Radiative forcing by contrails

A parametric study of the instantaneous radiative impact of contrails is presented using three different radiative transfer models for a series of model atmospheres and cloud parameters. Contrails are treated as geometrically and optically thin plane parallel homogeneous cirrus layers in a static atmosphere. The ice water content is varied as a function of ambient temperature. The model atmospheres include tropical, mid-latitude, and subarctic summer and winter atmospheres. Optically thin contrails cause a positive net forcing at top of the atmosphere. At the surface the radiative forcing is negative during daytime. The forcing increases with the optical depth and the amount of contrail cover. At the top of the atmosphere, a mean contrail cover of 0.1% with average optical depth of 0.2 to 0.5 causes about 0.01 to 0.03 Wm⁻² daily mean instantaneous radiative forcing. Contrails cool the surface during the day and heat the surface during the night, and hence reduce the daily temperature amplitude. The net effect depends strongly on the daily variation of contrail cloud cover. The indirect radiative forcing due to particle changes in natural cirrus clouds may be of the same magnitude as the direct one due to additional cover.     

Structure and Dynamical Influence of Water Vapor in the Lower Tropical Troposphere

In situ, airborne and satellite measurements are used to characterize the structure of water vapor in the lower tropical troposphere—below the height, \(z_*,\) of the triple-point isotherm, \(T_*.\) The measurements are evaluated in light of understanding of how lower-tropospheric water vapor influences clouds, convection and circulation, through both radiative and thermodynamic effects. Lower-tropospheric water vapor, which concentrates in the first few kilometers above the boundary layer, controls the radiative cooling profile of the boundary layer and lower troposphere. Elevated moist layers originating from a preferred level of convective detrainment induce a profile of radiative cooling that drives circulations which reinforce such features. A theory for this preferred level of cumulus termination is advanced, whereby the difference between \(T_*\) and the temperature at which primary ice forms gives a ‘first-mover advantage’ to glaciating cumulus convection, thereby concentrating the regions of the deepest convection and leading to more clouds and moisture near the triple point. A preferred level of convective detrainment near \(T_*\) implies relative humidity reversals below \(z*\) which are difficult to identify using retrievals from satellite-borne microwave and infrared sounders. Isotopologues retrievals provide a hint of such features and their ability to constrain the structure of the vertical humidity profile merits further study. Nonetheless, it will likely remain challenging to resolve dynamically important aspects of the vertical structure of water vapor from space using only passive sensors.     

Dielectrophoretic,thermal instability in a spherical shell of fluid

Abstract

This paper is concerned with the dielectrophoretic instability of a spherical shell of fluid. A dielectric fluid, contained in a spherical shell, with rigid boundaries is subjected to a simultaneous radial temperature gradient and radial a.c. electric field. Through the dependence of the dielectric constant on temperature, the fluid experiences a body force somewhat analogous to that of gravity acting on a fluid with density variations. Linear perturbation theory and the assumption of exchange of stabilities lead to an eighth order differential equation in radial dependence of the perturbation temperature. The solution to this equation, satisfying appropriate boundary conditions, yields a critical value of the electrical Rayleigh number and corresponding critical wave number at which convective motion begins. The dependence of each critical number is presented as a function of the gap size and temperature gradient. In the limit of zero shell thickness both the critical Rayleigh number and critical wave number agree with results for the case in the infinite plane problem.     

Numerical simulations of thermal convection in a rapidly rotating spherical shell cooled inhomogeneously from above

Abstract

Numerical simulations of thermal convection in a rapidly rotating spherical fluid shell with and without inhomogeneous temperature anomalies on the top boundary have been carried out using a three-dimensional, time-dependent, spectral-transform code. The spherical shell of Boussinesq fluid has inner and outer radii the same as those of the Earth's liquid outer core. The Taylor number is 10⁷, the Prandtl number is 1, and the Rayleigh number R is 5R_c (R_c is the critical value of R for the onset of convection when the top boundary is isothermal and R is based on the spherically averaged temperature difference across the shell). The shell is heated from below and cooled from above; there is no internal heating. The lower boundary of the shell is isothermal and both boundaries are rigid and impermeable. Three cases are considered. In one, the upper boundary is isothermal while in the others, temperature anomalies with (l,m) = (3,2) and (6,4) are imposed on the top boundary. The spherically averaged temperature difference across the shell is the same in all three cases. The amplitudes of the imposed temperature anomalies are equal to one-half of the spherically averaged temperature difference across the shell. Convective structures are strongly controlled by both rotation and the imposed temperature anomalies suggesting that thermal inhomogeneities imposed by the mantle on the core have a significant influence on the motions inside the core. The imposed temperature anomaly locks the thermal perturbation structure in the outer part of the spherical shell onto the upper boundary and significantly modifies the velocity structure in the same region. However, the radial velocity structure in the outer part of the shell is different from the temperature perturbation structure. The influence of the imposed temperature anomaly decreases with depth in the shell. Thermal structure and velocity structure are similar and convective rolls are more columnar in the inner part of the shell where the effects of rotation are most dominant.     

