Tropical Convection Overshoots the Cold Point Tropopause Nearly as Often Over Warm Oceans as Over Land

Tropical convection that overshoots the cold point tropopause can impact the climate by directly influencing water vapor, temperatures, and thin cirrus in the upper troposphere-lower stratosphere (UTLS) region. The distribution of cold point overshoots between land and ocean may help determine how the overshoots will affect the UTLS in a changing climate. Using 4 years of satellite and reanalysis data, we test a brightness temperature proxy calibrated by radar/lidar data to identify cold point-overshooting convection across the global tropics. We find evidence of cold point-overshooting convection throughout the tropics, though other cirrus above the cold point cover an area 100 times larger than overshooting tops. Cold point-overshooting convection occurs 30%-40% more often over convectively active land areas than over the warmest oceans. This proxy can be generalized to evaluate the fidelity of cold point overshoots simulated by storm-resolving models. Extremely deep convection in the tropics that overshoots the cold point, the coldest temperature level between the upper troposphere and lower stratosphere, influences the vertical temperature structure of this region and water vapor in the lower stratosphere, where it acts as a greenhouse gas. Overshooting cloud tops appear cold in infrared satellite imagery, so they can be identified from the difference between their brightness temperature and the nearby cold point temperature. We calibrate this brightness temperature proxy using satellite measurements of cloud ice. Cold point overshoots occur almost as often over the warmest oceans as over moist tropical land areas. Overshooting tops comprise only 1% of satellite-detectable cloud above the cold point, most of which is very thin ice cloud. Our proxy can be used as a real-world observational test of cold point overshoots simulated by the most realistic global atmospheric models, which resolve individual thunderstorm systems. We identify likely tropical cold point overshoots using a radar/lidar calibrated cold point-relative brightness temperature proxyIn a 4-year climatology, cold point overshoots only modestly favor convectively active land areas over the Indo-Pacific warm poolThin cirrus above the cold point covers over 100-fold more tropical area than cold point overshoots

Automated summary

Tropical convection that overshoots the cold point tropopause can directly impact the climate by affecting water vapor, temperatures, and thin cirrus in the upper troposphere-lower stratosphere region. The distribution of these overshoots between land and ocean can determine their influence in a changing climate. Cold point-overshooting convection occurs more frequently over convectively active land areas than over warm oceans. This proxy method can evaluate the accuracy of simulated cold point overshoots. Cold point overshoots have an influence on the temperature structure and water vapor in the lower stratosphere.