An analysis of vertical velocity spectra obtained in the bomex fair-weather, trade-wind boundary layer

Abstract

Spectra of vertical velocity fluctuations measured on board an aircraft flying over the sea along and across the wind during the Barbados Oceanographic and Meteorological Experiment (BOMEX) are stratified and composited according to estimates of the Monin-Obukhov stability length.This was done to test the hypothesis that a hierarchy of physical mechanisms, responding to wind shear and buoyancy, is active in the turbulent transfer processes of the oceanic subcloud layer. Despite the possibility that the data contain a heading-independent bias, it is concluded that a major change of eddy structure occurs over a narrow range of stability. This agrees well with an early theory on convection over land and observations of herring gull flight characteristics.The vertical variation of the spectral-composites compares favorably with other observations over land and sea. Physical models are suggested to explain the data. One of these models is in agreement with theoretical results concerning ringlike convection. The spectral data, which begin to lose confidence at about 10 km, suggest that a limiting size ofeddies over the ocean is approximately twice the depth of the subcloud layer (in this case 600 m) regardless of the kind of eddy structure.

The role of 1-D and 3-D radiative heating in the organization of shallow cumulus convection and the formation of cloud streets

“…Furthermore, we find a spread in the development of cloud streets depending on the magnitude of the prescribed Q net , with correlation ratios ranging from 1 to 5. The fact that buoyancy-driven cloud street organization is favored in slightly unstable conditions (low sun) compared to stronger instabilities (high sun) agrees well with observations (e.g., Woodcock, 1942;Priestley, 1957;Grossman, 1982;Weckwerth et al, 1997).…”

“…Following Grossman (1982), "At some point (increasing the wind speed/shear) the shearing becomes strong enough so that dynamic instability may interact with buoyancy to produce a hybrid roll vortex/convective cell mechanism. As the shear becomes stronger, shearing instability or roll vortex motion is predominant."…”

The Structure and Formation Mechanism of Transversal Cloud Bands Associated with the Japan-Sea Polar-Airmass Convergence Zone

“…High values of Àz i =L are associated with free convection and the existence of cellular pattern consistent with thermals, and low values of Àz i =L are associated with forced convection and in some cases, the existence of roll circulations. Grossman (1982) observed that roll circulations tended to form when Àz i =L was less than 21.4. Most of other previous studies (e.g., Deardor¤ 1972;LeMone 1973;Sykes and Henn 1989) showed that their criterions for Àz i =L were smaller than that of Grossman (1982), suggesting that roll circulations will be dominant within a slightly unstable mixed layer.…”

“…The previous studies have shown that an optimal combination of buoyancy and vertical wind shear is important for the existence of roll circulations in the mixed layer (e.g., Deardor¤ 1972;Grossman 1982;LeMone 1973;Sykes and Henn 1989). They used the Monin-Obukhov length L and the ratio Àz i =L, where z i is the depth of the mixed layer, to define the necessary conditions for the occurrence of roll circulations.…”

Possible observation of horizontal roll vortices over the Adriatic Sea during bora

“…The present analysis and the common presence of rolls in other cold-air outbreak studies suggests the presence of rolls or rolls+cells. Also, the strong winds and surface buoyancy flux combined with the unusually shallow ABL depth would likely put the present case in the ABL depth/Obukhov length regime associated with rolls+cells, or less likely, rolls, rather than pure cellular convection (Grossman, 1982). Note that roll-cell patterns are often referred to as rolls in the literature, because they produce well-defined cloud lines if the updrafts on the crosswind sides of the cells merge into clearly-defined bands of updraft air.…”