Tropopause Laminar Cirrus and Its Role in the Lower Stratosphere Total Water Budget

Abstract

Laminar cirrus are thin, extensive, isolated layers of ice clouds frequently observed in the tropical tropopause layer. Widespread laminar cirrus significantly affects tropical tropopause layer total water and thermal budget. In this study, we extract laminar cirrus from the Cloud‐Aerosol Lidar with Orthogonal Polarization Level 1 attenuated total backscatter images for January 2009, in order to characterize statistical properties of laminar cirrus cloud length, base, thickness, optical depth, and layer partial ice water path. These characteristics are used to develop an algorithm identifying laminar cirrus automatically from the Cloud‐Aerosol Lidar with Orthogonal Polarization Level 2 layer product for 2008–2017. The nearly 10‐year records reveal that tropopause laminar cirrus occurrence (30–40% of total cirrus) is strongly anticorrelated with the tropopause temperatures in that colder tropopause in frequent (super)saturation during boreal winter favors in situ formation of clouds. Interannually, anomalously warmer troposphere temperature (ΔT), easterly shear of the quasi‐biennial oscillation, and stronger upwelling branch of the Brewer‐Dobson circulation enhance laminar cirrus formation via cooling of the tropopause. The tropopause laminar cirrus carries ~0.05 mg/m3 (~0.5 ppmv) of ice water content during boreal winter and <0.01 mg/m3 during summer, which is anticorrelated with the seasonal variations of water vapor (H2O) observed by the Microwave Lime Sounder, indicating a temperature‐regulated partition between vapor and ice. Interannually, in cirrus‐rich region 1 ppmv decrease in H2O corresponds to 0.2–0.3 ppmv increase in ice water content. Frequently situated in (super)saturated air, tropopause laminar cirrus are likely to survive multiple lifecycles of the sublimation‐deposition processes, and may contribute up to 10% to the total water budget in the lower stratosphere.

The diurnal cycle of the clouds extending above the tropical tropopause observed by spaceborne lidar

“…The spatiotemporal distribution of the stratospheric clouds is in very good agreement with the 4-year climatology of Pan and Munchak (2011) from CALIPSO observations. The DJF distribution also matches the CALIPSO cirrus detection at 100 hPa reported by Wang et al (2019) for January 2009 very well. We report lower cloud frequencies than , which can be explained by the fact that we investigated slightly higher altitudes.…”

“…Both studies deplored that the fixed overpass local time of the CALIPSO dataset is far from the late af-ternoon, when land convection is at its maximum. More recently, Wang et al (2019) documented the presence of laminar cirrus in 10 years of CALIPSO data and reported a nonnegligible cloud amount above the tropopause. Because of the sun-synchronous orbit of CALIPSO, none of these studies were able to document the diurnal cycle of the stratospheric clouds.…”

Long‐Term Observations of Upper‐Tropospheric Cloud Ice From the MLS

“…Figure 6 as an example shows the evolution of 12-20 km pIWP in latitude-time section (Figures 6a and 6b), compared to the vertical pressure velocities (Figure 6c) and the convective available potential energy (CAPE, Figure 6d (SH, 0-15°S) is always found to have the least pIWP; and vice versa. This is in contrast to the thin tropopause-level cirrus ice, which is always the most abundant during boreal winter due to colder air favoring in situ formation of clouds (Wang and Dessler, 2012;Wang et al, 2019). This on the other hand suggests different formation mechanisms for upper troposphere and tropopause-level clouds: The former is more affected by deep convections whereas the latter is dominated by ambient cold temperatures that cause freeze-drying.…”

“…Obviously, both pIWP and CAPE/ bear clear hemispheric symmetry: While the Northern Hemisphere (NH, 0–15°N) has the largest enhancement of pIWP due to intense convections (larger CAPE, stronger updraft ) during boreal summer, the Southern Hemisphere (SH, 0–15°S) is always found to have the least pIWP ; and vice versa. This is in contrast to the thin tropopause‐level cirrus ice, which is always the most abundant during boreal winter due to colder air favoring in situ formation of clouds (Wang and Dessler, 2012; Wang et al., 2019). This on the other hand suggests different formation mechanisms for upper troposphere and tropopause‐level clouds: The former is more affected by deep convections whereas the latter is dominated by ambient cold temperatures that cause freeze‐drying.…”

Lidar observations of Cirrus clouds at Palau island (7°33′ N, 134°48′ E)

“…The role of Kelvin as well as gravity-and Rossby waves and their link to extensive, persistent laminar cirrus has then been addressed extensively (Pfister et al, 2001;Garrett et al, 2004). Wang et al (2019) shows from 10 years of lidar satellite observations that these optically thin laminar cirrus occurs frequently in the west/central tropical Pacific, equatorial western Africa, and northern South America, thus preferably in the tropical large-scale ascending zones. Similarly, Wang and Dessler (2012) have shown with satellite observations that the convective fractions of cirrus increase with height until the cold point tropopause is reached, peaking in their geographical occurrence over equatorial Africa, the tropical western Pacific, and South America.…”