Showing papers in "Journal of the Atmospheric Sciences in 1996"
TL;DR: In this paper, an extension of classical stability theory to address the stability of perturbations to time-dependent systems is described, where nonnormality is found to play a central role in determining the stability.
Abstract: An extension of classical stability theory to address the stability of perturbations to time-dependent systems is described. Nonnormality is found to play a central role in determining the stability of systems governed by nonautonomous operators associated with time-dependent systems. This pivotal role of nonnormality provides a conceptual bridge by which the generalized stability theory developed for analysis of autonomous operators can be extended naturally to nonautonomous operators. It has been shown that nonnormality leads to transient growth in autonomous systems, and this result can be extended to show further that time-dependent nonnormality of nonautonomous operators is capable of sustaining this transient growth leading to asymptotic instability. This general destabilizing effect associated with the time dependence of the operator is explored by analysing parametric instability in periodic and aperiodic time-dependent operators. Simple dynamical systems are used as examples including th...
528 citations
TL;DR: In this article, a simple two-layer diagnostic balance model is used to provide an alternate explanation of the effect of vertical shear on tropical cyclone intensity change, where heat and moisture at upper levels are advected away from the low-level circulation, which inhibits development.
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...
524 citations
TL;DR: In this paper, it was shown that the primary variables governing a hydrometeor's terminal velocity were its mass, its area projected to the flow, and its maximum dimension.
Abstract: Based on boundary layer theory and a comparison of empirical power laws relating the Reynolds and Best numbers, it was apparent that the primary variables governing a hydrometeor's terminal velocity were its mass, its area projected to the flow, and its maximum dimension. The dependence of terminal velocities on surface roughness appeared secondary, with surface roughness apparently changing significantly only during phase changes (i.e., ice to liquid). In the theoretical analysis, a new, comprehensive expression for the drag force, which is valid for both inertial and viscous-dominated flow, was derived. A hydrometeor's mass and projected area were simply and accurately represented in terms of its maximum dimension by using dimensional power laws. Hydrometeor terminal velocities were calculated by using mass- and area-dimensional power laws to parameterize the Best number, X. Using a theoretical relationship general for all particle types, the Reynolds number, Re, was then calculated from the Be...
518 citations
TL;DR: In this paper, simulations of scattering and polarization properties for randomly oriented polyhedral ice crystals are presented based on the geometric optics and the far-field diffraction approximation, and the effects of particle size are studied by applying observationally derived aspect-ratio parameterizations to the individual particle types.
Abstract: Simulations of scattering and polarization properties for randomly oriented polyhedral ice crystals are presented based on the geometric optics and the far-field diffraction approximation. Particle shapes range from various hexagonal symmetric particles to highly complex shaped deterministic and random fractals. All calculations are performed at a wavelength of 0.55 µm. Hexagonal symmetric particles show several narrow scattering peaks besides the well known 22° and 46° halos. Column-like ice crystals provide neutral points (NP) at larger scattering angles than plate-like ice crystals. The ranges of NPs for column-like and plate-like crystals are separated at a scattering angle of about 156°, which may allow a polarimetric distinction between these two crystal types. The effects of particle size are studied by applying observationally derived aspect-ratio parameterizations to the individual particle types. Differences in the asymmetry parameter versus size relations for column-like particle types...
490 citations
TL;DR: In this article, the authors investigated the manner in which the cloud clusters were organized on time and space scales ranging from the seasonal mean pattern over the whole warm-pool region to the scale of individual cloud clusters and their relationship to the large-scale circulation and sea surface temperature (SST).
