Showing papers by "Geophysical Fluid Dynamics Laboratory published in 1981"

Parameterization of Vertical Mixing in Numerical Models of Tropical Oceans

Abstract: Measurements indicate that mixing processes are intense in the surface layers of the ocean but weak below the thermocline, except for the region below the core of the Equatorial Undercurrent where vertical temperature gradients are small and the shear is large. Parameterization of these mixing processes by means of coefficients of eddy mixing that are Richardson-number dependent, leads to realistic simulations of the response of the equatorial oceans to different windstress patterns. In the case of eastward winds results agree well with measurements in the Indian Ocean. In the case of westward winds it is of paramount importance that the nonzero heat flux into the ocean be taken into account. This beat flux stabilizes the upper layers and reduces the intensity of the mixing, especially in the cast. With an appropriate surface boundary condition, the results are relatively insensitive to values assigned to constants in the parameterization formula.

Summer dryness due to an increase of atmospheric CO2 concentration

Abstract: To investigate the hydrologic changes of climate in response to an increase of CO2-concentration in the atmosphere, the results from numerical experiments with three climate models are analyzed and compared with each other. All three models consist of an atmospheric general circulation model and a simple mixed layer ocean with a horizontally uniform heat capacity. The first model has a limited computational domain and simple geography with a flat land surface. The second model has a global computational domain with realistic geography. The third model is identical to the second model except that it has a higher computational resolution. In each numerical experiment, the CO2-induced change of climate is evaluated based upon a comparison between the two climates of a model with normal and four times the normal concentration of carbon dioxide in air. It is noted that the zonal mean value of soil moisture in summer reduces significantly in two separate zones of middle and high latitudes in response to the increase of the CO2-concentration in air. This CO2-induced summer dryness results not only from the earlier ending of the snowmelt season, but also from the earlier occurrence of the spring to summer reduction in rainfall rate. The former effect is particularly important in high latitudes, whereas the latter effect becomes important in middle latitudes. Other statistically significant changes include large increases in both soil moisture and runoff rate in high latitudes of a model during most of the annual cycle with the exception of the summer season. The penetration of moisture-rich, warm air into high latitudes is responsible for these increases.

The Morning Glory of the Gulf of Carpentaria: An Atmospheric Undular Bore

Abstract: This paper presents the results of a field expedition mounted in late September/early October 1979 to investigate the structure and origin of the “morning glory” of the Gulf of Carpentaria in northern Australia. The morning glory is a line wind squall, accompanied by a pressure jump, and often by a long roll-cloud or series of such clouds. It frequently occurs in the early morning, especially in October, in the Gulf area. A light aircraft, fitted with a temperature and humidity probe, was flown in two glories to determine their thermodynamic structure, and wind fields wore obtained principally by tracking pilot balloons using the double theodolite method. Data also were obtained from a network of surface stations, recording wind velocity and pressure, installed at locations across Cape York Peninsula, which is believed to be the area of genesis. The morning glory is identified as an internal undular bore propagating on the nocturnal and/or maritime inversion. Its origin appears to lie frequently ...

The oceanic response to cross‐equatorial winds (with application to coastal upwelling in low latitudes)

Abstract: Sea surface temperature variations observed in the eastern equatorial Atlantic and Pacific Oceans on seasonal, and possibly interannual (El Nino) time scales, may to a large extent be due to the variability of the local meridional winds. In a numerical model of the ocean, southerly winds cause low sea surface temperatures in the southeastern part of the basin because the coastal upwelling zone is extended far westward by (1) advection and (2) Rossby wave propagation which is important on time scales greater than a month. North of the equator sea surface temperatures are high. The thermocline has a trough near 3° N where there is an intense eastward jet. A relaxation of the southerly wind causes a warming in the southeastern part of the basin primarily because of a zonal redistribution of heat by the South Equatorial Current and Countercurrent. DOI: 10.1111/j.2153-3490.1981.tb01744.x

The Response of Equatorial Oceans to a Relaxation of the Trade Winds

Abstract: The trade winds over the central Pacific are observed to weaken several months after the appearance of anomalously warm surface waters in the eastern equatorial Pacific Ocean. The following results obtained with a numerical model indicate how this relaxation of the winds affect the later stages of El Nino. A weakening of the westward trade winds causes a zonal redistribution of heat in the equatorial oceans and a warming of the eastern part of the basin. The warming depends on the zonal extent of the region over which the winds relax, and on the length of time T for. which the winds relax. As T increases the warming in the east increases until it asymtotes to a maximum value when T exceeds the adjustment time of the basin (which is ∼400 days in the case of the Pacific Ocean). Maximum heating is associated with a permanent weakening of the winds, unless the winds reverse direction and become eastward. Even weak eastward winds for a short period can cause disproportionately large temperature increa...

