Abstract
Abstract. The increase in computing power and recent model developments allow for the use of global kilometer-scale weather and climate models for routine forecasts. At these scales, deep convective processes can be partially resolved explicitly by the model dynamics. Next to horizontal resolution, other aspects such as the applied numerical methods, the use of the hydrostatic approximation, and time step size are factors that might influence a model's ability to resolve deep convective processes. In order to improve our understanding of the role of these factors, a model intercomparison between the nonhydrostatic COSMO model and the hydrostatic Integrated Forecast System (IFS) from ECMWF has been conducted. Both models have been run with different spatial and temporal resolutions in order to simulate 2 summer days over Europe with strong convection. The results are analyzed with a focus on vertical wind speed and precipitation. Results show that even at around 3 km horizontal grid spacing the effect of the hydrostatic approximation seems to be negligible. However, time step proves to be an important factor for deep convective processes, with a reduced time step generally allowing for higher updraft velocities and thus more energy in vertical velocity spectra, in particular for shorter wavelengths. A shorter time step is also causing an earlier onset and peak of the diurnal cycle. Furthermore, the amount of horizontal diffusion plays a crucial role for deep convection with more diffusion generally leading to larger convective cells and higher precipitation intensities. The study also shows that for both models the parameterization of deep convection leads to lower updraft and precipitation intensities and biases in the diurnal cycle with a precipitation peak which is too early.
Highlights
The Earth’s atmosphere is home to processes ranging from scales as large as the planet itself, such as the trade winds, down to scales of angstroms (10−10 m), such as Rayleigh scattering of sunlight by an air molecule
Time step proves to be an important factor for deep convective processes, with a reduced time step generally allowing for higher updraft velocities and more energy in vertical velocity spectra, in particular for shorter wavelengths
Integrated Forecast System (IFS) produces more light precipitation than Consortium for Small-scale Modeling (COSMO) in all configurations and generally produces more precipitation. For both models, parameterized deep convection leads to more light precipitation but less medium-to-heavy precipitation
Summary
The Earth’s atmosphere is home to processes ranging from scales as large as the planet itself, such as the trade winds, down to scales of angstroms (10−10 m), such as Rayleigh scattering of sunlight by an air molecule. Resolving all these processes in an atmospheric model is virtually impossible, even in the distant future. Deep convection is an important process for the redistribution of heat, moisture, and momentum with a subsequent large impact on the general circulation in the atmosphere (Houze and Betts, 1981; Held and Soden, 2006)
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