Engineering topics

Nathan, Terrence R.

Publications and source records attributed to Nathan, Terrence R..

10 records

NASA NTRS1994

The global atmospheric response to low-frequency tropical forcing: Zonally averaged basic states

The extratropical response to localized, low-frequency tropical forcing is examined using a linearized, non-divergent barotropic model on a sphere. Zonal-mean basic states characterized by solid-body rotation or critical latitudes are considered. An analytical analysis based on WKB and ray tracing methods shows that, in contrast to stationary Rossby waves, westward moving, low-frequency Rossby waves can propagate through the tropical easterlies into the extratropics. It is shown analytically that the difference between the stationary and low-frequency ray paths is proportional to the forcing frequency and inversely proportional to the zonal wavenumber cubed. An expression for the disturbance amplitude is derived that shows the ability of the forced waves to maintain their strength well into middle latitudes depends on their meridional wave scale and northward group velocity, both of which are functions of the slowly varying background flow. A local energetics analysis shows that the combination of energy dispersion from the forcing region and energy extraction from the equatorward flank of the midlatitude jet produces disturbances that have the greatest impact on the extratropical circulation. Under the assumption that the forcing amplitude is independent of frequency, this impact is largest when the tropical forcing period is in the range 10-20 days.

Li, Long↗

NASA NTRS1994

Ozone heating and the destabilization of traveling waves during summer

The effects of ozone heating on the linear stability of lower stratospheric traveling waves of the summertime, extratropical circulation are examined. Based on coupled equations for the quasigeostrophic potential vorticity and ozone volume mixing ratio, it is shown that the diabatic heating arising from ozone advection can offset the damping due to Newtonian cooling, leading to wave amplification and significant changes in the structure and zonally rectified fluxes of the wave fields in both the lower stratosphere and troposphere. The vertical profile of the zonal mean wind plays a crucial role in determining whether the ozone heating destabilizes eastward and/or westward traveling disturbances.

Nathan, Terrence R.↗

NASA NTRS1994

Low-frequency oscillations of forced barotropic flow

Jin and Ghil demonstrate that for topographically resonant flow, low-frequency finite-amplitude oscillations may arise from wave -- wave interactions and topographic form drag. Their model is extended to include a zonally asymmetric vorticity source, which is shown to interact with the perturbation field to produce zonally rectified wave fluxes that dramatically alter the Hopf bifurcation from stationary solutions to low-frequency oscillations. The frequency, intensity, and general character of these oscillations are shown to depend crucially upon the phasing and relative strength of the forcings.

Nathan, Terrence R.↗

NASA NTRS1993

On radiating baroclinic instability of zonally varying flow

A quasi-geostrophic, two-layer, beta-plane model is used to study the baroclinic instability characteristics of a zonally inhomogeneous flow. It is assumed that the disturbance varied slowly in the cross-stream direction, and the stability problem was formulated as a 1D initial value problem. Emphasis is placed on determining how the vertically averaged wind, local maximum in vertical wind shear, and length of the locally supercritical region combine to yield local instabilities. Analysis of the local disturbance energetics reveals that, for slowly varying basic states, the baroclinic energy conversion predominates within the locally unstable region. Using calculations of the basic state tendencies, it is shown that the net effect of the local instabilities is to redistribute energy from the baroclinic to the barotropic component of the basic state flow.

Finley, Catherine A.↗

NASA NTRS1991

Linear stability of free planetary waves in the presence of radiative-photochemical feedbacks

The diabatic effects of Newtonian cooling and ozone-dynamics interaction on the linear stability of free planetary waves in the atmosphere have been studied using a simple beta-plane model. The model couples radiative transfer, ozone advection, and ozone photochemistry with the quasi-geostrophic dynamical circulation. An analytical expression is derived which demonstrates the following: (1) the influence of meridional ozone advection on wave growth or decay depends on the wave and basic state vertical structures; and (2) photochemically accelerated cooling, which predominates in the upper stratosphere, augments the Newtonian cooling rate and is stabilizing. Attention is also given to the 1D linear stability problem which is numerically solved for a Charney basic state and for zonal mean basic states. It is shown that ozone heating generated by ozone-dynamics interaction in the stratosphere can reduce (enhance) the damping rates due to Newtonian cooling by as much as 50 percent for planetary waves of large vertical scale and maximum amplitude in the stratosphere.

