Showing papers by "Geophysical Fluid Dynamics Laboratory published in 1989"

Interpretation of Cloud-Climate Feedback as Produced by 14 Atmospheric General Circulation Models

Abstract: Understanding the cause of differences among general circulation model projections of carbon dioxide-induced climatic change is a necessary step toward improving the models An intercomparison of 14 atmospheric general circulation models, for which sea surface temperature perturbations were used as a surrogate climate change, showed that there was a roughly threefold variation in global climate sensitivity Most of this variation is attributable to differences in the models' depictions of cloud-climate feedback, a result that emphasizes the need for improvements in the treatment of clouds in these models if they are ultimately to be used as climatic predictors

The Influence of Soil Wetness on Near-Surface Atmospheric Variability

Abstract: The influence of land surface processes on near-surface atmospheric variability on seasonal and interannual time scales is studied using output from two integrations of a general circulation model. In the first experiment of 50 years duration, soil moisture is predicted, thereby taking into consideration interactions between the surface moisture budget and the atmosphere. In the second experiment, of 25 years duration, the seasonal cycle of soil moisture is prescribed at each grid point based upon the results of the first integration, thereby suppressing thew interactions. The same seasonal cycle of soil moisture is prescribed for each year of the second integration. Differences in atmospheric variability between the two integrations are due to interactions between the surface moisture budget and the atmosphere. Analyses of monthly data indicate that the surface moisture budget interacts with the atmosphere in such a way as to lengthen the time scales of fluctuations of near-surface relative humi...

A Variational Continuous Assimilation Technique

Abstract: A variational assimilation technique is presented which continuously adjusts a model solution by introducing a correction term to the model equations. The technique is essentially a modification of the adjoint technique. The Variational Continuous Assimilation (VCA) technique optimizes the correction to the model equations rather than the initial conditions as is done in the adjoint technique. The VCA-technique characteristics were examined by inserting independent analyses into a simple quasi- geostrophic model using both the VCA technique and the adjoint technique. Because the model equations do not have to be satisfied exactly in the VCA technique, some of the effects of systematic model errors can be removed from the assimilation. Thus, the VCA technique was able to consistently fit the data better than the adjoint technique. Predictions from the results from the assimilation techniques showed that the forecast from the adjoint technique's solution was consistently inferior to those from the ...

Simulated global distribution and deposition of reactive nitrogen emitted by fossil fuel combustion

Abstract: We use the medium resolution (- 265 km horizontal grid) GFDL general circulation transport model to simulate the global spread and deposition of reactive nitrogen emitted by fossil fuel combustion. The nitrogen species are transported as a single tracer with no explicit chemistry. Chemical reactions are only present implicitly in the bulk coefficients for dry and wet removal. The observed wet deposition of nitrogen over North America is used to determine the global parameter for wet deposition, and constant bulk coefficients for dry deposition over land and sea are pre-calculated from measured concentrations and deposition velocities. The simulated yearly depositions in Western Europe and at regional export sites, as well as simulated yearly concentrations and their seasonal variation over the North Pacific, are compared with available observations. The agreement is generally quite good and almost always within a factor of 2. This model is then used to identify a number of important source regions and long-range transport mechanisms: (1) Asian emissions supply two-thirds of the soluble nitrogen compounds over the North Pacific. In the summer, North American emissions are important over the subtropical North Pacific. (2) Nitrogen emissions from Europe dominate the nitrogen component of Arctic haze in the lower troposphere, while North American and Asian emissions are only important locally. The model predicts a large gradient in the Arctic with average winter mixing ratios ranging from less than 0.1 ppbv over Alaska to more than 1 ppbv over eastern Russia. (3) Throughout the Southern Hemisphere, the emissions from fossil fuel combustion account for 10% or less of the observed soluble nitrogen at remote sites, an amount less than a previously simulated contribution from stratospheric injection. The long-range transport of PAN, NO, production by lightning and biomass burning, and some, as yet, unknown marine biogenic source may all supply part of this background soluble nitrogen. However, the similarity between the seasonal cycles observed at Samoa for soluble nitrogen and for 03. a species known to be supplied from the stratosphere, suggests a major r6le for either stratospheric injection or an upper tropospheric source.

