Showing papers on "Convection published in 1996"

Bulk parameterization of air‐sea fluxes for Tropical Ocean‐Global Atmosphere Coupled‐Ocean Atmosphere Response Experiment

Abstract: This paper describes the various physical processes relating near-surface atmospheric and oceanographic bulk variables ; their relationship to the surface fluxes of momentum, sensible heat, and latent heat ; and their expression in a bulk flux algorithm. The algorithm follows the standard Monin-Obukhov similarity approach for near-surface meteorological measurements but includes separate models for the ocean's cool skin and the diurnal warm layer, which are used to derive true skin temperature from the bulk temperature measured at some depth near the surface. The basic structure is an outgrowth of the Liu-Katsaros-Businger [Liu et al., 1979] method, with modifications to include a different specification of the roughness/stress relationship, a gustiness velocity to account for the additional flux induced by boundary layer scale variability, and profile functions obeying the convective limit. Additionally, we have considered the contributions of the sensible heat carried by precipitation and the requirement that the net dry mass flux be zero (the so-called Webb correction [Webb et al., 1980]). The algorithm has been tuned to fit measurements made on the R/V Moana Wave in the three different cruise legs made during the Coupled Ocean-Atmosphere Response Experiment. These measurements yielded 1622 fifty-min averages of fluxes and bulk variables in the wind speed range from 0.5 to 10 m s -1 . The analysis gives statistically reliable values for the Charnock [1955] constant (a = 0.011) and the gustiness parameter (β = 1.25). An overall mean value for the latent heat flux, neutral bulk-transfer coefficient was 1.11 x 10 -3 , declining slightly with increasing wind speed. Mean values for the sensible and latent heat fluxes were 9.1 and 103.5 W m -2 ; mean values for the Webb and rain heat fluxes were 2.5 and 4.5 W m -2 . Accounting for all factors, the net surface heat transfer to the ocean was 17.9 ± 10 W m -2 .

Cool-skin and warm-layer effects on sea surface temperature

Abstract: To obtain bulk surface flux estimates approaching the ±10 W m−2 accuracy desired for the Tropical Ocean-Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (COARE) program, bulk water temperature data from ships and buoys must be corrected for cool-skin and diurnal warm-layer effects. In this paper we describe two simple scaling models to estimate these corrections. The cool-skin model is based on the standard Saunders [1967] treatment, including the effects of solar radiation absorption, modified to include both shear-driven and convectively driven turbulence through their relative contributions to the near-surface turbulent kinetic energy dissipation rate. Shear and convective effects are comparable at a wind speed of about 2.5 m s−1. For the R/V Moana Wave COARE data collected in the tropical western Pacific, the model gives an average cool skin of 0.30 K at night and an average local noon value of 0.18 K. The warm-layer model is based on a single-layer scaling version of a model by Price et al. [1986]. In this model, once solar heating of the ocean exceeds the combined cooling by turbulent scalar heat transfer and net longwave radiation, then the main body of the mixed layer is cut off from its source of turbulence. Thereafter, surface inputs of heat and momentum are confined to a depth DT that is determined by the subsequent integrals of the heat and momentum. The model assumes linear profiles of temperature-induced and surface-stress-induced current in this “warm layer.” The model is shown to describe the peak afternoon warming and diurnal cycle of the warming quite accurately, on average, with a choice of a critical Richardson number of 0.65. For a clear day with a 10-m wind speed of 1 m s−1, the peak afternoon warming is about 3.8 K with a warm-layer depth of 0.7 m, decreasing to about 0.2 K and 19 m at a wind speed of 7 m s−1. For an average over 70 days sampled during COARE, the cool skin increases the average atmospheric heat input to the ocean by about 11 W m−2; the warm layer decreases it by about 4 W m−2 (but the effect can be 50 W m−2 at midday).

