Thunderstorm activity in many land regions peaks in summer, when surface heat fluxes and the atmospheric moisture content reach an annual maximum. Modelling studies as well as satellite and ground-based observations alike suggest that especially under quiescent synoptic conditions the timing and vigour of many summer thunderstorms is influenced by the presence of a triggering mechanism. Mountain ranges and isolated hills often act as such a trigger by generating thermal circulations and by providing a lifting mechanism for air parcels. On the other hand, the soil-moisture content and distribution plays a crucial role in the occurrence and distribution of convective cells by partitioning the available energy into sensible and latent surface heat fluxes. Soil-moisture heterogeneities provide a further means to trigger convective cells by generating thermal circulations.


In a combined system, where orography and soil moisture interact a number of phenomena occur. As one example, orography influences the soil-moisture distribution by gravitational effects. Another example is the interplay of thermal circulations generated by orography and land-surface inhomogeneities. In this colloquium talk the combined impact of soil moisture and orography on moist convection will be reviewed and illustrated by recent studies.