Mean Temperature Fields Research Articles

Reconstructing Northern Hemisphere upper-level fields during World War II

Monthly mean fields of temperature and geopotential height (GPH) from 700 to 100 hPa were statistically reconstructed for the extratropical Northern Hemisphere for the World War II period. The reconstruction was based on several hundred predictor variables, comprising temperature series from meteorological stations and gridded sea level pressure data (1939-1947) as well as a large amount of historical upper-air data (1939-1944). Statistical models were fitted in a calibration period (1948-1994) using the NCEP/NCAR Reanalysis data set as predictand. The procedure consists of a weighting scheme, principal component analyses on both the predictor variables and the predictand fields and multiple regression models relating the two sets of principal component time series to each other. According to validation experiments, the reconstruction skill in the 1939-1944 period is excellent for GPH at all levels and good for temperature up to 500 hPa, but somewhat worse for 300 hPa temperature and clearly worse for 100 hPa temperature. Regionally, high predictive skill is found over the midlatitudes of Europe and North America, but a lower quality over Asia, the subtropics, and the Arctic. Moreover, the quality is considerably better in winter than in summer. In the 1945-1947 period, reconstructions are useful up to 300 hPa for GPH and, in winter, up to 500 hPa for temperature. The reconstructed fields are presented for selected months and analysed from a dynamical perspective. It is demonstrated that the reconstructions provide a useful tool for the analysis of large-scale circulation features as well as stratosphere-troposphere coupling in the late 1930s and early 1940s.

Importance of Turbulence-Radiation Interactions in Turbulent Diffusion Jet Flames

Traditional modeling of radiative transfer in reacting flows has ignored turbulence-radiation interactions (TRI). Radiative fluxes, flux divergences and radiative properties have been based on mean temperature and concentration fields. However, both experimental and theoretical work have suggested that mean radiative quantities may differ significantly from those predictions based on the mean parameters because of their strongly nonlinear dependence on the temperature and concentration fields. The composition PDF method is able to consider many nonlinear interactions rigorously, and the method is used here to study turbulence-radiation interactions. This paper tries to answer two basic questions: (1) whether turbulence-radiation interactions are important in turbulent flames or not; and (2) if they are important, then what correlations need to be considered in the simulation to capture them. After conducting many flame simulations, it was observed that, on average, TRI effects account for about 1/3 of the total drop in flame peak temperature caused by radiative heat losses. In addition, this study shows that consideration of the temperature self correlation alone is not sufficient to capture TRI, but that the complete absorption coefficient-Planck function correlation must be considered.

Unsteady Thermally Driven Flows on Gentle Slopes

Abstract The theoretical and laboratory studies on mean velocity and temperature fields of an unsteady atmospheric boundary layer on sloping surfaces reported here were motivated by recent field observations on thermally driven circulation in very wide valleys in the presence of negligible synoptic winds. The upslope (anabatic) flow on a long, shallow, heated (with a buoyancy flux Fbs) slope of inclination α located adjoining a level plane and the effects of cooling of the slope on this flow during the evening transition are studied for the case of a gentle slope for which the length of the sloping plane far exceeds the thickness h of the convective boundary layer. First, a theoretical analysis is presented for the mean upslope flow velocity UM, noting that the turbulence but not the mean flow structure therein is similar to that on a level terrain. The analysis, which is based on mean momentum and heat equations as well as closure involving level-terrain turbulence parameterizations, shows that UM is pro...

Coral mortality and recovery in response to increasing temperature in the southern Arabian Gulf

Analysis of a new data set for sea surface temperature shows that the seawater temperature rise in 1998 in the southern Arabian Gulf was the largest since 1870; it also showed that there is an underlying rising trend of sea surface temperature across the whole of the measured area spanning 5 degrees longitude and 3 degrees latitude. Over the last 50 years, sea surface temperature rise was about 0.2°C per decade, though this has accelerated to about 0.45°C per decade in the last 20 years. The pattern in temperature values in each 1 degree ‘cell’ are used to show that, at any one time, temperature rises towards the east of this area, which supports the concept of a general anti-clockwise circulation in the southeastern half of the Arabian Gulf. Spatial and temporal patterns in sea surface temperature rises also show that the mean temperature field at any location is similar to that which existed in the one degree cell immediately to its east approximately 30 years earlier.Coral cover in shallow water (<3 m depth) was <1 percent immediately following the 1998 event, due to near total mortality of vast areas of shallow Acropora (stagshorn) corals. Deeper than 3 m there was increasing survival of the large Porites (boulder) corals, survival increasing with depth. Coral survival improved towards the east, though overall, less than twenty coral species were found altogether. Juvenile recruitment also increased towards the east, and at any one site, recruitment increased with increasing depth. The identity of new juveniles was mainly faviid corals, not the Acropora or Porites which dominated these reefs before the 1998 event. This suggests that there may be a phase shift to a different stable state in terms of coral communities. We also speculate that, because the Arabian Gulf contains an extreme warm-tolerant subset of the Indian Ocean coral fauna, the changes seen in the Gulf may reflect future patterns across a greater part of the Indian Ocean.

