Boundary Layer Late Afternoon and Sunset Turbulence: Analysis of multi-layering – BLLAST_A

Fundamental turbulence analysis will be based on both observations and on numerical simulations with meso-scale modeling and idealized and real case large eddy simulations.


(1) Understanding the vertical structure during the afternoon transition:

- Data analysis of the observed mean vertical profiles and of the turbulence intensity and scales from in situ and remote sensing observations.

- Idealized numerical study and sensitivity tests on the role of wind shear, entrainment, boundary-layer top inversion, large scale subsidence and advection on the vertical structure evolution.

- Explicit real case numerical simulation with sensitivity studies.

- Synergetic analysis of the numerical studies and of the observations, for an accurate definition of the layers that come into play and a better understanding of the link between the evolving vertical structure and the processes involved.


(2): Study of the impacts on boundary layer scaling and parameterisations:

- Evaluation of the French NWP models (ARPEGE and AROME) behavior during the transition phase for the field campaign period using the observations of the campaign.

- Evaluation of the capacity of the scaling laws to represent the processes during the transition. Research of new scaling laws that can cover the transition from the mixed layer to the residual layer.

- Theoretical analysis of the limitations of typical boundary-layer parameterizations schemes used in 1-D or 3-D meso-scale models in the context of the afternoon and evening transition.

To be followed

To be followed

To be followed

As an interface between the earth surface and the atmosphere, the planetary boundary layer (PBL) is a critical component of the earth system. It controls transfers of heat, momentum, humidity, and trace species between the surface and the atmosphere. BLLAST_A deals with the transition that occurs in late afternoon.
The transition from the mixed convective boundary layer to a residual layer overlying a stably-stratified surface layer in late afternoon is still not well understood, and there are no simple scaling laws that fit during this period. Yet this transition to the nocturnal boundary layer plays an important role in such diverse atmospheric phenomena as transport and diffusion of trace constituents, nocturnal jet setting, and mountain-valley or sea-land breeze reversal.
A group of research scientists from several countries in Europe (France, Spain, the Netherlands, Italy, Germany, Poland, UK) and US decided to work together on those issues, with the purpose of improving our understanding of those late afternoon turbulence processes, and testing their representation in numerical weather forecast models. On a French initiative, the BLLAST (Boundary-Layer Late Afternoon and Sunset Turbulence) project has thus emerged 3 years ago. BLLAST_A (BLLAST: Analysis) is a key section of the project, since it aims at analyzing the observations that have been collected in summer 2011 during the BLLAST field campaign and at running numerical simulations, in order to better understand the turbulent processes that occur during the late afternoon transition, with a focus on one core question of BLLAST: the complexity of the vertical structure during this phase.
The objective of the present proposal is to support the young research French team of BLLAST_A, in order to ensure the continuation of BLLAST coordination and an optimized use of the data collected and of the numerical simulations, for a better understanding of this question. BLLAST_A will be based on the synergy between observations and Large Eddy Simulation (LES) modeling, to (1) understand the vertical structure of mean and turbulence variables and its evolution, and (2) evaluate the impact on the boundary layer scaling and representation. To achieve this, the following steps will be pursued: (i) analyze and understand the vertical structure from analyses of the observations, (ii) use idealized numerical simulation to test the role of several boundary layer governing processes, (iii) use explicit numerical simulation of specific cases to understand the vertical structure, (iv) estimate the impact of this transition on current typical parameterizations of the boundary layer in meso-scale models. The budget asked in BLLAST_A proposal corresponds to the salary of a postdoctoral fellow who will work on the core question that we address here, to travel fees and publication fees associated with this work.