Abstract: Deep convection over the Indo-Pacific oceanic warm pool in the Tropical Ocean Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (TOGA COARE) occurred in cloud clusters, which grouped together in regions favoring their occurrence These large groups of cloud clusters produced large-scale regions of satellite-observed cold cloud-top temperature This paper investigates the manner in which the cloud clusters were organized on time and space scales ranging from the seasonal mean pattern over the whole warm-pool region to the scale of individual cloud clusters and their relationship to the large-scale circulation and sea surface temperature (SST) The dominant convective variability was associated with the intraseasonal oscillation (ISO) A large eastward propagating ensemble of cloud clusters marked the ISO's progress The meridional structure of the ISO was strongly affected by the seasonal cycle with a southward shift from the Northern Hemisphere in October-November to the Southern Hemisphere in January-February The SST had an intraseasonal signal in lagged quadrature with the cold cloudiness and rainfall in COARE The SST increased (decreased) during the convectively suppressed (active) phases of the ISO, Enhanced low-level westerly winds occurred toward the later stages of the enhanced-convection periods of the ISO, though not always centered at the equator The strongest westerlies tended to be located between two synoptic-scale cyclonic gyres, which were often not symmetric about the equator in the low-level wind field This asymmetry in the anomalous equatorial low-level westerlies may have different implications for the oceanic response in the coupled atmosphere-ocean system than those centered on the equator The cyclonic gyres contained highly concentrated deep convection, and, in four cases, the gyres developed into tropical cyclones Within the envelope marking the convectively active phase of the ISO, cloud clusters were frequently concentrated into westward-propagating disturbances with a local periodicity of similar to 2 days These 2-day disturbances have been identified in earlier spectral studies and appear to be related to westward propagating inertio-gravity waves In COARE, they typically contained numerous cloud clusters, which underwent a distinct diurnal cycle Most of the cloud clusters embedded in the 2-day disturbances moved westward, though some were stationary, and a few moved eastward A cloud-duster tracking program identified groups of clusters (lime dusters) that exhibited continuity in time and space In the most convectively active period of the ISO, the tracking program identified almost the entire ISO cloud ensemble as a long-lasting, trackable superconvective system This observation indicates the lack of a distinct scale-separation between convection and large-scale disturbances during the most intense convective periods in COARE
384 citations
TL;DR: In this paper, a semi-empirical cloudiness parameterization using the large-scale average condensate (cloud water and cloud ice) mixing ratio as the primary predictor was proposed.
Abstract: Data produced from explicit simulations of observed tropical cloud systems and subtropical stratocumuli are used to develop a “semiempirical” cloudiness parameterization for use in climate models. The semiempirical cloudiness parameterization uses the large-scale average condensate (cloud water and cloud ice) mixing ratio as the primary predictor. The large-scale relative humidity and cumulus mass flux are also used in the parameterization as secondary predictors. The cloud amount is assumed to vary exponentially with the large-scale average condensate mixing ratio. The rate of variation is, however, a function of large-scale relative humidity and the intensity of convective circulations. The validity of such EL semiempirical approach and its dependency on cloud regime and horizontal-averaging distance are explored with the simulated datasets.
328 citations
TL;DR: In this article, the authors investigated the mechanism by which a vertically sheared zonal flow affects large-scale, low-frequency equatorial waves with two-level equatorial, β-plane and spherical coordinates models.
Abstract: The mechanism by which a vertically sheared zonal flow affects large-scale, low-frequency equatorial waves is investigated with two-level equatorial,β-plane and spherical coordinates models. Vertical shears couple barolinic and barotropic components of equatorial wave motion, affecting significantly the Rossby wave and westward propagating Yanai wave but not the Kelvin wave. This difference results from the fact that the barotropic component is a modified Rossby mode and can be resonantly excited only by westward propagating internal waves. The barotropic components emanate poleward into the extratropics with a pronounced amplitude, while the baroclinic components remain equatorially trapped. A westerly vertical shear favors the trapping of Rossby and Yanai waves in the upper troposphere, whereas an easterly shear tends to confine them in the lower troposphere. As such, their westward propagation is slowed down by both westerly and easterly shears. When the strength of the vertical shear varies w...
