Exploring the Slow Large-scale updraft Pathway into the Stratosphere over the Tropical Western Pacific

There are two major pathways for the air in the tropical tropopause Layer (TTL) transport into the stratosphere: overshooting convection and slow large-scale updrafts. Here we present further evidence of the latter pathway, the large-scale slow upwelling over the Tropical Western Pacific (TWP) region. The TWP region is known for its coldest tropopause, considered to be the region where water vapour is freeze-dried to a minimum value based on saturated vapour pressure before it eventually enters the stratosphere through a slow ascent. During this process, persistent subvisible cirrus clouds (SVC) with an optical thickness of less than 0.3 are formed in the region and the presence of SVC is taken as an indication of transport into the stratosphere. An important consequence of the slow upwelling pathway over the TWP region is the vertical transport of trace constituents. This pathway will cause the trace gases to transport into the stratosphere and therefore affect the composition of the stratospheric air (Müller et al., 2023; Rex et al., 2014).  

Motivated by this, we used ground-based COMpact Cloud-Aerosol Lidar (COMCAL) observations in Koror, Palau (7.34°N, 134.47°E, in the heart of the Pacific warm pool) and combined trajectory model simulations by Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) to study the transport pathway, with a special focus on this slow large-scale updrafts over this key region. 

We present measurements of cirrus clouds by the gound-based COMCAL Lidar from 2018 to 2022. The annual cycle shows that cloud layer height peaks with the highest Cold Point Tropopause (CPT) in NH winter and reaches its minimum with the lowest CPT in NH summer.  Compared with similar cirrus cloud measurements obtained in other tropical sites, our measurements reveal that cirrus clouds detected over TWP are the coldest and highest. The prevalence of the coldest cirrus cloud layer detected over Palau corresponds to the cold trap, a region of exceptionally cold air, in UTLS over the TWP region. In order to build the relationship between the transport path in the UTLS region and measurements, we conducted trajectory analysis by HYSPLIT model simulations based on cirrus cloud layer measurements. Our measurements and analysis of trajectories reveal that only in winter with high supersaturation at the altitude where the SVCs are detected, the air masses are further dehydrated and slowly ascend into the stratosphere. This sheds light on the pathway of slow ascend of the tropospheric air entering into the stratosphere during the NH winter over the TWP region.

Reference:

K. Müller, I. Wohltmann, P. von der Gathen, and M. Rex, “Air mass transport to the tropical west pacific troposphere inferred from ozone and relative humidity balloon observations above palau,” EGUsphere, vol. 2023, pp. 1–37, 2023.
M. Rex, I. Wohltmann, T. Ridder, R. Lehmann, K. Rosenlof, P. Wennberg, D. Weisenstein, J. Notholt, K. Krüger, V. Mohr, and S. Tegtmeier, “A tropical west pacific oh minimum and implications for stratospheric composition,” Atmos. Chem. Phys., vol. 14, no. 9, pp. 4827–4841, 2014.