Engineering topics

Dunkerton, Timothy J.

Publications and source records attributed to Dunkerton, Timothy J..

At least 19 records

NASA NTRS2012

Application of the Marsupial Paradigm to Tropical Cyclone Formation from Northwestward-Propagating Disturbances

A wave-tracking algorithm is developed for northwestward-propagating waves that, on occasion, play a role in tropical cyclogenesis over the western oceans. To obtain the Lagrangian flow structure, the frame of reference is translated obliquely at the same propagation speed with the precursor disturbance. Trajectory analysis suggests that streamlines in the obliquely translated frame of reference can be used to approximate flow trajectories. The algorithm was applied to Super Typhoon Nakri (2008), Tropical Cyclone Erika (2009), and a few other examples. Diagnoses of meteorological analyses and satellite-derived moisture and precipitation fields show that the marsupial framework for tropical cyclogenesis in tropical easterly waves is relevant also for northwestward-propagating disturbances as are commonly observed in the tropical western Atlantic, the Gulf of Mexico, and the western North Pacific. Finally, it is suggested that analysis of the global model data and satellite observations in the marsupial framework can provide useful guidance on early tropical cyclone advisories.

Wang, Zhuo↗

NASA NTRS2003

Investigation of Tropical Dynamics and Transport with UARS Data

Our research focused on Kelvin waves in the tropical lower stratosphere, and resulted in three papers published or submitted to the Journal of Geophysical Research. The first of these, published in 2002, used temperature data from the Microwave Limb Sounder to examine the amplitude, frequency, phase, and spatial structure of leading modes of Kelvin waves. The second and third, submitted late in 2002 and currently in revision, described the response to Kelvin waves of various trace constituents measured by MLS and CLAES (methane, nitrous oxide, CFC-12, and ozone in the second paper; water vapor in the third paper). Water vapor is a special case because the vertical structure induced by Kelvin waves is convolved with water vapor's seasonally varying vertical profile induced by seasonal variations in temperature at the tropical tropopause. Forward modeling indicated that the vertical resolution of MLS was indeed adequate to capture this complicated structure, yet it was not visible in the MLS data, though the Kelvin wave signature was clear on certain UARS levels. The effects of Kelvin waves on the tropical tropopause and on stratosphere- troposphere exchange cannot be quantified from UARS data because of poor vertical resolution and sensitivity in that region. It is recommended that this analysis be repeated using data from the new MLS and HIRDLS instruments aboard Aura, and that priority be given to fine-scale retrievals of temperature, water vapor, and ozone in the tropical tropopause region.

Jackman, Charles↗

NASA NTRS2000

Decadal Variability and Temperature Trends in the Middle Atmosphere From Historical Rocketsonde Data

Observational studies were performed using historical rocketsonde data to investigate long-term temperature trends, solar-cycle variations, and interactions between tropical and extratropical latitudes in the middle atmosphere. Evidence from tropical, subtropical, and midlatitude North American rocketsonde stations indicated a consistent downward trend over 25 years, with a solar cycle component superposed. The trend is about -1.4 to -2.0 K per decade and the amplitude of the decadal oscillation is about 1.1 K. Prior to trend derivation it was necessary for us to correct temperatures for aerodynamic heating in the early years. The empirically derived correction profile agrees well with a theoretical profile of Krumins and Lyons. A study was also performed of the correlation between equatorial winds and north polar temperatures in winter, showing that the entire stratospheric wind profile near the equator -- including the quasi-biennial oscillation (QBO) and stratopause semiannual oscillation (SAO) -- is important to the extratropical flow, not merely the QBO component as previously thought. A strong correlation was discovered between winter polar temperatures and equatorial winds in the upper stratosphere during the preceding September, suggesting a role for the second cycle of the SAO.

Dunkerton, Timothy J.↗

NASA NTRS1999

Investigation of Tropical Transport with UARS Data

Measurements of trace constituents obtained by instruments aboard the Upper Atmosphere Research Satellite (UARS) have been used to study transport processes associated with the quasi-biennial oscillation, laterally propagating Rossby waves, and upward propagating Kelvin waves in the tropical and subtropical upper troposphere and stratosphere. Mean vertical motions, vertical diffusivities and in-mixing rates were inferred from observations of the 'tape recorder' signal in near-equatorial stratospheric water vapor. The effect of the quasi-biennial oscillation (QBO) on tracer distributions in the upper half of the stratosphere was seen in a spectacular 'staircase' pattern, predominantly in the winter hemisphere, revealing the latitudinally asymmetric nature of QBO transport due to induced mean meridional circulations and modulation of lateral mixing associated with planetary Rossby waves. The propagation of Rossby waves across the equator in the westerly phase of the QBO was seen in tracer fields and corroborating United Kingdom Meteorological Office (UKMO) analyses; a modeling study of the effect of these waves on typical QBO wind profiles was performed. Water vapor in the upper troposphere and lower stratosphere was found to exhibit signatures of the tropical intraseasonal oscillation (TIO) and faster Kelvin waves in the two regions, respectively.