324 citations
TL;DR: In this article, a comprehensive general circulation model of the Martian atmosphere is used to simulate the thermotidal oscillations in simulations performed with a newly developed comprehensive GCS model, which produces both realistic zonal-mean temperature distributions and a diurnal surface pressure oscillation of at least roughly realistic amplitude.
Abstract: This paper discusses the thermotidal oscillations in simulations performed with a newly developed comprehensive general circulation model of the Martian atmosphere. With reasonable assumptions about the effective thermal inertia of the planetary surface and about the distribution of radiatively active atmospheric aerosol, the model produces both realistic zonal-mean temperature distributions and a diurnal surface pressure oscillation of at least roughly realistic amplitude. With any reasonable aerosol distribution, the simulated diurnal pressure oscillation has a very strong zonal variation, in particular a very pronounced zonal wavenumber-2 modulation. This results from a combination of the prominent wave-2 component in the important boundary forcings (topography and surface thermal inertia) and from the fact that the eastward-propagating zonal wave-1 Kelvin normal mode has a period near 1 sol (a Martian mean solar day of 88 775 s). The importance of global resonance is explicitly demonstrated w...
303 citations
TL;DR: In this article, the area between the tracer contour and a reference contour is defined as the horizontal coordinate, where the area is defined by the square of the equivalent length of the contour, and k is the constant microscale diffusion coefficient.
Abstract: Two-dimensional mixing of a tracer is diagnosed using A, the area between the tracer contour and a reference contour, as the horizontal coordinate. In the absence of sources or sinks, the tracer distribution in the area coordinate is governed bywhereis the square of the equivalent length of the tracer contour and k is the constant microscale diffusion coefficient. While diffusion is necessary to invoke transport, the large-scale kinematics regulate the evolution of q through the stretching and redistribution of Le2. It is argued that Le2 is a useful, easy to compute diagnostic for irreversible transport, especially for identifying a barrier. Typical behaviors of Le2 in various flow regimes are illustrated using the numerically simulated Kelvin-Helmholtz billow. Signature of mixing is found in the irreversible growth of Le2, but the precise time dependence is complex due to interplay between advection and diffusion. Formation of the edges (concentrated gradients) and their permeability to mass are...
294 citations
TL;DR: In situ microphysical measurements of three anvils were made 17 March, 1 April, and 4 April 1993 during the Central Equatorial Pacific Experiment for several constant altitude penetrations, in the same direction or opposite the ambient wind, from 7 to 14 km as mentioned in this paper.
Abstract: In situ microphysical measurements of three anvils were made 17 March, 1 April, and 4 April 1993 during the Central Equatorial Pacific Experiment for several constant altitude penetrations, in the same direction or opposite the ambient wind, from 7 to 14 km. Ice water contents (IWCs), derived from an optical array probe, increased downward in the anvil; concentrations also increased, but not as quickly. Median mass dimensions, defined as the ice crystal maximum dimension below which half of the mass is contained, increased from averages of less than 0.1 mm near the tops to averages of about 0.5 mm near the bases. Substantial variations in cloud parameters occurred in the horizontal direction as well, with larger IWCs and concentrations closer to the convective core and its remnants. Small crystals were measured and categorized according to their shapes with the Video Ice Particle Sampler, which detects particles with dimensions greater than 5 µm. For small IWCs, especially prevalent at cloud tops...
283 citations
TL;DR: In this article, the authors used rawinsonde and satellite IR data from the Coupled Ocean Atmosphere Response Experiment (COARE) to investigate mean and transient behavior and horizontal variability of the atmosphere over the western Pacific warm pool.