The Effect of Vertical Shear on Tropical Cyclone Intensity Change

Abstract: The effect of vertical shear on tropical cyclone intensity change is usually explained in terms of “ventilation” where heat and moisture at upper levels are advected away from the low-level circulation, which inhibits development. A simple two-layer diagnostic balance model is used to provide an alternate explanation of the effect of shear. When the upper-layer wind in the vortex environment differs from that in the lower layer, the potential vorticity (PV) pattern associated with the vortex circulation becomes tilted in the vertical. The balanced mass field associated with the tilted PV pattern requires an increased midlevel temperature perturbation near the vortex center. It is hypothesized that this midlevel warming reduces the convective activity and inhibits the storm development. Previous studies have shown that diabatic heating near the storm center acts to reduce the vertical tilt of the vortex circulation. These studies have also shown that there is an adiabatic process that acts to redu...

Statistical patterns of high‐latitude convection obtained from Goose Bay HF radar observations

Abstract: We have derived patterns that describe the statistical interplanetary magnetic field (IMF) dependencies of ionospheric convection in the high-latitude region of the northern hemisphere. The observations of plasma motion were made with the HF coherent backscatter radar located at Goose Bay, Labrador, over the period September 1987 to June 1993. The area covered by the measurements extended poleward of 65°Λ to a working limit of about 85°Λ. Distributions of electrostatic potential have been derived and expressed as series expansions in spherical harmonics. The patterns are the first derived from direct ground-based observations of ionospheric convection that approach in completeness and level of detail the patterns derived in recent satellite studies [Rich and Hairston, 1994; Weimer, 1995]. We show the dependence of the convection on IMF angle in the GSM y–z plane for three intervals of IMF magnitude in this plane. Except for predominantly northward IMF, the convection is primarily two-cell. The dusk cell is larger in terms of both spatial extent and potential variation/The effect of IMF By is apparent in the global shaping of the cells and the orientation of the overall pattern in MLT; for By + (By−) the dusk (dawn) cell is more round (crescent-shaped) and the pattern more rotated toward earlier MLTs. The By effect on the nightside convection is pronounced and is hemispherically antisymmetric, like the well-known day side By effect. For IMF increasingly northward, the convection trajectories on the dayside become increasingly distorted, evolving through a three-cell to a four-cell circulation. The additional cells appear on either side of the noon meridian and result in sunward flow. The overall agreement with the results of the satellite studies is good and extends to quite fine detail in the case of the comparison with Weimer [1995]. There are significant differences with the statistical patterns derived from magnetometer measurements, which tend to show domination by the dawn rather than the dusk cell.

Surface Meteorology and Air-Sea Fluxes in the Western Equatorial Pacific Warm Pool during the TOGA Coupled Ocean-Atmosphere Response Experiment

Abstract: A major goal of the Coupled Ocean-Atmosphere Response Experiment (COARE) was to achieve significantly more accurate and complete descriptions of the surface meteorology and air-sea fluxes in the western equatorial warm pool region. Time series of near-surface meteorology from a buoy moored near the center of the COARE Intensive Flux Array (IFA) are described here. The accuracies of the measurements and the derived fluxes are quantified; agreement between average net heat fluxes at the buoy and two nearby research ships is better than 10 W m−2 during three intercomparisons. Variability in the surface meteorology and fluxes associated with westerly wind bursts, periods of low winds, and short-lived, deep convective events characteristic of the region was large compared to the 4-month means. The ECMWF (European Centre for Medium-Range Weather Forecasts) analysis and prediction fields differed most from the buoy data during periods of short-lived, deep convective events, when several day averages of ...

Sensitivity of Moist Convection Forced by Boundary Layer Processes to Low-Level Thermodynamic Fields

Abstract: The sensitivity of moist convection to a number of low-level thermodynamic parameters is examined with a high-resolution, nonhydrostatic numerical model. The parameters examined are the surface temperature dropoff (defined as the difference between the potential temperature measured at the surface and that in the well-mixed boundary layer), the surface moisture dropoff (defined similarly for moisture), the boundary layer moisture dropoff (defined as the vertical decrease in moisture within the boundary layer), and the depth of the moisture. The typical variability in these parameters is estimated from two field experiments in northeastern Colorado. Sensitivity is then defined relative to this typical observational variability. Two convection initiation cases from northeastern Colorado are examined. In both cases, convection initiation is found to be most sensitive to the surface temperature dropoff and the surface moisture dropoff. It is found that variations in boundary layer temperature and moi...

