Showing papers on "Atmospheric wave published in 1996"

Global Observations of Oceanic Rossby Waves

Abstract: Rossby waves play a critical role in the transient adjustment of ocean circulation to changes in large-scale atmospheric forcing. The TOPEX/POSEIDON satellite altimeter has detected Rossby waves throughout much of the world ocean from sea level signals with ≲10-centimeter amplitude and ≳500-kilometer wavelength. Outside of the tropics, Rossby waves are abruptly amplified by major topographic features. Analysis of 3 years of data reveals discrepancies between observed and theoretical Rossby wave phase speeds that indicate that the standard theory for free, linear Rossby waves is an incomplete description of the observed waves.

Observation of a quasi-2-day wave during TOGA COARE

Abstract: Detailed structure of the quasi-2-day oscillation observed in the active phase of the Madden–Julian oscillations during the intensive observation period of Tropical Ocean and Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE IOP) was described. A variety of observational platforms is used including high-resolution GMS infrared histogram, rain-rate estimate from TOGA and MIT radar measurements, upper-air soundings, and boundary layer profiler winds from the Integrated Sounding System and surface data from the IMET buoy. The quasi-2-day mode had a westward propagation speed of 12°–15° day −1, a horizontal wavelength of 25°–30° longitude. A coupling with the westward-propagating n = 1 inertio–gravity waves was hypothesized from the space–time power spectral distribution of the cloud field. The wind disturbance structure was consistent with the hypothesis. The vertical wave structure had an eastward phase tilt with height below 175 hPa and vice versa above, indicating the wav...

Atmospheric Lee Waves

Abstract: The atmospheric lee wave is a disturbance propagated by buoyancy and arising from an isolated source, usually by flow over ridges and mountains. Part of this review treats two-dimensional solutions, both Boussinesq and non-Boussinesq, linear and nonlinear. These discussions emphasize trapped waves, the downslope windstorm, the drag on the earth and the upward momentum flux, the hydro- static approximation and its limitations, effects of critical layers, and middle atmospheric wave breaking. Three-dimensional Boussinesq linear and nonlinear solutions are also discussed; shown are the variety of regimes possible, from ship waves to shedding vortices. Photographs of natural phenomena are pre- sented as realizations, together with relevant numerical simulation graphics. The difficulties and achievements of simulation models are also outlined.

Excitation and evolution of the quasi-2-day wave observed in UARS/MLS temperature measurements

Abstract: The quasi-2-day wave is known as a strong and transient perturbation in the middle and upper atmosphere that often occurs shortly after solstice. The excitation mechanisms of this transient wave have been discussed for years, but no clear answer has yet been attained. In this paper, propagating characteristics of the 2-day wave are studied based on 8-mon temperature measurements from the Microwave Limb Sounder onboard the Upper Atmosphere Research Satellite. The studies are focused on the wave events that happened in January 1993 and in July-August 1993. The observations suggest that winter planetary waves could be responsible for triggering the summer 2-day wave through long penetration into the summer stratosphere. A connection is evident in the evolution of the wave amplitude between the summer 2-day wave generation and winter wave penetration. The data also suggest that the enhancement of the wave amplitude is a manifestation of both a local unstable wave and a global normal-mode Rossby wave.

The two‐day wave in a middle atmosphere GCM

Abstract: A strong signal of the two-day wave is diagnosed in a middle atmosphere GCM, with characteristics very similar to those of the observed two-day wave. It results from an instability, but its global structure shows similarities to a Rossby-gravity planetary normal mode. It has a remarkable potential vorticity structure in wave 3 and sometimes wave 4 and higher wavenumbers. It is shown that gravity wave drag is important in maintaining the unstable zonal mean state.

