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A. G. Williams, J. M. Hacker, and H. Kraus

Abstract

The structure of the intertropical convergence zone ITCZ cloud-topped marine atmospheric boundary layer away from the most intense mesoscale convective systems during the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) is investigated. Eight vertical profiles taken by the Australian Cessna research aircraft are analyzed, representing the successive influence of a growing small cluster of precipitating cumulus upon the subcloud layer. On the basis of conclusions from a spectral analysis in Part I of this study, results are partitioned into contributions from three distinct categories: (a) small-scale (<2 km) processes, corresponding to small eddies contained and forced mainly within the subcloud layer and weakly active cumulus motions; (b) cloud-scale (>2 km) processes, corresponding to meso-γ-scale motions associated mainly with the action of precipitating cumulus clouds and larger motions; and (c) extreme processes, corresponding to contributions from events at the tail of the small-scale statistical flux distribution. Such events are associated with downdrafts below precipitating cumulus, updrafts at gustfronts, and the effects of moisture contamination on thermodynamic data, and can act to significantly skew the flux distribution. In the presence of vigorous cumuli, cloud root circulations (including compensating downdrafts) force significant cloud-scale fluxes in the upper subcloud layer. When conditions become highly disturbed, these fluxes dominate and the vast majority of small-scale humidity transport is concentrated into the cloud root regions. Precipitation produces strong downdrafts and outflows of evaporatively cooled air in the lower subcloud layer, markedly increasing temperature and velocity variances. Neither cloud root circulations nor outflows are supported by cloud-scale buoyancy, with the former being fed by pressure and momentum forces while the latter are formed via small-scale (extreme) buoyancy effects. Small-scale (surface forced) processes moisten and slow the subcloud layer as a whole, while cloud processes cause drying and often acceleration due to enhanced cloud–subcloud-layer exchanges. Processes on all scales lead to net warming of the subcloud layer in the present dataset. Although in zero or low precipitation cases the mean structure of the mixed layer may still be represented to some degree by existing simple zero-order jump models, significant adjustments are required to such models in order to account for the effects of cloud-scale processes under disturbed conditions. In particular, the enhancement of cloud–subcloud-layer exchanges by cloud root processes and the effects of increased horizontal wind variances upon surface fluxes requires attention. A new velocity scale is suggested, based on large-scale vertical velocity at cloud base, which may be useful in the formulation of newparameterizations.

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A. G. Williams, H. Kraus, and J. M. Hacker

Abstract

Spectral analysis of high-resolution turbulence data from the South Australian Cessna research aircraft is performed in an investigation of the multiscale nature of vertical transport processes in the atmospheric boundary layer (ABL) during TOGA COARF. The flights were conducted in the vicinity of large cloud cluster systems in the intertropical convergence zone, but away from the most intense mesoscale (100s of km) convective systems within the clusters. A number of very long (up to 430 km) and low (20-70 m) continuous data runs, composing an excellent dataset for studying the spectral composition of near-surface fluxes, are complemented by eight “stack” patterns providing important information regarding vertical variations. The ABL in these regions is found to be highly horizontally heterogeneous, due to the intrusion of cool air masses associated with precipitating cumulus and cumulonimbus clouds, and the action of lines of convention on a range of scales. Not only does this lead to large variations in the surface turbulent flux field, but it can also generate significant direct fluxes in a submesoscale (20–50 km) range at low altitudes, which are not expected to be controlled by ABL parameters. That is, enhanced motions resulting from the action of precipitating cumulus clouds in the presence of wind shear can lead to strong entrainment of air into the subcloud layer, and, in addition, gravity waves generated above the ABL can also influence subcloud motion. Analysis of the form and consistency of the cospectra suggests that, despite the absence of a clear “gap” in the power spectra of the major variables, it is nevertheless possible to achieve a reasonable partitioning between “ABL turbulence” and the larger-scale processes via a simple spectral separation with a crossover wavelength at around 2 km. This useful characteristic appears to reflect an ability of the ABL turbulence to maintain a high degree of coherency in spite of the changing conditions imposed by the mesoscale disturbances.

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