Mesoscale shallow convection in the atmosphere

Abstract: This paper is a review of the observational, experimental, theoretical, and numerical studies of mesoscale shallow convection (MSC) in the atmosphere. Typically, MSC is 1 to 2 km deep, has a horizontal length scale of a few to a few tens of kilometers, and takes distinctive planforms: linear and hexagonal. The former is called a cloud street, roll, or band, while the latter is called mesoscale cellular convection (MCC), comprising three-dimensional cells. MSC is characterized by its shape, horizontal extent, convective depth, and aspect ratio. The latter is the ratio of the horizontal extent to that in the vertical. For cells the horizontal extent is their diameter, whereas for rolls it is their spacing. Rolls usually align along or at angles of up to 10° from the mean horizontal wind of the convective layer, with lengths from 20 to 200 km, widths from 2 to 10 km, and convective depths from 2 to 3 km. The typical value of aspect ratio ranges from 2 to 20. Rolls may occur over both water surface and land surfaces. Mesoscale convective cells may be divided into two types: open and closed. Open-cell circulation has downward motion and clear sky in the cell center, surrounded by cloud associated with upward motion. Closed cells have the opposite circulation. Both types of cell have diameters ranging from 10 to 40 km and aspect ratios of 5 to 50, and both occur in a convective layer with a depth of about 1 to 3 km. Both the magnitude and direction of horizontal wind in the convective layer change little with height. MSC results from a complex and incompletely understood mix of processes. These processes are outlined, and their interplay is examined through a review of theoretical and laboratory analyses and numerical modeling of MSC.

Effect of depth-dependent viscosity on the planform of mantle convection

Abstract: LITHOSPHERIC plate motions at the Earth's surface result from thermal convection in the mantle1. Understanding mantle convection is made difficult by variations in the material properties of rocks as pressure and temperature increase from the surface to the core. The plates themselves result from high rock strength and brittle failure at low temperature near the surface. In the deeper mantle, elevated pressure may increase the effective viscosity by orders of magnitude2–5. The influence of depth-dependent viscosity on convection has been explored in two-dimensional numerical experiments6–8, but planforms must be studied in three dimensions. Although three-dimensional plan-forms can be elucidated by laboratory fluid dynamic experiments9,10, such experiments cannot simulate depth-dependent rheology. Here we use a three-dimensional spherical convection model11,12 to show that a modest increase in mantle viscosity with depth has a marked effect on the planform of convection, resulting in long, linear downwellings from the upper surface boundary layer and a surprisingly 'red' thermal heterogeneity spectrum, as observed for the Earth's mantle13. These effects of depth-dependent viscosity may be comparable to the effects of the plates themselves.

Stagnant lid convection on Venus

Abstract: The effect of strongly temperature-dependent viscosity on convection in the interior of Venus is studied systematically with the help of finite element numerical models. For viscosity contrasts satisfying experimental constraints on the rheology of rocks, Venus is likely to be in the regime of stagnant lid convection. This regime is characterized by the formation of a slowly creeping, very viscous lid on top of the mantle-Venusian lithosphere and is in agreement with the tectonic style observed on Venus. Stagnant lid convection explains large geoid to topography ratios on Venus by the thermal thinning of a thick lithosphere. The thickness of the lithosphere can be as large as 400-550 km for Beta Regio and 200-400 km on average. Geoid and topography data and experimental data on the rheology of rocks provide constraints on the viscosity of the mantle, 10 20 -10 21 Pa s ; the convective stresses in the interior, 0.2-0.5 MPa ; the stresses in the lid, 100-200 MPa ; the velocity in the interior, 0.5-3 cm yr -1 ; and the heat flux beneath the lithosphere, 8-16 mW m -2 . Parameterized convection calculations of thermal history of Venus are difficult to reconcile with a thick present-day lithosphere. However, a sufficiently thick lithosphere can be formed if a convective regime with mobile plates was replaced by stagnant lid convection around 0.5 b.y. ago. One of the possible explanations for the cessation of Venusian plate tectonics is that during the evolution of Venus, stresses in the lid dropped below the yield strength of the lithosphere. This model predicts a drastic drop in the heat flux, thickening of the lithosphere, and suppression of melting and is consistent with the hypothesis of cessation of resurfacing on Venus around 0.5 b.y. ago.

