Why is it so difficult to represent stably stratified conditions in numerical weather prediction (NWP) models?

Abstract

[1] In the 1990s, scientists at European Centre for Medium-Range Weather Forecasts (ECMWF) suggested that artificially enhancing turbulent diffusion in stable conditions improves the representation of two important aspects of weather forecasts, i.e., near-surface temperatures and synoptic cyclones. Since then, this practice has often been used for tuning the large-scale performance of operational numerical weather prediction (NWP) models, although it is widely recognized to be detrimental for an accurate representation of stable boundary layers. Here we investigate why, 20 years on, such a compromise is still needed in the ECMWF model. We find that reduced turbulent diffusion in stable conditions improves the representation of winds in stable boundary layers, but it deteriorates the large-scale flow and the near-surface temperatures. This suggests that enhanced diffusion is still needed to compensate for errors caused by other poorly represented processes. Among these, we identify the orographic drag, which influences the large-scale flow in a similar way to the turbulence closure for stable conditions, and the strength of the land-atmosphere coupling, which partially controls the near-surface temperatures. We also take a closer look at the relationship between the turbulence closure in stable conditions and the large-scale flow, which was not investigated in detail with a global NWP model. We demonstrate that the turbulent diffusion in stable conditions affects the large-scale flow by modulating not only the strength of synoptic cyclones and anticyclones, but also the amplitude of the planetary-scale standing waves.Citation: Sandu, I., A. Beljaars, P. Bechtold, T. Mauritsen, and G. Balsamo, Why is it so difficult to represent stably stratified conditions in numerical weather prediction (NWP) models?, J. Adv. Model. Earth Syst., 5, 117-133,

Impact of planetary boundary layer turbulence on model climate and tracer transport

“…This scheme requires the specification of a turbulent length scale, which is formulated using a Blackadar (1962) style interpolation between the height above the surface and a vertical scale based on the PBL height from the previous time step. Although many AGCMs specify the length scale a priori to a constant global value (e.g., Sandu et al, 2013), the GEOS-5 formulation estimates this scale using the PBL depth diagnosed from the atmospheric profile from the previous model time step, adding "memory" and a dependence on the atmospheric state to the turbulence parameterization. This study modifies the PBL depth definition used within the Louis-scheme turbulent length-scale calculation and examines the model response.…”

ECMWF Atmospheric Profiles in Maroua, Cameroon: Analysis and Overview of the Simulation of Downward Global Solar Radiation

“…In numerical weather prediction (NWP) models such as ECMWF, turbulent diffusion under stable conditions is artificially enhanced to improve the representation of near-surface temperatures and synoptic cyclones [44]. This practice, which has often been used for tuning the large-scale performance of operational NWP models, is widely recognized to be detrimental for an accurate representation of stable atmospheric boundary layers [44][45][46]. Thus, it is not surprising that ECMWF data are greater than measurements, but the substantial biases observed and the way measured wind data appear, as shown in Figure 2c, bring us to suspect a problem with the anemometer used.…”

The impact of changes in parameterizations of surface drag and vertical diffusion on the large-scale circulation in the Community Atmosphere Model (CAM5)

“…However, the longer tail stability functions still have rather small values at high stabilities, compared to many other operational models. For comparison, the revised Louis functions, used by the IFS model at the European Centre for Medium-Range Weather Forecasts (ECMWF, Sandu et al 2013), are included in Fig. 1.…”

“…There is a lack of understanding of some of the relevant atmospheric processes and it is still unclear how observations from point measurements should be translated into parameterized grid-box averages of turbulence and drag. The issue of both representing the largescale circulation and the near-surface climate accurately and doing so for the right reasons has long been recognized as a challenge for GCMs and numerical weather prediction (NWP) models (Sandu et al 2013;Holtslag et al 2013).…”

“…It has been documented, for example in GABLS1, that the wind turning in the boundary layer becomes too small, the planetary boundary layer height (PBLH) too high and low level jets tend to be smoothed out and located too far from the surface (Brown et al 2005(Brown et al , 2008Cuxart et al 2006;Svensson and Holtslag 2009). The ECMWF model employs enhanced diffusion and Sandu et al (2013) concluded that while reducing the diffusion improved the simulation of the stable boundary layer, the near-surface temperature and the large-scale circulation were degraded. Brown et al (2008) reported an improvement in near-surface winds without a degradation of the large-scale circulation after reducing the diffusivity over sea in stable conditions and including a non-local momentum mixing in convection conditions in the UK Met Office Model.…”