Variability in Updraft Mass Flux Across the Top of the Atmospheric Boundary Layer Over the Tropical Western North Pacific Associated With a Tropical Depression

Over tropical oceans, cumulus convection is triggered by updrafts from the atmospheric boundary layer (BL). Given mutual interactions between cumulus convective ensembles and synoptic‐ to global‐scale atmospheric phenomena, it seems important to quantitatively evaluate the updraft on spatiotemporal scales relevant to the interactions and understand its variability, which have been rarely explored through observational studies. Therefore, this study attempts to estimate daily mean updraft mass flux across the BL top from in situ observation data obtained during a shipborne observation campaign over the tropical Western North Pacific and examines its variation associated with changes in circulation fields. It is found that the updraft mass flux ranges from a few to several tens of g m −2 s −1 and is tightly correlated with low cloud amount. From this relationship, the mean updraft intensity, the updraft mass flux averaged only over updraft areas, can be roughly estimated at ∼80 g m −2 s −1 . Variation of the daily mean mass flux associated with the passage of a northwestward‐migrating tropical depression is examined in detail. The updraft mass flux starts to increase ∼5 days before the passage and reaches its maximum at the time of the passage. Large‐scale (∼100‐km scale) horizontal convergence in the BL primarily leads to the increase of the flux in the first half of the increasing period. In contrast, variation of local‐scale mass exchange should be considered for understanding the increase in the second half.