Characteristics of atmospheric waves in the tidal period range derived from zenith observations of O2(0–1) Atmospheric and OH(6‐2) airglow at lower midlatitudes

Abstract: From about 100 nights of ground-based measurements of fluctuations in airglow brightness and rotational temperature, taken from two sites at 32°S and 37°N, a considerable number of new determinations of complex values of Krassovsky's η are derived, and compared with observations by other investigators and model predictions. Our findings support the model of Hines, Tarasick, and Shepherd (HTS), which leads to practical consequences regarding the usefulness of airglow observations for deriving vertical propagation of atmospheric waves in the mesopause region. It is shown, at least in part, that the vertical wave propagation can be inferred from zenith observations of one airglow emission, alone, and that consistent information can be obtained simultaneously for the two airglow layers. The analysis presented comprises the range of observed periods between 3 and 24 hours. The salient feature is that η values are essentially limited to the fourth quadrant, for both the O2 and OH emissions, which means, according to the HTS model, that the majority of the waves observed propagate upward, with vertical wavelengths between 20 and 60 km, and only a few are possibly evanescent. This would not contradict the interpretation that most if not all of the wave signatures may be due to the semidiurnal tide, or tidal transients.

High-Frequency Internal Waves in Lake Geneva

Abstract: Trains of high-frequency internal waves have been observed in the thermocline of Lake Geneva within a few hours after the passing of disturbances which cause a rapid increase or ‘jump’ in the depth of the thermocline Three possible mechanisms for the source of the internal waves are proposed and compared with available data, (i) that the waves are a soliton packet following the jump, (ii) that the waves are generated as the jump passes over or around a local region of rough topography, and (iii) that the jump produces a moving disturbance near the shoreline with internal waves forming a wake behind it The investigation of the last possibility involves the solution of the problem of the form of an internal wave wake in the presence of mean currents which vary in depth both in their strength and direction The wave pattern of the wake in the observed current distribution and stratification of the lake is described and compared with that in the absence of shear An expression for the energy of internal waves propagating in a shear flow is also derived, and it is found that the rate of energy loss from the jump to the waves is small The minimum Richardson number resulting from the co-existence of the mean shear and the internal waves is marginally greater than 1/4, suggesting that the waves may be limited by shear-flow instability However, none of the mechanisms for their generation and propagation is fully compatible with the available data

Tidal generation of the phase-locked 2-day wave in the southern hemisphere summer by wave-wave interactions

Abstract: The quasi-2-day wave is a prominent dynamical feature of the mesopause region. The onset of the large-amplitude summertime 2-day wave in the southern hemisphere is characterized by a sudden increase in amplitude and by phase locking so that the wind maximum occurs at nearly the same local time from cycle to cycle. The phase locking suggests a linkage to a diurnally repeatable phenomenon. We suggest that the phase-locked 2-day wave is a nearly resonant wave forced parametrically by the zonally symmetric diurnal tides. Recent observations and modeling support significant tidal forcing. It may also be possible for the migrating tides to generate the 2-day wave nonlinearly through a set of mutually reinforcing interactions. The process involves the self-excitation of the 2-day wave by an interaction with a diurnal zonal wavenumber 6 oscillation, itself generated nonlinearly in a two-step process initiated by the interaction of the 2-day wave and the migrating tides. Sudden onset, large amplitude, and phase locking are all consistent with the instability of a wave that has become resonantly tuned to a subharmonic frequency of the tides by the background wind system.

Observation of a nonlinear wave disturbance in the marine atmosphere by the synthetic aperture radar aboard the ERS 1 satellite

Abstract: A nonlinear wave disturbance in the marine atmosphere associated with an atmospheric cold front has been identified on a spaceborne radar image of the North Sea by its imprint on the sea surface This nonlinear wave disturbance is either a solitary wave disturbance or an undular bore propagating in the lower marine atmosphere The radar image showing sea surface manifestations of this atmospheric wave disturbance was acquired over the North Sea by the synthetic aperture radar (SAR) aboard the first European Remote Sensing Satellite ERS 1 on March 8, 1992 Atmospheric waves become visible on radar images of the sea surface because they are associated with a varying wind field at the sea surface, which modulates the sea surface roughness, and, thus, the backscattered radar power The identification of the wave pattern visible on the ERS 1 SAR image as a sea surface manifestation of an atmospheric nonlinear wave disturbance has been possible because this disturbance was also captured by meteorological sensors mounted on an 80-m high meteorological mast located on the island of Heligoland Geophysical parameters characterizing the atmospheric wave disturbance are derived from the ERS 1 SAR image and from the in situ meteorological data The wave period in the reference frame moving with the average wind is 95 min, the average wavelength is 2300 m, and the propagation velocity is 45–49 m s−1 Estimates of the amplitude of the wind speed fluctuations in the wave propagation direction derived from the meteorological measurements at Heligoland and from the gray level variations in the SAR image yield values between 13 and 30 m s−1 The experimental data are interpreted in terms of theories of atmospheric nonlinear gravity waves