Simulation of Atmospheric Variability

Abstract: A spectral atmospheric circulation model is time-integrated for approximately 18 years. The model has a global computational domain and realistic geography and topography. The model undergoes an annual cycle as daily values of seasonally varying insolation and sea surface temperature are prescribed without any interannual variation. It has a relatively low computational resolution with 15 spectral components retained in both zonal and meridional directions. Analysis of the results from the last 15 years of the time integration indicates that, in middle and high latitudes, the model approximately reproduces the observed geographical distribution of the variability (i.e., standard deviation) of daily, monthly and yearly mean surface pressure and temperature. In the tropics, the model tends to underestimate the variability of surface pressure, particularly at longer time scales. This result suggests the importance of processes with long time scales such as ocean–atmosphere interaction, in maintainin...

A combined laboratory and numerical study of fully developed steady baroclinic waves in a cylindrical annulus

Abstract: A description is given of a numerical model which has been used to simulate the steady baroclinic waves which may be observed in a fluid rotating about a vertical axis and subjected to a radial temperature gradient. the model is designed to provide accurate simulations of the observed flow without recourse to parametrization of small eddies. the model is verified by comparing the results of integrations with observed flows; many of the differences between the simulated and observed flows may be ascribed to the limited resolution of the model. the numerical model is used to provide data on the non-linear interactions between different scales of motion present in the steady flows. It is demonstrated that the predominant non-linear interaction, in the numerical model, involving the sidebands of the dominant wave is an interaction with the first harmonic of the dominant wave; this contrasts with an interaction with the long wave which was suggested by earlier workers. Experimental evidence is advanced to show that the sidebands may be enhanced by weak asymmetrical forcing and this sensitivity has been verified through the results the further numerical integrations.

The Quasi-Hydrostatic Approximation

Abstract: Second-order expansion of the aspect ratio gives rise to simple equations with a quasi-hydrostatic approximation that perform far better than the classical hydrostatic system in the simulation of moist convection in a mesoscale model. It also suggests that a simple modification to this system may extend the validity of schemes for aspect ratios larger than 1.

Some vorticity theorems and conservation laws for non-barotropic fluids

Abstract: Some theorems concerning the vorticity in barotropic flows of perfect fluids are generalized for non-barotropic flows. The generalization involves replacing the velocity in certain parts of the equations by a time-dependent quantity which is a function of the velocity and thermodynamic properties of the fluid. Results which are generalized include Kelvin's circulation theorem and conservation laws for potential vorticity and helicity. It is shown how the results can be further generalized to include dissipative effects. The possibility of using some of the results in deriving a complete set of Lagrangian conservation laws for perfect fluids is discussed.

On the determination of the size of the Earth's core from observations of the geomagnetic secular variation

Abstract: When the magnetic field of a planet is due to self-exciting hydromagnetic dynamo action in an electrically conducting fluid core surrounded by a poorly-conducting 'mantle', a recently proposed method (Hide I 978, 1979) can in principle be used to find the radius rc of the core from determinations of secular changes in the magnetic field B in the accessible region above the surface of the planet, mean radius rs, with a fractional error in rc of the order of, but somewhat larger than, the reciprocal of the magnetic Reynolds number of the core It will be possible in due course to apply the method to Jupiter and other planets if and when magnetic measurements of sufficient accuracy and detail become available, and a preliminary analysis of Jovian data (Hide & Malin I 979) has already given encouraging results The 'magnetic radius' rc of the Earth's molten iron core has been calculated by using one of the best secular variation models available (which is based on magnetic data for the period 1955-75), and compared with the 'seismological' value of the mean core radius, rc = 3486 + 5 km Physically plausible values of rc, are obtained when terms beyond the centred dipole (n = 1) and quadrupole (n = 2) in the series expansion in spherical harmonics of degree n = 1, , A, , n* are included in the analysis (where 2 < A <, n* < oo) Typical values of the fractional error

Large Mesoscale Convection and Sea Breeze Circulation. Part I: Linear Stability Analysis

Abstract: The interactions between the sea breeze circulation and trapeze instability are investigated using a set of linearized equations. The results show that mesoscale waves associated with trapeze instability can be easily triggered by the sea breeze circulation and can propagate far inland, but no mesoscale waves are observed over the ocean. The wavelength is a few hundred kilometers. The period of the waves, which depends on the Coriolis parameter, eddy viscosity and the strength of land-sea contrast, can be either one day or two days, or a combination of one and two days. The waves obtained here are similar to the cloud bands observed in West Africa and South America.

