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Abstract:
This paper develops a method to estimate the shallow-convective mass flux M at the top of the sub-cloud layer as a residual of the sub-cloud layer mass budget. Application of the method to dropsonde data from the NARVAL2 field campaign yields the first measurements of the mass flux derived from the mass budget. The ability of M to reproduce the mass flux diagnosed directly from the cloud-core area fraction and vertical velocity is tested using real-case large-eddy simulations over the tropical Atlantic. We find that M reproduces well the magnitude, diurnal cycle, and day-to-day variability of the core-sampled mass flux, with an average root-mean square error of less than 30% of the mean. The average M across the four winter days analyzed is 12 mm/s, where the entrainment rate (E) contributes on average 14 mm/s and the large-scale vertical velocity (W) -2 mm/s. We find that day-to-day variations in M are mostly explained by variations in W, whereas E is very similar among
the different days analyzed. E instead exhibits a pronounced diurnal cycle, with a minimum of about 10 mm/s around sunset and a maximum of about 18 mm/s around sunrise. The results suggest a strong coupling between the day-to-day variability in shallow convective mixing (as measured by M) and the large-scale circulation (W). Application of the method in the EUREC4A field campaign will help evaluate this coupling, and assess its implications for cloud-base cloudiness.
