Adel Imamovic, ETH Zürich – University of Copenhagen

Atmospheric complexity > Complex Fluids Seminar > 2017 > Adel Imamovic, ETH Zurich

Adel Imamovic, ETH Zürich

Isolated and combined impact of orographic and soil-moisture heterogeneities on the soil-moisture precipitation feedback

Thunderstorm activity in many extratropical regions peaks in the warm season, when the surface warming and the atmospheric humidity reach their annual maximum. Deep convection is an inherently turbulent process and as such challenging to predict, especially under weak synoptic forcing. Nonetheless, numerous observations suggest that deep convection persistently responds to the presence of orographic and soil-moisture heterogeneities. I will introduce basic theoretical concepts for deepconvection triggering in the vicinity of soil-moisture and orographic heterogeneities, such as soilmoisture controlled partitioning of surface latent and sensible heat fluxes, land breezes triggered along soil-moisture gradients, mountain-valley circulations and the resonant response of deep convection to surface hot spots. Furthermore, I will use numerical simulations with idealized boundary conditions to study deep convection initiation and invigoration processes driven by orographic and soil-moisture heterogeneities Results from the simulations show that substantial rain enhancement is found in the vicinity of localized dry soil-moisture anomalies. Moist soil-moisture anomalies, on the other hand, suppress rain amounts, which is indicative for a negative soil-moisture precipitation feedback A negative soilmoisture precipitation feedback is also found in settings with a shallow mountain: mountains drier (moister) than their forelands enhance (suppress) rain. The relative importance of the soil-moisture heterogeneity however decreases for increasing mountain height: a 500 m high mountain is already sufficient to overpower the impact of the soil-moisture heterogeneity. The non-linear breakdown of the additivity will be discussed as a result of the sensitive dependence of deep-convective activity on time scales associated with the growth of the planetary boundary layer, the propagation of internal gravity waves, and the onset of rain.