LARGE-EDDY SIMULATION OF HEAT TRANSFER OVER A BACKWARD-FACING STEP

A large-eddy simulation (LES) of the flow over a backward-facing step was conducted to investigate the heat transfer phenomena in the reattachment zone. The Navier-Stokes equations for an incompressible fluid with the temperature field considered as a passive scalar are solved using a second-order accurate scheme in space and time. An original coupling is used with a previous simulation to impose a fully turbulent flow at the entrance of the domain. The mixed scale subgrid model is used for the momentum equations while a scalar subgrid diffusivity model is employed for the temperature equation. The Reynolds number based on the velocity at the entrance of the domain and the step height is 7,432. The mean velocity field shows clearly that the shear layer issued from the step impacts the wall defining a recirculation zone, in which the reversed flow spreads into the original shear layer. A second recirculation region is found behind the step and acted like an obstacle for the first reversal flow. The mean reattachment length is well correlated to the maximum Nusselt number. The examination of the mean temperature field proves that the mixing behind the step is weak. The temperature fluctuations are important in the shear layer and in a zone issued from the reattachment point and stretched toward the region of reversed flow. Two main frequencies are identified: a high one due to the vortex shedding of the shear layer and a low one corresponding to the drift of the reattachment point.

An application of statistical downscaling to estimate surface air temperature in Japan

In this study, a statistical downscaling model based on singular value decomposition was applied to estimate the monthly mean temperature field in Japan for January and July. The regression model estimated surface air temperature in Japan from upper air temperature in east Asia with root‐mean‐square errors of around 1.0°C for an independent verification period. The method was applied to the output of a CO2 transient run of NCAR‐CSM, and the result was compared with the output of NCAR‐RegCM2.5 nested in CSM. The statistical model reproduced the spatial distribution of the temperature more realistically than the CSM output. In January the results corresponded well between the models except that the climate simulated by RegCM was generally cooler than that estimated using the statistical method mainly due to the unrealistic RegCM topography. In July the results did not correspond well between the methods, since the climate is more complex and difficult to be estimated solely from the upper air temperature field. Temperature rise from 1CO2 climate to 2CO2 climate was larger in RegCM than in the statistical method for both months reflecting the influence of sea surface temperature rise. It was concluded that this statistical downscaling method can be applied to estimate the January mean temperature in Japan, although other predictor variables such as sea surface temperature should be included to improve the estimation in July.

Diagnosing and Picturing the North Atlantic Segment of the Global Conveyor Belt by Means of an Ocean General Circulation Model

The monthly mean velocity, salinity, and temperature fields of a numerical simulation of the World Ocean climatological circulation are used to study the intensity and pathways associated with the meridional overturning in the North Atlantic. Lagrangian diagnostics based on the computation of several hundreds of thousands of individual three-dimensional trajectories are combined with an appropriate study of water mass potential densities in order to describe the warm and cold limbs of the so-called conveyor belt. Circulation schemes are established for both limbs of the overturning, and can be easily compared with schemes or transport estimates deduced from direct measurements, as the model temperature and salinity fields are constrained to remain close to the observed climatology. Diagnostics emphasize most typical pathways as well as main mass transfers that lead to the establishment of such numerical circulation schemes.

Model intercomparison in the Mediterranean: MEDMEX simulations of the seasonal cycle

The simulation of the seasonal cycle in the Mediterranean by several primitive equation models is presented. All models were forced with the same atmospheric data, which consists in either a monthly averaged wind-stress with sea surface relaxation towards monthly mean sea surface temperature and salinity fields, or by daily variable European Centre for Medium Range Weather Forecast (ECMWF) reanalysed wind-stress and heat fluxes. In both situations models used the same grid resolution. Results of the modelling show that the model behaviour is similar when the most sensitive parameter, vertical diffusion, is calibrated properly. It is shown that an unrealistic climatic drift must be expected when using monthly averaged forcing functions. When using daily forcings, drifts are modified and more variability observed, but when performing an EOF analysis of the sea surface temperature, it is shown that the basic cycle, represented similarly by the models, consists of the seasonal cycle which accounts for more than 90% of its variability.

