Showing papers in "Journal of the Atmospheric Sciences in 1992"

On the correlated k-distribution method for radiative transfer in nonhomogeneous atmospheres

Abstract: The paper discusses the correlated k(absorption coefficient)-distribution (CKD) method for radiative transfer in nonhomogeneous atmospheres, in terms of the physical and mathematical conditions under which this method is valid. The CKD approach is theoretically formulated, and the mathematical and physical conditions under which this method is valid are identified. The parameterization for incorporating pressure and temperature effects in the k distribution is presented. The errors of CKD for absorption bands in the solar and thermal infrared spectra are examined, and the errors due to both the CKD method and the use of the optimum number of quadrature points are identified. Finally, an overall treatment for various absorption bands is developed, which will provide high efficiency and accuracy.

Midwinter Suppression of Baroclinic Wave Activity in the Pacific

Abstract: Seasonal variations in baroclinic wave activity and jet stream structure in the Northern Hemisphere are investigated based upon over 20 years of daily data. Baroclinic wave activity at each grid point is represented for each day by an envelope function, the lowpass-filtered time series of the squared highpass-filtered geopotential height. Baroclinic wave activity over the Atlantic exhibits a single maximum in January, whereas in the Pacific it exhibits peaks in late autumn and in early spring and a significant weakening in midwinter, which is more evident at the tropopause level than near the surface. This suppression occurs despite the fact that the low-level baroclinity and the intensity of the jet stream are strongest in midwinter.Based on the analysis of 31-day running mean fields for individual winters, it is shown that over both the oceans baroclinic wave activity is positively correlated with the strength of the upper-tropospheric jet for wind speeds up to ∼45 m s−1. When the strength of t...

A radar and electrical study of tropical hot towers

Abstract: Radar and electrical measurements for deep tropical convection are examined for both “break period” and “monsoonal” regimes in the vicinity of Darwin, Australia. Break period convection consists primarily of deep continental convection, whereas oceanic-based convection dominates during monsoonal periods, associated with the monsoon trough over Darwin. Order-of-magnitude enhancements in lightning flash rates for the “break period” regime are associated with 10–20-dB enhancements in radar reflectivity in the mixed-phase region of the convection compared with the monsoonal regime. The latter differences are attributed to the effect of convective available potential energy (CAPE) and its nonlinear influence on the growth and accumulation of ice particles aloft, which are believed to promote charge separation by differential particle motions. CAPE, in turn, is largely determined by the boundary-layer wet-bulb temperature. Modest differences (1°–3°C) in wet-bulb potential temperature between land and s...

Numerical simulations of convectively generated stratospheric gravity waves

Abstract: A 2D model of a mesoscale convective flow is used to simulate the excitation and vertical propagation of gravity waves. Data obtained show that, in the absence of storm-relative mean winds in the stratosphere, the primary mode of excitation of gravity waves is by mechanical forcing owing to oscillatory updrafts. The stratospheric response consists of waves whose periods match the primary periods of the forcing. Due to the tendency of the oscillating updrafts to propagate toward the rear of the storm, gravity wave propagation is limited primarily to the rearward direction. Results suggest that squall-line-generated gravity waves arise from mechanical forcing rather than thermal effects.

Dual-Aircraft Investigation of the Inner Core of Hurricane Norbert. Part I: Kinematic Structure

Abstract: On 24–25 September 1984 air motions and physical processes in the eyewall region of Hurricane Norbert were investigated with two National Oceanic and Atmospheric Administration (NOAA) Aircraft Operations Center (AOC) WP-3D research aircraft. One aircraft, equipped with the airborne Doppler radar, flew repeated radial penetrations of the eyewall, mapping the three-dimensional wind field throughout a 40-km radius of the storm center. These data provide the first complete documentation of the three-dimensional wind field of the inner core of a hurricane on spatial scales of 10–25 km in the horizontal and 1 km in the vertical and temporal scales of 1–2 h. The wind field was asymmetric, and the asymmetry varied with altitude. As altitude increased the location of the maximum tangential wind sloped upwind, from left of the storm track at 1-km altitude to right of the storm track above 3 km. The radial wind at 1-km altitude was inward in front of the storm and outward behind. This pattern in the radial ...

