupled model

The dynamics of a low-order coupled wind-driven ocean–atmosphere system is investigated with emphasis on its predictability properties. The low-order coupled deterministic system is composed of a baroclinic atmosphere for which 12 dominant dynamical modes are only retained (Charney and Straus in J Atmos Sci 37:1157–1176, 1980) and a wind-driven, quasi-geostrophic and reduced-gravity shallow ocean whose field is truncated to four dominant modes able to reproduce the large scale oceanic gyres (Pierini in J Phys Oceanogr 41:1585–1604, 2011). The two models are coupled through mechanical forcings only. The analysis of its dynamics reveals first that under aperiodic atmospheric forcings only dominant single gyres (clockwise or counterclockwise) appear, while for periodic atmospheric solutions the double gyres emerge. In the present model domain setting context, this feature is related to the level of truncation of the atmospheric fields, as indicated by a preliminary analysis of the impact of higher wavenumber (“synoptic” scale) modes on the development of oceanic gyres. In the latter case, double gyres appear in the presence of a chaotic atmosphere. Second the dynamical quantities characterizing the short-term predictability (Lyapunov exponents, Lyapunov dimension, Kolmogorov–Sinai (KS) entropy) displays a complex dependence as a function of the key parameters of the system, namely the coupling strength and the external thermal forcing. In particular, the KS-entropy is increasing as a function of the coupling in most of the experiments, implying an increase of the rate of loss of information about the localization of the system on its attractor. Finally the dynamics of the error is explored and indicates, in particular, a rich variety of short term behaviors of the error in the atmosphere depending on the (relative) amplitude of the initial error affecting the ocean, from polynomial (at 2 + bt 3 + ct 4) up to exponential-like evolutions. These features are explained and analyzed in the light of the recent findings on error growth (Nicolis et al. in J Atmos Sci 66:766–778, 2009).

The Mutual Interaction between External Rossby Waves and Thermal Forcing: The Subpolar Regions

Abstract The authors hypothesize a simple feedback mechanism between external Rossby waves and diabatic heating from convection. This mechanism could explain the large amplitude that external Rossby waves attain as they propagate to mid- and high latitudes. A series of experiments has been carried out with a core dynamic global spectral model. These simulations with the idealized atmospheric GCM and a simple parameterization of thermal forcing proportional to the low-level wave meridional velocity suggest that external Rossby waves can be enhanced by convection, which they themselves induce. It is shown that in the tropospheric upper levels the amplitude of the external waves can be twice as large with feedback as for a control simulation that does not allow feedback.

The mechanism for the impact of sea surface temperature anomaly on the ridgeline surface of Western Pacific

Based on the atmospheric circulation data provided by ECMWF and the sea surface temperature data by NOAA, we studied the mechanism for the impact of sea surface temperature anomaly on the ridgeline surface of western Pacific using an improved high truncated spectral model. Our results show that the wave-wave interaction and the wave-mean flow interactions are weaker in the inner dynamic process of atmospheric circulation, when atmospheric circulation is forced by the sea surface temperature of El Nino pattern. With the external thermal forcing changed from winter to summer pattern, the range of ridgeline surface of western Pacific moving northward is smaller, which causes the ridgeline surface of western Pacific on south of normal. On the contrary, the wave-wave interaction and the wave-mean flow interaction are stronger, when atmospheric circulation is forced by the sea surface temperature of La Nina pattern. With the external thermal forcing turning from winter to summer pattern, the ridgeline surface of western Pacific shifts northward about 19 latitude degrees, which conduces the ridgeline surface of western Pacific on north of normal. After moving to certain latitude, the ridgeline surface of western Pacific oscillates with the most obvious 30–60 d period and the 4°–7° amplitude. It is one of the important reasons for the interannual variation of ridgeline surface of Western Pacific that the atmospheric inner dynamical process forced out by different sea surface temperature anomaly pattern is different.