On the Role of Large-Scale Transient Eddies in the Maintenance of the Vorticity and Enstrophy of the Time-Mean Atmospheric Flow

Abstract: The global distribution of the forcing of the time-mean flow due to large-scale, horizontal Reynolds stresses (u′ u′, v′ v′, u′ v′) is determined from upper wind statistics for the period 1968–73. The role of this forcing in the maintenance of the vorticity and enstrophy of the time-mean flow is discussed. The most striking effect of transient eddy stresses is the tendency to shift the subtropical maxima in the time-mean flow and the associated vorticity patterns poleward. However, significant longitudinal Variations in forcing occur, also. Calculations of the dominant terms in vorticity budgets of the North Pacific Low, the North Atlantic Low, and the Siberian High, which may he called the centers of action of winter-time circulation at sea level in the Northern Hemisphere, are presented. In all three cases, transient eddies are found to be important in maintaining the centers against the dissipative action of surface friction. In terms of the enstrophy budget, the hemispheric and global-mean ef...

Albedo Feedback, the Meridional Structure of the Effective Heat Diffusivity, and Climatic Sensitivity: Results from Dynamic and Diffusive Models

Abstract: The sensitivity of a two-level primitive equation atmospheric model to solar constant perturbations is examined in the presence of surface albedo feedback. The model is simplified to the point that a large number of numerical experiments can be performed and statistically steady states defined with relative case. Exceptionally sensitive equilibrium states are found that are unrelated to the large and small ice-cap instabilities obtained in the simplest diffusive energy balance models. Similar results are produced in a two-level diffusive model closely patterned after the dynamic model, and in a more highly idealized one-level model, by choosing a diffusivity with pronounced meridional structure resembling that of the effective diffusivity of the dynamic model. Sensitive states occur in the diffusive models when the albedo gradient enters the region equatorward of 60° in which the effective heat diffusivity of the atmosphere increases with increasing latitude.

Self-exciting dynamos and geomagnetic polarity changes

Abstract: Cosmical magnetic fields are produced and maintained against the effects of ohmic decay by inductive interactions with fluid motions. But neither a steady nor a non-steady magnetic field can be supported by this so-called ‘self-exciting homogeneous dynamo’ process if the field possesses an axis of symmetry. This suggests that palaeomagnetic and archaeomagnetic data might be expected to show evidence that departures from axial symmetry are systematically less during the decay phase of a geomagnetic polarity ‘reversal’ or ‘excursion’ than during the growth or recovery phase. The recent proof that neither fluid compressibility nor thermoelectric effects of the Nernst–Ettinghausen type can prevent the ohmic decay of axisymmetric magnetic fields is also discussed here.

Some effects of boundary conditions and fluid properties on vacillation in thermally driven rotating flow in an annulus

Abstract: A series of laboratory experiments have been carried out which demonstrate the conditions favourable for vacillation in baroclinic thermally driven flows in a rotating annular convection chamber. As earlier studies have indicated amplitude vacillation occurs close to the transitions between dominant wave numbers in the regime under which regular waves are observed. Amplitude vacillation is shown to be less common when the depth of the system is decreased; it is not greatly affected by the use of a free or rigid upper boundary condition. It is however much more frequent when the Prandtl number of the convecting fluid is increased from 11 to 74 although in these experiments other parameters of the system, eg the isotherm slopes, were necessarily changed. Shape vacillation is only significant in the experiments when the upper surface is free; it is not influenced by the nature of the fluid. An heuristic argument is advanced to account for the observations of amplitude vacillation. This is shown to b...

The Effects of Condensational Heating on Midlatitude Transient Waves in Their Mature Stage: Control Experiments with a GFDL General Circulation model

Abstract: The effects of condensational heating on midlatitude transient waves in their mature stage are re-examined by comparing moist and dry GFDL spectral general circulation models, both of which have all ocean surfaces with prescribed zonally uniform temperature The zonal mean states of both models are fixed in time so as to he identical throughout the time integration It is found that the transient eddy kinetic energy is significantly enhanced for all wavenumbers by the effect of latent beat release This increase is primarily due to an increase in baroclinic conversion from the zonal available potential energy and only partly due to the generation of eddy available potential energy by condensational heating

Vertical-Zonal Propagation of a Stationary Planetary Wave Packet.

Abstract: In order to explain why the Aleutian high stands out in the winter stratosphere, a complex Fourier analysis is made of simulated and observed stationary waves. It is found that in the troposphere the envelope of the time mean geopotential height consisting of wavenumbers 1 ∼ 3 attains its major and minor maxima in the Pacific and Atlantic, respectively. The major maximum is dominated by wavenumbers 1 ∼ 2 and shifts eastward with height in the stratosphere in the approximate direction of the group velocity and strengthens the Aleutian high. The minor maximum is dominated by wavenumber 3 and is confined in the troposphere.