Abstract: Rawinsonde and satellite infrared radiation (IR) data from the Tropical Ocean Global Atmosphere (TOGA) Coupled Ocean Atmosphere Response Experiment (COARE) are used to investigate mean and transient behavior and horizontal variability of the atmosphere over the western Pacific warm pool. Infrared data for the 4-mo Intensive Observing Period (IOP) and vertical motion fields indicate that the intensity of convection, height of maximum upward motion, and SST all increased from west (140°E) to east across the COARE domain. IOP-mean IR data show a double ITCZ (Intertropical Convergence Zone) structure north and south of the Intensive Flux Array (IFA, centered at 2°S, 156°E), although marked variability in the patterns occurred on a month to month basis. Three prominent westerly wind bursts occurred over the IFA during the 4-mo IOP in association with the intraseasonal oscillations (ISOs). Strong upward motion usually occurred 1–3 weeks prior to the peak low-level westerlies. Subsidence dominated when ...
TL;DR: In this article, the scaling argument for eddy amplitudes and fluxes in a horizontally homogeneous, two-layer model on an f plane is extended to a β plane.
Abstract: The scaling argument developed by the authors in a previous work for eddy amplitudes and fluxes in a horizontally homogeneous, two-layer model on an f plane is extended to a β plane. In terms of the nondimensional number ξ=U/(βλ2), where λ is the deformation radius and U is the mean thermal wind, the result for the rms eddy velocity V, the characteristic wavenumber of the energy-containing eddies and of the eddy-driven jets kj, and the magnitude of the eddy diffusivity for potential vorticity D, in the limit ξ ≫ 1, are as follows: V/U ≈ ξ, kjλ ≈ ξ−1, D/(Uλ) ≈ ξ2.Numerical simulations provide qualitative support for this scaling but suggest that it underestimates the sensitivity of these eddy statistics to the value of ξ. A generalization that is applicable to continuous stratification is suggested that leads to the estimates V ≈ (βT2)−1, kj ≈ βT, D ≈ (β2T3)−1,where T is a timescale determined by the environment; in particular, it equals λU−1 in the two-layer model and N(f∂zU)−1 in a continuous fl...
TL;DR: In this article, a formal framework is established for the way in which cloud-resolving numerical models are used to investigate the role of precipitating cloud systems in climate and weather forecasting models.
Abstract: A formal framework is established for the way in which cloud-resolving numerical models are used to investigate the role of precipitating cloud systems in climate and weather forecasting models. Emphasis is on models with periodic lateral boundary conditions that eliminate unrealistic numerically generated circulations caused by open boundary conditions in long-term simulations. Defined in this formalism is the concept of large-scale forcing and the cloud-environment interactions that are consistent with the periodic boundary conditions. Two-dimensional numerical simulations of the evolution of cloud systems during 1–7 September 1974 in Phase III of the Global Atmospheric Research Program Atlantic Tropical Experiment (GATE) are conducted. Based on the above formalism, a simple technique is used to force an anelastic cloud-resolving model with evolving large-scale horizontal wind field and large-scale forcing for the temperature and moisture obtained from the GATE data. The 7-day period selected i...
TL;DR: In this paper, a set of 500 simulated trajectories and a simple parcel model are used to evaluate the performance of a large eddy simulation model coupled with a detailed representation of the droplet spectrum.
Abstract: A set of 500 simulated trajectories and a simple parcel model are used to (i) evaluate the performance of a large eddy simulation model coupled to a detailed representation of the droplet spectrum (the LES-BM model) and (ii) gain insight into the microphysical structure of numerically simulated nonprecipitating stratocumulus. The LES-BM model reasonably reproduces many observed features of stratocumulus. The largest sources of error appear to be associated with limited vertical resolution, the neglect of gas kinetic effects and the inability of the model to properly represent mixing across cloud interfacial boundaries. The first two problems have simple remedies; for instance, a condensation–nucleation scheme is derived that includes gas–kinetic effects thus obviating the second problem. The third source of error poses a more vexing, and as yet unsolved, problem for models of the class described herein. Trajectories timescales are analyzed and in-cloud residence times are found to be, in the mean...
TL;DR: In this article, 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.
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).