Sudden changes in Arctic atmospheric aerosol concentrations during summer and autumn

Abstract: The International Arctic Ocean Expedition of 1991 measured number concentrations of condensation nuclei and cloud condensation nuclei from 1 August to 6 October between latitudes 70°N and 90°N. Changes in concentration of more than a factor of 2 in 1 h or less were frequent in spite of the long distances from significant sources and often appeared in groups separated by times of the order of 1 h. These observations provide a unique opportunity to study interacting meteorological mechanisms in the stable marine boundary layer. It is suggested that most of these changes were due to two factors: (1) large vertical concentration gradients caused mainly by interaction of aerosol and clouds and (2) intermittent localised mixing into the shallow surface mixed layer, caused by atmospheric wave motions or roll vortices. Quasi-periodic changes of smaller amplitude were present for about 35% of the total time with mean periods in the range 60–90 min. About 1/6 of the sudden large changes, usually involving an isolated decrease and recovery in accumulation mode particle number concentrations, is attributed to mixing caused by shear-induced Kelvin-Helmholtz breaking waves. Regular sequences of maxima and minima were about as common but involved 2/3 of all such changes because of the number of events in each sequence. Satellite images, radiosonde wind profiles and the typical mean periods suggested that roll vortices were common in the high Arctic. The associated mixing changes seemed to account adequately for the alternating maxima and minima in aerosol number concentrations. The special conditions of atmospheric stability and high frequencies of low cloud cover in the high Arctic combined with the absence of strong local sources of particles have allowed us to identify processes of mixing that often drastically modified aerosol size distributions and concentrations near the surface which are likely to have a far more general application. These factors have to be added to studies of chemical and physical processes influencing the life cycle of the aerosol and air transport, in order to understand the sources, sinks and to define the radiative forcing by the atmospheric aerosol in the Arctic and elsewhere. DOI: 10.1034/j.1600-0889.1996.t01-1-00009.x

The role of atmospheric waves in the laminated structure of ozone profiles at high latitude

Abstract: A relationship between the laminated structure of ozone profiles and atmospheric waves has been found from some EASOE ozonesonde data. This result is derived first from the correlation of ozone and potential temperature fluctuations. Then, the analysis of simultaneous measurements shows that the same wave can explain the structure found at different sites. When the wind profile is available, it may be related to the ozone structure, confirming the role of waves in this phenomenon. DOI: 10.1034/j.1600-0870.1996.t01-2-00006.x

Processes Leading to the Formation of Mesoscale Waves in the Midwest Cyclone of 15 December 1987

Abstract: On 15 December 1987, several long-lived, large-amplitude mesoscale wave disturbances accompanied a rapidly developing extratropical cyclone in the midwest United States. Previous observational and modeling studies have suggested that the disturbances had large amplitudes and long lifetimes as a result of a wave-CISK-type instability occurring within an imperfect wave duct and were initiated by convection. However. infrared (IR) satellite imagery and radar echoes shortly before the wave disturbances formed suggest that convection was not the primary feature in the wave genesis region at that time. Instead, a meso-β-scale comma-shaped cloud was present and appeared to evolve into the wave disturbances. The origins of the comma cloud can be traced back to a cloud streak and precipitation maximum in the left exit region of an approaching jet streak over northern Mexico 15 h earlier. In this study, satellite observations are examined in conjunction with numerical simulations of the case to explore a n...

