Factors that govern the spatial distribution of tropical cirrus clouds on the seasonal time scale

Abstract:

In this research, ten-year (2007–2016) data from the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) and the fifth generation reanalysis product (ERA5) of the European Centre for Medium-Range Weather Forecasts are corroborated to investigate the influences of convection and local air temperature on tropical (20°S–20°N) cirrus clouds. We consider the convective rain rate (CRR) from ERA5 as an indicator of convection. The monthly averaged CRR and cirrus cloud fraction below 15.5 km are found to be well correlated. On the annual mean, both the CRR and cirrus cloud fraction below 15.5 km are distributed asymmetrically about the equator with larger values in the Northern Hemisphere. Seasonally, both variables obtain their maxima in the boreal summer at latitudes around 8°N–10°N. However, the correlation between the CRR and cirrus clouds decreases rapidly with height above 15.5 km. Cirrus clouds above 15.5 km are more strongly correlated with co-located air temperature. The annual mean latitudinal profiles of cirrus cloud fraction and temperature above 15.5 km are both nearly symmetric about the equator. The maximum cirrus cloud fraction and the minimum temperature above 15.5 km occur near the equator in the boreal winter, consistently with the seasonal cycle of the Brewer-Dobson circulation. The 15.5 km level can therefore be identified as the level above which the influence of convection on cirrus clouds diminishes. Above this level, in situ formation of cirrus clouds, which is regulated by air temperature, is more important to cloud occurrence than convection.