Department Seminar: Kaighin McColl

Title: Land surface controls on weather: new insights from theory, models and observations

Abstract: The land surface plays a major role in determining weather over inland continental regions, from the onset of severe storms to the temperature and humidity experienced by humans in the lower atmosphere. I will present new results from two sets of projects related to this topic. First, I will discuss two new mechanisms by which the land surface controls rainfall, based on cloud-permitting simulations conducted in our group. We show that dry soils cause the atmosphere to dry out, causing more raindrops to evaporate as they fall, resulting in less rainfall at the surface. We also show that rough surfaces – such as cities, wind farms and forests – can trigger storms; unlike similar proposed mechanisms, the new mechanism we identify occurs even in the absence of a mean wind. Second, I will present a simple theory for the spatial structure of near-surface relative humidity (RH) over land. Why is RH never much lower than 35% over land, on average? And, why is the latitudinal profile of RH over land shaped like the letter ‘W’, when both specific humidity and saturation specific humidity essentially decline monotonically from the equator to the poles? The theory provides simple answers to these questions by showing that near-surface RH is largely controlled by the land surface, particularly soil moisture.