How Storm Cloud Top Dynamics Impacts Global Climate

IPCC reports have indicated that the cloud/aerosol forcing is the largest uncertainty factor in the climate predictions by models. Most of the studies on the impact of clouds on climate trying to resolve this problem appear to focus on the clouds with large coverage areas such as cirrus or stratocumulus clouds. Very little attention is paid to the role that may be played by deep convective clouds.

This paper reports a study by us that points to a possible important impact on the global climate process by deep convective storms. It is known that water vapor concentration in the stratosphere have great impact on the climate due to their strong interaction with infrared radiation. This interaction is especially strong in the stratosphere because of the very cold temperature there. It is also known that water vapor concentration in the stratosphere fluctuates as much as 50% in the last 40 years according to NOAA balloon borne measurements. Because water vapor residence time in the stratosphere is longer than a year, its concentration fluctuation should be an important climate forcing that needs to be considered and assessed carefully.

We perform cloud resolving model simulation of thunderstorms and show that water substance from the storm cloud can be pumped into the stratosphere by the internal gravity wave breaking mechanism at the storm top. The deep convection induces strong updraft that excites internal gravity waves at the storm top, and under suitable conditions wave breaking can occur that transports storm water substance (mainly ice particles and water vapor) into the stratosphere irreversibly.

There are both satellite and aircraft observations showing the presence of above-anvil cirrus plumes and jumping cirrus at the storm top that give strong support for the model results, and all three will be presented in the meeting. We have also made an estimate of the amount of water substance transported into the stratosphere by this mechanism. The result shows that it can provide adequate water substance to moisturize the present stratosphere. We will present the calculations of this result in the meeting also.