The effect of radiative heat loss on steady cellular convection

Summary In the atmosphere there may be layers undergoing cellular convection with a much larger heat flux through the base of the layer than through the top. This may be either because there is a steady loss of heat by radiation from the body of the fluid or because the temperature is everywhere rising. In this latter case the temperature gradients could remain constant so that the mechanics would be the same as if the heat were being lost and the temperature kept steady. The fluid is considered incompressible as in the classical theory of cellular convection, and we determine the critical Rayleigh number for the onset of convection and the width to height ratio of the cells as functions of the heat loss. The problem, is in some respects analogous to that of the motion of a viscous fluid between rotating cylinders but in this case there are two non-dimensional-numbers-the Rayleigh number (g h ⁴/K v) and a number representing the ratio of the heat loss by radiation to the heat flux. It is found that the critical Rayleigh number is decreased and the cells widened as had already been found for the case of a fluid with transfer coefficients having a spatial variation, with free boundaries, but the cells are made more narrow if the boundaries are rigid.     

The Role of Radiative Transfer in Maintaining the Indian Summer Monsoon Circulation

—The radiative-convective feedback and land-sea thermal forcing play significant roles in maintenance of the summer monsoon circulation over the Indian sub-continent. In this study, the role of radiative transfer in maintaining the monsoon circulation is examined with numerical sensitivity experiments. For this purpose, a sixteen layer primitive equation limited area model is used to perform numerical simulations with and without atmospheric radiative transfer processes parameterized in the model. The initial values and boundary conditions for the numerical integrations of the model are derived from operational analyses of the ECMWF, UK. The results show that the radiative transfer is essential in maintaining the intensity of the low level Somali Jet as well as the upper level Tropical Easterly Jet (TEJ) over the Indian sub-continent and adjoining seas. The meridional circulation over the region is also well simulated. As a result, enough moisture transports from the warm equatorial region to simulate more realistic orographic precipitation in the windward side of the mountains along the West coast of India. Without radiative transfer processes in the model atmosphere the simulated monsoon circulation weakens, moisture transport decreases and the precipitation lessens.     

Intermittent convection

Abstract

A theoretical analysis of pseudo two-dimensional, finite-amplitude, thermal convection is made for an infinite Prandtl number fluid which is subjected to a constant heat flux out of the top boundary and insulated at the bottom. For large Rayleigh numbers the convective flow becomes intermittent and the system is characterized by the following cyclic process: the formation of a thermal boundary layer by diffusion, the instability of this layer when it becomes sufficiently thick, the destruction of the layer by the convective flow, the dying down of the convection, and the reforming of the thermal boundary layer by diffusion. The periodicity and the horizontal wave number of the intermittent convective flow are found to be independent of the depth of the fluid layer but depend on the rate of cooling and the properties of the fluid.     

A simple theory for the vertical structure of the tropical atmosphere

It is suggested that the gross mean vertical structure of the undisturbed tropical atmosphere may be understood in terms of convective boundary layers driven in different ways and on different time scales by the evaporation of water from the sea surface. The mixed layer on a short time scale is driven partly by the buoyancy produced by the light weight of the water vapor; the trade cumulus layer on an intermediate time scale by the buoyancy (but not heating) produced by the condensation of the water vapor in shallow trade cumulus clouds; and the troposphere itself on a long time scale by the buoyancyand heating produced by the condensation of the water vapor in the deep cumulonimbus clouds.May 1985This paper was issued as a Harvard University report in 1974. For this version only Section 5 has been rewritten. There has been sufficient interest in this work over the years to warrant making it more widely available through the open literature.Contribution No. 783 from NOAA/Pacific Marine Environmental Laboratory     