A Mechanism for the Interannual Variation of Summer Northeast Asian Blocking High

AbstractBased on the spatial spectral functions describing the atmospheric circulation variation retrieved with observational data, the mechanism for the interannual variation of summer Northeast Asian Blocking High were studied with improved high truncated spectral model. The results showed that when the external thermal forcing distributes as El Niño pattern, there are wave‐wave interaction of wave 2 in zonal direction and wave‐mean flow interactions between wave 2 and wave 3 in meridianal direction and mean flow of wave 1 in meridianal direction. Under such case, there is positive abnormity over Northeast Asia on 500 hPa geopotential height field, which provides advantage atmospheric circulation background for the frequently occurring of Northeast Asia Blocking High during summer. When the external thermal forcing distributes as La Niña pattern, there exist wave‐wave interaction between wave 1 in zonal direction and wave‐mean flow interactions between wave 2 and wave 4 in meridianal direction and mean flow of wave 2 in meridianal direction. Under such case, there is negative abnormity over Northeast Asia on 500hPa geopotential height field, which restrains the occurring of Northeast Asia Blocking High during summer.

Similarity theory and parameterization of mixing in the upper ocean

Boundary layers with small thermal and mechanical inertia are close to steady-state condi- tions. This underlies the Monin-Obukhov similarity theory and explains why the surface values of the fluxes can be chosen as external parameters. For fluids with large thermal inertia, such as the ocean, the thermal time scale is relatively large, and the density flux is a complex function of depth; thus, the external thermal forcing is no longer a governing parameter. However, the mechanical inertia of the upper ocean is about three orders of magnitude smaller than the thermal inertia. Consequently, the upper ocean can be considered as steady-state in the dynamic sense, to any dynamic property depends primarily on the depth, the surface momentum flux, and the vertical density structure. This property allows us to suggest an alternative formulation of the similarity theory for the stratified boundary layers through specification of a new stratification parameter which characterizes the internal density structure instead of the external density flux. The turbulent mixing coefficient is derived as dependent on the stratification parameter. The latter includes the surface stress and the integral density deficit for the entire layer above. The general form and the asymptotic behavior of the nondimensional tur- bulent mixing coefficient as a function of the stratification parameter are obtained using dimensional considerations. Determination of numerical parameters is based on 8 years of temperature profiles acquired at the Ocean Weather Ship (OWS) PAPA. Finally, a method for calculating the profile of the turbulent mixing coefficient is obtained. This approach reproduces the 8-year evolution of the upper ocean with the maximum rms difference of approximately 1C and the bias of 1C over the depth range 0-150 m. Additional 1-year simulation of the upper ocean at OWS CHARLEY and 9-year simulation at OWS NOVEMBER confirms reasonable applicability of this approach. The proposed simple turbulent mixing scheme reproduces the evolution of the upper ocean with accuracies similar to those obtained using much more complicated models.

Progress in the study on the formation of the summertime subtropical anticyclone

The studies in China on the formation of the summertime subtropical anticyclone on the climate timescale are reviewed. New insights in resent studies are introduced. It is stressed that either in the free atmosphere or in the planetary boundary, the descending arm of the Hadley cell cannot be considered as a mechanism for the formation of the subtropical anticyclone. Then the theories of thermal adaptation of the atmosphere to external thermal forcing and the potential vorticity forcing are developed to understand the formation of the subtropical anticyclone in the three-dimensional domain. Numerical experiments are designed to verify these theories. Results show that in the boreal summer, the formation of the strong South Asian High in the upper troposphere and the subtropical anticyclone over the western Pacific in the middle and lower troposphere is, to a great extent, due to the convective latent heating associated with the Asian monsoon, but affected by orography and the surface sensible heating over the continents. On the other hand, the formation of the subtropical anticyclone at the surface over the northern Pacific and in the upper troposphere over North America is mainly due to the strong surface sensible heating over North America, but affected by radiation cooling over the eastern North Pacific. Moreover, in the real atmosphere such individual thermal forcing is well organized. By considering the different diabatic heating in synthesis, a quadruple heating pattern is found over each subtropical continent and its adjacent oceans in summer. A distinct circulation pattern accompanies this heating pattern. The global summer subtropical heating and circulation may be viewed as “mosaics” of such quadruplet heating and circulation patterns respectively. At last, some important issues for further research in understanding and predicting the variations of the subtropical anticyclone are raised.

