Precipitation Types in the Positive and Negative Low-level Moisture Convergence Regimes in the Tropics

Low-level Moisture Convergence (LLMC) has been used as a diagnostics of tropical precipitation. Using a combination of reanalysis ERA-5 data and observational GPM and GPCP data, we show that the LLMC-precipitation relationship has two regimes. When LLMC is positive, precipitation increases with increasing LLMC. When LLMC is negative, the relationship is opposite: precipitation increases with decreasing LLMC, i.e., as LLMC becomes more negative. Precipitation events in the positive LLMC regime are mainly associated with convective precipitation, while those in the negative LLMC regime are mainly associated with stratiform precipitation. The sources of the convective precipitation, convective clouds, tend to develop in the positive LLMC regime. In the positive LLMC regime, the atmosphere below 600hPa is unstable and strong upward motions predominate. Based on the continuity equation, the upward motions must be associated with low-level convergence and therefore positive LLMC values. The sources of the stratiform precipitation, nimbostratus clouds, don't occur alone but following the well-developed deep convection. Even though negative LLMC events may start with a degree of low-level convergence similar to positive LLMC events, the development of nimbostratus clouds subsequently produces downward motions at low levels due to the melting of ice crystals and the evaporation of falling condensates. The heating/cooling structure leads to divergence at low levels, corresponding to negative LLMC values on average.