Modeling the atmospheric boundary layer in stably stratified conditions and over complex terrain areas: from mesoscale to LES

Abstract

The atmospheric boundary layer in stably-stratified conditions and over non-homogeneous terrain becomes a complex system with many interactions of physical processes occurring in a wide range of different spatial and temporal scales. During clear sky night-time or in any stably-stratified conditions intermittent turbulent events and gravity waves are usually present in the stable boundary layer (SBL), which can substantially modify the flow structure. In addition, the circulations in stable flows can be strongly driven by the underlying and surrounding topography, generating katabatic winds, density currents and low level jets, which in turn, trigger gravity waves and turbulence. This thesis aims to contribute to a better comprehension of some of the processes and phenomena occurring in the SBL and over complex terrain areas.

In order to understand and quantify the unknown atmospheric processes one can distinguish three different procedures that are very well connected: theoretical descriptions, experimental campaigns and numerical modeling. The numerical models allow us to further understand the experimental data, to test the theoretical relationships or to simulate processes which are very difficult to measure. Principally, in this thesis we have used numerical models to deal with the uncertainties that arise in stably-stratified flows and over heterogeneous terrain and to explore the model capabilities and limitations to resolve them.

These numerical weather prediction models (NWP) contain the primitive equations of the atmosphere to describe and forecast the flow motions and properties. In this thesis we have employed one of the worldwide known NWP model, the Weather Research and Forecasting (WRF) model, using two different approaches: the mesoscale approximation and the large eddy simulation (LES). While the mesoscale methodology has allowed us to investigate the flow circulation patterns in a wide range of scales, the LES approximation has enabled us to explicitly resolve the turbulence and describe its structure. In this thesis each methodology has been applied to investigate these different purposes.

Using the WRF model with the mesoscale approach we have determined the origin of a density current that generated internal gravity waves over the "Centro de Investigaciones de la Baja Atmosfera"(CIBA) site. We have seen that the long distance mesoscale sea-breeze circulation and the night-time katabatic flows originated at the surrounding complex topography were the origin of the density current which generated displacement in the air parcels and periodic oscillations.

In this thesis we have also investigated the vertical turbulence structure using the LES approximation of the WRF model. As a previous step, we have first validated the WRF-LES model in the SBL with a reference case by a comparison of the first and second order moments profiles.

Using different wind speed initial conditions we reproduce neutrally and stably stratified flows. However, different from the reality, stably stratified flows are strongly coupled with the surface and turbulence is always maintained. We have shown how the turbulence intensity increases sharply with the wind speed at each height above ground but the rate of increase (slope) is not maintained, as we would expect. It seems that the the top domain potential temperature inversion affects the flow turbulence structure over the whole domain.

Finally, we have studied topographically generated gravity waves over the Pyrenees and specifically simulated a trapped lee wave event using the mesoscale approximation with WRF. We have seen that the model is able to reproduce the gravity waves at the lee side of the mountain range with periodic oscillations in all magnitudes. We have seen that 1-km horizontal resolution is necessary to capture the wave field. We have also showed that upstream conditions have to be well represented to capture the adequate wave characteristics.

La capa límit atmosfèrica és la part més baixa de l'atmosfera terrestre on s'hi desenvolupa la vida humana. En condicions d'estratificació estable i sobre terreny no homogeni esdevé un sistema molt complex amb múltiples interaccions dels processos físics que hi tenen lloc.

Per a entendre i quantificar algunes de les incerteses que planteja l'atmosfera a la capa límit en aquesta tesi principalment hem utilitzat eines de simulació numèrica. Els models numèrics permeten la comprensió més enllà de les dades experimentals, així com testejar les descripcions teòriques, a més de simular fenòmens que són molt difícils de mesurar. L'objectiu és, doncs, contribuir a la comprensió dels fenòmens que tenen lloc a la capa límit en condicions d'estratificació estable i sobre àrees de terreny complex i explorar les capacitats i les limitacions de la seva modelització numèrica.

D'entre els principals resultats, fent ús del model WRF en l'aproximació de mesoscala, hem determinat l'origen d'una corrent de densitat que va donar lloc a ones de gravetat interna en la zona del Centro de Investigaciones de la Baja Atmósfera (CIBA). Hem vist que una massa d'aire amb origen de brisa marítima juntament amb els vents catabàtics originats a les cadenes muntanyoses del voltant són l'origen de la corrent de densitat que genera ones de gravetat al seu pas per l'àrea del CIBA.

Per altra banda, hem explorat l'estructura vertical de la turbulència en condicions neutrals i estables fent ús del model WRF en l'aproximació LES (WRF-LES). S'han investigat els règims de intensitat de turbulència en funció de la velocitat del vent i s'ha obtingut una relació semblant a les observacions en situació de forta turbulència. Veiem les condicions de contorn del model a la superfície i al límit superior poden afectar molt significativament l'estructura dels remolins.

Finalment, l'estudi de les ones de muntanya sobre la orografia complexa del Pirineu amb el model WRF en el mode mesoscalar ha permès avaluar la capacitat del model per a representar l'esdeveniment i la variació en els resultats en funció de les seves diferents opcions físiques i de configuració.