Rapidly rotating turbulent Rayleigh-Bénard convection

Abstract: Turbulent Boussinesq convection under the influence of rapid rotation (ie with comparable characteristic rotation and convection timescales) is studied The transition to turbulence proceeds through a relatively simple bifurcation sequence, starting with unstable convection rolls at moderate Rayleigh (Ra) and Taylor numbers (Ta) and culminating in a state dominated by coherent plume structures at high Ra and Ta Like non-rotating turbulent convection, the rapidly rotating state exhibits a simple power-law dependence on Ra for all statistical properties of the flow When the fluid layer is bounded by no-slip surfaces, the convective heat transport (Nu − 1, where Nu is the Nusselt number) exhibits scaling with Ra2/7 similar to non-rotating laboratory experiments When the boundaries are stress free, the heat transport obeys ‘classical’ scaling (Ra1/3) for a limited range in Ra, then appears to undergo a transition to a different law at Ra ≈ 4 × 107 Important dynamical differences between rotating and non-rotating convection are observed: aside from the (expected) differences in the boundary layers due to Ekman pumping effects, angular momentum conservation forces all plume structures created at flow-convergent sites of the heated and cooled boundaries to spin-up cyclonically; the resulting plume/cyclones undergo strong vortex-vortex interactions which dramatically alter the mean state of the flow and result in a finite background temperature gradient as Ra → ∞, holding Ra/Ta fixed

Mechanisms of Heat Exchange: Biophysics and Physiology

Abstract: The sections in this article are: 1 Human Heat Balance Equation 2 Independent Variables Affecting the Thermal Environment 2.1 Ambient Temperature 2.2 Dew Point Temperature and Ambient Vapor Pressure 2.3 Air and Fluid Movement 2.4 Mean Radiant Temperature and Effective Radiant Field 2.5 Clothing Insulation 2.6 Barometric Pressure 3 Peripheral Factors to Heat Exchange 3.1 Mean Skin Temperature 3.2 Skin Wettedness 3.3 Body Heat Storage and Rate of Change of Mean Body Temperature 3.4 Metabolic Energy 4 Sensible Heat Exchange by Radiation and Convection 4.1 Operative Temperature 4.2 Clothing Properties Effective in Sensible Heat Exchange 5 Radiation Exchange 5.1 Mean Radiant Temperature and Effective Radiant Field 6 Convective Heat Exchange 6.1 Heat Transfer Theory 6.2 Measurement of the Convective Heat Transfer Coefficient 6.3 Effect of Altitude (Barometric Pressure) on Convective Heat Loss 7 Evaporative Heat Exchange 7.1 Direct Measurement of Evaporative Heat Loss 8 Psychrometrics of the Human Heat Balance Equation 8.1 Lewis Relation: Interpretation of Wet-Bulb Temperature and Enthalpy 8.2 Generalization to Energy Transfer between Humans and the Environment 8.3 Enthalpy of the Human Environment 8.4 Enthalpy and the Rational Indices of the Human Environment 9 Pierce Two-Node Model of Thermoregulation 9.1 The Passive State 9.2 The Control System 9.3 Initial Warm and Cold Signals 9.4 Control of Skin Blood Flow 9.5 Control of the Whole-Body Sweating Drive 9.6 Control of the Skin Shell Properties 9.7 Control of Shivering 10 Heat Loss Factors in Warm and Cold Environments: Physiology 10.1 Heat and Mass Transfer from the Body to the Environment 10.2 Body Motion 10.3 Thermal Aspects 10.4 Thermoregulatory Control Relative to Heat Loss Factors 10.5 Physiological Responses of Heat Loss by Thermal Radiation and Adaptive Response 11 Heat Loss in Special Environments 11.1 Hypobaric Environments 11.2 Hyperbaric Environments 11.3 Aquatic Environments 11.4 Heat Exchange in Spacecraft

