From AMS Glossary
(Also called countergradient flux.) The transport of any quantity S from small to large values, contrary to that expected by diffusion theory.
Upgradient flux is typically observed in the top fifth of the atmospheric boundary layer, where heat flux is often observed to flow from colder to warmer values of potential temperature caused by nondiffusion transport processes such as convective thermals. Some theories that account for this countergradient flux include top-down/bottom-up diffusion and transilient turbulence theory.