A Numerical Study Of Nocturnal Wavelike Motion In Forests

In this paper, we use a two-dimensional eddy-resolved model to investigate the instability of a parallel shear flow in a stably stratified boundary layer whose lower domain is occupied by a canopy. The results support our contention that wave motion in the canopy is initiated by shear in an air layer near the treetops. Significant modification by the wave motion of the mean velocity and temperature fields is found even before the wave reaches saturation. The wave fluxes of momentum and heat are not constant with height. Downwind tilting braids are found at the finite amplitude stage of the wave growth and could persist after wave breaking; these downwind tilting structures are believed to be the same as the temperature microfronts reported in the literature. We also present an analysis of the velocity and temperature fields of an observed wave event in the time-height domain and show that the simulation has captured the broad features of the observation.

Seasonal Variations in the Sea off Sanriku Coast, Japan

The temperature and salinity data obtained by the Iwate Fisheries Technology Center for the 25-year period from 1971 to 1995 were analyzed to clarify the seasonal variations in the sea off Sanriku, Japan. The variations of three typical waters found in this region, the Tsugaru Current water, the Oyashio water, and the Kuroshio water are discussed in terms, of a T-S scatter diagram referring to the water mass classification proposed by Hanawa and Mitsudera (1986). The mean temperature and salinity fields averaged for each month show clear seasonal variation. Distributions of the Tsugaru Current water and the Oyashio water can barely be distinguished in the fields deeper than 200 m since the Tsugaru Current has a shallow structure; however, the fields at 100 m depth indicate remarkable seasonal variation in the area of the Tsugaru Current. At 100 m depth, the temperature and salinity fronts between the Tsugaru Current water and the Oyashio water gradually disappear in January through April, appear again in May, then become clearest in September.

The effect of non‐uniform radiative damping on the zonal‐mean dynamics of the extratropical middle atmosphere

AbstractThe effect of spatial and temporal variations in the radiative damping rate on the response to an imposed forcing or diabatic heating is examined in a zonal‐mean model of the middle atmosphere. Attention is restricted to the extratropics, where a linear approach is viable. It is found that regions with weak radiative damping rates are more sensitive in terms of temperature to the remote influence of the diabatic circulation. The delay in the response in such regions can mean that ‘downward’ control is not achieved on seasonal time‐scales. A seasonal variation in the radiative damping rate modulates the evolution of the response and leaves a transient‐like signature in the annual mean temperature field.Several idealized examples are considered, motivated by topical questions. It is found that wave drag outside the polar vortex can significantly affect the temperatures in its interior, so that high‐latitude, high‐altitude gravity‐wave drag is not the only mechanism for warming the southern hemisphere polar vortex. Diabatic mass transport through the 100 hPa surface is found to lag the seasonal evolution of the wave drag that drives the transport, and thus cannot be considered to be in the downward control regime. On the other hand, the seasonal variation of the radiative damping rate is found to make only a weak contribution to the annual mean temperature increase that has been observed above the ozone hole. Copyright © 2002 Royal Meteorological Society.

Heat transfer enhancement by Görtler instability

Measurements of mean velocity field and mean temperature field were performed in a concave boundary layer. The growth of the induced Görtler vortices embedded in this boundary layer has been determined in two ways by a criterion based equally on spanwise distortion of velocity or temperature profile. The streamwise and spanwise evolution of heat transfer coefficient has been determined by measurement of wall temperature. It was found that the steady growth of these structures induces a characteristic plateau in the evolution of Stanton number versus Reynolds number. This plateau starts with the enhancement of the amplification criterion and finishes when the transition occurs.

