Joint Seminar: SCALE INTERACTION to understand the tropical rainbelt: a MODELING framework
Atmospheric convection involves the interaction among scales and components: going from microscale processes (< 1 km) transferring energy in the air-surface interface, passing by the development of convective clouds and their interaction with mesoscale circulation (1-2000 km) to the influence of the large-scale flow (>2000 km). A global km-scale grid permits an explicit representation of convective storms by coupling the mesoscale and large-scale circulation. This makes km-scale climate models the adequate tools to verify the hypothesis that the representation of the meso- and large-scale circulation coupling is sufficient to represent the tropical rainbelt. ICON simulations in coupled (ocean-atmosphere-land) and uncoupled (atmosphere-land) configurations show that this hypothesis is false over tropical oceans. In both configurations, a double precipitation band is represented in the Western Pacific instead of a single one, as observations show. In the uncoupled configuration, the double precipitation band in the Western Pacific is related to a reduced precipitation over the warm pool, raising up two questions: what is the mechanism explaining the warm-pool precipitation bias? and if increasing warm-pool precipitation gives a single tropical rainbelt in the Western Pacific? To address the warm-pool precipitation bias, we use an energetic framework to set up three hypotheses, and they are related to 1) high-cloud radiative effect, 2) moisture import into the deep tropics, and 3) surface heat fluxes in light wind regimes. Our experiments show that increasing surface heat fluxes in light wind regimes increased precipitation over the warm pool, accompanied by a series of feedback. In the Western Pacific, more surface heat fluxes increase the entropy in the boundary layer together with an intensify and more frequent the top-heavy circulation (more diabatic heating). More deep convection over the Western Pacific reduce the atmospheric pressure, accelerating near-surface easterly winds, having a positive feedback on surface heat fluxes. Moreover, there is a single tropical rainbelt over the Western Pacific. Increasing the radiative effect of high clouds or reducing the precipitation efficiency of shallow clouds to import more moisture into the deep tropics do not increase warm pool precipitation. Thus, our result indicates that boundary layer processes involved in the transfer of energy in light wind regime are crucial for warm pool precipitation and the structure of the tropical rainbelt.
The result of this investigation gives the basis for the origin of the Scale Interaction Modeling (SIM) group. SIM’s objective is to understand the scale interaction during atmospheric convection and to identify the ones important in shaping climatic features. In the first step, the group focuses on identifying the role of the different scales in the entropy budget of the boundary layer over tropical oceans, in particular where light winds and warm sea surfaces interact.
Date
09.04.2025
Time
13:30–14:30 h
Place
- Bundesstr. 53, room 022/023
- Seminar Room 022/023, Ground Floor, Bundesstrasse 53, 20146 Hamburg, Hamburg