The Neighboring Column Approximation (NCA)—a Method for the Calculation of 3D Thermal Heating and Cooling Rates in Structured and Unstructured Grids

In the thermal spectral range, emission and absorption of radiation by clouds and the atmosphere are the main radiative interactions. For stratiform clouds, this results in cloud top cooling and a modest warming at the cloud bottom. Finite clouds show additional cloud side cooling. These heating and cooling rates were shown to have an impact on cloud formation, and to cause changes in the dynamics of the system, in cloud field organization or the strength of extra tropical cyclones.

Cloud side cooling effects in the thermal spectral range are often neglected in current large eddy simulation (LES) models where radiation is generally calculated (if at all) with the independent column approximation. For the accurate calculation of these thermal heating and cooling rates, fast 3-dimensional radiative transfer models are necessary: These models (usually Monte Carlo models) are far too expensive to be used in large eddy simulation models which shows the need for less expensive 3D approximations. The Neighboring Column Approximation (NCA) was, to our knowledge, the first approach to overcome this lack in 3D thermal radiative transfer parameterizations. Developed in 2015, it is by now implemented in several LES models and was applied successfully. The speed and thus the only modest increase of computational cost (a factor of 1.5-2 compared to a 1D approximation) made it possible to investigate the impact of 3D thermal radiative effects on clouds in a large domain for the first time. Here it could be shown that 3D thermal radiative effects can change the organization of the cloud field and cloud dynamics (e.g. updraft velocities). With the continuing development of cloud models and the increase in computational power, model resolution increases further, which enhances the necessity of 3D radiative transfer parameterizations.

Here, we present an update of the original NCA: We will show a new and even faster approach, based on the original NCA and our experience with it. This new version of the NCA is suitable to be used on structured (rectangular) and unstructured (triangular) grids (e.g. for the ICON-LEM model).