Understanding 3D light transfers through a Koch shape cloud
Radiative heating rate (W m-3μm-1) horizontal distributions in the cloud‐containing model layer from the 4th order Koch shape cloud layer control simulations with SZAs set to 15º, 30º, 45º, and 60º respectively. Solar azimuth angle is set to 270º, which sheds sunlight from the left to the right.
Figure 1 from Tong et al. (2024), https://doi.org/10.1029/2024GL110469
We perform 3D Monte Carlo simulations of the behavior of light as it transfers through a flat cloud layer with a Koch-shaped lateral boundary. The results show that
- radiative transfer through Koch-shaped cloud sides correlates more significantly with cloud fraction than with cloud perimeter;
- increased cloud vertical extent often enhances cloud‐side leak of radiation more than cloud‐side interception of radiation; and
- cloud‐free‐to‐cloudy region horizontal radiative transfer increases with increasing cloud mass at low sun elevationns.
Modeling radiative transfer in a 3D cloudy atmosphere matters for super high-resolution global climate simulation (e.g., ~1km spatial resolution). A recently developed fast 3D radiation parameterization scheme gains some success in quantifying horizontal radiative transfer through cloud sides using cloud area fraction. Why this approach can succeed requires more in-depth examination. Our results support the use of cloud area fraction to parameterize the effect of horizontal radiative transfers through cloud sides in the radiation scheme in Earth system models.
Modeling radiative transfer in a 3D cloudy atmosphere is critical to climate projections. A recently developed fast 3D radiation parameterization scheme gains some success in quantifying horizontal radiative transfer through cloud sides using cloud area fraction. Based on 3D Monte Carlo simulations of radiative transfer through an idealized single-layer cloud with Koch-shaped fractal geometry edges, here we show that radiative energy transport through cloud sides correlates more significantly with cloud area fraction than with cloud perimeter length. The results exemplify the importance of accounting for the horizontal radiative energy exchanges between cloud-free and cloudy regions with cloud area fraction. Results from additional sensitivity simulations show that increased cloud vertical extent often enhances cloud-side sunlight leak more significantly than cloud-side sunlight interception. At low sun elevations, cloud-side sunlight interception is enhanced more than cloud-side sunlight leak does with the increase of cloud mass.