Two-Point Mixing, Buoyancy Sorting and Updraft Dilution Observed in the RACORO Campaign

Abstract:

Aircraft measurements of shallow cumulus cases from the RACORO campaign are studied in a transformed Paluch diagram. Previous studies have interpreted samples in Paluch diagrams in terms of two-point mixing and buoyancy sorting. The two-point mixing behavior is frequently seen in the RACORO observations and can be unambiguously distinguished from buoyancy sorting. The latter is not found in the RACORO observations and is also rare in a large-eddy simulation (LES) of a RACORO case. As opposed to often assumed gradual dilution to updrafts, around half of the cases of two-point mixing show no clear evidence of updraft dilution. We also find substantial spread in the properties of parcels that have adjusted to their level of neutral buoyancy and that the inclusion of surface heterogeneity in a LES significantly improves the agreement of its horizonal moisture variations with the aircraft measurements.