Dunkerton, Timothy J.↗

NASA NTRS1994

Observed correlations between winter-mean tropospheric and stratospheric circulation anomalies

It is shown that interannual variability of the northern winter stratospheric flow in 1964-1993 was closely linked to large-scale circulation anomalies in the middle troposphere. Of the known tropospheric teleconnection patterns, the one having the strongest relation to the DJF (December-February) zonal-mean stratospheric flow was the North Atlantic Oscillation (NAO). Singular value decomposition between the 500 and 50-hPa geopotential heights produced a 500-hPa structure containing elements of the NAO pattern, but including an anomaly in eastern Siberia. During this time period, the correlation of NAO-related modes to the polar lower stratosphere exceeded that of the equatorial quasi-biennial oscillation.

Baldwin, Mark P.↗

NASA NTRS1993

Observation of 3-6 day meridional wind oscillations over the tropical Pacific, 1973-1992: Vertical structure and interannual variability

Rawinsonde data from tropical Pacific stations were examined for westward-propagating 3-6-day meridional wind oscillations in the troposphere and lower stratosphere, 1973-1992. Four types were identified from cross-spectrum and principal component analysis. (1) The dominant oscillation, near 250 mb, had a period slightly greater than 5 days, zonal wavenumber 4-6, and modified Rossby-gravity structure near the date line. (2) In the western Pacific lower troposphere there was broadband activity with short zonal scale, coupled to upper-tropospheric waves in NH summer. (3) In the central Pacific, during NH autumn, there was a well-defined approximately 4 1/2-day oscillation with maximum amplitude in the lower troposphere and baroclinic phase tilt above. The vertical structure suggested coupling to deep tropical convection; this interpretation was supported by correlation of meridional wind with antisymmetric outgoing longwave radiation. (4) In the stratosphere, Rossby-gravity waves had periods less than or equal to 4 days and zonal wavenumber 3-4. Unlike tropospheric waves, these disturbances were coherent in a shallow layer, largest in west phase of quasi-biennial oscillation (QBO) and annual cycle (NH winter-spring).

Dunkerton, Timothy J.↗

NASA NTRS1992

Analytic and numerical models of wave-CISK with conditional heating

Wave-CISK with conditional heating is investigated in the equatorial zonal-height plane by analytic and numerical techniques. For two- and three-level models, previous results are extended to give additional evidence that the most unstable mode has a single wet region of infinitesimal width. A three-level model has qualitatively similar behavior as the two-level model except that propagating solutions are possible due to coalescence of internal vertical modes. Phase speeds with conditional heating are found to be slightly greater than those for unconditional heating. The structure has one circulation cell in the vertical and is asymmetric in longitude with stronger motion on the leading edge. Growth rate is inversely proportional to the width of the single wet region. That width can be limited by second-order diffusion. A general integral relationship between growth rate, viscosity, phase speed, and heating is derived. The main conclusion is that the linear wave-CISK catastrophe is modified by conditional heating but not eliminated. The preferred mode of instability has one wet region, but it occurs on the smallest possible scale. It is likely that numerical models that use conditional heating are sensitive to resolution, especially for the commonly used spectral truncations, unless there is sufficiently strong damping at the smallest scales.

Crum, Francis X.↗

NASA NTRS1992

Modes of interannual variability in the stratosphere

During 1964-91, stratospheric temperature and circulation in Northern Hemisphere winter varied interannually on time scales from 2 to about 12 years. A substantial percentage of December-February interannual variance was correlated with the quasi-biennial oscillation (QBO). Additional monthly variance could be accounted for by quasi-decadal oscillation and QBO/low-frequency modulation. The QBO was the largest and most consistent of these signals, and its decadal modulation explains an apparent correlation with the solar cycle depending on the sign of the QBO - an interpretation supported by principal component analysis.

