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DOE OSTI2025

Projected changes in African easterly wave activity due to climate change

African easterly waves significantly influence regional hydroclimate, making it crucial to understand how global warming will impact their activity. Here, we investigate future changes in wave activity and assess the underlying mechanisms using an ensemble of Earth system models. We find a robust increase in wave activity over the Sahel–Sahara region by the end of the 21st century under two emission scenarios. This intensification is linked to increased baroclinicity associated with a strengthening of the meridional temperature gradient between the Guinea Coast and the Sahara. Our results also indicate that low-level warming enhances the waves by reinforcing monsoon flow, leading to increased convergence and vertical motion along the intertropical discontinuity. These energetic alterations significantly modify the conditions that currently produce these waves. Overall, our findings suggest that changes in wave activity could impact the transport of Saharan dust and mesoscale convective activity over the Sahel.

54 ENVIRONMENTAL SCIENCES↗

NASA NTRS1983

Non-planar MHD model for solar flare-generated disturbances in the heliospheric equatorial plane

An analysis, with a representative (canonical) example of solar-flare-generated equatorial disturbances, is made for the temporal and spatial changes in the solar wind plasma and magnetic field environment between the sun and 1 AU. The goal is to search for first-order global consequences rather than to make a parametric study. The analysis treats all three plasma velocity and magnetic field components in any convenient heliospheric plane of symmetry. The representative disturbance is examined for the canonical case in which the temporal and spatial changes in a homogeneous solar wind caused by a solar-flare-generated shock wave are described. All plasma and field parameters at three radial locations are examined. These are the central meridian and 33 deg W and 90 deg W of the flare's central meridian. It is found that the incorporation of a small meridional magnetic field in the ambient magnetic spiral field has negligible effect on the results. The magnetic field exhibits strong kinking within the interplanetary shocked flow, even reversed polarity that, coupled with low temperature and low density, suggests a plausible explanation for magnetic clouds' with accompanying double-streaming of electrons observed at directions approximately 90 deg to the heliocentric radius.

Wu, S. T.↗

NASA NTRS1986

Baroclinic instability in the presence of barotropic jets

A formalism is developed for the calculation of baroclinic instability for barotropically stable jets. The formalism is applied to jet versions of both the Eady and Charney problems. It is found that jets act to confine instabilities meridionally, thus internally determining meridional wave scales. Once this internally determined meridional scale is taken into account, results correspond plausibly to classical results without a jet. Consideration of the effect of such instabilities on the mean flow shows that they act to concentrate the jet barotropically while simultaneously reducing baroclinicity.

Ioannou, Petros↗

NASA NTRS1991

Circulation of the gyres of the world ocean: Observation modeling using TOPEX/Poseidon altimeter data

The overall objectives of the proposed investigation are to study the dynamics of the large-scale recirculating cells of water in the ocean, which are loosely defined as 'gyres' in this study. A gyre is normally composed of a swift western boundary current (e.g., the Gulf Stream and the Kuroshio), a tight recirculating cell attached to the current, and a large-scale sluggish return flow. The water, of course, is not entirely recirculating within a gyre. The exchange of water among gyres is an important process in maintaining the meridional heat transport of the ocean. The gyres constitute a major mode of water movement in the ocean and play significant roles in the global climate system.

Fu, Lee-Lueng↗

NASA NTRS1993

Nearly steady flows in GONG prototype data

Doppler velocity images obtained with the GONG prototype instrument were analyzed to measure the nearly steady photospheric flows. The data consists of 88 images each of velocity, intensity, and modulation obtained at 20:00 UT on 88 days from July 1992 to February 1994. Each velocity image was temporally filtered to remove the p-mode oscillations, masked to exclude active regions, and then analyzed using spherical harmonics and orthogonal functions as described by Hathaway (1992). The spectral coefficients show very consistent results for the entire time interval with some evidence of year-to-year variations. The rotation profile agrees well with previous results and exhibits a north-south asymmetry that reverses sign during the 20 month interval. The residual rotation velocities exhibit structures with amplitudes of approximately 5 m/s that may be related to torsional oscillations. The meridional circulation is directed from the equator toward the poles with a peak velocity in the photosphere of approximately 50 m/s. The higher order components are very weak but indicate a divergent flow from the mid-latitudes (opposite that found for the June 1989 data). The convective limb shift is well fit by a 3rd order polynomial. The convection spectrum has a prominent peak at spherical harmonic degrees of l approximately 150 with very little signal in the low degree modes. Analysis of this signal shows that there is no evidence for giant cell convection at the level of approximately 10 m/s for all modes up to l = 32.