TL;DR: In this paper, the relative importance of three specific longwave radiative mechanisms by comparing cloud-resolving models with and without one or more of these processes was determined by using a two-dimensional version of the Goddard Cumulus Ensemble model.
Abstract: Radiative forcing and latent heat associated with precipitation are the two most important diabatic processes that drive the circulation of the atmosphere. Clouds can affect radiation and vice versa. It is known that longwave radiative processes can enhance precipitation in cloud systems. This paper concentrates on determining the relative importance of three specific longwave radiative mechanisms by comparing cloud-resolving models with and without one or more of these processes. Three of the ways that longwave radiation is thought to interact with clouds are as follows: 1) cloud-top cooling and cloud-base warming may alter the thermal stratification of cloud layers, 2) differential cooling between clear and cloudy regions might enhance convergence into the cloud system, and 3) large-scale cooling could change the environment. A two-dimensional version of the Goddard Cumulus Ensemble model has been used to perform a series of sensitivity tests to identify which is the dominant cloud-radiative fo...
TL;DR: In this article, the motion and evolution of tropical cyclone-like vortices in an environmental flow with vertical shear were investigated using a baroclinic primitive equation model.
Abstract: The motion and the evolution of tropical cyclone-like vortices in an environmental flow with vertical shear are investigated using a baroclinic primitive equation model. The study focuses on the fundamental dynamics of a baroclinic vortex in vertical shear, the influence of vortex structure, and the role of diabatic heating. The results show that the initial response of the vortex to the vertical shear is to tilt downshear. As soon as the tilt develops, the upper-level anticyclonic and lower-level cyclonic circulations begin to interact with each other. As a result of these interactions, the tilted axis of the vortex reaches a stable state after an initial adjustment, which varies with the structure of the vortex, its environmental flow shear, and the cumulus convective heating. The motion of an adiabatic vortex in vertical shear is controlled by both the steering of the environmental flow and vertical coupling mechanisms. Most of the vortices move with the environmental flow at about 650 hPa or ...
TL;DR: In this paper, an atmospheric surface-layer experiment over a nearly uniform plowed field was performed to determine the constants in the flux-profile similarity formulas, particularly the von Karman constant.
Abstract: An atmospheric surface-layer experiment over a nearly uniform plowed field was performed to determine the constants in the flux-profile similarity formulas, particularly the von Karman constant. New instruments were constructed to minimize flow distortion effects on the turbulence measurements and to provide high-resolution gradient measurements. In addition, a hot-wire anemometer directly measured the turbulent kinetic energy dissipation rate. An average value of the von Karman constant of 0.365 ± 0.015 was obtained from 91 runs (31 h) in near-neutral stability conditions. However, four near-neutral runs when snow covered the ground gave an average value of 0.42. This result suggests that the von Karman constant depends on the roughness Reynolds number, which may resolve some of the differences in previous determinations over different surfaces. The one-dimensional Kolmogorov inertial subrange constant was found to have a value of 0.54 ± 0.03, slightly larger than previous results. The flux-prof...
TL;DR: In this paper, the authors examined the behavior of the atmosphere and the response of the ocean surface before, during, and after the passages of westerly wind bursts and found that the vertical wind shear associated with tropical lowfrequency oscillations strongly modulated convective beating and moistening.
Abstract: Rawinsonde, surface, and satellite data collected from the Tropical Ocean Global Atmosphere (TOGA) Coupled Ocean Atmosphere Response Experiment (COARE) are used to investigate the distributions of beating, moistening, precipitation, and evaporation over the western Pacific warm pool. The behavior of the atmosphere and the response of the ocean surface before, during, and after the passages of westerly wind bursts are examined. The tropospheric vertical wind shear associated with tropical low-frequency oscillations strongly modulated convective beating and moistening. Heavy precipitation usually fell 1 to 3 weeks prior to the peak westerly wind bursts. SSTs reached their maximum during the undisturbed phase of the intraseasonal oscillations (ISOs), gradually decreased as convection intensified, and reached their minimum during the periods of peak westerly winds when deep convection was generally suppressed over the intensive flux array (IFA). Surface latent heat fluxes were positively correlated w...
