Midlatitude cirrus clouds and multiple tropopauses from a 2002–2006 climatology over the SIRTA observatory

Abstract

This study presents a comparison of lidar observations of midlatitude cirrus clouds over the Site Instrumental de Recherche par Télédétection Atmosphérique (SIRTA) observatory between 2002 and 2006 with multiple tropopauses (MT) retrieved from radiosounding temperature profiles. The temporal variability of MT properties (frequency, thickness) are discussed. Results show a marked annual cycle, with MT frequency reaching its lowest point in May (∼18% occurrence of MT) and slowly rising to more than 40% in December, January, and February (DJF). The average thickness of the MT also follows an annual cycle, going from less than 1 km in spring to 1.5 km in late autumn. Comparison with lidar observations show that cirrus clouds show a preference for being located close below the first tropopause. When the cloud top is above the first tropopause (7% of observations), in 20% of cases, the cloud base is above it as well, resulting in a cirrus cloud “sandwiched” between the two tropopauses. Compared with the general distribution of cirrus, cross‐tropopause cirrus clouds show a higher frequency of large optical depths, while intertropopause cirrus clouds show almost exclusively low optical depths (τ < 0.03 in 90% of cases) typical of subvisible clouds. Results suggest that the occurrence of intertropopause cirrus clouds is correlated with the frequency of multiple tropopauses.