Hathaway, David H.↗

NASA NTRS1986

The behavior of planetary wave 2 in preconditioned zonal flows

The response of linear planetary wave 2 to changes in the isentropic zonally symmetric distribution of potential vorticity (PV) is investigated numerically. Wave 2 is sensitive to the width and position of a region where the meridional derivative of PV is weak, denoted the 'surf zone', in the middle stratosphere. A narrow surf zone leads to an amplification of wave 2, and confines some of its Eliassen-Palm wave activity in high latitudes. For wider surf zones the wave activity is concentrated near the associated critical surface, and the amplitude of the wave decreases. Large changes in the wave amplitude and in the distribution of its activity are associated with the subtle changes in the zonal winds produced by modest modifications in the distribution of PV. Basic states that include regions of reversed meridional gradients of PV lead to wave overreflection and strong poleward focusing of wave activity. The amplitude of wave 2 is enhanced in the presence of negative gradients, with large responses occurring for eastward traveling waves.

Robinson, Walter A.↗

NASA NTRS1988

Equatorial semiannual oscillation in zonally averaged temperature observed by the Nimbus 7 SAMS and LIMS

Zonally averaged equatorial temperatures obtained aboard Nimbus 7 by the stratospheric and mesospheric sounder (SAMS) are compared to comparable data obtained from the limb IR monitor of the stratosphere. The SAMS data are shown to confirm the seasonal asymmetry in semiannual wind regimes previously noted in rocketsonde observations near the equator. Two explanations for the asymmetry are considered: (1) an improved Kelvin and gravity wave transmissivity in stronger equatorial easterlies (resulting from planetary Rossby wave momentum transport), implying stronger westerly mean flow acceleration in the first cycle than in the second; and (2) evidence of strong polar-tropical coupling in the northern winter indicating that mean meridional circulations are present on a global scale.

Delisi, Donald P.↗

NASA NTRS1993

The Influence of Swirl Brakes and a Tip Discharge Orifice on the Rotordynamic Forces Generated by Discharge-to-Suction Leakage Flows in Shrouded Centrifugal Pumps

Recent experiments conducted in the Rotor Force Test Facility at the California Institute of Technology have examined the effects of a tip leakage restriction and swirl brakes on the rotordynamic forces due to leakage flows on an impeller undergoing a prescribed circular whirl. The experiments simulate the leakage flow conditions and geometry of the Alternate Turbopump Design (ATD) of the Space Shuttle High Pressure Oxygen Turbopump and are critical to evaluating the pump's rotordynamic instability problems. Previous experimental and analytical results have shown that discharge-to-suction leakage flows in the annulus of a shrouded centrifugal pump contribute substantially to the fluid induced rotordynamic forces. Also, previous experiments have shown that leakage inlet (pump discharge) swirl can increase the cross-coupled stiffness coefficient and hence increase the range of positive whirl for which the tangential force is destabilizing. In recent experimental work, the present authors demonstrated that when the swirl velocity within the leakage path is reduced by the introduction of ribs or swirl brakes, then a substantial decrease in both the destabilizing normal and tangential forces could be achieved. Motivation for the present research is that previous experiments have shown that restrictions such as wear rings or orifices at pump inlets affect the leakage forces. Recent pump designs such as the Space Shuttle Alternate Turbopump Design (ATD) utilize tip orifices at discharge for the purpose of establishing axial thrust balance. The ATD has experienced rotordynamic instability problems and one may surmise that these tip discharge orifices may also have an important effect on the normal and tangential forces in the plane of impeller rotation. The present study determines if such tip leakage restrictions contribute to undesirable rotordynamic forces. Additional motivation for the present study is that the widening of the leakage path annular clearance and the installation of swirl brakes in the ATD has been proposed to solve its instability problems. The present study assesses the effect of such a design modification on the rotordynamic forces. The experimental apparatus consists of a solid or dummy impeller, a housing instrumented for pressure measurements, a rotating dynamometer and an eccentric whirl mechanism. The solid impeller is used so that leakage flow contributions to the forces are measured, but the main throughflow contributions are not experienced. The inner surface of the housing has been modified to accommodate meridional ribs or swirl brakes within the leakage annulus. In addition, the housing has been modified to accommodate a discharge orifice that qualitatively simulates one side of the balance piston orifice of the Space Shuttle ATD. Results indicate the detrimental effects of a discharge orifice and the beneficial effects of brakes. Plots of the tangential and normal forces versus whirl ratio show a substantial increase in these forces along with destabilizing resonances at some positive whirl ratios when a discharge orifice is added. When brakes are added, some of the detrimental effects of the orifice are reduced. For the tangential force, a plot versus whirl ratio shows a significant reduction and a destabilizing resonance appears to be eliminated. For the normal force, although the overall force is not reduced, again a destabilizing resonance appears to be eliminated.