Effects of strongly variable viscosity on three-dimensional compressible convection in planetary mantles

Abstract: A systematic investigation into the effects of temperature dependent viscosity on three-dimensional compressible mantle convection has been performed by means of numerical simulations in Cartesian geometry using a finite volume multigrid code, with a factor of 1000–2500 viscosity variation, Rayleigh numbers ranging from 105–107, and stress-free upper and lower boundaries. Considerable differences in model behavior are found depending on the details of rheology, heating mode, compressibility, and boundary conditions. Parameter choices were guided by realistic Earth models. In Boussinesq, basally heated cases with viscosity solely dependent on temperature and stress-free, isothermal boundaries, very long wavelength flows (∼25,000 km, assuming the depth corresponds to mantle thickness) with cold plumes and hot upwelling sheets result, in contrast to the upwelling plumes and downwelling sheets found in small domains, illustrating the importance of simulating wide domains. The addition of depth dependence results in small cells and reverses the planform, causing hot plumes and cold sheets. The planform of temperature-dependent viscosity convection is due predominantly to vertical variations in viscosity resulting from the temperature dependence. Compressibility, with associated depth-dependent properties, results in a tendency for broad upwelling plumes and narrow downwelling sheets, with large aspect ratio cells. Perhaps the greatest modulation effect occurs in internally heated compressible cases, in which the short-wavelength pattern of time-dependent cold plumes commonly observed in constant-viscosity calculations completely changes into a very long wavelength pattern of downwelling sheets (spaced up to 24,000 km apart) with time-dependent plumelike instabilities. These results are particularly interesting, since the basal heat flow in the Earth's mantle is usually thought to be very low, e.g., 5–20% of total. The effects of viscous dissipation and adiabatic heating play only a minor role in the overall heat budget for constant-viscosity cases, an observation which is not much affected by the Rayleigh number. However, viscous dissipation becomes important in the stiff upper boundary layer when viscosity is temperature dependent. This effect is caused by the very high stresses occurring in this stiff lid, typically 2 orders of magnitude higher than the stresses in the interior of the domain for the viscosity contrast modeled here. The temperature in the interior of convective cells is highly sensitive to the material properties, with temperature-dependent viscosity and depth-dependent thermal conductivity strongly increasing the internal temperature, and depth-dependent viscosity strongly decreasing it. The sensitivity of the observed flow pattern to these various complexities clearly illustrates the importance of performing compressible, variable-viscosity mantle convection calculations with rheological and thermodynamic properties matching as closely as possible those of the Earth.

Energy balance for turbulent flow around a surface mounted cube placed in a channel

Abstract: Results of an experimental investigation of the inhomogeneous, three‐dimensional flow around a surface mounted cube in a channel are presented. LDA measurements of single‐point velocity correlations are used to determine the production, convection and transport of the turbulence kinetic energy, k, in the obstacle wake. The turbulence dissipation rate is obtained as a closing term to the balance of the k‐transport equation. The results provide some insight to the evolution of the turbulence dissipation rate from the near field recirculation zone to the asymptotic wake. Also presented is a comparison between measured and modeled transport terms.

