Providing a physical basis to SPPT through the identification of weather features as coherent objects of potential vorticty anomalies: application to Mediterranean cyclogenesis
Authors: Flaounas Emmanouil , Patlakas Platon 
Published: June 2022

&lt;p&gt;&lt;span&gt;The Mediterranean Basin is a region characterized by intense cyclonic activity associated with events of extreme and adverse weather, often having high social and economic impact. The genesis and &lt;/span&gt;&lt;span&gt;further development&lt;/span&gt;&lt;span&gt; of these cyclones is &lt;/span&gt;&lt;span&gt;strongly dependent to &lt;/span&gt;&lt;span&gt;preceding&lt;/span&gt;&lt;span&gt; Rossby wave breaking over &lt;/span&gt;&lt;span&gt;the Atlantic Ocean&lt;/span&gt;&lt;span&gt;. However, the accurate reproduction&lt;/span&gt; &lt;span&gt;of &lt;/span&gt;&lt;span&gt;such &lt;/span&gt;&lt;span&gt;Rossby wave breaking is&lt;/span&gt; &lt;span&gt;a challenging issue for numerical weather prediction (NWP) &lt;/span&gt;&lt;span&gt;limit&lt;/span&gt;&lt;span&gt;ing&lt;/span&gt; &lt;span&gt;the predictability&lt;/span&gt;&lt;span&gt; of Mediterranean cyclones to&lt;/span&gt; &lt;span&gt;lead times of &lt;/span&gt;&lt;span&gt;typically no more than &lt;/span&gt;&lt;span&gt;5 days&lt;/span&gt;&lt;span&gt;. &lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;span&gt;To &lt;/span&gt;&lt;span&gt;better&lt;/span&gt;&lt;span&gt; understand the sensitivity of forecast systems in predicting Mediterranean cyclo&lt;/span&gt;&lt;span&gt;genesis and for further &lt;/span&gt;&lt;span&gt;advancing with&lt;/span&gt;&lt;span&gt; adequate ensemble approaches &lt;/span&gt;&lt;span&gt;for&lt;/span&gt;&lt;span&gt; NWP, we have developed a new&lt;/span&gt; &lt;span&gt;scheme of &lt;/span&gt;&lt;span&gt;stochastic parameter perturbation&lt;/span&gt;&lt;span&gt; of physical tendencies&lt;/span&gt;&lt;span&gt; (SPP&lt;/span&gt;&lt;span&gt;T&lt;/span&gt;&lt;span&gt;)&lt;/span&gt;&lt;span&gt;.&lt;/span&gt; &lt;span&gt;The new SPPT scheme &lt;/span&gt;&lt;span&gt;has been implemented into WRF and &lt;/span&gt;&lt;span&gt;consist&lt;/span&gt;&lt;span&gt;s&lt;/span&gt;&lt;span&gt; of &lt;/span&gt;&lt;span&gt;random &lt;/span&gt;&lt;span&gt;perturbations &lt;/span&gt;&lt;span&gt;which are &lt;/span&gt;&lt;span&gt;applied &lt;/span&gt;&lt;span&gt;solely &lt;/span&gt;&lt;span&gt;to &lt;/span&gt;&lt;span&gt;coherent 3D objects&lt;/span&gt; &lt;span&gt;of&lt;/span&gt; &lt;span&gt;potential vorticity (PV) &lt;/span&gt;&lt;span&gt;component&lt;/span&gt;&lt;span&gt;s that are p&lt;/span&gt;&lt;span&gt;roduced by physical parametrisations. Given the invertibility trait of PV, these 3D objects are expected to significantly affect the forecasted atmospheric state with inherent uncertainties to physical parametrisations and consequent model errors.&lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;span&gt;A scenario tested for the needs of the study is the Mediterranean tropical-like cyclone (&lt;/span&gt;&lt;span&gt;m&lt;/span&gt;&lt;span&gt;edicane) Zorbas that took place in September 2018; an event characterized by difficulties in &lt;/span&gt;&lt;span&gt;adequately &lt;/span&gt;&lt;span&gt;forecasting&lt;/span&gt;&lt;span&gt; the cyclogenesis stage&lt;/span&gt;&lt;span&gt;. Overall, &lt;/span&gt;&lt;span&gt;our&lt;/span&gt; &lt;span&gt;results&lt;/span&gt; &lt;span&gt;quantify&lt;/span&gt;&lt;span&gt; the &lt;/span&gt;&lt;span&gt;sensitivity of NWP of Mediterranean cyclogenesis and highlight our scheme as&lt;/span&gt;&lt;span&gt; an attractive alternative to classic approaches &lt;/span&gt;&lt;span&gt;producing significant ensemble spread&lt;/span&gt;&lt;span&gt;.&lt;/span&gt;&lt;/p&gt;