Quarterly journal of the Royal Meteorological Society, 2019-04, Vol.145 (721), p.1427-1443
Convection‐resolving models (CRMs) are established as a solid framework to simulate moist convection in both numerical weather prediction and regional‐scale climate projections. However, capturing the different scales of the governing processes is challenging. Previous studies have shown that the size and properties of individual clouds and updraughts do not converge until horizontal grid spacings (Δx) of
(100 m). We refer to this as structural convergence. On the other hand, a few recent studies have demonstrated that domain‐averaged and integrated tendencies related to a large ensemble of convective cells converge at the kilometre scale. We refer to this as bulk convergence.
This study investigates both the bulk convergence of the mean diurnal cycle and spatial distribution of precipitation, clouds and convective transport, and structural convergence of cloud‐scale statistics in real‐case convection‐resolving simulations. Two nine‐day episodes of quasi‐periodic diurnal moist convection are simulated at Δx = 8.8, 4.4, 2.2, 1.1 km and 550 m over the Alps and over Central Germany to compare the results in the presence and in the absence of a mesoscale orographic forcing.
Results reveal that bulk convergence is systematically achieved in both episodes for the spatial distribution of the analysed quantities. For their mean diurnal cycle, bulk convergence is generally observed in simulations over the Alps, but not over Central Germany, indicating that the presence of a mesoscale orographic forcing reduces the resolution sensitivity of the bulk flow properties. Structural convergence is confirmed to be not yet fully achieved at the kilometre scale. In particular, the size and strength of the simulated convective updraughts and the size of the smallest clouds are largely determined by Δx.
This study investigates both the bulk convergence of the mean diurnal cycle and spatial distribution of precipitation, clouds and convective transport, and structural convergence of cloud‐scale statistics in real‐case convection‐resolving simulations over flat and mountainous terrain. Bulk convergence is systematically obtained for the spatial distribution of the analyzed variables. For their mean diurnal cycle, bulk convergence is attained only in the presence of a mesoscale orographic forcing. Structural convergence is not yet achieved at the kilometre scale.
convection‐resolving models ; grey zone of convection ; convergence ; resolution sensitivity
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