Fluctuations in the Earth's rotation and the topography of the core-mantle interface

Abstract: As arguments in favour of the notion that very slow convection in the highly viscous mantle is confined to the upper 700 km gradually weakened over the past 20 years, so geophysicists have increased their willingness to entertain the idea that significant horizontal variations in temperature and other structural parameters occur at all levels in the lower mantle. Concomitant density variations, including those caused by distortions in the shape of the core-mantle interface, would contribute substantially to long-wavelength features of the Earth’s gravity held and also affect seismic travel times. The implied departures from axial symmetry in the thermal and mechanical boundary conditions thus imposed by deep mantle convection on the underlying low-viscosity liquid metallic core would affect not only spatial variations in the long-wavelength features of the main geomagnetic held (which is generated by dynamo action involving comparatively rapid chaotic magnetohydrodynamic how in the core) but also temporal variations on all relevant timescales, from decades and centuries characteristic of the geomagnetic secular variation to tens of millions of years characteristic of changes in the frequency of polarity reversals. Core motions should influence the rotation of the ‘solid’ Earth (mantle, crust and cryosphere), and in the absence of any quantitatively reasonable alternative line of attack, geophysicists have long supposed that irregular ‘decade’ fluctuations in the length of the day of about 5 x 10 -3 s must be manifestations of angular momentum exchange between the core and mantle produced by time-varying torques at the core-mantle interface. The stresses responsible for these torques comprise ( a ) tangential stresses produced by viscous forces in the thin Ekman-Hartmann boundary layer just below the interface and also by Lorentz forces associated with the interaction of electric currents in the weakly conducting lower mantle with the magnetic held there, and (b) normal stresses produced largely by dynamical pressure forces acting on irregular interface topography (i.e. departures in shape from axial symmetry). The hypothesis that topographic stresses might provide the main contribution to the torque was introduced by the author in the 1960s and the present paper gives details of his recently proposed method for using Earth rotation and other geophysical data in a new test of the hypothesis. The method provides a scheme for investigating the consistency of the hypothesis with various combinations of ‘models’ of ( a ) motions in the outer reaches of the core based on geomagnetic secular variation data, and ( b ) core-mantle interface topography based on gravity and seismic data, thereby elucidating the validity of underlying assumptions about the dynamics and structure of the Earth’s deep interior upon which the various ‘models’ are based. The scheme is now being applied in a complementary study carried out in collaboration with R. W. Clayton, B. H. Hager, M. A. Spieth and C. V. Voorhies.

Barotropic Decay of Baroclinic Waves in a Two-Layer Beta-Plane Model

Abstract: A two-layer quasi-geostrophic model is used to study the effects of a meridionally sheared zonal flow on the life cycle of a weakly unstable baroclinic wave. In most of the cases analyzed, the fluid is inviscid with the exception of scale-selective fourth-order horizontal diffusion. The initial zonal flow is identically zero in the lower layer. The character of the eddy life cycle in the limit of weak supercritically is shown to depend on whether or not the meridional shell in the upper layer is strong enough to produce a critical latitude for the wave. If the shear is sufficiently weak, the wave undergoes periodic amplitude vacillation characterized by symmetric growth and baroclinic decay. However, when the meridional shear is strong enough to allow for the existence of a critical layer, the flow undergoes an asymmetric life cycle which resembles that found by Simmons and Hoskins in a primitive equation model on the sphere: the wave grows baroclinically but decays barotropically toward a wave-f...

