Overshooting Convection, Cirrus, and the Cold Point Tropopause in Global Storm-Resolving Models and Satellite Observations

Abstract

Processes occurring in upper troposphere-lower stratosphere region (UTLS), especially overshooting convection and radiative heating from thin cirrus, play a key role in the Earth's climate system. The cold point tropopause, defined as the coldest temperature between the upper troposphere and lower stratosphere, is particularly important for determining the moisture content of the lower stratosphere, which feeds back onto the surface warming rate. This work aims to address how the cold point tropopause, overshooting convection, and cirrus are related using five 40-day simulations of global storm-resolving models in addition to 4 years of active and passive satellite observations. We evaluate how convection that overshoots the cold point is distributed across the tropics and find that overshoots only somewhat favor warm land areas over warm ocean regions. The general geographic distribution in cold point overshoots is reproduced by the GSRMs. We also look at radiatively active cirrus near the cold point that are capable of producing a radiative heating rate strong enough to loft and cool the cold point. The GSRMs tend to simulate radiatively active cold point cirrus, as well as thin stratospheric cirrus, much less often than in the observations.

Still, despite the differences in the details of how the models simulate cold point overshoots and cirrus, we find consistent relationships between the cold point, overshooting convection, and cirrus in the GSRMs over oceans. Areas with more frequent cold point overshoots were associated with cooler mean cold point temperatures and higher cold point heights. No significant relationship was found between the mean cold points and areas of more frequent radiatively active cold point cirrus, suggesting that this mechanism does not strongly influence the cold point temperature or height. Further work examining longer GSRM simulations is needed to better understand the role of cold point-overshooting convection and cirrus lofting in setting the cold point tropopause in the context of other UTLS processes.