Sivo, Joseph M.↗

NASA NTRS1986

Effects of orography on planetary scale flow

The earth's orography is composed of a wide variety of scales, each contributing to the spectrum of atmospheric motions. A well studied subject (originating with Charney and Eliassen) is the direct forcing of planetary scale waves by the planetary scale orography: primarily the Tibetan plateau and the Rockies. However, because of the non-linear terms in the equations of dynamic meteorology, even the smallest scales of mountain induced flow can contribute to the planetary scale if the amplitude of the small scale disturbance is sufficintly large. Two possible mechanisms for this are illustrated. First, preferentially located lee cyclones can force planetary waves by their meridional transport of heat and momentum (Hansen and Chen). Recent theories are helping to explain the phenomena of lee cyclogenesis (e.g., Smith, 1984, J.A.S.). Second, mesoscale mountain wave and severe downslope wind phenomena produce such a large local drag, that planetary scale waves can be produced. The mechanism of upscale transfer is easy to understand in this case as the standing planetary scale wave has a wavelength which depends on the mean structure of the atmosphere, and not on the width of the mountain (just as in small scale lee wave theory). An example of a theoretical description of a severe wind flow with very large drag is shown.

Smith, R. B.↗

NASA NTRS1978

On the mean meridional mass motions of the stratosphere and mesosphere

Using a simplified, approximate 'Lagrangian-mean' dynamical formulation, the mean meridional mass circulation of the stratosphere and mesosphere is discussed. Under solstice conditions, it is shown that this Lagrangian-mean circulation may be inferred, as a first approximation, from the Eulerian-mean diabatic heating. Diabatic heating rates for the solstices, originally derived by Murgatroyd and Goody (1958), result in Lagrangian-mean rising motion at the tropical tropopause, subsidence across the extra-tropical tropopause, and a very strong summer-to-winter pole flow in the mesosphere. This circulation is exactly that obtained by Murgatroyd and Singleton (1961) for the solstices. Those authors, however, attempted to identify this circulation as the Eulerian-mean motion, and were later criticized for their neglect of the meridional eddy heat flux in the calculation, which proved to be extremely important in the winter hemisphere. The present study, nevertheless, indicates that Murgatroyd and Singleton's circulation may in fact be representative of actual air parcel motions in the stratosphere and mesosphere.

Dunkerton, T.↗

NASA NTRS1984

Observations and theory of the solar semidiurnal tide in the mesosphere of Venus

The 1978 Pioneer Venus mission, in addition to producing strong IR radiometer experiment evidence for the existence of a semidiurnal solar tide in the Venus mesosphere, has yielded radiative flux and cloud particle distribution data about the structure of solar heating and IR cooling which facilitate calculations of tidal forcing functions and estimates of radiative dissipation rates. It is presently shown that a physically reasonable mean flow can be found which leads to theoretically predicted tides that are in excellent agreement with those observed. The zonal mean flow obtained exhibits a maximum velocity of about 130 m/sec at 70 km altitude, decreasing to about zero at 88 km, together with a jet-like meridional structure that strongly deviates from solid body rotation.

Fels, S. B.↗

NASA NTRS1994

Evolution of the Southern Hemisphere subpolar middle atmosphere during summer and autumn

The evolution of zonal wind and zonal wavenumber one (wave 1) in the Southern Hemisphere subpolar middle atmosphere is described for the period December 1978 - May 1979 using temperature and ozone measurements from the Limb Infrared Monitor of the Stratosphere (LIMS) experiment. In late December maximum zonal easterlies of approx. -70 m/s are observed at 0.1 mb, 60 deg S. A zonal flow reversal occurs during late February and westerlies subsequently increase to 60-70 m/s in the upper stratosphere by April - May. LIMS zonal winds are compared with rocketsonde measurements and nadir sounder (derived) winds for summer and autumn. Although quantitative agreement is found at stratospheric levels, substantial discrepancies are evident in the mesosphere, most likely a reflection of sampling and resolution differences in the respective datasets. Stationary and traveling wave 1 temperature disturbances (amplitudes approx. 1 - 2 K at 60 deg S) are observed by LIMS during summer. The stationary wave is confined to the lower stratosphere near the level of zero zonal- mean wind flow, whereas the traveling wave is prominent in the middle stratosphere moves west at a rate similar to the zonal-mean wind, and exhibits a vertical - meridional structure similar to a P(sub 4)(sup 1) normal mode Rossby wave. A substantial intensification of wave 1 activity occurs during autumn (amplitudes approx. 5 - 10 K), which is found to be associated with an upward-directed Eliasse - Palm flux near the subpolar tropopause level. Evidence relating wave 1 activity in the lower - middle stratosphere to the occurrence of zonal ozone perturbations of 10% - 20% amplitude is presented for summer and autumn.

