Modeling real convective boundary layers in the terra incognita:
evaluation of different approaches
Abstract
Turbulent motions regulate vertical transport of momentum, heat,
moisture and pollutants in the atmospheric boundary layer. From a
numerical perspective, modeling such motions becomes challenging at
kilometer and sub-kilometer resolutions, as the horizontal grid spacing
of the model approaches the size of the most energetic convective eddies
in the boundary layer. In this range of resolutions, typically termed
‘terra incognita’ or ‘gray zone’, partially resolved convective
structures are grid-dependent and neither traditional 1D mesoscale
parametrizations nor 3D closures typical of Large Eddy Simulations are
theoretically appropriate. However, accurate numerical modeling at gray
zone resolutions is a key aspect in several practical applications, such
as proper coupling of mesoscale and microscale simulations. While some
progress has been achieved in recent years through idealized simulations
and theoretical considerations, the evaluation of different approaches
in real convective boundary layers (CBL) is still very limited.
Leveraging on a new set of one-way nested, full-physics multiscale
numerical experiments, we quantify the magnitude of the errors
introduced at gray zone resolutions and provide new perspectives on
recently proposed modeling approaches. The new set of experiments is
forced by real time-varying boundary conditions, spans a wide range of
scales and includes traditional 1D schemes, 3D closures, scale-aware
parametrizations and strategies to suppress resolved convection at gray
zone resolutions. The study area is Riyadh (Saudi Arabia), where deep
CBLs develop owing to strong convective conditions. Detailed analyses of
our experiments, including validation with radiosonde data, calculations
of spectral characteristics and partitioning of turbulent fluxes between
resolved and subgrid scales, show that (i) grid-dependent convective
structures entail minor impacts on first order statistics of the flow
due to some degree of ‘implicit scale-awareness’ of 1D parametrizations
and (ii) 3D closures outperform traditional and scale-aware 1D schemes
especially in the surface layer, among other findings. The new
simulation suite provides a benchmark of real simulations that can be
extended to assess how new techniques for simulations at gray zone
resolutions perform.