Three‐Dimensional Radiative Effects By Shallow Cumulus Clouds on Dynamic Heterogeneities Over a Vegetated Surface

M A Veerman,X Pedruzo‐Bagazgoitia,J Vilà‐Guerau De Arellano,F Jakub,C C Van Heerwaarden

doi:10.1029/2019ms001990

Abstract

AbstractOne‐dimensional radiative transfer solvers are computationally much more efficient than full three‐dimensional radiative transfer solvers but do not account for the horizontal propagation of radiation and thus produce unrealistic surface irradiance fields in models that resolve clouds. Here, we study the impact of using a 3‐D radiative transfer solver on the direct and diffuse solar irradiance beneath clouds and the subsequent effect on the surface fluxes. We couple a relatively fast 3‐D radiative transfer approximation (TenStream solver) to the Dutch Atmosphere Large‐Eddy Simulation (DALES) model and perform simulations of a convective boundary layer over grassland with either 1‐D or 3‐D radiative transfer. Based on a single case study, simulations with 3‐D radiative transfer develop larger and thicker clouds, which we attribute mainly to the displaced clouds shadows. With increasing cloud thickness, the surface fluxes decrease in cloud shadows with both radiation schemes but increase beneath clouds with 3‐D radiative transfer. We find that with 3‐D radiative transfer, the horizontal length scales dominating the spatial variability of the surface fluxes are over twice as large as with 1‐D radiative transfer. The liquid water path and vertical wind velocity in the boundary layer are also dominated by larger length scales, suggesting that 3‐D radiative transfer may lead to larger convective thermals. Our case study demonstrates that 3‐D radiative effects can significantly impact dynamic heterogeneities induced by cloud shading. This may change our view on the coupling between boundary‐layer clouds and the surface and should be further tested for generalizability in future studies.

Highlights

The transfer of solar and thermal radiation is well understood (e.g., Liou, 2002), the extremely high computational costs of full 3‐D radiative transfer solvers have necessitated trade‐offs between accuracy and speed in atmospheric models
The liquid water path and vertical wind velocity in the boundary layer are dominated by larger length scales, suggesting that 3‐D radiative transfer may lead to larger convective thermals
We coupled a 3‐D radiative transfer solver to a large‐eddy simulation model and performed a case study to investigate the impact of 3‐D radiative effects on the coupling between clouds, solar radiation, and the vegetated land surface

Summary

Introduction

The transfer of solar and thermal radiation is well understood (e.g., Liou, 2002), the extremely high computational costs of full 3‐D radiative transfer solvers have necessitated trade‐offs between accuracy and speed in atmospheric models. These include coarsening the temporal resolution of radiation calculations compared to the dynamical time step or solving radiative transfer in only one dimension, neglecting the horizontal transfer of energy. Horn et al (2015) showed that dynamic heterogeneities due to cloud shading decrease boundary layer turbulence and result in smaller and shorter lived clouds Both studies used a 1‐D radiative transfer scheme that only considered the vertical propagation of radiation. It is important to investigate whether these 3‐D effects can alter the horizontal length scales of clouds, dynamic heterogeneities at the surface, and convective thermals

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