Midlatitude Cirrus Clouds and Multiple Tropopauses from a 2002-2006 Climatology over the SIRTA Observatory

TL;DR

This study analyzes midlatitude cirrus clouds and multiple tropopause occurrences over several years, revealing seasonal patterns and their relationship with cloud optical properties using lidar and radiosounding data.

Contribution

It provides a detailed climatology of cirrus clouds and multiple tropopauses, highlighting their seasonal variability and correlation, based on multi-year lidar and radiosounding observations.

Findings

MT frequency varies seasonally, lowest in May (~18%) and highest in DJF (>40%.

Cirrus clouds tend to be located just below the 1st tropopause.

Inter-tropopause cirrus are mostly low optical depth, subvisible clouds.

Abstract

This study present a comparison of lidar observations of midlatitude cirrus clouds over the 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 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 1st tropopause. When the cloud top is above the 1st 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 to the general distribution of cirrus, cross-tropopause cirrus show a higher frequency of large optical depths, while inter-tropopause cirrus show almost exclusively low optical depths (Tau < 0.03 in 90% of cases) typical of subvisible clouds. Results suggest the occurrence of inter-tropopause cirrus clouds is correlated with the frequency of multiple tropopauses.