Modeling Rotating Stratified Turbulent Flows with Application to Oceanic Mixed Layers

Abstract: Rotational effects on turbulence structure and mixing are investigated using a second-moment closure model. Both explicit and implicit Coriolis terms are considered. A general criterion for rotational effects to be small is established in terms of local turbulent Rossby numbers. Characteristic length scales are determined for rotational effects and Monin–Obukhov type similarity theory is developed for rotating stratified flows. A one-dimensional version of the closure model is then applied to simulate oceanic mixed layer evolution. It is shown that the effects of rotation on mixed layer depth tend to be small because of the influence of stable stratification. These findings contradict a hypothesis of Garwood et al. that rotational effects on turbulence are responsible for the disparity in the mixed-layer depths between the eastern and western regions of the equatorial Pacific Ocean. The model is also applied to neutrally stratified flows to demonstrate that rotation can either stabilize or destab...

Nonlinear Equilibration of Two-Dimensional Eady Waves

Abstract: The initial-value problem for Eady's model is reexamined using a two-dimensional (x–z) primitive equation model. It is generally accepted that a finite amplitude instability of Eady's basic state will produce a frontal discontinuity in a finite time. When diffusion prevents the frontal discontinuity from forming, the wave amplitude eventually stops growing and begins to oscillate. We analyze this equilibration and suggest that it is a result of enhanced potential vorticity in the frontal region that is mixed into the interior from the boundaries. The dynamics of equilibration is crudely captured in a modified quasi-geostrophic model in which the zonal-mean static stability is allowed to vary. The magnitude of the meridional wind speed of the equilibrated wave is O(N0H), where N0 is the initial buoyancy frequency and H is the depth of the fluid. This is of the same order as the amplitude of the wave predicted by semigeostrophic theory at the point of frontal collapse. Scaling arguments are present...

Rossby wave packets in baroclinic mean currents

Abstract: A WKB description of the propagation of Rossby wave packets in a shallow water model of the tropical oceans indicates that the presence of the baroclinic mean currents can modify the characteristics of wave propagation significantly. For currents with weak latitudinal shear the effect of the current itself is less important than the effect of the associated variations in the depth of the thermocline, except near critical layers where waves are absorbed. For example, a westward current, and the associated shoaling of the thermocline towards the equator, can cause the speed of the long Rossby waves to decrease with decreasing latitude. (The speed increases towards the equator in the absence of mean currents.) Westward currents inhibit meridional propagation, but eastward currents enhance it. The amplification and decay of a wave packet as it propagates through a mean current are described in terms of these conservation of wave action. Implications of these results for the propagation of Rossby waves in the real ocean are discussed.

The effect of horizontal resolution on gravity waves simulated by the GFDL "SKYHI" general circulation model

Abstract: To examine the effects of horizontal resolution on internal gravity waves simulated by the 40-level GFDL “SKYHI” general circulation model, a comparison is made between the 3° and 1° resolution models during late December. The stratospheric and mesospheric zonal flows in the winter and summer extratropical regions of the 1° model are much weaker and more realistic than the corresponding zonal flows of the 3° model. The weaker flows are consistent with the stronger Eliassen-Palm flux divergence (EPFD).

Influence of long-range transport of combustion emissions on the chemical variability of the background atmosphere

Abstract: MAUNA Loa Observatory, located 3,400 m above sea level on the island of Hawaii in the middle of the Pacific Ocean, is a critical site for determining the background chemical reactivity of the unpolluted atmosphere and for monitoring its rate of change on a global scale. However, recent measurements of soluble nitrogen (principally HNO3) at the observatory find mixing ratios rising from their expected background values of 0.02–0.03 parts per 109 by volume (p.p.b.v.) in the winter to 0.07–0.12 p.p.b.v. in late summer with three-hour events as high as 0.25 p.p.b.v. (ref. 1). This raises the specific question of contamination by the long-range transport of pollution1 and a broader question of the chemical variability of the background atmosphere. Here we show that a general circulation transport model which simulates the global spread and deposition of emissions from fossil-fuel combustion can reproduce the nitrogen measurements at the Mauna Loa Observatory. By isolating individual source regions, we show that US emissions are responsible for the late summer increase and that Asian emissions cause a smaller increase in the spring. These simulations, together with the earlier observations, indicate frequent contamination of the Mauna Loa Observatory by the long-range transport of reactive trace gases, such as HNO3, and suggest a highly variable background atmosphere. It is essential that we are aware of such variability in order to discern anthropogenic effects on the atmosphere.