Relative particle motion in capillary waves.

Abstract: When a container of fluid is oscillated vertically, capillary waves develop on the surface if the amplitude exceeds a critical value. Experimentally one finds that the motion of small particles on the surface of the fluid is close to Brownian. Here we study the relative motion of particle pairs. The experiment establishes that particle motion is strongly correlated over macroscopic distances. Our observations are in striking agreement with upper-ocean studies, and with theories that appear applicable to this “weak turbulence” problem, and in disagreement with experimental and theoretical results for two-dimensional large-scale atmospheric turbulence. [S0031-9007(96)00448-6]

Lee vorticity Production by Large-Scale Tropical Mountain Ranges. Part II: A Mechanism for the Production of African Waves

Abstract: A mechanism that acts to produce vorticity in the lee of large-scale mountain ranges embedded in an easterly flow in a stably stratified rotating atmosphere is investigated as it applies to the production of westward-propagating African waves. Three-dimensional numerical simulations of a dry, adiabatic flow using the PSU/NCAR MM4 model show that the Hoggar and Atlas Mountains of west-central Africa block low-level easterly flow, resulting in a barotropically unstable jet that is associated with the continuous production of lee vortices, which separate from the mountain and propagate downstream. The wavelength of the disturbances is roughly 1600 km, and they propagate to the west with a period of about 2.5 days. These characteristics correspond to those of observed waves in the Africa-Atlantic region. It is also shown that flow interaction with the topography of north-central Africa results in a midtropospheric easterly jet having a maximum wind speed near 10 ms−1 between 15° and 20°N, 0° and 10°E...

Sudden changes in arctic atmospheric aerosol concentrations during summer and autumn

Abstract: The International Arctic Ocean Expedition of 1991 measured number concentrations of condensation nuclei and cloud condensation nuclei from 1 August to 6 October between latitudes 70°N and 90°N. Changes in concentration of more than a factor of 2 in 1 h or less were frequent in spite of the long distances from significant sources and often appeared in groups separated by times of the order of 1 h. These observations provide a unique opportunity to study interacting meteorological mechanisms in the stable marine boundary layer. It is suggested that most of these changes were due to two factors: (1) large vertical concentration gradients caused mainly by interaction of aerosol and clouds and (2) intermittent localised mixing into the shallow surface mixed layer, caused by atmospheric wave motions or roll vortices. Quasi-periodic changes of smaller amplitude were present for about 35% of the total time with mean periods in the range 60–90 min. About 1/6 of the sudden large changes, usually involving an isolated decrease and recovery in accumulation mode particle number concentrations, is attributed to mixing caused by shear-induced Kelvin-Helmholtz breaking waves. Regular sequences of maxima and minima were about as common but involved 2/3 of all such changes because of the number of events in each sequence. Satellite images, radiosonde wind profiles and the typical mean periods suggested that roll vortices were common in the high Arctic. The associated mixing changes seemed to account adequately for the alternating maxima and minima in aerosol number concentrations. The special conditions of atmospheric stability and high frequencies of low cloud cover in the high Arctic combined with the absence of strong local sources of particles have allowed us to identify processes of mixing that often drastically modified aerosol size distributions and concentrations near the surface which are likely to have a far more general application. These factors have to be added to studies of chemical and physical processes influencing the life cycle of the aerosol and air transport, in order to understand the sources, sinks and to define the radiative forcing by the atmospheric aerosol in the Arctic and elsewhere. DOI: 10.1034/j.1600-0889.1996.t01-1-00009.x

Stellar scintillations as a remote atmospheric wave-front sensor

Abstract: Stellar scintillations are considered noise in adaptive-optics sensors and are measured for calibration purposes only. We propose to use scintillations to provide direct instantaneous information about the structure of the atmosphere. As a result it will be possible to increase the field of view provided by adaptive optics. The scintillation pattern is created when stellar light is diffracted by high-altitude turbulence. Alternatively, this pattern can be viewed as a Laplacian of this turbulence and can thus be inverted to estimate it. The measurement is limited by the intensity and the angular size of the reference star, by the height distribution of the atmospheric turbulence, and by the detector resolution and spectral response.