Miles, T.↗

NASA NTRS1991

GEOSIM: A numerical model for geophysical fluid flow simulation

A numerical model which simulates geophysical fluid flow in a wide range of problems is described in detail, and comparisons of some of the model's results are made with previous experimental and numerical studies. The model is based upon the Boussinesq Navier-Stokes equations in spherical coordinates, which can be reduced to a cylindrical system when latitudinal walls are used near the pole and the ratio of latitudinal length to the radius of the sphere is small. The equations are approximated by finite differences in the meridional plane and spectral decomposition in the azimuthal direction. The user can specify a variety of boundary and initial conditions, and there are five different spectral truncation options. The results of five validation cases are presented: (1) the transition between axisymmetric flow and baroclinic wave flow in the side heated annulus; (2) the steady baroclinic wave of the side heated annulus; (3) the wave amplitude vacillation of the side heated annulus; (4) transition to baroclinic wave flow in a bottom heated annulus; and (5) the Spacelab Geophysical Fluid Flow Cell (spherical) experiment.

Butler, Karen A.↗

NASA NTRS1985

On the interaction between the quasi-2-day wave and the mean flow

Studies of the quasi-2-day wave show that it is a summertime phenomenon. Long-term measurement of the winds in the 60 to 100 km height range at Adelaide show that the wave appears in the late December but that the peak amplitudes are not reached until mid-January. In the summer of 1983 to 1984 the main phase of the wave appeared as a pulse in mid-January which lasted about 7 cycles (14 days). Little or no amplification with height was observed; peak amplitudes were about 60 m/s for the meridional and 20 m/s for the zonal component. Coincident with the onset of the pulse, a temporary but substantial change occurred throughout a deep layer of the upper mesosphere over a period of about 3 days. A change of more than 10 m/s occurred in the northward flow while the change in the zonal flow (about 30 m/s westward) actually caused a reversal of the prevailing zonal circulation.

Craig, R. L.↗

NASA NTRS1986

Atmospheric tidal forcing of the zonal-mean circulation - The Martian dusty atmosphere

Classical atmospheric tidal theory has been used to compute the bilinear tidal zonal-mean forcing per unit mass of the zonal-mean meridional and zonal winds, together with the tidal zonal-mean heating per unit mass for the dusty Martian atmosphere. The convergences of the tidal Eliassen-Palm (EP) flux have been computed for both clear and dusty atmospheric conditions, including the special case of a 'dusty corridor' in the summer southern subtropics that is meant to simulate the early stages of a planetary-scale Martian dust storm. The calculation of the tidal EP zonal forcing differs from Hamilton in that more realistic thermotidal forcings and basic state temperatures are used. The zonal-mean convergences of the tidal fluxes of heat and momentum are large during a Martian great dust storm and should alter significantly the zonal-mean circulation and its residual component driven by the zonal-mean heating. In particular, the tidal forcing of the meridional wind, which is an order of magnitude greater than its zonal counterpart, is likely to give rise to a complex pattern of significantly ageostrophic zonal-mean flow in the Martian tropics.

Zurek, R. W.↗

NASA NTRS1989

Large-scale dynamics of the stratosphere and mesosphere during the MAP/WINE campaign winter 1983 to 1984 in comparison with other winters

For the MAP/WINE winter temperature and wind measurements of rockets were combined with SSU radiances (Stratospheric Sounder Unit onboard the NOAA satellites) and stratopause heights from the Solar Mesosphere Explorer (SME) to get a retrieved data set including all available information. By means of this data set a hemispheric geopotential height, temperature and geostrophic wind fields eddy transports for wave mean flow interaction and potential vorticity for the interpretation of nonlinear wave breaking could be computed. Wave reflection at critical lines was investigated with respect of stratospheric warmings. The meridional gradient of the potential vorticity and focusing of wave activity is compared with derived data from satellite observations during other winters.