A Solar Radiation Model for Use in Climate Studies

Abstract: A solar radiation routine is developed for use in climate studies that includes absorption and scattering due to ozone, water vapor, oxygen, carbon dioxide, clouds, and aerosols. Rayleigh scattering is also included. Broadband parameterization is used to compute the absorption by water vapor in a clear atmosphere, and the k-distribution method is applied to compute fluxes in a scattering atmosphere. The reflectivity and transmissivity of a scattering layer are computed analytically using the delta-four-stream discrete-ordinate approximation. The two-stream adding method is then applied to compute fluxes for a composite of clear and scattering layers. Compared to the results of high spectral resolution and detailed multiple-scattering calculations, fluxes and heating rate are accurately computed to within a few percent. The high accuracy of the flux and heating-rate calculations is achieved with a reasonable amount of computing time. With the UV and visible region grouped into four bands, this solar radiation routine is useful not only for climate studies but also for studies on photolysis in the upper atmosphere and photosynthesis in the biosphere.

Sources of Mesoscale Variability of Gravity Waves. Part II: Frontal, Convective, and Jet Stream Excitation

Abstract: We present studies of four cases of mesoscale variance enhancements of horizontal velocity and temperature due to frontal activity, nonfrontal convection, and wind shear. These data were obtained aboard commercial aircraft during the Global Atmospheric Sampling Program (GASP) in 1978 and 1979 and from the corresponding meteorological analyses and satellite imagery. Additional GASP data were used to permit a statistical assessment of the importance of various sources of enhanced variances. Our results, and those in a companion paper addressing the variance enhancements associated with topography, represent refinements of previous source analyses using the GASP dataset. Significant findings include mean variance enhancements of velocity and temperature due to convection and jet-stream flow ranging from ∼2 to 8 for 64-km and 256-km data segments, and enhancements for individual segments as high as ∼20 to 100. The mean 64-km variance enhancement for all variables and source types, relative to a quies...

The Role of Convectively Generated Rear-Inflow Jets in the Evolution of Long-Lived Mesoconvective Systems

Abstract: In this study, the structure of convectively generated rear-inflow jets and their role in the evolution of long-lived mesoconvective systems are investigated through an analysis of idealized three-dimensional simulations using a nonhydrostatic cloud model. Rear-inflow jets are generated within these systems in response to the upshear-tilting of the convective circulation, as the horizontal buoyancy gradients along the back edge of the expanding system create a circulation that draws midlevel air in from the rear. Within this framework, a wide range of rear-inflow strengths and structures are produced, depending on the magnitude of the ambient convective available potential energy (CAPE) and the vertical wind shear. In general, for environments characterized by weak-to-moderate vertical wind shear and weak-to moderate CAPE, the rear-inflow jet descends and spreads along the surface well behind the leading edge of the gust front, and the subsequent convective activity becomes weaker. However, for e...

Piecewise potential vorticity inversion

Abstract: The treatment of the potential vorticity (PV) distribution as a composite of individual perturbations is central to the diagnostic and conceptual utility of PV. Nonlinearity in the inversion operator for Ertel's potential vorticity renders quantitative piecewise inversion (inversion of individual portions of the potential vorticity field) ambiguous. Several methods of piecewise inversion are compared for idealized and observed potential vorticity anomalies of varying strengths. Even as the Rossby number of the balanced solutions increases well past unity, relative differences among the more plausible methods do not increase significantly near the anomaly. These relative differences are also found to be smaller than those obtained by comparing any of the methods to quasigeostrophic inversion. However, differences above and below anomalies increase with increasing Rossby number, suggesting that one cannot uniquely diagnose the interaction of large amplitude PV anomalies.

