Observational analysis of thermally-driven topographic flows and their turbulence-waves interaction

Thermally-driven flows (TDFs) are mesoscale circulations driven by horizontal thermal contrasts in scales ranging from 1 and 100-200 km. The presence of mountains can generate a kind of these TDFs called thermally-driven topographic flows, with a typical daily cycle which is observed when weak synoptic conditions are present. These flows impact the turbulence features in the Atmospheric Boundary Layer (ABL), as well as different scalars (temperature, CO₂, water vapor, pollutants, etc.). Moreover, these circulations, which can be of different scales (from small-scale shallow drainage flows to for example the larger Mountain – Plain flows) can generate gravity waves (GWs) along the transition to the stable boundary layer (SBL) and during the night. In this work, 88 days belonging to an extended period of the BLLAST field campaign^[1] have been analysed. The corresponding nocturnal TDFs have been detected through a systematic and objective algorithm which considers both synoptic and local meteorological conditions. The main objectives of the study are: to characterize the TDFs at CRA (which is placed on a plateau near the Pyrenees in France); to evaluate the performance of the objective algorithm^[2] in obtaining the events of interest; to establish different categories of TDFs and search for driving mechanisms (local, synoptic,..); and finally to explore the connections between TDFs and the generation of Gravity Waves (GWs), often observed in the nocturnal SBL^[3]. Their interaction with turbulence is also analysed using different multiscale techniques, such as wavelets applied to pressure measurements obtained from high accurate microbarometers, and MultiResolution Flux Decomposition –MRFD- applied to sonic anemometer data. The contribution of different scales to turbulent parameters will be deeply evaluated and related to the arrival of TDFs and to the presence of GWs.

 

[1] Lothon, M., Lohou, F. et al (2014): The BLLAST field experiment: Boundary-Layer Late Afternoon and Sunset Turbulence. Atmos. Chem. Phys., 14, 10931-10960.

 [2] Román-Cascón, C., Yagüe, C., Arrillaga, J.A., Lothon, M., Pardyjak, E,R., Lohou, F., Inclán, R.M., Sastre, M., Maqueda, G., Derrien, S., Meyerfeld, Y., Hang, C., Campargue-Rodríguez, P. & Turki, I. (2019): Comparing mountain breezes and their impacts on CO2 mixing ratios at three contrasting areas. Atmos. Res., 221, 111-126.

[3] Sun, J., Nappo, C.J., Mahrt, L., Belusic, D., Grisogono, B., Stauffer, D.R., Pulido, M., Staquet, C., Jiang, Q., Pouquet, A., Yagüe, C. Galperin, B., Smith, R.B., Finnigan, J.J., Mayor, S.D., Svensson, G., Grachev, A.A. & Neff., W.D.: (2015): Review of wave-turbulence interactions in the stable atmospheric boundary layer, Rev. Geophys., 53, 956–993.