A Case Study of Cumulus Convection Over Land in Cloud‐Resolving Simulations With a Coupled Ray Tracer

Abstract

Solar radiation enters the atmosphere following the direct beam of the sun, and can be absorbed or scattered in any direction by gas molecules, aerosols, cloud droplets, and the surface. This 3D nature of radiation affects the spatial structure of atmospheric radiative heating and global horizontal irradiance (GHI). Clouds intercept more radiation at their sides as the solar zenith angle increases, which enhances the size of cloud shadows (side illumination; Hogan & Shonk, 2013), but may also result in more diffuse irradiance (side escape; Hogan & Shonk, 2013). Additionally, clouds can intercept radiation reflected by the surface or by neighboring clouds (entrapment;Hogan et al., 2019). These solar radiation-cloud interactions create complex surface patterns with cloud shadows and regions where GHI exceeds clear-sky radiation.Ideally, one would capture these 3D effects in the radiation computations of cloud-resolving simulations. However, the actual status quo in weather and climate models is the use of relatively efficient two-stream methods that solve radiation in the vertical direction only (Cahalan et al., 2005;Meador & Weaver, 1980). These approaches are computationally affordable, but lack the aforementioned 3D radiative effects.The development of the TenStream solver (Jakub & Mayer, 2015), which reduces the photon propagation to a limited set of directions, allowed for detailed studies into 3D radiative effects in a coupled cloud-resolving model. These studies Veerman et al., 2020) revealed clear impacts of 3D radiative effects on cloud development, mainly driven by GHI patterns that strongly deviate from those in simulations with two-stream solvers. However, while emphasizing the importance of surface-radiation feedbacks relative to in-cloud processes, the GHI patterns produced by the TenStream solver still deviate from those produced by ray tracing (Jakub & Mayer, 2015, their Figure 8).Monte Carlo ray tracing is widely regarded as the most accurate technique to solve radiative transfer in 3D and can produce patterns in GHI that closely resemble field observations (Gristey et al., 2020). Furthermore, ray tracing is often used as reference for the development of 3D radiative transfer approximations (Hogan et al., 2016;

Reconciling Observations of Solar Irradiance Variability With Cloud Size Distributions

“…Apart from directly negative consequences for solar energy applications, these errors become more pronounced the better such models resolve individual clouds, and can feed back to other components of the model. Most notably, heterogeneity in surface irradiance can feed back through surface heat fluxes to alter the cloud field and domain‐mean irradiance (Jakub & Mayer, 2017; Lohou & Patton, 2014; Veerman et al., 2022). Given the aforementioned importance of accurately resolved or forecast irradiance variability, it is imperative to understand on more fundamental level how irradiance variability is characterized and where it originates from.…”

An Efficient Parameterization for Surface Shortwave 3D Radiative Effects in Large‐Eddy Simulations of Shallow Cumulus Clouds

“…To this end, we took the cloud field from our simulation with 1D radiative transfer and performed Monte Carlo ray tracing, as described in Veerman et al. (2022) but with delta‐scaled cloud optical properties. The surface irradiance fields obtained with the ray tracing are shown in Figures 5d–5f.…”

“…Veerman et al. (2022) showed for the case of 15 August 2016 that a similar cloud cover is modeled when 3D radiative transfer calculations are used.…”

“…Although the modeled cloud structures will never be exactly as observed, the average cloud cover is well simulated for both days. Veerman et al (2022) showed for the case of 15 August 2016 that a similar cloud cover is modeled when 3D radiative transfer calculations are used.…”

“…This global radiation field shows cloud enhancements in addition to the cloud shadows and clear sky patches.For comparison, we performed a 3D radiative transfer calculation for this time step. To this end, we took the cloud field from our simulation with 1D radiative transfer and performed Monte Carlo ray tracing, as described inVeerman et al (2022) but with delta-scaled cloud optical properties. The surface irradiance fields obtained with the ray tracing are shown in Figures5d-5f.…”