Modelling convectively induced secondary circulations in the terra incognita with TerrSysMP

Advances in high-performance computing have led to kilometre and even sub-kilometre scale regional simulations with numerical weather prediction models. This range of grid resolution also termed 'terra incognita' approaches the length-scale of the most energetic eddies of the convective atmospheric boundary layer (ABL), which accordingly can only be inadequately resolved. This dilemma becomes particularly obvious when simulating convectively induced secondary circulations (CISCs) in the terra incognita, because the modelled CISC-like circulations also termed M-CISCs are poorly resolved and exhibit a grid-resolution dependence. Previous studies have pointed out this problem using different modelling platforms and also suggested options for attenuating poorly resolved M-CISCs with methods specific to the applied ABL schemes. This study examines M-CISCs at horizontal grid resolutions of 0(1 km), using the Terrestrial Systems Modelling Platform (TerrSysMP) for idealized and real case studies. The analysis of simulations using different surface heterogeneity and grid resolutions of O(1 km) shows that the presence of a superadiabatic layer near the surface and the increase of the horizontal grid resolution allow the critical Rayleigh number to be exceeded, generating poorly resolved M-CISCs, whose amplitudes are strongly dependent on the horizontal grid resolution. We show that the asymptotic turbulent mixing length-scale in the used ABL scheme can be tuned in a way that M-CISCs are attenuated while the non-resolved turbulence dealt with by the ABL scheme effectively propagates the surface fluxes into the ABL and sustains reasonable ABL profiles.