Nathan, Terrence R.↗

NASA NTRS1990

Barotropic stability of westward-moving waves in realistic stratospheric zonal flows

A linearized barotropic model on a sphere is used to study the stability of a basic state composed of a westward-moving wave and a zonal mean jet. It is found that the inclusion of a very weak jet in the basic state dramatically alters the stability of the flow. The alteration of stability characteristics for realistic summer and winter jets is discussed.

Manney, Gloria L.↗

NASA NTRS1989

On the role of ozone in the stability of Rossby normal modes

The role of ozone in the linear stability of Rossby normal modes is examined in a continuously stratified, extratropical baroclinic atmosphere. The flow is described by coupled equations for the quasi-geostraphic potential vorticity and ozone volume mixing ratio. A perturbation analysis is carried out under the assumption of weak diabatic heating, which is generated by Newtonian cooling and dynamics-ozone interaction. An expression for the propagation and growth characteristics is obtained analytically in terms of the vertically averaged wave activity, which depends explicitly on the wave spatial structure, photochemistry, and basic state distributions of wind, temperature, and ozone mixing ratio. Calculations show that stationary internal modes, whose amplitudes are largest in the stratosphre, are destabilized by dynamics-ozone interaction and Newtonian cooling, with e-folding times on the order of 20-40 days.

Nathan, Terrence R.↗

NASA NTRS1989

Barotropic instability of basic states with a realistic jet and a wave

The stability of basic states consisting of a jet similar to the stratospheric polar night jet and a traveling wave with a single zonal wavenumber is examined in a linearized nondivergent barotropic model on a sphere. Basic state waves are chosen to resemble observed traveling and stationary features in the winter stratosphere. Results are presented for disturbance growth rates, propagation characteristics, and energy conversion as a function of the basic state wave amplitude. The effects of small amplitude basic state waves on unstable disturbances arising from a zonally symmetric jet are discussed; results are shown where a small amplitude basic state wave dramatically affects the stability characteristics. Evidence is shown that the presence of a traveling wave may favor the appearance of disturbances that include other zonal wavenumbers which move with the basic state wave; this result is discussed in relation to the origin of observed quasi-nondispersive features in the polar winter stratosphere. Results for a stationary wavenumber 1 basic state wave suggest that a distorted polar vortex may be unstable to disturbances that would lead to further distortion. An unstable disturbance for a basic state with an eastward moving wavenumber 2 has components which resemble, in period and location, traveling waves that are observed in the winter stratosphere.

Manney, Gloria L.↗

NASA NTRS1989

Use of satellite data and modeling to asses the influence of stratospheric processes on the troposphere

The research is comprised of the following tasks: use of simple analytical and numerical models of a coupled troposphere-stratosphere system to examine the effects of radiation and ozone on planetary wave dynamics and the tropospheric circulation; use of satellite data obtained from the Nimbus 7 Limb Infrared Monitor of the Stratosphere (LIMS) instrument and Solar Backscattered Ultraviolet (SBUV) experiment, in conjunction with National Meteorological Center (NMC) data, to determine the planetary wave vertical structures, dominant wave spectra, ozone spectra, and time variations in diabatic heating rate; and synthesis of the modeling and observational results to provide a better understanding of the effects that stratospheric processes have on tropospheric dynamics.

Nathan, Terrence R.↗

NASA NTRS1988

Barotropic stability of realistic stratospheric jets

The stability of realistic jets is examined in a linearized barotropic model on a sphere. Approximately non-dispersive modes associated with a region of negative basic state absolute vorticity gradient on the poleward side of the jet are examined in detail. As in previous studies, broader jets and those which peak at higher latitudes produce poleward modes that are less dispersive. Jet profiles derived from observational data at 10, 5, and 2 mb for three Southern Hemisphere winter months are used in the model, and the results are compared with quasi-nondispersive features (QNDF) which have been observed in satellite data in the Southern Hemisphere winter stratosphere. Characteristics of the barotropically unstable modes compare remarkably well with those of the observed modes. The barotropic model results for a month in which these features are not observed indicate the presence of equatorward modes at wavenumbers 3 and 4 which grow considerably faster than the quasi-nondispersive poleward modes. The appearance of westward moving modes in the summer hemisphere during June and in analytical profiles with a realistic global structure is also noted.

Manney, Gloria L.↗