Surface wind speed retrieval using passive microwave polarimetry: the dependence on atmospheric stability

Abstract: The results of the Joint US/Russia Internal Wave Remote Sensing Experiment are presented. An airborne Ka-band radiometer-polarimeter was used for surface wind vector retrieval based upon the polarizational anisotropy effect. The anisotropy versus wind speed curve was noted to be approximately twice as steep under unstable atmospheric stratification than under stable or neutral stratification. The dependence of anisotropy on both friction velocity and stability parameter z/L was examined. The correlation between anisotropy and stability parameter is significant, especially for slightly stable stratification. The paper concludes that atmospheric stratification should be taken into consideration to improve the wind speed retrieval algorithm.

Atmospheric tides and rotation of the Earth

Abstract: The six-hourly values of the atmospheric angular momentum (AAM) functions computed by the U.S. National Meteorological Center (NMC) were used to estimate the effects of the atmospheric tides on the Earth's rotation. Variations of the equatorial componentsχ1 andχ2 of the AAM have periods close to gravitational tidesP1 andK1.The amplitudes of the detected variations inχ1 andχ2 functions have been found to be much larger than the theoretical ones, the reason of this amplification remains unexplained. According to theoretical formulations, these waves can be expressed only as retrograde motions. Because of frame effects, there is a correspondance between diurnal retrograde polar motion and precession-nutations and the atmospheric effect on polar motion cannot be detected from observations.

Baroclinic wave variations observed in MLS upper tropospheric water vapor

Abstract: Upper tropospheric water vapor measurements from the UARS Microwave Limb Sounder are used to investigate the structure and evolution of eastward traveling medium-scale wave features in Southern Hemisphere summertime. The extratropical Southern Hemisphere summer circulation pattern is frequently dominated by medium scale waves which exhibit life cycles of baroclinic growth and barotropic decay. The water vapor field during such life cycles is examined here and found to be well correlated with meteorological fields derived from European Centre for Medium Range Weather Forecasts global analyses. From mid January to mid February 1992 several episodes of growth and decay in the amplitude of eastward traveling waves are found in the water vapor and meteorological data at levels of the upper troposphere, with zonal waves four, five and six being predominant modes. The water vapor data are compared with derived potential vorticity (PV) fields, with strong anticorrelation observed in middle and high latitudes. The results are consistent with model paradigms for the structure and evolution of baroclinic disturbances, coupled with the known characteristics of high PV and low water vapor mixing ratios in lower stratospheric air parcels and the reverse for upper tropospheric air.

COAST WATCH-95: ERS-1/2 SAR applications of mesoscale upper ocean and atmospheric boundary layer processes off the coast of Norway

Abstract: The ERS SAR sensors have demonstrated their capability for imaging ocean waves, internal waves, underwater topography, eddies, fronts, natural slicks, oil slicks, and atmospheric features in the marine boundary layer such as wind. In order to quantify the mesoscale upper ocean and atmospheric boundary layer processes, a tandem ERS-1/2 ESA AO experiment was carried out off the south-western coast of Norway during the month of September 1995. A unique integrated data set was collected, including extensive SAR coverage in the ERS-1/2 tandem mode, an extensive set of oceanographic and meteorological variables from the R/V Hakon Mosby of the University of Bergen, metocean parameters from state-of-the-art metocean buoys, and natural slick sampling with two radio-controlled "mini research vessels". Analysis from this very comprehensive integrated experiment is reported, quantifying coastal jets, ocean fronts, natural slicks, wind velocity, wind fronts and rain showers from SAR imaging.

Excitation of Plasma Waves in the Ionosphere Caused by Atmospheric Acoustic Waves

Abstract: The transformation of atmospheric acoustic waves into plasma waves in the ionosphere is investigated. The transformation mechanism is based on plasma wave exitation by growing acoustic waves, when a frequency/wavelength matching situation is reached. The interaction of acoustic and plasma waves occurs through collisions of neutral particles with ions. For the case of ion-sound waves, oscillations on ion cyclotron frequency and Alfven waves is considered. A peculiarity of Alfven waves is the wide frequency band which may be stimulated through wave-wave interaction.

