Abstract
AbstractThe stratosphere is one of the main potential sources for subseasonal to seasonal predictability in midlatitudes in winter. The ability of an atmospheric model to realistically simulate the stratospheric dynamics is essential in order to move forward in the field of seasonal predictions in midlatitudes. Earlier studies with the ICOsahedral Nonhydrostatic atmospheric model (ICON) point out that stratospheric westerlies in ICON are underestimated. This is the first extensive study on the evaluation of Northern Hemisphere stratospheric winter circulation with ICON in numerical weather prediction (NWP) mode. Seasonal experiments with the default setup are able to reproduce the basic climatology of the stratospheric polar vortex. However, westerlies are too weak and major stratospheric warmings too frequent in ICON. Both a reduction of the nonorographic, and a reduction of the orographic gravity wave and wake drag lead to a strengthening of the stratospheric vortex and a bias reduction, in particular in January. However, the effect of the nonorographic gravity wave drag scheme on the stratosphere is stronger. Stratosphere‐troposphere coupling is intensified and more realistic due to a reduced gravity wave drag. Furthermore, an adjustment of the subgrid‐scale orographic drag parameterization leads to a significant error reduction in the mean sea level pressure. As a result of these findings, we present our current suggested improved setup for seasonal experiments with ICON‐NWP.
Highlights
The ICOsahedral Non-hydrostatic atmospheric model (ICON) has been developed jointly by the German weather service (DWD) and the Max Planck Institute for Meteorology (MPI-M), and is the central piece of the new unified modeling approach in Germany (Bonaventura, 2004; Zängl et al, 2015)
The dynamical core has been tested (Zängl et al, 2015), the ICOsahedral Nonhydrostatic atmospheric model (ICON)-numerical weather prediction (NWP) forecasts are constantly verified by DWD and ICON-A has been described (Giorgetta et al, 2018) and evaluated (Crueger et al, 2018), but there is no extensive study on the evaluation of Northern Hemisphere (NH) stratospheric winter circulation in ICON
ICON is able to reproduce the climatological characteristics of the different vortex phases, the standard deviation and extreme values are in a realistic range
Summary
Borchert et al (2019) show that stratospheric westerlies in winter are too weak in ICON, despite using an upper atmosphere extension of ICON They suggest a retuning of the gravity wave drag parameterizations in ICON. Parameterizations for the orographic drag were included into atmospheric models to reduce the systematic westerly biases in the Northern Hemisphere wintertime flow (Palmer et al, 1986), and more recently, the nonorographic gravity wave drag was originally implemented to further improve stratospheric processes in the tropics, Southern Hemisphere and the mesosphere (Dunkerton, 1997; Orr et al, 2010). How the different reduced gravity wave drag parameterizations affect the stratospheric circulation and the coupling to the troposphere in ICON will be discussed in the second part of this study (cf Section 3.2). In the third part of this study, we will investigate the surface pressure patterns in the troposphere and how they are impacted by changes to the parameterizations (cf. Section 3.3)
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