A Class of Semi-Lagrangian Approximations for Fluids

Abstract: This paper discusses a class of finite-difference approximations to the evolution equations of fluid dynamics These approximations derive from elementary properties of differential forms Values of a fluid variable ψ at any two points of a space-time continuum are related through the integral of the space-time gradient of ψ along an arbitrary contour connecting these two points (Stokes' theorem) Noting that spatial and temporal components of the gradient are related through the fluid equations, and selecting the contour composed of a parcel trajectory and an appropriate residual, leads to the integral form of the fluid equations, which is particularly convenient for finite-difference approximations In these equations, the inertial and forcing terms are separated such that forces are integrated along a parcel trajectory (the Lagrangian aspect), whereas advection of the variable is evaluated along the residual contour (the Eulerian aspect) The virtue of this method is an extreme simplicity of t

A Study on the “Runaway Greenhouse Effect” with a One-Dimensional Radiative–Convective Equilibrium Model

Abstract: A simple one-dimensional radiative–convective equilibrium model is used to investigate the relationship between the surface temperature and the outgoing infrared radiation at the top of the atmosphere. The model atmosphere has a gray infrared absorption coefficient and is composed of a radiative equilibrium stratosphere and a moist adiabat troposphere. An upper limit of the outgoing infrared radiation is found to exist. The existence of the upper limit is characterized by the radiation limits that appear when the optical depth of the entire atmosphere becomes sufficiently deep and the temperature structure around the levels where the optical depth is about unity approaches a fixed profile. This appearance of an upper limit differs from that found by Komabayashi and Ingersoll, which is obtained from the constraint of the stratospheric radiation balance. As one of those radiation limits, the outgoing infrared radiation has an asymptotic limit as the surface temperature increases. This is caused by ...

Sources of Mesoscale Variability of Gravity Waves. Part I: Topographic Excitation

Abstract: Aircraft measurements of winds and temperatures collected during the GASP program are used to study the effects of topography as a source of mesoscale variability. Variances of fluctuations at the mesoscale over rough terrain are enhanced up to nearly two orders of magnitude compared to nonsource regions in some cases and are frequently enhanced by an order of magnitude. The implications of these episodic enhancements of variances for the vertical transports of energy and momentum are considered in the framework of gravity wave theory. The observed flight data are used to estimate the momentum flux u′w′ on several flight segments. Results show that the flux is generally negative with mean value −0.26 m2 s−2 and with magnitudes ranging up to −1.5 m2 s−2. Spectral analysis shows that the largest contributions to the net flux come from horizontal scales of ∼25 < λx <60 km. Finally, the observed momentum fluxes are used to infer the anisotropy factor of gravity waves over rough terrain, which is foun...

The Maintenance of Low-Frequency Atmospheric Anomalies

Abstract: A linear diagnostic analysis of four prominent low-frequency atmospheric perturbation patterns in a GCM is carried out to determine the mechanism responsible for the maintenance of these patterns. Results indicate that interactions between the low-frequency patterns and the time-mean zonal asymmetries of the model climate are crucial to the maintenance of the patterns. Of equal importance are anomalous fluxes from transients. Vorticity fluxes due to bandpass (1-7 days) fluctuations ranging over broad sectors of the globe were found to be the most important components of the maintenance of flux anomalies. The nonlinear interactions of the low-frequency deviations with themselves were only of secondary importance.

The Response of a Zonally Symmetric Atmosphere to Subtropical Thermal Forcing: Threshold Behavior

Abstract: We consider the response of a zonally symmetric atmosphere to a thermal forcing that is localized in the subtropics. Specifically, the equilibrium temperature distribution has a local subtropical peak and is flat elsewhere, including at the equator. On the basis of inviscid steady-state theory, it is argued that the response to such forcing is one of two distinct types. Below a threshold forcing, the atmosphere adopts a steady state of thermal equilibrium with no meridional flow. With supercritical forcing, this state breaks down and a strong meridional circulation is predicted. The threshold forcing value is that at which the absolute vorticity of the zonal flow (in gradient balance with the equilibrium temperatures) vanishes at the upper boundary. These inviscid predictions are tested in a zonally symmetric numerical model; while the model viscosity shifts the threshold and otherwise modifies the response, the threshold is clearly evident in the model behavior.

