Non-hydrostatic simulation of frontogenesis in a moist atmosphere. Part II: Equivalent potential vorticity budget and wide bands
Simulation non-hydrostatique de la frontogénèse dans une atmosphère humide. Partie II: bilan de vorticité potentielle équivalente et bandes larges
Bénard, P. ; Lafore, J.-P. ; Redelsperger, J.-L.
The different processes responsible for the occurrence of wide <br>rainbands, as obtained by high-resolution (5-km) nonhydrostatic <br>two-dimensional simulations of frontogenesis induced by shear, with an <br>explicit representation of the convection are discussed. The study is <br>restricted to a case of strong friction at surface without any surface <br>heat flux.</p><div align="justify"> </div><p align="justify">A budget of the moist potential vorticity (<i>q<sub>e</sub></i>)<br> has been implemented for a rigorous investigation of generating <br>mechanisms of wide rainbands. The balance between sources, transport, <br>and evolution of <i>q<sub>e</sub></i> in the model is first successfully validated. The parameterized turbulent subgrid-scale processes represent the main <i>q<sub>e</sub></i> source in these simulations, especially at the PBL top.</p><div align="justify"> </div><p align="justify">It is shown that friction acts as a source of intense <i>q<sub>e</sub></i> vertical flux at the ground, maximum below the alongfront low-level jets in both warm and cold air masses. An intense positive <i>q<sub>e</sub></i><br> anomaly is obtained in the warm sector, appears to be generated by <br>frictional processes in the far prefrontal zone, and is then transported<br> towards the frontal system. This anomaly induces an intensification of <br>the alongfront low-level jet on its warm flank. In the present <br>shear-driven case, this jet corresponds to a maximum of warm moist <br>advection: the warm conveyor belt, resulting in the formation of an <br>intense warm sector wide rainband located 300 km ahead the surface cold <br>front, lies in a region of strong to weak moist symmetric stability.</p><div align="justify"> </div><p class="last" align="justify">Wide<br> cold-frontal rainbands, on the other hand, occur in a region of moist <br>symmetric instability, which thus seems to enhance the circulation <br>forced by the geostrophic shearing deformation and frictional <br>convergence in the frontal zone and favors the development of these <br>bands. They efficiently transport the lowest <i>q<sub>e</sub></i> values<br> upward and are thus diffusive. It is suggested that these bands were <br>initiated by the dissipation of convective cells generated during the <br>previous convective stage.</p>
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