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A major uncertainty in accretion disk theory is the nature and properties ofgas turbulence, which drives transport in protoplanetary disks. The commonlyused viscous prescription for the Maxwell-Reynolds stress tensor gives rise toa meridional circulation where flow is outward near the midplane and inwardaway from it. This meridional circulation has been proposed as an explanationfor the presence of high-temperature minerals (believed to be of inner solarsystem provenance) in comets. However, it has not been observed in simulationsof magnetohydrodynamical (MHD) turbulence so far. In this study, we evaluatethe extent to which the net transport of solids can be diagnostic of theexistence of meridional circulation. To that end, we propose and motivate aprescription for MHD turbulence which has the same free parameters as theviscous one. We compare the effects of both prescriptions on the radialtransport of small solid particles and find that their net, verticallyintegrated radial flux is actually quite insensitive to the flow structure fora given vertical average of the turbulence parameter $\alpha$, which weexplain. Given current uncertainties on disk turbulence, one-dimensional modelsare thus most appropriate to investigate radial transport of solids. Acorollary is that the presence of high-temperature material in comets cannot beconsidered an unequivocal diagnostic of meridional circulation. In fact, weargue that outward transport in viscous disk models is more properly attributedto turbulent diffusion rather than to the mean flows of the gas.

On vertical variations of gas flow in protoplanetary disks and their impact on the transport of solids