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20/6/2019: Séminaire de David Coppin (Département de physique, Université d’Auckland, Auckland, Nouvelle Zélande)

David Coppin donnera un séminaire sur le thème "Coastal precipitation, low frequency variability and physical mechanisms"
Quand ? Le 20/06/2019,
de 10:30 à 12:30
Où ? Salle de réunion du LMD (T45-55, 3e étage)
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Téléphone 0144272799
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As an increasing number of people lives near the coast, especially in the tropics, understanding the processes that control coastal climates becomes crucial for public safety and well-being. However, how coastal precipitation influences the long-term climate and interacts with lower-frequency variability is still an open question. The physical mechanisms responsible for its nocturnal offshore propagation around tropical islands also remain unclear.

To tackle these questions, we use two different approaches. The first one implies looking at 18 years of high-resolution (8 km - 30 mins) satellite precipitation data and tracking coastal precipitation systems. The algorithm developed for this purpose identifies the Maritime Continent and Central America as regions where coastal rainfall contributes the most to total rainfall. Coastal precipitation also controls the diurnal cycle of precipitation, with relatively short-lived (6-12h) systems during the day and longer-lived systems peaking in the early evening. We investigate the hypothesis that coastal precipitation is enhanced prior to the arrival of the Madden-Julian (MJO) envelope over the Maritime Continent. Our results support this hypothesis and show that, when considering only coastal precipitation, the diurnal cycle appears reinforced even earlier over islands than previously reported. We discuss the respective roles of coastal and large-scale precipitation in the propagation of the MJO over the Maritime Continent.

The second approach uses idealized cloud-resolving simulations with an island placed in the middle of a long channel oceanic domain with constant sea surface temperature, without rotation and without exchange with the atmosphere outside of the domain. A strong diurnal cycle typical of a tropical island is simulated, with a thermally forced sea breeze in daytime and the associated inland propagation of precipitation. Nocturnal offshore propagation of convective systems and a land breeze are simulated every night, but with varying extent. Detailed budgets of momentum, temperature and humidity show that a fast-propagating gravity wave triggers convection far from the coast if the large-scale conditions are favorable. This affects the propagation speed of the land-breeze as it reduces the onshore large-scale wind. The distance of propagation is particularly sensitive to humidity and temperature at the top of the boundary layer. The impact of topography on these mechanisms is also investigated by adding different topography on the island.