TL;DR: In this article, a hydrodynamic nonhydrostatic anelastic numerical model of an axisymmetric convective cloud is described in which the microphysical processes are treated in detail for different species of hydrometeors: drops.
Abstract: A hydrodynamic nonhydrostatic anelastic numerical model of an axisymmetric convective cloud is described in which the microphysical processes are treated in detail for different species of hydrometeors: drops. ice crystals, graupel, and snow particles. The size distribution function for each type of particle is divided into 34 spectral bins. In each spectral category two physical moments of the distribution function (number and mass concentrations are independently calculated using the method of moments. The following physical processes are computed: nucleation of drops and ice crystals, freezing of drops, diffusional growth/evaporation of drops and ice particles, collisional coalescence of drops and ice particles, binary breakup of drops, melting of ice particles, and sedimentation. The model describes the different stages of cloud development, the formation of ice, its growth by deposition and riming, the formation of graupel, and the precipitation stage. Analysis of the distribution functions ...
TL;DR: The Independent Pixel approximation (IPA) is one of the simplest methods of computing solar radiative fluxes for inhomogencous clouds as discussed by the authors, which can be used to compute mean cloud albedo.
Abstract: The Independent Pixel approximation (IPA) is one of the simplest methods of computing solar radiative fluxes for inhomogencous clouds. It claims that if p(τ) is a normalized probability density function for cloud optical depth τ and Rpp(τ) is plane-parallel, homogeneous (PPH) albedo, mean cloud albedo can be approximated by integrating p(τ)Rpp(τ) over all τ. The purpose of this study is to assess the ability of the gamma distribution function to represent p(τ) for marine boundary layer clouds and to examine the accuracy of the ensuing gamma IPA albedos. In a separate study, pixel values of τ were inferred from high spatial resolution Landsat imagery of marine boundary layer clouds. The present study utilizes 45 images, each measuring (58 km)2. For each image, a density function pobs(τ) is estimated, and, using the mean τ¯ and variance of τ, a corresponding truncated gamma distribution function pγ(τ) is defined. For a diverse range of clouds, pγ(τ) usually approximate pobs(τ) well. The best result...
TL;DR: In this paper, the TOGA COARE field program was examined for humidity drops (layers in which relative humidity decreases rapidly with height), indicative of the bases of dry layers.
Abstract: Dry layers are frequently observed in atmospheric soundings from the climatologically humid western Pacific warm pool region. Some 2400 soundings from the TOGA COARE field program were objectively examined for humidity drops (layers in which relative humidity decreases rapidly with height), indicative of the bases of dry layers. These occur throughout the lower and middle troposphere, with frequency peaks near 550 (the 0°C level), 800, and 950 mb. A composite constructed from these sounding data indicates the following. Almost all dry layers are too dry (and not warm enough) to be interpreted as conservative vertical displacements. Rather, they apparently consist of filaments or tongues of low moist static energy air advected into the column, often from the subtropics. Dry tongues are anomalously virtually warm near their bases with a slight cool layer below; that is, they sit atop sharp stable layers or inversions. The authors hypothesize that radiation is responsible for the thermal structure o...
TL;DR: In this paper, the internal structure of marine stratocumulus (Sc) using the spatial fluctuations of liquid water content (LWC) measured along horizontal flights off the coast of southern California during the First ISCCP Regional Experiment (FIRE) in summer of 1987.