Evolution mechanism of the western Pacific subtropical high

To surmount the deficiency in studying the multiple equilibrium states in the atmosphere motion with highly truncated spectral method, the trigonometric functions for describing the prototypical 500 hPa height fields and the outgoing long wave radiation (OLR) fields are retrieved respectively for the northward bias and the southward bias years of the western Pacific subtropical high with corresponding observational data and the optimum subset retrieval method for four factors. Then the evolution mechanism of the western Pacific subtropical high is studied by means of multiple equilibrium state theory. The results show that the cause of inducing the abnormal location of the western Pacific subtropical high is differences in the early external thermal forcing, which evoke different waveforms in atmosphere. If the early meridional and zonal external thermal forcing differences are stronger, there are wave-mean flow and wave-wave interactions between the response waveforms in atmosphere. In such a case, the western Pacific subtropical high shifts northward obviously. On the contrary, when the early meridional and zonal external thermal forcing differences are weaker, there is no wave-mean flow interaction between the response waveforms in atmosphere, and accordingly the position of the western Pacific subtropical high oscillates with the external thermal forcing oscillation, and is on the south of normal.

Vertical circulation structure,interannual variation features and variation mechanism of western pacific subtropical high

The paper investigates the vertical circulation structure of the western Pacific subtropical high (STH) and its interannual variation features in relation to East Asian subtropical summer monsoon and external thermal forcing by using the high-resolution and good-quality observations from the 1998 South China Sea Summer Monsoon Experiment (SCSMEX), the NCEP 40-year reanalysis data and relevant SST and the STH parameters. It is found that the vertical circulation structures differ greatly in features between quasi-stationary and transient components of the western Pacific STH. When rainstorms happen in the rainband of East Asian subtropical monsoon on the STH north side, the downdrafts are distinct around the ridge at a related meridian. The sinking at high (low) levels comes from the north (south) side of the STH, thereby revealing that the high is a tie between tropical and extratropical systems. The analyses of this paper suggest that the latent heat release associated with subtropical monsoon precipitation, the offshore SST and East Asian land-sea thermal contrast have a significant effect on the STH interannual anomaly. Our numer-ical experiment shows that the offshore SSTA-caused sensible heating may excite an anomalous anticyclonic circulation on the west side, which affects the intensity (area) and meridional position of the western Pacific STH.

A Dynamical Model of “Quasi-stationary” States in Large-Scale Atmospheric Motions

Abstract A theoretical study is made to establish the concept of “quasi-stationary (QS)” states in irregular, large-scale atmospheric motions by using a simple model. It is a truncated, two-layer, quasi-geostrophic model in a β-channel with surface topography, external thermal forcing (i.e., meridional differential heating) and dissipative processes. We define a QS state as a period when the magnitude of the time derivative of spectral components, |x˙(t)|, is smaller than a prescribed threshold value. For a wide range of parameter values of the thermal forcing, each QS state is found to occur when the trajectory x(t) passes immediately next to a local minimum point (MP) of |x˙| in phase space; the MP is either a stationary solution or a nonstationary minimum point bifurcating from a limit point of a stationary solution. Moreover, we give the statistical significance to this relation between each QS state and an MP. The relation is almost independent of the threshold value defining the QS states. Therefore...

A nonlinear convective system with oscillatory behaviour for certain parameter regimes

The behaviour of a fluid system governed by a quadratic equation of state for the temperature is studied. The model consists of two well-mixed and interconnected vessels subjected to external thermal forcing. If inertial effects are neglected the temperature response of the fluid is governed by two autonomous ordinary differential equations in time. An investigation of these equations revealed that, depending upon the choice of parameters, the system has two possible final states: one stationary, the other a relaxation oscillation. The transitions between these states occur as sub- and supercritically unstable Hopf bifurcations. For certain parameter ranges, a stationary solution can thus coexist with a relaxation oscillation, the initial conditions determining which of these is realized.

地形を含む二層準地衡低次モデルにおける非線型相互作用

A two-layer, quasi-geostrophic, low-order model is constructed to investigate the nonlinear interactions between the zonal flow, topographically forced waves and free baroclinic waves. In Part I free baroclinic waves are excluded to direct our attention to the zonal flow-forced wave interactions.In the conservative case, without external thermal forcing and dissipation, equilibrium solutions are obtained and the resonance condition in the present two-layer model is examined. Multiple flow equilibria are also obtained in the non-conservative case. However, unlike the barotropic model of Charney and DeVore (1979), there do not exist two stable equilibria. A multiplicity of the time-dependent solutions is found in a certain range of the external parameters. There exist two or more stable periodic (or aperiodic) solutions for the same external conditions. The selection of a solution depends on the initial condition.As the external thermal forcing parameter increases or decreases by bits, the period of a stable periodic solution can become doubled, quadrupled, and so on. Finally aperiodic solutions appear through the period-doubling sequence phenomenon.