The effect of stochastically perturbed parametrization tendencies on rapidly ascending air streams

<p><span>Most of the precipitation formation in extratropical cyclones occurs in the warm sector along an elongated air stream ahead of the cold front - the so-called warm conveyor belt (WCB). The WCB ascends slantwise from the planetary boundary layer into the upper troposphere, where its outflow interacts with the upper-level jet and modifies the Rossby wave structure. <span>The ascent of WCBs is strongly driven by cloud-condensational processes, which are parametrized in numerical weather prediction models, and </span><span>is</span><span> therefore associated with forecast uncertainty. In the </span><span>European Centre for Medium-Range Weather Forecasts (</span><span>ECMWF</span><span>)</span><span> ensemble prediction system </span><span>(EPS)</span><span>, model uncertainty related to parametrizations is represented by the so-called stochastically perturbed parametrization tendencies (SPPT)-scheme, which </span><span>introduces</span><span> multiplicative noise to the physics tendencies.</span></span></p><p>&#160;</p><p><span>In this study, we investigate the systematic effect of the SPPT-scheme on rapidly ascending air streams </span><span>in the extratropics </span><span>(i.e. WCBs</span><span>)</span><span> and </span><span>on</span><span> tropical convection by conducting sensitivity experiments with the ECMWF </span><span>EPS</span><span> based on the </span><span>Integrated Forecasting System (IFS) model</span><span>. The comparison of an experiment with an operational setup (initial condition and model physics perturbations) to </span><span>one </span><span>where</span><span> model physics perturbations </span><span>are switched off</span><span> demonstrates that the SPPT-scheme systematically influences the activity of WCBs and tropical convection.</span></p><p>&#160;</p><p><span>Globally, rapidly ascending air streams, which are detected by applying trajectory analysis </span><span>in each ensemble member</span><span>, are enhanced by about 37% when SPPT is activated. Also </span><span>the</span><span> dynamical and physical characteristics of the trajectories are systematically modified: the latent heat release and the ascent speed are increased, while the outflow latitude is decreased. This systematic modulation is </span><span>stronger</span><span> in the tropics and weaker in the extratropics. </span><span>A detailed investigation</span><span> of vertical velocities indicate</span><span>s</span><span> that SPPT increases the </span><span>frequency</span><span> </span><span>of </span><span>relatively strong</span><span> upward </span><span>motion related to WCBs and tropical convection</span><span>, while slower upward </span><span>motion</span><span> </span><span>is</span><span> suppressed compared to the </span><span>unperturbed</span><span> experiment. </span><span>Despite the symmetric, zero-mean nature of the perturbations, the response of rapidly ascending air streams to the SPPT-scheme is systematically unidirectional, </span><span>pointing</span><span> towards non-linearities in the underlying </span><span>processes.</span></p><p>&#160;</p><p><span>This study </span><span>shows</span><span> that process-oriented diagnostics of weather systems help to advance the understanding of upscale impacts of the ensemble configuration on the representation of the large-scale circulation in numerical models.</span></p>