Natural convection in the annulus between concentric horizontal circular and square cylinders

Abstract: Numerical solutions are presented for natural convection heat transfer between a heated horizontal cylinder placed concentrically inside a square enclosure. Three different aspect ratios (R/L 0.1, 0.2, and 0.3), and four different Rayleigh numbers (Ra = 10, 10, 10, and 10), are considered. The governing elliptic conservation equations are solved in a boundary-fitted coordinate system using a control volumebased numerical procedure. Results are displayed in the form of streamlines, isotherms, maximum stream function estimates, and local and average normalized Nusselt number values. At constant enclosure aspect ratio, the total heat transfer increases with increasing Rayleigh number. For constant Rayleigh number values, convection contribution to the total heat transfer decreases with increasing values of R/L. For convection-dominated flows, the average Nusselt number correlation is expressed as Nu = 0.92Ra(R/ L)*. Generated results are in good agreement with previously published experimental and numerical data.

On the thermal evolution of the Earth's core

Abstract: The Earth's magnetic field is sustained by dynamo action in the fluid outer core. The energy sources available to the geodynamo are well established, but their relative importance remains uncertain. We focus on the issue of thermal versus compositional convection, which is inextricably coupled to the evolution of the core as the Earth cools. To investigate the effect of the various physical processes on this evolution, we develop models based on conservation of energy and the assumption that the core is well mixed by vigorous convection. We depart from previous numerical studies by developing an analytical model. The simple algebraic form of the solution affords insight into both the evolution of the core and the energy budget of the geodynamo. We also present a numerical model to compare with the quantitative predictions of our analytical model and find that the differences between the two are negligible. An important conclusion of this study is that thermal convection can contribute significantly to the geodynamo. In fact, a modest heat flux in excess of that conducted down the adiabatic gradient is sufficient to power the geodynamo, even in the absence of compositional convection and latent-heat release. The relative contributions of thermal and compositional convection to the dynamo are largely determined by the magnitude of the heat flux from the core and the inner-core radius. For a plausible current-day heat flux of Q = 3.0 × 1012 W and the current inner-core radius, we find that compositional convection is responsible for approximately two thirds of the ohmic dissipation in the core and thermal convection for the remaining one third. The proportion of ohmic dissipation produced by thermal convection increases to 45% with an increase in Q to 6.0 × 1012 W. In the early Earth, when the inner core was smaller and the heat flux probably greater than the present values, thermal convection would have been the dominant energy source for the dynamo. We also calculate the history of inner-core growth as a function of the heat flux. For example, the inner core would have grown to its present size in 2.8 × 109 years if the average heat flux was Q = 4.0 × 1012 W. The model does not require the heat flux to be constant.

Advances in Numerical Heat Transfer

Abstract: 1.High-Perfomance Computing for Fluid Flow and Heat Transfer 2.Unstructured Finite Volume Methods for Multi-Mode Heat Transfer 3.SpectralElement Methods for Unsteady Fluid Flow and Heat Transfer in Complex Geometries:Methodology and Applications 4.Finite-Volume Method for Radiation Heat Transfer 5.Boundary Element Methods for Heat Conduction 6.Molecular Dynamics Method for Microscale Heat Transfer 7.Numerical Methods in Microscale Heat Transfer:Modeling of Phase-Change and Laser Interactions with Materials 8.Current Status of the Use of Parallel Computing in Turbulent Reacting Flows:Computations Involving Sprays, Scalar Monte Carlo Probability Density Function and Unstructured Grids 9.Overview of Current Computational Studies of Heat Transfer in Porous Media and Their Applications-Forced Convection and Multiphase Heat Transfer 10.Overview of Current Computational Studies of Heat Transfer in Porous Media and Their Applications-Natural and Mixed Convection 11.Recent Progress and Some Challenges in Thermal Modeling of Electronic Systems 12.Index