Angular Momentum Cycle in the Atmosphere–Ocean–Solid Earth System*

Abstract: Some of the contributions of Victor Paul Starr (1909–76) as a scholar and teacher at the Massachusetts Institute of Technology are described. His work on the atmospheric branch of the earth's angular momentum cycle is emphasized. Certain recent efforts to include the oceanic and solid earth branches of the cycle are discussed.

Two Different Simulations of the Southern Oscillation and El Nino with Coupled Ocean--Atmosphere General Circulation Models

Abstract: Two different coupled ocean-atmosphere models simulate irregular interannual fluctuations that in many respects resemble El Nino Southern Oscillation phenomena. For example, the spatial structure of various fields at the peaks of the warm El Nino and cold La Nina phases of the oscillation are realistic. This success indicates that the models capture certain aspects of the interactions between the ocean and atmosphere that cause the Southern Oscillation. The principal difference between the models, namely the prominence of oceanic Kelvin waves in one but not the other, causes the two models to differ significantly in the way El Nino episodes evolve, and in the mechanisms that cause a turnabout from El Nino to La Nina and vice versa. It is possible that the different processes that determine the properties of the simulated oscillations all play a role in reality, at different times and in different regions. Each of the models captures some aspects of what is possible. However, reality is far more complex than any model developed thus far and additional processes not yet included are also likely to have a significant influence on the observed Southern Oscillation.

Structure and evolution of North Australian cloud lines observed during AMEX phase I

Abstract: North Australian Clouds Lines are distinctive, squall-line phenomena that occur in easterly flow across northern Australia. Three basic types have been identified, ranging from a long, narrow line of convective clouds (Type 1) to a severe squall line (Type 3). In this paper we examine a group of Type 1 lines, which occurred during the first phase of the Australian Monsoon Experiment (AMEX). The lines occurred in an ambient easterly flow with a distinct maximum near 850 hPa. Most of the lines developed on the western side of deep convective cells along the sea-breeze front in a manner that had substantial similarities to the African squall-line development described by Bolton. The resolvable structure resembled a shallow version of the Moncrieff–Miller squall line.

Report on SEQUAL/FOCAL

Abstract: In Eos (April 6, 1982), United States plans for a program to study the dynamic response of the equatorial Atlantic to seasonally varying surface winds were described. Now, 6 years later, we report on progress toward our goal “to describe accurately, and to model correctly” the changes in the currents and density field of the upper equatorial Atlantic Ocean during a 2-year period. A major effort toward this goal was the field phase of SEQUAL (Seasonal Response of the Equatorial Atlantic) and the closely coordinated French program FOCAL (Francais Ocean et Climat dans l'Atlantique Equatorial). Between February 1983 and September 1984 changes in the surface winds and in oceanic conditions in the equatorial Atlantic were monitored continuously with a variety of instruments. Figure 1 shows key deployments and sections. The resulting data include six current meter moorings, 15 inverted echo sounders and island tide gauges, 57 near-surface drifters, and 18 French and five U.S. cruises that made 1200 hydrographic stations and 800 current profiles. All these data, as well as nearly 10,000 expendable bathythermographs (XBTs) (from the cruises, from air-dropped expendable bathythermograph (AXBT) programs, and from repeated ship of opportunity lines run between 1980 and 1985) were combined and documented by George Heimerdinger, of the National Oceanographic Data Center, and are available from NODC on request.