Observations of Helmholtz Waves in the Lower Atmosphere with an Acoustic Sounder

Acoustic sounder and tower data obtained at the Boulder Atmospheric Observatory (BAO) are used to examine several features of the wave and turbulence structure associated with a disturbed nocturnal inversion. General features, including mean fields and Richardson number, for the case selected for this study are presented. Spectral analysis of the tower data reveals a separation of energy into wavelike and turbulent fluctuations. Analysis of the heat flux, however, shows upward counter-gradient fluxes in the vicinity of a low-level jet and near the top of the inversion. Cospectral analysis shows that the major contribution to the upward heat flux occurs at frequencies that would normally be considered characteristic of waves. In some cases, the upward flux is associated with a phase shift between vertical velocity w and fluctuating temperature 0 different from the quadrature relation that would be expected of internal waves. Time series analysis reveals that these unexpected positive fluxes occur in relatively short bursts. Analysis of time series of 0 and w in other cases, as well as inspection of acoustic sounder records, shows that sometimes such upward fluxes can result from a combination of wave motion and horizontal temperature advection. In this case the advection is associated with a shallow cold front.

Section: Introductionmentioning

confidence: 99%

Wave and turbulence structure in a disturbed nocturnal inversion

Naiping

Neff

Kaimal

1983

“…Especially the well-known Kelvin-Helmholtz (K-H) waves, generated by shear of the basic flow within stable stratification, are a frequent phenomenon. A great deal of our knowledge on these waves has been derived from observations at meteorological towers (e.g., Merrill, 1977;Einaudi and Finnigan, 1981;De Baas and Driedonks, 1985) and with remote sensing instruments (e.g., Gossard et al, 1970;Emmanuel et al, 1972) in connection with microbarograph pressure arrays at the ground (Hooke et al, 1972;Lu Nai-Ping et al, 1983).…”

Section: Introductionmentioning

confidence: 99%

A case study of Kelvin-Helmholtz waves within an off-shore stable boundary layer: Observations and linear model

Busack¹,

Brümmer²

1988

A case study of Kelvin-Helmholtz waves which were observed by two aircraft in a warm off-shore stable boundary-layer flow over the North Sea is presented. During the one-hour flight mission within an area of 40 x 40 km2, the waves were intermittent both in space and time. They were centered around two levels, at 90 and 330m, where inflection points in the mean profile of the cross-wave wind component occurred together with Richardson numbers smaller than the critical value of 0.25. Observed wave amplitudes were on the order of 0.1 K for the potential temperature, 0.15 m s-l for the vertical wind component, 0.3 m s-l for the cross-wave wind component and 0.15 m SF' for the along-wave wind component. Horizontally averaged vertical wave transports were down-gradient.Based on the observed wind and temperature profiles, wave simulations with a linear model are performed. Different diffusion coefficient estimates are tested. The model produces two types of Kelvin-Helmholtz waves with maximum amplitudes at the above mentioned two heights. The modeled wavelengths are about 30% shorter than the observed ones. Adjusting the modeled to the observed temperature variations, the modeled vertical wind variance and the vertical transports agree well with the observations, whereas the modeled horizontal wind variances are smaller than the observed ones.

“…Acoustic soundings (Emmanuel et al, 1972;Hooke et al, 1973) and FM-CW radars have been used for several years to observe and study these perturbations in inversion layers.…”

Section: Introductionmentioning

confidence: 99%

A study of gravity waves in the planetary boundary layer by acoustic sounding

Eymard

Weill

1979

Gravity waves associated with stably stratified layers were observed in the planetary boundary layer at two locations in France. Using an array of three monostatic Doppler acoustic sounders, the wave speeds and directions were found. A quantitative study of the waves has been performed to determine their physical characteristics. Based on their dynamics, a classification into three types was possible. Most were found to be trapped beneath a critical level.