The rapid dissipation of magnetic fields in highly conducting fluids

Abstract

This paper treats the dynamical conditions that obtain when long straight parallel twisted flux tubes in a highly conducting fluid are packed together in a broad array. It is shown that there is generally no hydrostatic equilibrium. In place of equilibrium there is a dynamical nonequilibrium, leading to neutral point reconnection and progressive coalescence of neighboring tubes (with the same sense of twisting), forming tubes of larger diameter and reduced twist. The magnetic energy in the twisting of each tube declines toward zero, dissipated into small-scale motions of the fluid and thence into heat.

The physical implications are numerous. For instance, it has been suggested that the subsurface magnetic field of the sun is composed of close-packed twisted flux tubes. Any such structures are short lived, at best.

The footpoints of the filamentary magnetic fields above bipolar magnetic regions on the sun are continually shuffled and rotated by the convection, so that the fields are composed of twisted rubes. The twisting and mutual wrapping is converted directly into fluid motion and heat by the dynamical nonequilibrium, so that the work done by the convection of the footpoints goes directly into heating the corona above. This theoretical result is the final step, then, in understanding the assertion by Rosner, Tucker, and Valana, and others, that the observed structure of the visible corona implies that it is heated principally by direct dissipation of the supporting magnetic field. It is the dynamical nonequilibrium that causes the dissipation, in spite of the high electrical conductivity. It would appear that any bipolar magnetic field extending upward from a dense convective layer into a tenuous atmosphere automatically produces heating, and a corona of some sort, in the sun or any other convective star.     

Internal waves in a fluid of finite Prandtl number

Abstract

It is shown, through calculation and physical arguments, that in a smoothly stratified fluid thermal diffusion plays a damping role comparable to that of viscosity only in the interior of the fluid, and not in the boundary layers.     

Numerical analysis for cooling effect of open boundary ripped-rock embankment on Qinghai-Tibetan railway

At present, the Qinghai-Tibetan railway is being built, and it will pass across more than 550-km perma-frost regions. Therefore, the key to the stability of therailway embankment lies in solving the permafrost problem. Because global warming and existence of railway tend to degrade the permafrost in these re-gions[1], more difficulties and problems are induced in the construction and maintenance of railway. In the area where the mean annual air temperature is higher than a certain value, the …     

A model of thermal convection in the major planets with a strongly density-stratified atmospheric layer

Abstract

As an extension of a model by Busse (1983a), a two-layer model of thermal convection in the self-gravitating rotating spherical fluid is considered. The upper layer with arbitrary vertical distributions of density and potential temperature representing the atmospheric layer of major planets is imposed on the spherical Boussinesq fluid. The Prandtl number P and the ratio of the mass of the upper layer to that of the lower layer are used as small expansion parameters. The modification of the critical Rayleigh number by imposing the upper layer are clearly separated into two parts, proportional to (1) the mass of the upper layer and to (2) an integral representing a measure of convective instability of the upper layer. Some implications for atmospheric dynamics of the major planets are also presented.     

云覆盖对流边界层顶部湍流结构参数的研究

应用飞机探测资料分析研究云覆盖对流边界层顶部温度和湿度湍流结构,在考虑对流边界层顶部夹卷过程的基础上得到计算温度和湿度结构参数的公式。应用实际观测资料分析了云覆盖对流边界层顶部的湍流特征．资料分析表明,云外晴空区温度和湿度结构函数值明显高于云内的值．云顶边界清晰,通过界面温度和湿度具有明显的跃变特征．应用观测资料检验了温度和湿度结构参数计算公式,计算结果与观测结果符合较好．     

Inertial theory of the mean Hadley circulation under the condition of nonstatic equilibrium

1 Introduction The mean meridional circulation plays an important role in the transportation and balance of heat, momen- tum, vortex and vapor between different latitudes. According to the data analysis, there are three circula- tions from the equator to the polar area: the Hadley circulation, a heat-driven circulation which rises around the equator and sinks at a certain latitude; the subpolar circulation, another heat-driven circulation around the polar area; and the Ferrel circulation, an i…     