Slowly Modulated Baroclinic Waves in a Three-Layer Model

Abstract: A coupled pair of envelope equations is derived which describe the nonlinear evolution of slowly varying wave packets in a three-layer model of baroclinic instability on a βplane. The equations are identical in form to those obtained by Pedlosky (1972) to study wave-packet evolution in a two-layer model. They are transformable to the Self-Induced Transparency equations of nonlinear optics for complex wave amplitude, and to the sine-Gordon equation for real wave amplitude. Both are known to possess solution solutions, with associated highly predictable behavior. The three-layer model therefore is another example of a mathematical model of baroclinic instability to exhibit solution behavior. The significance of such solutions to meteorology and oceanography is discussed.

Mean Surface Stress Curl over the Oceans as Determined from the vorticity Budget of the Atmosphere

Abstract: The vertically integrated atmospheric vorticity budget over the oceans offers, in principle, a possibility of determining the surface stress curl from upper wind data without the need to specify a relationship between the surface stress and surface wind. Results for the wind stress curl obtained by this vorticity method, using upper wind data for the period 1968–73, are compared with the recent stress-curl calculations by Hellerman from surface data. The two completely independent methods give basically similar mean latitudinal distributions of the stress curl. In the midlatitudes of the Southern Hemisphere, where the transient eddies are the main mechanism of vorticity transfer. the two estimates of the basin-wide longitudinal averages of the stress curl do not deviate from each other. by ≳20%. However, in the Northern Hemisphere the agreement is less. This seemingly strange result appears to be due to the sensitivity of the vorticity method to errors in the estimates of vorticity advection by t...

Cumulative Results of Extended Forecast Experiment. III: Precipitation

Abstract: A diagnostic analysis and an appraisal of the precipitation calculation by the GFDL (Geophysical Fluid Dynamics Laboratory) 1967 version prediction model are presented, using two-week forecasts of 12 January and 12 July cases. The geographical distribution of predicted rainfall, moisture, and snow over the Northern Hemisphere and the contiguous United States was investigated in comparison with climatological maps published by other authors. The agreement of precipitation and dew-point temperature is marginal. The major causes for the deficiencies are 1) a specification of excessive soil moisture over land, 2) probably an improper treatment of moisture diffusion associated with topography, and 3) an inadequate rain generation process in the model. However, the predicted snow distribution over the United States was reasonable.

CO2 and Climate - A Continuing Story

Abstract: The increased use of fossil fuels is providing an intense unnatural source of CO2 to the atmosphere (24). That the level of CO2 has been rising rapidly in the atmosphere (Figures 1a, 1b) must mean that other components of the terrestrial biogeochemical system have not fully taken up the new CO2 (25). We can see from Figure 1c that the seasonally adjusted trends of Mauna Loa and Antarctica indicate that the CO2 seems to be well mixed with about 5% increase over a twenty year period. It has been estimated that the CO2 concentration at the beginning of the industrial revolution, about 130 years ago, was 290 ppm, while projections for the future are for 365–395 ppm by the year 2000 and for a doubling by the mid twenty-first century (26).

On the rotation of Jupiter

Abstract: Summary. Jupiter's non-uniform rotation is reflected in the use by observers of three systems for measuring longitude: system I which rotates in the period 9 h 50 m 30.003 s, system II in 9 h 55 m 40.632 s and system III in 9 h 55 m 29.710 s. Disturbances of Jupiter's visible surface of dense ammonia-cirrus cloud within a narrow equatorial zone of about 7° latitudinal half-width show comparatively little motion relative to system I, and most (but not all) higher-latitude atmospheric disturbances including the Great Red Spot and the three white ovals move only slowly in system II. Finding neither system I nor system II particularly convenient for monitoring sources of intense non-thermal radio-emission from Jupiter, radio-astronomers introduced system III with respect to which the radio sources show little systematic movement. The interpretation of Jupiter's non-uniform rotation involves considerations of the hydrodynamics of the atmosphere of the planet and the hydromagnetics of the deep interior. Jovian radio sources are extensive regions of plasma tied to the lines of force of the magnetic field B surrounding the planet. These in turn are intimately linked to material at the outer surface of Jupiter's electrically-conducting fluid core, mean radius rc, which consists largely but not entirely of metallic hydrogen. A method for determining rc from observations of secular changes in B in the accessible region above the visible surface of the planet, mean radius rs= 69 700 km, has recently been proposed and applied using the few available data, giving rc greater than about 50 000 km and possibly as large as 63 000 km. The difference in period between system I and system II reflects the presence at the upper cloud level of a sharply-bounded equatorial jet stream within which the atmosphere moves at about 100 ms−1 in a westerly (eastward) sense relative to higher latitude parts of the atmosphere and to the deep interior. The width of the equatorial jet can be accounted for satisfactorily by fairly general dynamical arguments, which predict that the apparently much stronger equatorial jet (400 m s−1) on Saturn will be found to be about twice as wide as that on Jupiter.