Distortion of gravity wave spectra of horizontal winds measured in atmospheric radar experiments

Abstract: Large disparities in upper mesospheric winds measured with different radar techniques have been attributed earlier to the possibility that MF radars are sensitive to the phase speed of waves, rather than to true winds. An alternative explanation attributes these disparities to biases due to horizontal gradients in the wind field. These biases are common to the Doppler beam swinging (DBS), spaced antenna (SA), and imaging Doppler interferometry (IDI) methods and arise because of spatial filtering of the wind field by the beam configuration. We develop a theoretical model for intrinsic frequency and vertical wave number spectra of DBS wind component estimates that includes the effect of spatial filtering on an ensemble of gravity waves. The model shows a strong enhancement of frequency components above a knee, at ∼1 hour intrinsic period. A similar model for the SA experiment is inherently intractable. We suggest, instead, that the DBS model should also hold for the spectra of SA wind component estimates. The knee is clearly evident in recently observed SA wind component spectra, but is less pronounced and occurs at a frequency lower than the DBS model predicts. Better agreement is expected with a model refined to include the effect of Doppler shifts due to background winds. Occurrence of the knee in observed SA wind spectra implies that MF radars are sensitive to winds rather than to phase speed of atmospheric waves.

Nonlinear interaction between acoustic gravity waves in a rotating atmosphere

Abstract: The influence of the Earth's rotation on the resonant interaction of atmospheric waves is investigated. The explicit expressions for the coupling coefficients are presented. They are derived by means of two different techniques; first, by a direct expansion derivation from a set of reduced equations, and second, by a Hamiltonian method.

Atmospheric solitary waves : some applications to the morning glory of the Gulf of Carpentaria

Abstract: A mathematical model is proposed to describe atmospheric solitary waves at the interface between a 'shallow' layer of fluid near the ground and a stationary upper layer of compressible air. The lower layer is in motion relative to the ground, perhaps as a result of a distant thunderstorm or a sea breeze, and possesses constant vorticity. The upper fluid is compressible and isothermal, so that its density and pressure both decrease exponentially with height. The profile and speed of the solitary wave are determined, for a wave of given amplitude, using a boundary-integral method. Results are discussed in relation to the 'morning glory', which is a remarkable meteorological phenomenon evident in the far north of Australia.

An investigation of wave-induced momentum flux through phase averaging of open ocean wind and wave fields

Abstract: This thesis presents an investigation of the influence of the dominant, wind-driven, surface waves on the vertical flux of horizontal momentum in the marine surface layer over open ocean conditions. Through a procedure which involves phase averaging the wind fields at the period of the dominant waves, the wave-induced component of the atmospheric fluctuations is isolated and vertical profiles of mean wave-induced momentum flux are computed. Previous investigators have used phase averaging to remove the turbulence from an oscillatory signal, but the absence of a monochromatic wave field in open ocean conditions complicates this approach. This difficulty is overcome by choosing only the time periods characterized by the most monochromatic-like waves present and filtering those sections of wind data with a narrow band-pass filter centered around the dominant wave frequency before beginning the phase averaging process. The analysis is carried a step further by investigating the dependence of wave-induced momentum flux on sea state through bin averaging according to wave age. This results in a set of profiles which express the ratio of wave-induced momentum flux to the total flux as a function of the wave age parameter, c/U 10 . These profiles all tend to zero with height, and remain negligible at all heights over fully developed seas. Over younger seas, this ratio becomes increasingly more positive (corresponding to a positive value for i ) with decreasing wave age; while over older seas, this ratio becomes increasingly more negative with increasing wave age. This provides strong quantitative evidence that there is a significant flux of momentum to the atmosphere from decaying waves, and a transfer of atmospheric momentum to developing waves. Thesis Supervisor: Dr. James B. Edson Title: Associate Scientist, WHOI