Transport Processes in the Tropical Warm Pool Boundary Layer. Part I: Spectral Composition of Fluxes

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.