Natural Convection as a Heat Engine: A Theory for CAPE

Abstract: On many planets there is a continuous heat supply to the surface and a continuous emission of infrared radiation to space by the atmosphere. Since the heat source is located at higher pressure than the heat sink, the system is capable of doing mechanical work. Atmospheric convection is a natural heat engine that might operate in this system. Based on the heat engine framework, a simple theory is presented for atmospheric convection that predicts the buoyancy, the vertical velocity, and the fractional area covered by either dry or moist convection in a state of statistical equilibrium. During one cycle of the convective heat engine, heat is taken from the surface layer (the hot source) and a portion of it is rejected to the free troposphere (the cold sink) from where it is radiated to space. The balance is transformed into mechanical work. The mechanical work is expended in the maintenance of the convective motions against mechanical dissipation. Ultimately, the energy dissipated by mechanical friction is transformed into heat. Then, a fraction of the dissipated energy is radiated to space while the remaining portion is recycled by the convecting air parcels. Increases in the fraction of energy dissipated at warmer temperature, at the expense of decreases in the fraction of energy dissipated at colder temperatures, lead to increases in the apparent efficiency of the convective heat engine. The volume integral of the work produced by the convective heat engine gives a measure of the statistical equilibrium amount of convective available potential energy (CAPE) that must be present in the planet's atmosphere so that the convective motions can be maintained against viscous dissipation. This integral is a fundamental global number qualifying the state of the planet in statistical equilibrium conditions. For the earth's present climate, the heat engine framework predicts a CAPE value of the order of 1000 J kg^−1 for the tropical atmosphere. This value is in agreement with observations. It also follows from our results that the total amount of CAPE present in a convecting atmosphere should increase with increases in the global surface temperature (or the atmosphere's opacity to infrared radiation).

Three-dimensional numerical study of shallow convective clouds and precipitation induced by land surface forcing

Abstract: A state-of-the-art mesoscale atmospheric model was used to investigate the three-dimensional structure and evolution of shallow convective clouds and precipitation in heterogeneous and homogeneous domains In general, the spatial distribution of clouds and precipitation is strongly affected by the landscape structure When the domain is homogeneous, they appear to be randomly distributed However, when the landscape structure triggers the formation of mesoscale circulations, they concentrate in the originally dry part of the domain, creating a negative feedback which tends to eliminate the landscape discontinuities, and spatially homogenize the land water content The land surface wetness heterogeneity of the domain and the toted amount of water vapor present in the atmosphere (locally evapotranspired and/or advected) affect the precipitation regime In general, the upward motion of mesoscale circulations generated by landscape heterogeneities is stronger than thermal cells induced by turbulence Furthermore, their ability to transport moist, warm air to higher elevations increases the amount of water that can be condensed and precipitated The evolution of shallow convective clouds and precipitation consists of a “build-up phase” during which turbulence is predominant and responsible for the moistening of the atmosphere In heterogeneous domains, it is also responsible for the creation of horizontal pressure gradients leading to the generation of mesoscale circulations This phase occurs during the morning hours From about 1200 until 1600 LST, clouds develop and most of the precipitation is produced This is the “active phase” After 1600 LST, the horizontal thermal and pressure gradients, which fed the energy necessary to create and sustain the mesoscale circulations, gradually disappear This is the “dissipation phase” The differences and similarities obtained between three-dimensional and two-dimensional simulations were also studied These simulations indicate that, unless the landscape presents a clear two-dimensional structure, the use of such a two-dimensional model is not appropriate to simulate this type of clouds and precipitation Conversely, two-dimensional simulations can be confidently used, provided that the simulated domain presents a two-dimensional heterogeneity

Thermo‐hydro‐mechanical analysis of partially saturated porous materials

Abstract: Presents a fully coupled numerical model to simulate the slow transient phenomena involving heat and mass transfer in deforming partially saturated porous materials. Makes use of the modified effective stress concept together with the capillary pressure relationship. Examines phase changes (evaporation‐condensation(, heat transfer through conduction and convection, as well as latent heat transfer. The governing equations in terms of gas pressure, capillary pressure, temperature and displacements are coupled non‐linear differential equations and are discretized by the finite element method in space and by finite differences in the time domain. The model is further validated with respect to a documented experiment on partially saturated soil behaviour, and the effects of two‐phase flow, as compared to the one‐phase flow solution, are analysed. Two other examples involving drying of a concrete wall and thermoelastic consolidation of partially saturated clay demonstrate the importance of proper physical modelling and of appropriate choice of the boundary conditions.