Abstract: This study investigates the internal structure of marine stratocumulus (Sc) using the spatial fluctuations of liquid water content (LWC) measured along horizontal flights off the coast of southern California during the First ISCCP Regional Experiment (FIRE) in summer of 1987. The results of FIRE 87 data analyses are compared to similar ones for marine Sc probed during the Atlantic Stratocumulus Transition Experiment (ASTEX) in summer 1992 near the Azores. In this first of two parts, the authors use spectral analysis to determine the main scale-invariant regimes, defined by the ranges of scales where wavenumber spectra follow power laws; from there, they discuss stationary issues. Although crucial for obtaining meaningful spatial statistics (e.g., in climate diagnostics), the importance of establishing stationarity—statistical invariance under translation—is often overlooked. The sequel uses multifractal analysis techniques and addresses intermittency issues. By improving our understanding of both...
TL;DR: In this article, a detailed structure of the global quasi-biennial oscillation (QBO) in ozone was analyzed using Stratospheric Aerosol and Gas Experiment II ozone and nitrogen dioxide data.
Abstract: Detailed structure of the global quasi-biennial oscillation (QBO) in ozone is analyzed using Stratospheric Aerosol and Gas Experiment II ozone and nitrogen dioxide data. Emphasis is placed on the midlatitude QBO, in particular its vertical structure and seasonal synchronization. The global QBO signal is isolated using a combination of singular-value decomposition and regression analyses, which combine to act as an accurate QBO digital filter. Results show that the midlatitude ozone QBO has a two-cell structure in the vertical (similar to that at the equator), with in-phase maxima in the lower and middle stratosphere. Both upper- and lower-level anomalies contribute important fractions to the midlatitude column amounts. The lower-level maxima have a broad latitudinal structure (˜15°–60°), and collocation with the strongest background gradients suggests that these anomalies result from mean vertical transport. The, middle stratosphere signal maximizes in the subtropics (10°–40°) and is likely due t...
TL;DR: In this article, the distribution of cloud microphysical properties measured from the University of Washington's aircraft in mostly unbroken stratocumulus (Sc) clouds in the vicinity of the Azores Islands during ASTEX was investigated.
Abstract: This observational study looks at the distribution of some cloud microphysical properties measured from the University of Washington's aircraft in mostly unbroken stratocumulus (Sc) clouds in the vicinity of the Azores Islands during ASTEX. The average behavior of the Sc includes the presence of two drizzle modes. In 85% of the Sc the amount of drizzle LWC (liquid water content in droplets with radius r > 20 µm) is less than 0.01 g m−3, while in the rest, drizzle LWC is much larger than 0.01 g m−3. The microphysics of light-drizzle Sc approach classical conditions, because measured and adiabatic LWC profiles are similar, and droplet spectral dispersions decrease with height. The lognormal function fits approximately measured droplet spectra of light-drizzle Sc. Standard deviations of these spectra remain about constant with height, vary over a small range, and average 1.74-µm radius. Drizzle LWC > 0.01 g m−3 is found on the average in Sc with spectra that have effective radii re &ge 16 µm. This h...
TL;DR: In this paper, the initial evolution of optical depth and ice water content under threshold contrail formation conditions is studied, and a lower bound is deduced for the number density of ice crystals initially present in contrails.
Abstract: Physicochemical processes that generate and transform aerosols in jet aircraft plumes are discussed on the basis of theoretical models and recent observations of young contrails in the upper troposphere. The initial evolution of optical depth and ice water content under threshold contrail formation conditions is studied. Constrained by the measurements, a lower bound is deduced for the number density of ice crystals initially present in contrails. This bound serves as a visibility criterion for young contrails. An analysis of the primary contrail particles (aqueous solution droplets nucleated in situ, emitted insoluble combustion aerosols, and entrained background aerosols) reveals that only soot must he involved as ice forming nuclei if the visibility criterion is to be fulfilled. Possible activation pathways of the soot aerosols are investigated, including an analysis of their wetting behavior and droplet scavenging and heterogeneous nucleation properties. To support these investigations, resul...
TL;DR: In this paper, a new radiation scheme for two-stream radiation transfer models was developed for cirrus clouds and analytical expressions were derived for the extinction and absorption coefficients and the asymmetry parameter.