Petzoldt, K.↗

NASA NTRS1994

Principal modes of atmospheric circulation anomalies associated with global angular momentum fluctuations

This paper provides a description of the variability of global atmospheric angular momentum (GAM) and its relationship with principal modes of three-dimensional atmospheric circulation anomalies. The data used are 5-day mean global wind fields from the European Centre for Medium-Range Weather Forecasts initialized dataset for 1980-1989. Significant seasonal variation of GAM is observed with maxima in April and November and a minimum during late July. The amplitude of the annual cycle is largest in the upper troposphere and decreases toward the surface. Although the lower tropospheric contribution to the total angular momentum is relatively small, its annual cycle is out of phase with those of the upper atmosphere and GAM. Also identified is a distinct semiannual component, with double peaks appearing in April and November. This signal is most noticeable in the upper troposphere above the 300-mb level. The principal modes of zonal-mean angular momentum and meridional circulation anomalies and their coupled modes are obtained by using empirical orthogonal function analysis and singular value decomposition. It is shown that the leading modes of the angular momentum and meridional circulation are coupled with each other and are responsible for much of the variability in GAM. The coupled modes represent fluctuations of upper-level subtropical zonal flow, which are linked to the modulation of Hadley circulation intensity in both hemispheres. It is found that GAM is highly correlated with the first eigenvector of upper-level streamfunction anomalies, which consists of a superrotational flow in the tropics and subtropics, except over the central Pacific where a 'blocked' flow with two subtropical anticyclonic circulation cells straddling the equator is found. Much of the blocked flow is due to the establishment of dipole anomalies in the velocity potential with centers over the central Pacific and the Maritime Continent on the interannual time scale. On the intraseasonal time scale, GAM fluctuation is dominated by superrotational flow in the tropics, with the blocked flow present to a much lesser extent. The associated velocity potential anomaly has a weak dipole structure with centers over the Indian Ocean and the eastern Pacific. The implications of the above results on the total angular momentum balance of the earth-atmosphere system are also discussed.

Kang, In-Sik↗

NASA NTRS2019

Extratropical Large-Scale Traveling Weather Systems in the Southern Hemisphere on Mars

From late-autumn through early-spring, the middle- and high-latitudes of both hemispheres of Mars and its predominantly carbon-dioxide atmosphere support mean equator-to-pole thermal contrasts, and then, support a strong mean westerly polar vortex. Observations from orbiting spacecraft indicate that this intense mean baroclinicity-barotropicity supports large-scale eastward traveling weather systems (i.e., transient, traveling synoptic-period waves, on the order of the Rossby deformation scale). On Earth, extratropical weather disturbances arise from wind-shear instabilities, and these are critical components of the terrestrial global circulation. So it is the case for Mars. Large-scale traveling weather systems on Mars serve as agents in the transport of heat, momentum and scalar and tracer quantities (e.g., atmospheric dust, watervapor, ice clouds, chemical species, etc). Such weather systems interact with other large-scale atmospheric circulation components, namely, quasi-stationary (i.e., forced Rossby) modes; global thermal tidal modes; and then, upon large-/continental- geographical scales, upslope/ down-slope flows amongst high relief, low relief, impact basins, and volcanic rises, and more. The character of Mars' traveling extratropical weather disturbances in its southern hemisphere during late winter through early spring is investigated using a high-resolution Mars global climate model (i.e., Mars GCM), and one from the Agency's Mars Climate Modeling Center (MCMC) based at the NASA Ames Research Center. The climate model includes several complex atmospheric physical packages. With such physics modules, our global climate simulations present comparatively well with observations of the planet's current water cycle (Haberle et al.,2019). The climate model is "forced" with an annual dust cycle (i.e., nudged based on MGS/TES observations). Compared to the northern-hemisphere counterparts, the southern synoptic-period weather disturbances and accompanying frontal waves have smaller meridional and zonal scales, and are less intense. Influences of the zonally asymmetric (i.e., east-west varying) topography on southern large-scale weather are investigated, in addition to large-scale up-slope/down-slope flows and the diurnal cycle. A southern storm zone in late winter and early spring presents in the western hemisphere via orographic influences from the Tharsis highlands, and the Argyre and Hellas impact basins. Geographically localized transient-wave activity diagnostics are constructed that illuminate dynamical differences amongst the simulations and these are presented.

Hollingsworth, Jeffery L.↗