Ocean circulation influences on sea surface temperature in the equatorial central Pacific

Velocity data from an array of acoustic Doppler current profilers moored about 0°, 140°W from May 1990 through June 1991 during the Tropical Instability Wave Experiment are used in conjunction with Tropical Atmosphere‐Ocean array data and a blended sea surface temperature (SST) product to study the processes that control SST variations. The horizontal velocity data allow us to calculate the vertical velocity component by vertically integrating the continuity equation. Given the three‐dimensional temperature flux divergence, we examine the role of the ocean circulation in SST. Upwelling and downwelling are found to be associated with cooling and warming, respectively, suggesting that a vertical velocity component of either sign affects SST. Both the temperature flux divergence and advective formulations for the ocean circulation's influence on the temperature budget show times when the ocean circulation appears to provide the primary control on SST and times when this is not the case, with the flux divergence formulation performing better than the advective formulation. Statistically, within a bandwidth encompassing the tropical instability waves and the intraseasonal variations, roughly half of the SST variation is accounted for by the ocean circulation. These results are encouraging, given that data sets with different spatial and temporal scales have been used. They suggest that future field experimentation which utilizes a flux divergence array with velocity and temperature data sampled at the same spatial and temporal scales will yield quantitatively improved results. The analyses also show that the ocean circulation, on average, provides a cooling effect requiring the net surface heat flux to be positive to maintain the mean background state. The cooling effect is mainly controlled by mean ocean circulation and temperature fields.

SIMULATION OF PARTICLE TRANSPORT AND DEPOSITION IN A COMBUSTOR

A computational model for Lagrangian particle tracking for studying dispersion and deposition of particles in a combustor with swirling flow and chemical reaction is developed. The model accounts for the effect of thermophoretic force, as well as the drag and lift forces acting on particles, in addition to the Brownian motion and gravitational sedimentation effects. The mean turbulent gas flow, temperature fields and chemical species concentration in the combustor are evaluated using the stress transport turbulent model of the FLUENT code. The instantaneous fluctuation velocity field is generated by a Gaussian filtered white noise model. The simulated axial, radial and tangential mean gas velocities are compared with the existing experimental data. Ensembles of particle trajectories are generated and statistically analyzed. The effects of size and initial distribution on particle dispersion and deposition are studied. The particle concentration at different sections are also evaluated and discussed. The results shows that the turbulence dispersion effect is quite important, while the thermophoresis effect is small.

Contribution numérique à l'étude hydrodynamique et thermique des écoulements turbulents induits par une turbine radiale en cuve agitée

The hydrodynamic and thermal behaviours induced by a six flat-bladed turbine, in a closed, unbaffled, batch stirred vessel are numerically predicted by means of Computational Fluid Dynamic (CFD) model. Heat transfer is applied on the jacketed wall of the stirred vessel. Solutions of the time-averaged Navier–Stokes and energy equations in conjunction with the standard k– ε turbulence model are developed using a control volume discretization method. A three-dimensional analysis of the hydrodynamic and thermal characteristics of the turbulent flow generated within the vessel are performed. Analyses concern the mean velocities and temperature fields, the turbulence kinetic energy and its production and dissipation rates. Turbulent flow characteristics are particularly examined between blades and in the impeller discharge stream. Also, the evolution in transient regime of the thermal state of the agitated vessel has been studied. Finally, Nusselt numbers numerically predicted are correlated by a dimensionless equation in order to be compared with ones found by other researchers.

Dissimilarity between the velocity and temperature fields in a perturbed turbulent thermal boundary layer

Dissimilarity between the streamwise velocity and temperature fields is investigated in a perturbed turbulent thermal boundary layer using direct numerical simulation. Perturbations are provided into a turbulent boundary layer by uniform blowing or suction from a spanwise slot. The skin-friction coefficient and Stanton number are significantly changed due to blowing and suction. The streamwise evolution of the Stanton number is very different from that of the skin-friction coefficient because the pressure gradient induced by blowing or suction plays an important role in the momentum transport. Before the slot, the mean streamwise velocity changes smoothly, as compared to the mean temperature, due to the pressure gradient, and a similar behavior is also observed in the variations of the streamwise velocity and temperature fluctuations. Above the slot, the change in the mean streamwise velocity is smaller than that in the mean temperature, and after the slot the mean streamwise velocity recovers slowly to the unperturbed flow as compared to the mean temperature. By investigating each term in the mean streamwise velocity and temperature budgets, it is concluded that the mean pressure gradient is the main source of dissimilarity between the mean streamwise velocity and temperature fields. The budgets for the variances of the streamwise velocity and temperature fluctuations show that the difference between the production terms in both budget equations, which results from the existence of the mean pressure gradient, is the primary source of dissimilarity between the streamwise velocity and temperature fields above the slot, whereas downstream of the slot, the velocity pressure-gradient term (i.e., pressure gradient fluctuation) is another important source of dissimilarity. Dissimilarity between the streamwise velocity and temperature fluctuations caused by blowing persists longer in the streamwise direction than that by suction.

