The effect of stochastically perturbed parametrization tendencies on rapidly ascending air streams
Authors: Pickl Moritz , Grams Christian M , Lang Simon T K , Leutbecher Martin 
Published: March 2021

&lt;p&gt;&lt;span&gt;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. &lt;span&gt;The ascent of WCBs is strongly driven by cloud-condensational processes, which are parametrized in numerical weather prediction models, and &lt;/span&gt;&lt;span&gt;is&lt;/span&gt;&lt;span&gt; therefore associated with forecast uncertainty. In the &lt;/span&gt;&lt;span&gt;European Centre for Medium-Range Weather Forecasts (&lt;/span&gt;&lt;span&gt;ECMWF&lt;/span&gt;&lt;span&gt;)&lt;/span&gt;&lt;span&gt; ensemble prediction system &lt;/span&gt;&lt;span&gt;(EPS)&lt;/span&gt;&lt;span&gt;, model uncertainty related to parametrizations is represented by the so-called stochastically perturbed parametrization tendencies (SPPT)-scheme, which &lt;/span&gt;&lt;span&gt;introduces&lt;/span&gt;&lt;span&gt; multiplicative noise to the physics tendencies.&lt;/span&gt;&lt;/span&gt;&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;&lt;span&gt;In this study, we investigate the systematic effect of the SPPT-scheme on rapidly ascending air streams &lt;/span&gt;&lt;span&gt;in the extratropics &lt;/span&gt;&lt;span&gt;(i.e. WCBs&lt;/span&gt;&lt;span&gt;)&lt;/span&gt;&lt;span&gt; and &lt;/span&gt;&lt;span&gt;on&lt;/span&gt;&lt;span&gt; tropical convection by conducting sensitivity experiments with the ECMWF &lt;/span&gt;&lt;span&gt;EPS&lt;/span&gt;&lt;span&gt; based on the &lt;/span&gt;&lt;span&gt;Integrated Forecasting System (IFS) model&lt;/span&gt;&lt;span&gt;. The comparison of an experiment with an operational setup (initial condition and model physics perturbations) to &lt;/span&gt;&lt;span&gt;one &lt;/span&gt;&lt;span&gt;where&lt;/span&gt;&lt;span&gt; model physics perturbations &lt;/span&gt;&lt;span&gt;are switched off&lt;/span&gt;&lt;span&gt; demonstrates that the SPPT-scheme systematically influences the activity of WCBs and tropical convection.&lt;/span&gt;&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;&lt;span&gt;Globally, rapidly ascending air streams, which are detected by applying trajectory analysis &lt;/span&gt;&lt;span&gt;in each ensemble member&lt;/span&gt;&lt;span&gt;, are enhanced by about 37% when SPPT is activated. Also &lt;/span&gt;&lt;span&gt;the&lt;/span&gt;&lt;span&gt; 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 &lt;/span&gt;&lt;span&gt;stronger&lt;/span&gt;&lt;span&gt; in the tropics and weaker in the extratropics. &lt;/span&gt;&lt;span&gt;A detailed investigation&lt;/span&gt;&lt;span&gt; of vertical velocities indicate&lt;/span&gt;&lt;span&gt;s&lt;/span&gt;&lt;span&gt; that SPPT increases the &lt;/span&gt;&lt;span&gt;frequency&lt;/span&gt;&lt;span&gt; &lt;/span&gt;&lt;span&gt;of &lt;/span&gt;&lt;span&gt;relatively strong&lt;/span&gt;&lt;span&gt; upward &lt;/span&gt;&lt;span&gt;motion related to WCBs and tropical convection&lt;/span&gt;&lt;span&gt;, while slower upward &lt;/span&gt;&lt;span&gt;motion&lt;/span&gt;&lt;span&gt; &lt;/span&gt;&lt;span&gt;is&lt;/span&gt;&lt;span&gt; suppressed compared to the &lt;/span&gt;&lt;span&gt;unperturbed&lt;/span&gt;&lt;span&gt; experiment. &lt;/span&gt;&lt;span&gt;Despite the symmetric, zero-mean nature of the perturbations, the response of rapidly ascending air streams to the SPPT-scheme is systematically unidirectional, &lt;/span&gt;&lt;span&gt;pointing&lt;/span&gt;&lt;span&gt; towards non-linearities in the underlying &lt;/span&gt;&lt;span&gt;processes.&lt;/span&gt;&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;&lt;span&gt;This study &lt;/span&gt;&lt;span&gt;shows&lt;/span&gt;&lt;span&gt; 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.&lt;/span&gt;&lt;/p&gt;