MLS RHi (colors) and CALIOP CF (black contours). During DJF (December–February), the peak in RHi (about 16 km) is located well above both the highest convective frequency and the peak in CF. This suggests that convective moistening has a direct impact only in the lower part of the Tropical Tropopause Layer (TTL) and is less important at higher altitudes. During JJA (June–August), in contrast, the peak in RHi and peak convective frequency have a closer correspondence.
Technical description of figure:
Zonal mean RHi (color). Black contours indicate the zonal mean CALIOP cloud fraction, and the red band shows the zone of the highest density of satellite convective tops; thick white line is the zonal mean tropopause. (a) DJF. (b) JJA. CALIOP = Cloud‐Aerosol Lidar with Orthogonal Polarization; CF = cloud fraction; MLS = Microwave Limb Sounder; RHi = relative humidity with respect to ice; DJF = December–February; JJA = June–August. See front page caption for interpretation.
Scientific significance, societal relevance, and relationships to future missions:
In addition to affecting the radiative balance of the TTL itself (see front page), the processes investigated here strongly control the humidity of air entering the stratosphere. This work accordingly provides additional insights into the future evolution of stratospheric water vapor, a parameter whose variability may account for as much as 30% of equilibrium climate sensitivity.
No future missions are currently planned to provide water vapor observations with the same daily near-global-coverage as Aura MLS. The European Altius mission will provide sparser (~monthly global coverage) water vapor observations using solar occultation. The Decadal Survey “Aerosols, Clouds, Convection, and Precipitation” (ACCP) designated observables will likely include measurements similar to those from CALIOP.
CALIOP, MLS, and MERRA2 data used in this study are publicly available from NASA. Satellite convection data sets are available through Lenny Pﬁster and/or Rei Ueyama (Leonhard.Pﬁster@nasa.gov, Rei.Ueyama@nasa.gov). Aircraft data are available from the NASA Earth Science Project Ofﬁce. Ticosonde water vapor soundings are archived with the Network for Detection of Atmospheric Composition Change (NDACC).
References: Schoeberl, M. R., Jensen, E. J., Pﬁster, L., Ueyama, R., Wang, T., Selkirk, H., et al. (2019). Water vapor, clouds, and saturation in the tropical tropopause layer. Journal of Geophysical Research: Atmospheres, 124, 3984–4003. doi:10.1029/2018JD029849