The phrase "mass flux" refers to the amount of mass that is flowing or transferring through a specific area or space. It measures the rate at which mass is moving from one place to another. Full definition
Its application to the C - grid reduces numerical errors in the vertical mass flux resulting in improvements in precipitation and other quantities. (pubs.giss.nasa.gov)
Prediction of mass fluxes of polar ice sheets, improved models of glacial isostatic adjustment, and better modeling and prediction of sea - level change. (polenet.org)
Kumar V. V., A. Protat, C. Jakob, C. R. Williams, S. Rauniyar, G. L. Stephens and P. T. May (May 2016): The estimation of convective mass flux from radar reflectivities. (esrl.noaa.gov)