Clouds and Climate
The Romps Group at the University of California, Berkeley
It has been known for some time that deep convection can penetrate into the stratosphere. More recently, it has been argued that this overshooting convection in the tropics can play an important role in setting the humidity of the stratosphere, which, in turn, has implications for climate. In this paper, we test the hypothesis that a large fraction of this overshooting tropical convection occurs within tropical cyclones. This is accomplished by combining over two-decades worth of satellite imagery with reanalysis tropopause data and best-track data for tropical cyclones.
We find that, although tropical cyclones account for only 7% of the deep convection in the tropics, they account for about 15% of the deep convection with cloud-top temperatures below the monthly averaged tropopause temperature and 29% of the clouds that attain a cloud-top temperature 15 K below the temperature of the tropopause. This suggests that tropical cyclones could play an important role in setting the humidity of the stratosphere.
The following animation shows the tropical brightness temperatures as measured by geostationary satellites and polar orbiters during the year 2005. Red areas are clear-sky regions where the sateliites measure infrared radiation from the warm surface; the region with the warmest temperature progresses around the globe with the diurnal cycle. The blue areas are the cold cloud tops. Overlaid on this animation are white circles centered on the best-track tropical-cyclone (TC) locations. These circles have a 1000-km radius and were used to distinguish TC convection from non-TC convection. Very similar results were obtained using circles with a 500-km radius.
Figure 14. Animation of 2005 brightness temperatures with best-track data