Increased Likelihood of Appreciable Afternoon Rainfall over Wetter Soils under Limited Atmospheric Dynamic Influence

The relationship between morning soil moisture and afternoon rainfall persists as an important yet unresolved challenge in land-atmosphere interaction study, complicated in part by atmospheric influence. Here, we address this relationship by utilizing NASA's satellite soil moisture (SMAP L4) and precipitation (GPM-IMERG) data for the warm season (June-September) of 2015-2018. Raining days are first identified, then partitioned into low, medium, and high dynamic regimes of atmospheric water vapor convergence as estimated from a reanalysis (MERRA2), and the probability of afternoon rainfall occurrence given positive and negative soil moisture anomalies is calculated for each regime. Afternoon rainfall tends to occur over drier soils for each dynamic regime, which is consistent with past studies. When atmospheric influence is suppressed, however, above median afternoon rainfall, which contributes to 95% of the total afternoon rainfall on average, is more likely to occur over wetter soils. For median and high dynamic regime days, conclusions for above median afternoon rainfall are similar to those for all afternoon rainfall, with preference over drier soils. These results highlight the importance of accounting for the magnitude of afternoon rainfall (i.e. eliminating negligible "drizzle" events) when considering the relationship between soil moisture and rainfall triggering, particularly for low dynamic regime days.