Formation and maintenance of subsiding shells around non-precipitating and precipitating cumulus clouds

Savre, Julien (2020): Formation and maintenance of subsiding shells around non-precipitating and precipitating cumulus clouds. In: Quarterly Journal of the Royal Meteorological Society, Bd. 147, Nr. 735: S. 728-745

Abstract

A shell of subsiding air is generally known to develop around cumulus clouds and shield them from their environment. We seek here to improve our understanding of such shells by (a) revealing the detailed vertical and horizontal structure of shells surrounding both shallow and deeper clouds, and (b) identifying the mechanisms responsible for in-shell subsidence generation and maintenance. To that end, a high-resolution Cloud Resolving Model simulation of the shallow-to-deep convection transition over a tropical land surface is analysed with an emphasis on the cloud's near environment. Shells surrounding shallow and deep clouds are found to possess surprisingly similar characteristics. However important differences are observed near cloud top where the deepest clouds are associated with stronger subsidence and broader shells. In the convective outflow region, stronger in-shell subsidence coincides with strong buoyancy reversal, but also with strong pressure gradients naturally generated by cloud-top vortex dynamics. A more delicate balance between various processes takes place below, and in-shell subsidence is only barely sustained as buoyancy reversal is largely compensated by pressure gradients. Finally, while evaporation is clearly the main source of buoyancy reversal everywhere around cloud edges, it is also shown that the downward transport of warmer air from aloft through the subsiding shells may compensate for evaporative cooling to slowly bring in-shell buoyancy to a near-neutral state. Overall, while it cannot be denied that evaporative cooling and buoyancy reversal play important roles in generating and sustaining in-shell subsidence, the present results also emphasise that mechanical forcing at cloud top and downward transport within the shells should not be overlooked.

• Dokumententyp: Zeitschriftenartikel
• Fakultät: Physik
• Themengebiete: 500 Naturwissenschaften und Mathematik > 530 Physik
• ISSN: 0035-9009
• Sprache: Englisch