Thermally direct mesoscale circulations driven by roughness gradients
Abstract
Deforestation, urbanization and constructions of wind farms can change the land-surface roughness length (z_0), which is an important parameter for computing turbulent fluxes in the atmospheric surface layer in weather and climate models. At synoptic scales, it is well-known that roughness gradients can dynamically trigger convergence that leads to precipitation. Here, we investigate an alternative mechanism which has received much less attention: roughness heterogeneities can induce differential heating at the surface, and thus trigger thermally direct mesoscale circulations and precipitation, even in the absence of a mean wind. To study this mechanism, we conduct cloud-permitting simulations over an idealized tropical land surface, where a surface roughness gradient is prescribed for two equal patches of the domain (625 km2). When the prescribed roughness gradient is significant, the sensible heat flux is larger over the high-roughness patch than that over the low-roughness patch, thus generating mesoscale circulations driven by differential heating. As a result, steady-state precipitation over the high-roughness patch is generally larger than that over the low-roughness patch, regardless of initial soil moisture conditions. To our knowledge, this is the first study of thermally direct mesoscale circulations driven by roughness gradients. This mechanism may be relevant to storm formation over wind farms, cities and forests.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2022
- Bibcode:
- 2022AGUFM.A26A..03C