On the utility of individual tendency output: Revealing interactions between parameterised processes during a marine cold air outbreak

Abstract

Forecasts of marine cold air outbreaks critically rely on the interplay of multiple parameterisation schemes to represent sub-grid scale processes, including shallow convection, turbulence, and microphysics. Even though such an interplay has been recognised to contribute to forecast uncertainty, a quantification of this interplay is still missing. Here, we investigate the tendencies of temperature and specific humidity contributed by individual parameterisation schemes in the operational weather prediction model AROME-Arctic. From a case study of an extensive marine cold air outbreak over the Nordic Seas, we find that the type of planetary boundary layer assigned by the model algorithm modulates the contribution of individual schemes and affects the interactions between different schemes. In addition, we demonstrate the sensitivity of these interactions to an increase or decrease in the strength of the parameterised shallow convection. The individual tendencies from several parameterisations can thereby compensate each other, sometimes resulting in a small residual. In some instances this residual remains nearly unchanged between the sensitivity experiments, even though some individual tendencies differ by up to an order of magnitude. Using the individual tendency output, we can characterise the subgrid-scale as well as grid-scale responses of the model and trace them back to their underlying causes. We thereby highlight the utility of individual tendency output for understanding process-related differences between model runs with varying physical configurations and for the continued development of numerical weather prediction models.

Spatial Variability of Nocturnal Stability Regimes in an Operational Weather Prediction Model

“…The individual tendency output provides the contributions of each parametrization to the prognostic variables such as temperature, specific humidity, and momentum. A detailed description of this diagnostic can be found in Kähnert et al (2021). To better frame this output, let us look at the evolution of temperature in the (dry) stable boundary layer in AROME-Arctic:…”

“…The first two terms on the right-hand-side of Eq. 4 are resolved by the model dynamics and are incorporated in the dynamical tendency, including horizontal diffusion (Kähnert et al 2021). The latter two terms represent the model physics and are calculated by the respective parametrizations.…”

“…4). We focus on the periods when the model internal boundary layer type (Kähnert et al 2021) is stable, i.e. when a downward surface-buoyancy flux is present.…”

“…Such output is enabled here using the tool diagnostics par domaines horizontaux (DDH, Météo-France, 2019), which provides the output of selected variables in a limited domain at every model integration timestep. To identify the contribution of different parametrized processes to the temperature evolution in the NWP model, we in addition employ the output of individual physics parametrization tendencies from AROME-Arctic (Kähnert et al 2021). This unique combination of two diagnostics provides us unprecedented insight into the SBL development in a three-dimensional NWP model.…”

Model development in practice: a comprehensive update to the boundary layer schemes in HARMONIE-AROME cycle 40

“…Even when the temperature inversion between surface and 700 hPa exceeds 20 • C, it still seems legitimate to presume the existence of an ensemble of relatively weak, dry updrafts and stronger, moist updrafts. Moreover, rigid and rather arbitrary thresholds in parameterisations, like the above-mentioned bulk temperature difference, should be avoided (Kähnert et al, 2021). Based on the considerations above, the removal of the stratocumulus regime with only a wet updraft is part of the cy40NEW configuration and therefore applies to all results of cy40NEW in this paper.…”