Surface thermal inversion evolution in the bottom of a Pyrenean valley studied by observations and mesoscale simulations

Two experimental campaigns have been carried out in the Cerdanya valley at south side of the Pyrenees (E-W oriented, 35 km long and 9 km wide) during fall 2015 (Cerdanya Cold Pool experiment, CCP’15) and winter 2017 (CCP’17, as a part of the Cerdanya-2017 experiment) to study the cold pool that usually forms there at night. The main site (Das) is placed in the central bottom part of the basin. Conangla et al (2018, IJOC) showed that most cold pool events reported there have a daily cycle, being formed in the evening and destroyed by solar heating of the surface the morning after.

The availability of vertical soundings performed by a tethered balloon and a WindRASS, together with measured surface fluxes of latent and sensible heat and momentum at the surface layer allows to inspect the establishment and evolution of the surface thermal inversion in Das. This area collects also downslope and downvalley flows accumulating cold air in the valley along the night. The organization of the flow at low levels is studied through mesoscale simulations of some selected Intensive Operational Periods (IOPs) and the surface observations at different locations along and across the valley.

The selected IOPs comprise nights with only locally-generated winds and small cloud cover, and with variable surface state including grass, fresh snow and patches of old snow. The evolution of the strength and depth of the surface inversion as seen by the model are compared to the available data. Besides, the organization of the flow at low levels and the contribution of the air from the tributary valleys is analyzed in terms of temperature and wind speed budgets to properly characterize the differences in the strength of the cold pool for the selected studied IOPs.