Abstract: A new radiation scheme, suitable for two-stream radiation transfer models, was developed for cirrus clouds. Analytical expressions were derived for the extinction and absorption coefficients and the asymmetry parameter. These are functions of the ice particle size distribution parameters, ice particle shapes, and wavelength. The ice particle shapes considered were hexagonal plates and columns, bullet rosettes, and planar polycrystals. These appear to be the principal crystal types found in cirrus clouds. The formulation of radiative properties accounts for the size distribution projected area and the distance radiation travels through ice particles. For absorption, refraction and internal reflection of radiation were parameterized. By assuming an idealized cirrus cloud, the dependence of the single scatter albedo, reflectance, and emissivity on wavelength, ice particle shape, and size distribution was demonstrated. Reflectance and emissivity exhibited a strong dependence on ice particle shape, wi...
TL;DR: In this paper, the vertical motions in seven Atlantic hurricanes are determined from data recorded by Doppler radars on research aircraft, which consists of velocities and reflectivities from vertically pointing radar rays collected along radial flight legs through the hurricane centers.
Abstract: Vertical motions in seven Atlantic hurricanes are determined from data recorded by Doppler radars on research aircraft. The database consists of Doppler velocities and reflectivities from vertically pointing radar rays collected along radial flight legs through the hurricane centers. The vertical motions are estimated throughout the depth of the troposphere from the Doppler velocities and bulk estimates of particle fallspeeds. Portions of the flight tracks are subjectively divided into eyewall, rainband, stratiform, and “other” regions. Characteristics of the vertical velocity and radar structure are described as a function of altitude for the entire dataset and each of the four regions. In all of the regions, more than 70% of the vertical velocities range from −2 to 2 m s−1. The broadest distribution of vertical motion is in the eyewall region where ∼5% of the vertical motions are >5 m s−1. Averaged over the entire dataset, the mean vertical velocity is upward at all altitudes. Mean downward mot...
TL;DR: In this article, relatively reflective ice crystals with dimensions smaller than about 100 µm near the tops of tropical cirrus clouds, produced by deep convection when the sea surface temperature exceeds 300 K, are principally responsible for the high albedos observed in this region.
Abstract: Recent studies suggest that extensive shields of cirrus clouds over the equatorial Pacific “warm pool” may have a significant influence on the global climate, yet details of the links between cloud microphysical properties, upper-tropospheric latent and radiative heating rates, and climate are poorly understood. This study addresses whether relatively reflective ice crystals with dimensions smaller than about 100 µm near the tops of tropical cirrus clouds, produced by deep convection when the sea surface temperature exceeds 300 K, are principally responsible for the high albedos observed in this region. In situ measurements of ice crystal size distributions and shapes, acquired during the Central Equatorial Pacific Experiment (CEPEX), are used to derive cloud ice water content (IWC), particle cross-sectional area (A), and other microphysical and optical properties from particles with sizes down to 5 µm. These measurements are needed to ascertain the microphysical properties primarily responsible ...
TL;DR: In this article, the equations of motion for a compressible atmosphere under the influence of gravity are reexamined to determine the necessary conditions for which the anelastic approximation holds.
Abstract: The equations of motion for a compressible atmosphere under the influence of gravity are reexamined to determine the necessary conditions for which the anelastic approximation holds. These conditions are that (i) the buoyancy force has an O (1) effect in the vertical momentum equation, (ii) the characteristic Vertical displacement of an air parcel is comparable to the density scale height, and (iii) the horizontal variations of the thermodynamic state variables at any height are small compared to the static reference value at that height. It is shown that, as a consequence of these assumptions, two additional conditions hold for adiabatic flow. These ancillary conditions are that (iv) the spatial variation of the base-state entropy is small, and (v) the Lagrangian time scale of the motions must be lager than the inverse of the buoyancy frequency of the base state. It is argued that condition (iii) is more fundamental than (iv) and that a flow can be anelastic even if condition (iv) is violated pr...