The flux ejection dynamo effect

Abstract

The mean-field effects of cyclonic convection become increasingly complex when the cyclonic rotation exceeds ½-π. Net helicity is not required, with negative turbulent diffusion, for instance, appearing in mirror symmetric turbulence. This paper points out a new dynamo effect arising in convective cells with strong asymmetry in the rotation of updrafts as against downdrafts. The creation of new magnetic flux arises from the ejection of reserve flux through the open boundary of the dynamo region. It is unlike the familiar α-effect in that individual components of the field may be amplified independently. Several formal examples are provided to illustrate the effect. Occurrence in nature depends upon the existence of fluid rotations of the order of π in the convective updrafts. The flux ejection dynamo may possibly contribute to the generation of field in the convective core of Earth and in the convective zone of the sun and other stars.     

Observation of Double-mixing Line Structure in the Convective Boundary Layer During the Summer Monsoon Season

v--vThe special aerological observations carried out as a part of Land Surface Processes Experiment (LASPEX) were used to investigate the thermodynamic structure of the convective boundary layer during the summer monsoon of 1997. The analysis suggested that the convective boundary layer top was found at 700 hPa which was associated with Š_e minimum and Š_es maximum values. Double-mixing line structure was noticed in the conserved variable diagrams which was possibly attributed to the radiative warming/evaporation of falling precipitation.     

A simple model of convection in the terrestrial planets

Abstract

In this paper we study analytically the simplest fluid mechanical model which can mimic the convective behavior which is thought to occur in the solid mantles of the terrestrial planets. The convecting materials are polycrystalline rocks, whose creep behavior depends very strongly on temperature and probably also on pressure. As a simple model of this situation, we consider the flow of a Newtonian viscous fluid, whose viscosity depends strongly on temperature (only), and in fact has an infinite viscosity below a certain temperature, and a constant viscosity above this temperature. This model would also be directly relevant to the convection of a melt beneath its own solid phase (e.g. water below ice, though in that case there are other physical complications).

As a consequence of this assumption, there is a vigorous convection zone overlain by a stagnant lid, as also observed in analogous laboratory experiments (Nataf and Richter, 1982). The analysis is then very similar to that of Roberts (1979), but the extension to variable viscosity introduces important differences, most notably that the boundary between the lid and the convecting zone is unknown, and not horizontal. The resulting buoyancy induced stresses near this boundary are much larger than the stresses produced by buoyancy in the side-wall plumes, and mean that the dynamics of this region, and hence also the heat flux, are independent of the rest of the cell. We give a first order approximation for the Nusselt number-Rayleigh number relationship.     

Structure of the inner core boundary

Abstract

A model of the inner-core boundary (ICB) is constructed which is consistent with current ideas of the dynamic and thermodynamic state of the core and which is capable of reflecting seismic waves with period of one second. This requires the mass fraction of solid below the ICB to grow to an appreciable fraction in roughly one kilometer. This rapid growth of solid with depth is a result of downward fluid flow from the outer core which is a part of the convective motions which sustain the geodynamo. The solid which crystallizes from this descending fluid after it crosses the ICB continually coats the dendrites which occur there. The gradual cooling of the outer core causes the ICB to advance by growth of dendrites at their tips. The balance of these two effects gives an equilibrium profile for the mass fraction of solid with depth below the ICB which is capable of yielding sharp reflection of seismic waves.     

Flows within the Ekman layer during spin-up of a thermally stratified fluid

Abstract

Finite-difference numerical solutions were obtained to present the flow and temperature field details within the transient Ekman layer during spin-up of a thermally stratified fluid in a cylinder. This complements the earlier studies on stratified spin-up which examined the flows in the interior core region. As the stratification increases, the following changes in the flow field are noticeable. The radial velocity in the Ekman layer decreases in magnitude. The azimuthal flows adjust smoothly from the interior region to the endwall boundary, and the Ekman layer in the azimuthal flow field fades. Vertical motions are inhibited, resulting in a weakened Ekman pumping. The axial vorticity field behaves similarly to the azimuthal flows. The temperature deviation from the equilibrium profile decreases, and the heat transfer flux from the endwall to the fluid decreases. The thickness of the thermal layer is larger than the velocity layer thickness. Illustrative comparisons of the relative sizes of the terms in the governing equations are conducted in order to assess the stratification effect in the adjustment process of the fluid.