Soil moisture-precipitation coupling over Central Europe: Interactions between surface anomalies at different scales and the dynamical implication

The sign of soil moisture-precipitation coupling and its scale dependence are controversial issues when examining soil-atmosphere interactions. Using the operational convection-permitting COSMO model with 2.8km horizontal grid spacing, the interplay of soil-moisture heterogeneities on different scales and the role of the midtropospheric background wind are addressed by performing a series of real-case simulations for 11 synoptically weakly and moderately forced summer precipitation events across Central Europe. The cumulative effect of soil-moisture bias and heterogeneities on different scales is investigated, with consideration of the prevailing synoptic forcing, by explicitly perturbing soil-moisture conditions. A soil-moisture bias of 25% is combined with different soil-moisture heterogeneity length-scales ranging from 30 to 140km introduced by chessboard patterns. For synoptically moderately forced cases, our experiments only show a small impact of soil-moisture perturbations on convective afternoon precipitation. Averaged over all seven synoptically weakly forced cases, however, we find (a) a positive coupling between the overall soil-moisture bias and the domain-averaged precipitation and (b) a negative local soil moisture-precipitation coupling. Increased precipitation over drier soils is related to an interaction between thermally induced vertical circulations and the background wind, causing a persistent updraft region at the downstream flank of the dry patches. Convective precipitation is triggered preferentially near these soil-moisture gradients. The circulation cells are most dominant for soil-moisture perturbations at scales between 40 and 80km. The spatial locking of convection at soil-moisture boundaries at these scales results in an earlier triggering of convection and an overall reduction of the day-to-day variability of area-averaged precipitation.