Propagation of Planetary Waves between the Troposphere and Stratosphere

Abstract: The propagation of planetary waves between the troposphere and stratosphere is investigated using a linear, time-dependent, primitive equation model. It is found that the tropopause acts like a valve for the propagation of planetary waves. The key parameters controlling the valve are the vertical gradient of buoyancy frequency and the vertical shear of the zonal winds at the tropopause. For a given wind profile smaller gradient of buoyancy frequency enhances the propagation of planetary waves. For a given profile of buoyancy frequency the larger the shear, the more the wave activity is trapped in the troposphere, and therefore, there is less left to propagate into the stratosphere. The transmission across the tropopause is, however, not sensitive to zonal winds in the upper stratosphere. The propagation of planetary waves is very sensitive to transience. More transient waves propagate more vertically within the troposphere, and for more transient waves, more wave activity is transferred into the stratosphere from the troposphere.

Gravity Wave Excitation by Geostrophic Adjustment of the Jet Stream. Part I: Two-Dimensional Forcing

Abstract: A previous study of inertio–gravity wave motions radiating from a two-dimensional ageostrophic Gaussian jet is extended here to a three-dimensional jet source. Fourier integral solutions of the initial-value problem are obtained in analytic form to facilitate comparison with the previous two-dimensional results. For an initial disturbance elongated along the jet axis, the wave solutions near the midpoint are nearly indistinguishable from those obtained in two dimensions and approach those solutions as the jet increases in length. At locations not symmetric with respect to the longitudinal jet axis, inertio–gravity wave structure departs increasingly from the two-dimensional results. In such cases, the early response is determined by the nearby jet structure and exhibits propagation primarily normal to the jet axis. At later times, however, the response is due to the initial disturbance at other locations along the jet axis and reveals a tendency for propagation parallel to the jet. The mean motio...

The Diagnosis of Mechanical Dissipation in the Atmosphere from Large-Scale Balance Requirements

Abstract: The momentum budget for January 1987 is evaluated with global observations analyzed at the European Centre for Medium-Range Weather Forecasts (ECMWF). The dissipation term is diagnosed from the budget as a balance requirement, that is, as that required to balance the sum of the advection, Coriolis, pressure gradient, and local tendency terms. This is then compared with the parameterized subgrid-scale effects in the ECMWF model's momentum equation, with a view of identifying possible errors in those parameterizations. The balance requirement does not support the high parameterized values of orographically induced gravity-wave drag in the lower stratosphere. A deeper analysis also does not suggest a major role for turbulent vertical transports above the boundary layer. On the other hand, our budget does indicate that more effort be spent on a better representation of the potential enstrophy cascade associated with Rossby wave breaking in the upper troposphere. These statements are qualified by the ...

A second-order bulk boundary-layer model

Abstract: Bulk mass-flux models represent the large eddies that are primarily responsible for the turbulent fluxes in the planetary boundary layer as convective circulations, with an associated convective mass flux. In order for such models to be useful, it is necessary to determine the fractional area covered by rising motion in the convective circulations. This fraction can be used as an estimate of the cloud amount, under certain conditions. 'Matching' conditions have been developed that relate the convective mass flux to the ventilation and entrainment mass fluxes. These are based on conservation equations for the scalar means and variances in the entrainment and ventilation layers. Methods are presented to determine both the fractional area covered by rising motion and the convective mass flux. The requirement of variance balance is used to relax the 'well-mixed' assumption. The vertical structures of the mean state and the turbulent fluxes are determined analytically. Several aspects of this simple model's formulation are evaluated using results from large-eddy simulations.

The Vertical Structure and Development of the ENSO Anomaly Mode during 1979–1989

Abstract: The latest two Pacific basinwide warm episodes (1982–83 and 1986–87) exhibit some common features in their development and vertical structure. These features are examined by multivariate empirical orthogonal function analysis of the interannual variability of the ocean-atmosphere system along equatorial Indian and Pacific oceans. The updraft and downdraft branches of the anomalous Walker circulation originate over the western Pacific and the eastern Indian Ocean, respectively. The early development of basinwide warming is characterized by the strengthening of a cross-equatorial low-level southerly component over the eastern Pacific and the enhancement of convection and boundary-layer westerlies over the western Pacific. The structure of the ENSO anomaly mode changes from the cold to the warm phase of the Southern Oscillation. This is attributed to its eastward migration and the intrinsic longitudinal dependence of the vertical structure. The latter results from the east–west contrast of the air–s...

