Recent research has described a ‘moist margin’ in the tropics, defined through a total column water vapor (TCWV) value of 48 kg m-2, that encloses most of the rainfall over the tropical oceans. Diagnosing the moist margin in the ERA5 reanalysis reveals that it varies particularly on synoptic time scales, which this study aims to quantify. We define ‘wet and dry perturbation’ objects based on the margin’s movement relative to its seasonal climatology. These perturbations are associated with a variety of features, such as tropical cyclones and lows, tropical waves, and extrusions of moisture towards the extratropics. Wet (dry) perturbations produce substantially more (less) rainfall compared to the seasonal average, confirming the clear link between moisture and precipitation. On synoptic scales we suggest that mid-tropospheric humidity plays a key role in creating these perturbations, while sea surface temperatures (SSTs) are relatively unimportant.

Most cumulus parametrizations today make use of a simple conceptual model of convection, called the mass-flux approach. This approach depicts convection as an ensemble of updrafts and downdrafts occurring within a model grid-box. The aim of this study is to determine convective mass-fluxes and their constituents on the scale of a 100 km GCM grid-box from a C-band polarimetric radar and thereafter investigate the relative role of area fraction and vertical velocity in determining the shape and magnitude of bulk mass-flux profiles. We make use of observational estimates of these quantities spanning 13 wet seasons in the tropical region of Darwin. Following a bulk approach, the results show that the distribution of mass-flux is positively skewed and its mean profile peaks at 4 km. This is the result of constant area fractions and increasing vertical velocities below that level. Above 4 km, in-cloud vertical velocity plays a marginal role compared to the convective area fraction in controlling mass-flux profiles.