Empirical evidence for deep convection related stratospheric cirrus clouds over North America

Abstract. Cirrus clouds in the lowermost stratosphere affect stratospheric water vapor and the Earth's radiation budget. The knowledge of its occurrence and driving forces is limited. To assess the distribution and possible formation mechanisms of stratospheric cirrus clouds (SCCs) over North America, we analyzed SCC occurrence frequencies observed by the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) instrument during the years 2006 to 2018. Possible driving forces such as deep convection are assessed based on Atmospheric Infrared Sounder (AIRS) observations during the same time. Results show that at nighttime, SCCs are most frequently observed during the thunderstorm season over the Great Plains from May to August (MJJA) with a maximum occurrence frequency of 6.2 %. During the months from November to February (NDJF), the highest SCCs occurrence frequencies are 5.5 % over the North-Eastern Pacific, western Canada and 4.4 % over the western North Atlantic. Occurrence frequencies of deep convection from AIRS, which includes storm systems, fronts, mesoscale convective systems and mesoscale convective complexes at mid- and high latitude, show similar hotspots like the SCCs, with highest occurrence frequencies being observed over the Great Plains in MJJA (4.4 %) and over the North-Eastern Pacific, western Canada and the western North Atlantic in NDJF (~2.5 %). Both, seasonal patterns and daily time series of SCCs and deep convection show a high degree of spatial and temporal relation. Further analysis indicates that the maximum fraction of SCCs related to deep convection is 74 % over the Great Plains in MJJA and about 50 % over the western North Atlantic, the North-Eastern Pacific and western Canada in NDJF. We conclude that, locally and regionally, deep convection is the leading factor related to occurrence of SCCs over North America. In this study, we also analyzed the impact of gravity waves as another important factor related to the occurrence SCCs, as the Great Plains is a well-known hotspot for stratospheric gravity waves. In the cases where SCCs are not directly linked to deep convection, we found that stratospheric gravity wave observations correlate with SCCs in as much as 30 % of the cases over the Great Plains in MJJA, about 50 % over the North-Eastern Pacific, western Canada and up to 90 % over eastern Canada and the north-west Atlantic in NDJF. Our results provide better understanding of the physical processes and climate variability related to SCCs and will be of interest for modelers as SCC sources such as deep convection and gravity waves are small-scale processes that are difficult to represent in global general circulation models.