Studies on the Bulk Transfer Coefficients over a Vegetated Surface with a Multilayer Energy Budget Model

Abstract: A multilayer energy budget model for vegetation canopy is developed to describe the fluxes of sensible and latent heat exchanged between the vegetated surface and the atmosphere The model gives satisfactory results when the calculated radiative surface temperature TR and fluxes of sensible and latent heat are compared with experimental data This model illustrates that values of the bulk transfer coefficients CH and CE are sensitive to meteorological conditions (incident radiation, wind speed, air humidity) when their values are evaluated using TR as the mean surface temperature Therefore, the use of TR is inadequate to determine the bulk transfer coefficients As an alternative, the effective surface temperatures for the energy exchange are defined and the values of CH and CE are determined with these temperatures These values of CH and CE are only dependent on the vegetation parameters: vegetation type and density, the leaf transfer coefficients (or stomatal resistance), and the moisture ava

Parameterizations of Condensational Growth of Droplets for Use in General Circulation Models

Abstract: Motivated by the importance of the effective radius (re) of the droplets to radiative transfer, this paper presents parameterization schemes, which provide a measure of re in stratiform liquid water clouds (in the −13° to +13°C temperature range), for use in general circulation models (GCMs) or mesoscale models. The first scheme developed here is based on theory and numerical calculations of droplet condensational growth, while the second is based on Twomey's analytical approach. Both methods are evaluated against detailed model calculations, and a method for implementing either scheme in general circulation models and remote sensing applications is described. The new parameterization produces accurate (within a few percent) estimates of the effective droplet radius as a function of height, while the cloud optical thickness compares favorably (often to within <10%) with the model calculations. Twomey's scheme gives reasonable estimates of optical thickness, but tends to underestimate the droplet ...

Rossby-wave breaking, microbreaking, filamentation, and secondary vortex formation-The dynamics of a perturbed vortex

Abstract: The behavior of an isolated vortex perturbed by topographically forced Rossby waves is studied using the method of Contour Dynamics. For a single-contour vortex a distinct forcing threshold exists above which the wave breaks in a dynamically significant way, leading to a disruption of the vortex. This breaking is distinguished from the process of weak filamentary breaking described by Dritschel and classified here as microbreaking; the latter occurs in nondivergent flow even at very small forcing amplitudes but does not affect the vortex in a substantial manner. In cases with finite Rossby deformation radius (comparable with the vortex radius) neither breaking nor microbreaking occurs below the forcing threshold. In common with previous studies using high-resolution spectral models, the vortex is not diluted by intrusion of outside air, except during remerger with a secondary vortex shed previously from the main vortex during a breaking event. The kinematics of the breaking process and of the vor...

The Macroscopic Behavior of Cumulus Ensembles Simulated by a Cumulus Ensemble Model

Abstract: A number of simulation experiments are performed with the 2D UCLA cumulus ensemble model to study the macroscopic behavior of cumulus convection under a variety of different large-scale and underlying surface conditions. Specifically, the modulation of cumulus activity by the imposed large-scale processes and the eddy kinetic energy (EKE) budget are investigated in detail. In all simulations, cumulus convection is rather strongly modulated by large-scale advective processes in spite of the existence of some nonmodulated high-frequency fluctuations. The modulation exhibits some phase delays, however, when the basic wind shear is strong. This is presumably associated with the existence of mesoscale convective organization. The EKE budget analysis shows that the net eddy buoyancy generation rate is nearly zero for a wind range of cumulus ensembles.

Polar Low Dynamics

Abstract: Polar lows are intense subsynoptic-scale cyclones that form over high-latitude oceans in association with deep cumulus convection and strong ambient baroclinicity. Recent observations indicate that polar lows are generally initiated by a nonaxisymmetric interaction between a surface disturbance and an upper-level mobile trough. Extant theories of polar low formation preclude study of such a process since they either constrain their models to be axisymmetric, or do not explicitly account for this transient interaction. In this work the physics of interacting upper- and lower-level potential vorticity structures is studied as an initial-value problem using a three-dimensional nonlinear geostrophic momentum model that incorporates moist processes and includes strong baroclinic dynamics. Model results illustrate the rapid formation of an intense small-scale cyclone whose structure is consistent with observations of mature polar lows. A conceptual model of polar low development is proposed. In the fir...

