Cirrus observations in the tropical tropopause layer over the western Pacific

Abstract

A polarization lidar was continuously operated aboard the research vessel Mirai in the tropical western Pacific over three northern winters: at 2.0°N, 138.0°E during November and December 2001; at 2.0°N, 138.5°E during November and December 2002; and at 7.5°N, 134.0°E during December 2004 and January 2005. Intensive radiosonde soundings were made from the vessel at 3‐h intervals during all three campaigns. The mechanisms that underlie the observed variations in cirrus in the tropical tropopause layer (TTL) are discussed from the viewpoint of large‐scale dynamics and transport. During the 2001 campaign, the tropopause region was cold, but the TTL was often clear, with only some subvisual cirrus. Potential vorticity data and trajectories show that the TTL during this period was strongly affected by dry air transport from the northern midlatitude lower stratosphere. During the 2002 campaign, a packet of large‐amplitude equatorial Kelvin waves was the primary control on the generation and disappearance of cirrus in the TTL. During the 2004–2005 campaign, a cold phase of large‐scale waves resulted in cirrus generation in the TTL in late December of 2004, similar to that observed during the 2002 campaign. Outflow from the South Pacific Convergence Zone (SPCZ) caused optically thick cirrus in the TTL, particularly during early January 2005, when we also observed regular diurnal variations in cirrus development within the TTL, that is, apparent sedimentation during the nighttime. We investigated two possible controlling processes, namely, horizontal advection together with diurnal variations in convective activity within the SPCZ and diurnal variations in local temperature due to tides and gravity waves. In the equatorial western Pacific, equatorial Kelvin waves are the important dynamical process that controls cirrus variations in the TTL. Dry‐air horizontal transport from the midlatitude lower stratosphere and wet‐air vertical transport near the tropical convergence regions should also be considered in fully explaining the cirrus observations in the TTL.

Tropical tropopause ice clouds: a dynamic approach to the mystery of low crystal numbers

“…There is now evidence from several studies (Boehm and Verlinde, 2000;Immler et al, 2008;Fujiwara et al, 2009;Virts et al, 2010;Flury et al, 2012) that there is a correlation between cold anomalies in the TTL and cirrus clouds. From recent studies on cirrus clouds in the TTL using the lidar in space onboard CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation), the vertical motions in the regions of cirrus cloud occurrence were estimated (Virts et al, 2010).…”

Dehydration in the tropical tropopause layer estimated from the water vapor match

“…According to Fig. 3 of Fujiwara et al (2009), cirrus clouds were observed at around the 355 K potential temperature level (about 15.5 km height) when the second observation of this match was made. The cirrus clouds may have resulted from dehydration of the match air mass shown in Fig.…”

“…Because the default soundings in previous SOWER campaigns did not include any sensors for particles, we could not estimate the total water content in which the air mass has ice particles. In addition, for the campaign region and period (i.e., over the western Pacific in the boreal winter), cirrus clouds are common in the TTL; Shibata et al, 2007;Fujiwara et al, 2009;Massie et al, 2010;Yang et al, 2010;Inai et al, 2012;Shibata et al, 2012). If we could observe ice particles and measure their concentration and size distribution, we could estimate their growth rate under the assumption that a critical relative humidity leads to the initiation of ice nucleation, using the same technique as that employed to estimate the efficiency of dehydration (Sect.…”

Cloud-Top Height Variability Associated with Equatorial Kelvin Waves in the Tropical Tropopause Layer during the Mirai Indian Ocean cruise for the Study of the MJO-Convection Onset (MISMO) Campaign

“…The variability of TTL clouds depends on cloud microphysics , cloud dynamics (Boehm et al 1999), and large-scale dynamics (Fujiwara et al 2009). On synoptic to subseasonal time scales, equatorial Kelvin waves are one of the most influential disturbances in the TTL (Suzuki and Shiotani 2008;Suzuki et al 2010), significantly affecting the cirrus variability there (Boehm and Verlinde 2000;Immler et al 2008;Fujiwara et al 2009). …”