Midday Boundary-Layer collapse in the Altiplano Desert: the combined effect of Advection and Subsidence

Abstract

Observations in the Altiplano region of the Atacama Desert show that the atmospheric boundary layer (ABL) suddenly collapses at noon. This behavior departs from the typical convective ABL normally reproduced by weather and climate models. The collapse occurs simultaneously to the entrance of a thermally driven and topographically enhanced regional flow, characterized by strong winds that produce mechanical turbulence and advect cold air. To identify the main drivers that cause such ABL collapse and the impact on the potential temperature diurnal variability, we use a land-atmosphere coupled model, observations from a comprehensive field campaign, and the Weather and Research Forecasting regional model. We also address the question of how local (surface-atmosphere interactions) and non-local processes (entrainment/ advection/ subsidence) contribute to the surface and ABL dynamics in the region.A suite of numerical experiments were performed to disentangle the boundary-layer collapse by increasing the level of complexity: from only considering local land-atmosphere interactions to systematically including the non-local contributions of mass advection, cold air advection, and subsidence. Our results show that during the morning the local surface-atmosphere interactions are the dominant contributions and lead to increase the heat exchange that, together with the entrainment processes, warm the atmosphere and allow the ABL to grow. However, this regime abruptly changes at noon and turns into a boundary-layer regime mainly controlled by non-local phenomena. Two interconnected processes lead to a strong decrease of the ABL height (h ): the advection of a shallower boundary layer (~ -250 m h−1 at noon) that causes an immediate decrease of h at midday, and the arrival of a cold air mass which reaches a strength of ~ -3 K h−1 at 1400 LT, strong enough to stop the ABL growth by counteracting the large turbulence levels driven by the high surface fluxes. These two external forcings become dominant over entrainment and surface processes that warm the atmosphere and increase h. As a consequence, the ABL growth is capped during the afternoon. Finally, a wind divergence of ~ 8 x 10−5 s−1 contributes to the collapse by causing subsidence motions that provide additional downward push over the ABL from 1200 LT onward. Without these non-local processes, the ABL would be continuously growing to 3.5 km by the end of the afternoon. Our findings show the relevance of treating large and small processes as a continuum to be able to understand the ABL dynamics and reproduce them adequately in weather and climate models.