Barotropic Instability and Optimal Perturbations of Observed Nonzonal Flows

Abstract: An eigenvalue analysis of a divergent barotropic model on a sphere is extended to the formulation of a global optimization problem, whose solution selects an initial perturbation that evolves into the most energetic structure at a finite time interval, τ. The evolution of this perturbation is obtained from companion linear and nonlinear global spectral time-dependent models, and the optimization prediction of perturbation size at time τ is verified. Two zonally asymmetric flows defined by time-mean ECMWF global 300-mb analyses during winter 1985/86 are used to illustrate the application and insights provided by the optimization problem. The dependence of the optimal perturbations on the parameter τ is examined. The optimal perturbations become increasingly localized as τ is decreased to periods on the order of three days. The initial growth rates of these perturbations greatly exceed that of the most unstable normal mode, and also exceed the growth rate of a disturbance with maximum projection on...

The influence of concentrated heating on the Hadley circulation

Abstract: The studies of Lindzen and Hou (1988) of Hadley circulation forced by global heating distributions are extended to situations where heating is latitudinally concentrated. The qualitative effect of narrow heating is examined by means of simple 'equal area' arguments of the type developed by Held and Hou (1980). Numerical calculations for a continuous fluid are carried out, showing that the intensity of the resulting Hadley circulation depends on how the heating is concentrated. It is shown that only a mild concentration is needed to achieve agreement with observations.

Seasonal Variations of Tropical Intraseasonal Oscillations: A 20–25-Day Oscillation in the Western Pacific

Abstract: Spectral analysis within a movable time window is used to investigate the frequency and intensity of tropical oscillations with intraseasonal time scales Surface and rawinsonde time series from several locations in the western Pacific and Indian oceans are used Evidence is provided that the Madden–Julian Oscillation (MJO) in the Indian Ocean region shows a seasonal variation in its preferred period from 50 days during Northern Hemisphere winter to about 35 days during summer, though this evidence is weak and stretches the limits of our ability to define a precise period for the oscillation In the western tropical Pacific during the September–December season, a pronounced spectral peak with a central frequency between 20 and 25 days is observed This spectral peak appears to reveal an oscillation that is distinct from the MJO but may also result from coupling between the large-scale motion and latent heat release on much smaller scales A relationship between the oscillation and the occurrence

The Slow Manifold—What Is It?

Abstract: Two studies that disagree as to whether a slow manifold is present in a particular low-order primitive equation model are compared. It is shown that the discrepancy occurs because of a difference of opinion as to what constitutes a slow manifold.

Turbulent Entrainment and Mixing in Clouds: A New Observational Approach

Abstract: A statistical analysis of cloud droplet interarrival times, measured using an aircraft-mounted forward-scattering spectrometer probe, is used to deduce spatial scales of inhomogeneity of droplet concentration in cumulus clouds. The analysis often indicates inhomogeneity of the droplet concentration at small scales (about 1 cm) within regions that appear homogeneous when viewed at larger scales. An explanation of the small-scale inhomogeneity is offered, and the larger-scale homogeneity of the droplet concentration is interpreted in terms of the processes of entrainment and mixing.

Hurricane vortex motion and evolution in a three-layer model

Abstract: A three-layer multinested numerical model is used to evaluate the asymmetric evolution of a hurricane and its interaction with the large-scale environment. The model uses a compressible fluid in isentropic coordinates. In 72 h the hurricane vortex on a beta plane moves northwest at an average speed of 2.4 m s−1. In the presence of a westerly zonal wind in the upper model layer, the hurricane on an f plane moves to the southeast at an average speed of 0.9 m s−1.A series of experiments establishes that the southeastward drift in the presence of westerly shear is primarily due to the southward isentropic gradient of background potential vorticity (PV) in the middle model layer that is associated with the background temperature field. The cyclonic circulation advects low PV air southward on the west side of the vortex, inducing a negative isentropic PV anomaly to the southwest. This anomaly is associated with a wind field that advects the vortex to the southeast, just as the northward isentropic grad...