Joint Seminar: Diurnal circulation adjustment and organized deep convection over ocean
The diurnal cycle of tropical deep convection over open ocean is investigated, with emphasis on understanding convection–circulation interaction in organized systems. Gravity wave phase speeds are invoked to estimate the time scales over which weak temperature gradient (WTG) balance is established. This estimation suggests that circulation adjustment towards WTG balance is achieved rapidly (<6 h) relative to diurnal diabatic forcing on horizontal scales typical of organized convection. Cloud-permitting numerical simulations of self-aggregation in diurnal radiative–convective equilibrium (RCE) are conducted to explore this further. These simulations depict a pronounced diurnal cycle of circulation in the organized (aggregated) state, which indeed maintains WTG balance to first order. This diurnal cycle is characterized by strong bottom-heavy circulation coinciding with enhanced convection nocturnally, resulting from longwave cloud–radiative forcing (CRF) that manifests in both cloud-top cooling and enhanced low-level differential warming. In daytime, the top-heavy heating structure of shortwave CRF suppresses convection and causes the development of top-heavy circulation. These findings are consistent with the observed diurnal pulsing of the Hadley cell driven by the ITCZ. In the case of disorganized convection and negligible large-scale circulation, the direct radiation--convection interaction (or lapse-rate) mechanism is found to be of primary importance to the diurnal cycle. In organized convection, however, the spatial variance of radiative heating between convective and dry regions is very large, and its diurnal variations lead to rapid circulation adjustments. This dynamic cloudy--clear differential radiative heating mechanism both amplifies and delays the nocturnal precipitation peak, by ~5 h. This mechanism therefore explains the tendency for tropical heavy rainfall to peak in the early morning.
Date
07.11.2017
Time
15:15 h
Place
- Bundesstr. 53, room 022/023
- Seminar Room 022/023, Ground Floor, Bundesstrasse 53, 20146 Hamburg, Hamburg
Speaker
James H. Ruppert, Max Planck Institute for Meteorology