Dunkerton, Timothy J.↗

NASA NTRS1991

Anomalous temperature and zonal wind in the tropical upper stratosphere, 1982/1983

Observed temperature and zonal wind anomalies during 1982-1983 are described in relation to the 20-yr rocketsonde data record. Temperature data from the Nimbus 7 SAMS indicate that in mid-1982, cooling occurred in the upper and middle tropical stratosphere, starting shortly after the eruption of El Chichon (April 1982). Rocketsonde data from Kwajalein gave additional evidence of cooling and revealed that zonal winds in the upper and middle stratosphere were anomalous in 1982 and also in 1983. Rocketsonde observations were consistent with balance winds derived from SAMS. Upper-level cooling was linked to anomalous easterlies near the equator and therefore could not be interpreted as a radiative (nondynamical) response to El Chichon. The structure of an expected 2D dynamical response to aerosol heating is examined with a numerical model and compared to the data. It is suggested that internal sources of atmospheric variability (eg., extratropical forcing and quasi-biennial oscillation) contributed to the equatorial cooling observed in 1982.

Dunkerton, Timothy J.↗

NASA NTRS1991

Intensity variation and coherence of 3-6 day equatorial waves

Meridional wind oscillations of 3-6 day period are common in the equatorial lower stratosphere. Upper air data obtained from 32 equatorial stations, 1973-87, show that 70 mb wave intensity over the Maritime continent varied annually with largest amplitude in NH late winter and spring, but the seasonal cycle of intensity was not uniform in the tropics. Time series of meridional velocity and its spatial coherence suggest that 70 mb wave packets were local in longitude and episodic in time.

Dunkerton, Timothy J.↗

NASA NTRS1991

Scale selection and propagation of wave-CISK with conditional heating

Analytic solutions are derived for simple models of equatorial wave-CISK with conditional (positive-only) heating. Conditional heating favors an unstable mode having a single wet region of infinitesimal width. This result helps explain model simulations of tropical superclusters and intraseasonal oscillations. Mechanisms of scale selection and propagation are discussed, and the model is generalized to include evaporation-wind feedback, multiple vertical-mode CISK, and scale-dependent damping. The first two mechanisms cause zonal propagation, without scale selection. Scale-dependent damping breaks the inviscid continuum (wherein modes exist as a continuous function of wet region width) into discrete modes with unique horizontal structure and phase speed. Results are contrasted with the hierarchy of scales in observed intraseasonal oscillations.

Dunkerton, Timothy J.↗

NASA NTRS1991

Quasi-biennial oscillation above 10 mb

It is shown that the quasi-biennial oscillation of the equatorial lower stratosphere was correlated with mean zonal wind in the upper stratosphere, 1979-1990. Correlations were positive near 60 deg N and 30 deg S during Northern Hemisphere (NH) winter and negative in the equatorial upper stratosphere during all seasons. Spatial autocorrelation of mean zonal wind during NH winter was actually largest in the upper stratosphere, between 10 deg S and 62 deg N, due to strong coupling between tropical and extratropical flow at upper levels.

Baldwin, Mark P.↗

NASA NTRS1991

Quasi-biennial modulation of planetary-wave fluxes in the Northern Hemisphere winter

Using 25 years of National Meteorological Center (NMC) data for 1964-88 the relation between tropical and extratropical quasi-biennial oscillations (QBOs) was examined for zonally averaged quantities and planetary-wave Eliassen-Palm fluxes in the Northern Hemisphere winter. The extratropical QBO discussed by Holton and Tan (1980) existed in both temporal halves of the dataset. Autocorrelation analysis demonstrated that it was an important mode of interannual variability in the extratropical winter stratosphere. Correlation with the tropics was strongest when 40-mb equatorial winds were used to define the tropical QBO. Easterly phase at 40 mb implied a weaker than normal polar night jet and warmer than normal polar temperature and vice versa. An opposite relationship was obtained using 10-mb equatorial winds. The association between tropical and extratropical QBOs was observed in about 90 percent of the winters and was statistically significant. It is shown that planetary-wave Eliassen-Palm fluxes were generally consistent with the extratropical QBO. These fluxes were more (less) convergent in the midlatitude (subtropical) upper stratosphere in the 40-mb east (= easterly) phase category relative to the west category.