Thermodynamic Variability within the Convective Boundary Layer Due to Horizontal Convective Rolls

Abstract: Data from the Convection and Precipitation/Electrification (CaPE) Experiment conducted during the summer of 1991 are used to examine and quantify the horizontal variability of temperature and moisture within the convective boundary layer (CBL). Potential temperature variations were only about 0.5 K, while variations in water vapor mixing ratio values of 1.5–2.5 g kg−1 were observed throughout the CBL. Using radar, aircraft, and sounding data, it is shown that horizontal convective rolls are the likely cause of these variabilities. The enhanced moisture occurred within the roll updraft regions, thus rolls were transporting moist air from the surface upward. The observed cloud-base heights, obtained from cloud photogrammetry, were produced from the highest moisture values within the roll updraft regions. Since the roll ascending branches contained moisture values that were most representative of the observed cloud-base heights, it is likely that measurements from within the roll updrafts would prov...

Dynamics of isolated convective regions in the ocean

Abstract: An initially resting ocean of stratification N is considered, subject to buoyancy loss at its surface of magnitude B0 over a circular region of radius r, at a latitude where the Coriolis parameter is f. Initially the buoyancy loss gives rise to upright convection as an ensemble of plumes penetrates the stratified ocean creating a vertically mixed layer. However, as deepening proceeds, horizontal density gradients at the edge of the forcing region support a geostrophic rim current, which develops growing meanders through baroclinic instability. Eventually finite-amplitude baroclinic eddies sweep stratified water into the convective region at the surface and transport convected water outward and away below, setting up a steady state in which lateral buoyancy flux offsets buoyancy loss at the surface. In this final state quasi-horizontal baroclinic eddy transfer dominates upright “plume” convection. By using “parcel theory” to consider the energy transformations taking place, it is shown that the de...

Tomographic imaging of the sun's interior

Abstract: A new method is presented of determining the three-dimensional sound-speed structure and flow velocities in the solar convection zone by inversion of the acoustic travel-time data recently obtained by Duvall and coworkers. The initial inversion results reveal large-scale subsurface structures and flows related to the active regions, and are important for understanding the physics of solar activity and large-scale convection. The results provide evidence of a zonal structure below the surface in the low-latitude area of the magnetic activity. Strong converging downflows, up to 1.2 km s-1, and a substantial excess of the sound speed are found beneath growing active regions. In a decaying active region, there is evidence for the lower than average sound speed and for upwelling of plasma.

CISK or WISHE as the Mechanism for Tropical Cyclone Intensification

Abstract: Examination of conditional instability of the second kind (CISK) and wind-induced surface heat exchange (WISHE), two proposed mechanisms for tropical cyclone and polar low intensification, suggests that the sensitivity of the intensification rate of these disturbances to surface properties, such as surface friction and moisture supply, will be different for the two mechanisms. These sensitivities were examined by perturbing the surface characteristics in a numerical model with explicit convection. The intensification rate was found to have a strong positive dependence on the heat and moisture transfer coefficients, while remaining largely insensitive to the frictional drag coefficient. CISK does not predict the observed dependence of vortex intensification rate on the heat and moisture transfer coefficients, nor the insensitivity to the frictional drag coefficient since it anticipates that intensification rate is controlled by frictional convergence in the boundary layer. Since neither conditional instability nor boundary moisture content showed any significant sensitivity to the transfer coefficients, this is true of CISK using both the convective closures of Ooyama and of Charney and Eliassen. In comparison, the WISHE intensification mechanism does predict the observed increase in intensification rate with heat and moisture transfer coefficients, while not anticipating a direct influence from surface friction.