Land-surface controls on afternoon precipitation diagnosed from observational data: uncertainties and confounding factors

The feedback between soil moisture and precipitation has long been a topic of inter-est due to its potential for improving weather and seasonal forecasts. The generallyproposed mechanism assumes a control of soil moisture on precipitation via the parti-tioning of the surface turbulent heat fluxes, as assessed via the Evaporative Fraction, EF, i.e. the ratio of latent heat to the sum of latent and sensible heat, in particular un-der convective conditions. Our study investigates the poorly understood link betweenEF and precipitation by investigating the impact of before-noon EF on the frequencyof afternoon precipitation over the contiguous US, using a statistical analysis of therelationship between multiple datasets of EF and precipitation. We analyze remote sensing data products (EF from GLEAM, Global Land Evaporation: the AmsterdamMethodology, based on satellite observations; and radar precipitation from NEXRAD,the NEXt generation weather RADar system), FLUXNET station data, and the NorthAmerican Regional Reanalysis (NARR). While most datasets agree on the existenceof regions of positive relationship between between EF and precipitation in the Eastern and Southwestern US, observation-based estimates (GLEAM, NEXRAD and to someextent FLUXNET) also indicate a strong relationship in the Central US which is notfound in NARR. Investigating these differences, we find that much of these relation-ships can be explained by precipitation persistence alone, with ambiguous results onthe additional role of EF in causing afternoon precipitation. Regional analyses reveal contrasting mechanisms over different regions. Over the Eastern US, our analyses sug-gest that the apparent EF-precipitation coupling takes place on a short day-to-day timescale and is either atmospherically controlled (from precipitation persistence and po-tential evaporation) or driven by vegetation interception and subsequent re-evaporation(rather than soil moisture and related plant transpiration/bare soil evaporation), in line with the high forest cover and the wet regime of that region. Over the Central andSouthwestern US, the impact of EF on convection triggering is additionally linked tosoil moisture variations, owing to the soil moisture–limited climate regime.ISSN:1680-7375ISSN:1680-736