Dunkerton, Timothy J.↗

NASA NTRS1991

LIMS (Limb Infrared Monitor of the Stratosphere) observation of traveling planetary waves and potential vorticity advection in the stratosphere and mesosphere

Eastward and westward traveling waves were observed by the Nimbus 7 Limb Infrared Monitor of the Stratosphere (LIMS) during the northern winter 1978-1979. Eastward waves were prevalent in early winter and were involved in a minor Canadian warming in December 1978. A large westward traveling wave, as described by previous authors, was observed in January 1979 during a series of minor warmings. By comparing these two events, it is shown that in both cases the superposition of traveling and quasi-stationary waves led to constructive interference that was responsible for the warmings. However, there was significant asymmetry between eastward and westward traveling components. A local Eulerian analysis of potential vorticity (PV) transport indicates that adiabatic, geostrophic advection by the resolvable scales of motion explains qualitatively (but not quantitatively) the observed potential vorticity tendencies in the LIMS Northern Hemisphere winter. In particular, calculated advection explains the eastward rotation of the main vortex, intrusion of low PV air into the polar cap, and formation of high PV filaments at the vortex periphery.

Dunkerton, Timothy J.↗

NASA NTRS1990

The role of the seasonal cycle in the quasi-biennial oscillation of ozone

The evidence for seasonal synchronization of the ozone quasi-biennial oscillation (QBO) is reviewed, showing how this behavior is not expected on the basis of QBO transport alone but may arise due to the seasonal cycle. Near the equator, the ozone QBO is closely tied to the dynamical QBO and exhibits the same period as that oscillation. Maximum column ozone is attained soon after the transition to westerlies at 50 mb. In the subtropics, the ozone QBO is strongly tied to the seasonal cycle, with anomalies centered in the winter-spring season irrespective of QBO phase. These anomalies result from the interaction of the annual cycle and the QBO. Numerical model results are reported which establish that hemispheric asymmetry in the ozone QBO does not require seasonal asymmetries in the strength or location of the Hadley circulation. The fundamental cause of hemispheric asymmetry in the subtropical ozone QBO is the timing of equatorial, ozone anomalies relative to the seasonal cycle.

Gray, Lesley J.↗

NASA NTRS1990

Annual variation of deseasonalized mean flow acceleration in the equatorial lower stratosphere

The quasi-biennial oscillation (QBO) in the equatorial lower stratosphere appears to be influenced by the seasonal cycle, as phase transitions at 50 mb occur primarily in the northern spring/summer season (April-August). Descent of east wind regimes varies widely from one QBO cycle to another. Most of this variation occurs because easterly shears slow down or 'stall' in their descent sometime between July and February. Minimum mean flow accelerations at 50 mb occur in the northern winter season, slightly before the annual minimum in equatorial tropopause temperature. Although a weak effect of the semiannual oscillation can be detected near 10 mb, the seasonal effect over most of the QBO region is annual. The seasonal cycle apparently modulates the onset of QBO phases, and slightly enhances the ability to predict the QBO, but is of insufficient strength or consistency to exactly synchronize the quasi-biennial oscillation with the seasonal cycle.

Dunkerton, Timothy J.↗

NASA NTRS1990

Eigenfrequencies and horizontal structure of divergent barotropic instability originating in tropical latitudes

A systematic investigation of eigenfunctions of the generalized Laplace tidal equation, for the case of tropical-latitude monotonic mean zonal wind profiles with a single narrow reversed vorticity gradient region, has revealed that, in the limit of low planetary zonal wavenumber, the barotropic instability modes bifurcate into weakly divergent modes of hemispheric scale, on the one hand, and on the other strongly divergent 'internal' modes trapped about the source region. These latter disturbances penetrate into the deep tropics as a Kelvin wave-type phenomenon not previously seen in this context. These results suggest that hemispheric barotropic instability need not be purely nondivergent, and that the growth of weakly divergent modes is preferred.

Dunkerton, Timothy J.↗

NASA NTRS1989

The stratospheric major warming of early December 1987

The NMC observations of temperature and geopotential carried out during the stratospheric major warming event of early December 1987 are analyzed. The December 1987 warming event displayed features typical of other major warmings, including vortex displacement, erosion, and splitting and, as such, was the earliest major warming yet observed in the Northern Hemisphere winter. The December 1987 major warming was unusual in that it took place during the (deep) westerly phase of the equatorial quasi-biennial oscillation and was not preceded by any significant preconditioning of the extratropical vortex. It is suggested that this unusually early major warming was mainly attributable to an anomalously large tropospheric forcing.

Baldwin, Mark P.↗