Turbulent Heat Transfer in a Square Duct with a Rough Wall.

Characteristics of the temperature field in forced convection turbulent heat transfer in a square duct with one rib-roughened wall have been investigated experimentally. Distributions of the mean temperature, primary flow velocity, intensities of the fluctuating temperature and velocity, correlation coefficients between the fluctuating temperature and velocity components, turbulent heat fluxes and turbulent shear stresses were measured, and they were compared with those obtained in a smooth-walled square duct to examine the influence of the rough wall on the characteristics of the temperature field. Near the rough wall, a weak dissimilarity appeared between the mean temperature field and the mean velocity field ; this suggested that the transport of heat by turbulence becomes less active than that of the momentum there. Based on the measured results, the mechanism of this dissimilarity in the mean fields was examined by evaluating the production terms in the transport equations of <u12>^- and <t2>^-.

Variability of the Stratification and Circulation on the Flemish Cap duringthe decades of the 1950s-1990s

Oceanographic observations on the Flemish Cap (around 47°N, 45°W) from the early-1990s to 1999 are compared to the long-term mean and to conditions during the past several decades. The mean temperature and salinity fields and their seasonal cycles were first computed and then used to construct T/S anomaly time series. The data shows a relatively warm time period from the 1950s through the 1960s and three colder-than-normal periods since the early-1970s. In general, it was found that variations in water properties on the Flemish Cap are highly correlated with those observed in the inshore branch of the Labrador Current at Station 27 and in other areas of the Newfoundland Continental Shelf. Studies have shown that these conditions are linked to the largescale atmospheric winter circulation, sea ice conditions, local atmospheric forcing and advection. An examination of local air-sea heat flux indicate that advection of Labrador Current water into the region may be the principle cause of oceanic variability over the Flemish Cap. An examination of recent acoustic Doppler current measurements and geostrophic current calculations indicated the predominance of an anticyclonic gyre circulation around the Flemish Cap during the summer and a significant and persistent Labrador Current component to the circulation. The results, however, exhibited a high degree of interannual variability.

Investigations in the TECFLAM swirling diffusion flame: Laser Raman measurements and CFD calculations

A standard burner for confined swirling natural gas flames is presented which was developed within the German TECFLAM cooperation. The aims of the TECFLAM research program are the establishment of an extensive experimental database from selective flames and the validation and improvement of mathematical combustion models. In this paper, results from joint PDF measurements of temperature, mixture fraction, and major species concentrations in a turbulent diffusion flame with 150 kW thermal load, equivalence ratio 0.833, and swirl number 0.9 are presented. Major aspects of the investigation are the general quantitative characterization of the flame and the study of the thermochemical state, e.g. effects of turbulence–chemistry interactions. Scatterplots of temperature, CH4, and CO mole fractions as well as mean mixture fraction and temperature fields are presented and discussed. Furthermore, CFD calculations have been performed using the code Fluent 5 as an example of a commercially available code that is frequently used for technical applications. The comparison between the calculated and measured results reveals some significant deviations which are discussed with respect to the applicability of this code to swirling turbulent flames.

Analysis Of Temperature Fluctuations In ATurbulent Flow Over A Backward-facing Step

A large-eddy simulation (LES) of the flow over a backward-facing step was conducted to investigate the heat transfer phenomena in the reattachment zone. The Navier-Stokes equations for an incompressible fluid with the temperature field considered as a passive scalar, are solved using a second-order accurate scheme in space and time. In order to have a fully turbulent flow in the domain, an auxiliary simulation is carried out to provide unsteady velocity and temperature fields at the entrance to the backward-facing step. The Mixed Scale Subgrid Model is used for the momentum equations while a scalar subgrid diffusivity model is employed for the temperature equation. The Reynolds number based on the bulk velocity (Ub) at the entrance and the step height (h) is 66 100. The mean reattachment point was predicted at x=5.4h which is close to the value calculated by Moss et al. [5] for the same expansion ratio and for a turbulent flow. Mean velocity field shows clearly that the shear layer issued from the step impacts the wall defining a recirculation zone in which the reversed flow spreads into the original shear layer. Just behind the step, a second recirculation region acts like an obstacle for the first reversal flow. The examination of the mean temperature field proves that the mixing behind the step is weak. From the shear layer to the reattachment zone, a large region exists containing warm fluid, but the fluid just behind the step remains cold. Apart from the fact that the temperature fluctuations are important in the shear layer, another region of high fluctuations was found to issue from the reattachment point and stretch toward the region of reversed flow.