On the formation and persistence of subvisible cirrus clouds near the tropical tropopause

Abstract

We have used a detailed cirrus cloud model to evaluate the physical processes responsible for the formation and persistence of subvisible cirrus near the tropical tropopause and the apparent absence of these clouds at midlatitudes. We find that two distinct formation mechanisms are viable. Energetic tropical cumulonimbus clouds transport large amounts of ice water to the upper troposphere and generate extensive cirrus outflow anvils. Ice crystals with radii larger than 10 -20 •um should precipitate out of these anvils within a few hours, leaving behind an optically thin layer of small ice crystals (•vis -0.01 -0.2, depending upon the initial ice crystal size distribution). Given the long lifetimes of the clouds, wind shear is probably responsible for the observed cloud thickness <_ I km. Ice crystals can also be generated in situ by slow, synoptic scale uplift of a humid layer. Given the very low temperatures at the tropical tropopause (_-85øC), synoptic-scale uplift can generate the moderate ice supersaturations (less than 10%) required for homogeneous freezing of sulfuric acid aerosols. In addition, simulations suggest that relatively large ice crystal number densities should be generated (more than 0.5 cm-a). The numerous crystals cannot grow larger than about 10-20/zm given the available vapor, and their low fall velocities will allow them to remain in the narrow supersaturated region for at least a day. The absorption of infrared radiation in the thin cirrus results in heating rates on the order of a few K per day. If this energy drives local parcel temperature change, the cirrus will dissipate within several hours. However, if the absorbed radiative energy drives lifting of the cloud layer, the vertical wind speed will be about 0.2 cm-s -•, and the cloud may persist for days with very little change in optical or microphysical properties. The fact that these clouds form most frequently over the tropical western Pacific is probably related (through the nucleation physics) to the very low tropopause temperatures in this region. Simulations using midlatitude tropopause temperatures near -65øC suggest that at the higher temperatures, fewer ice crystals nucleate, resulting in more rapid crystal growth and cloud dissipation by precipitation. Hence, the lifetime of thin cirrus formed near the midlatitude tropopause should be limited to a few hours after the synoptic-scale system that initiated cloud formation has passed. r Introduction Th•n, persistent ice clouds have been detected near the tropical tropopause by satellite measurements [e.g., (n•s) measurements, Prabhakara et al. [1993] found that the seasonal average cloud cover produced by thin cirrus is as large as 50% over the tropical western Pacific 21,361 21,362 JENSEN ET AL.-THIN CIRRUS DEHYDRATION in all seasons. Wang et al. [1994] also observed optically thin cirrus near the tropopause in excess of 50% of the time in this region using the Stratospheric Aerosol and Gas Experiment (SAGE) II solar occultation particle extinction measurements. Optical lidar m...

The role of tropical deep convective clouds on temperature, water vapor, and dehydration in the tropical tropopause layer (TTL)

“…"Cold trap dehydration" is the hypothesis that assumes a slow ascent by large-scale freeze-drying through a particularly low temperature area over the western Pacific. Lagrangian trajectory studies (Fueglistaler et al 2005) and the frequent existence of in-situ thin cirrus clouds near the TTL (Jensen et al 1996) support this hypothesis. The other hypothesis is based on "convective dehydration," which occurs mainly due to the effects of deep overshooting convection (Sherwood and Dessler, 2001).…”

“…This indicates that the radiative heating term is not dominant in determining the local temperature anomalies since there is local net radiative heating in thin cirrus clouds (Jensen et al, 1996;Rosenfield et al, 1998), while there is net radiative cooling in thick cloud tops and in cirrus clouds having thick clouds below them (McFarquhar et al, 2000;Hartmann et al, 2001). This radiative heating contributes only to the reduction of the magnitude of the cooling in thin clouds.…”

Stratosphere‐troposphere exchange: A review, and what we have learned from STACCATO

“…Furthermore, slow ascent associated with the stratospheric fountain would likely cause large cirrus sheets in the tropics, which could not be observed for a long time [Russell et al, 1993]. Note, though, that large subvisible cirrus clouds have been detected recently [Jensen et al, 1996;McFarquhar et al, 2000], which are difficult to observe. Also, several studies Sherwood, 2000] indicate that there is net subsidence at the tropopause in the stratospheric fountain region, not ascent.…”

Evidence of impact of aviation on cirrus cloud formation

“…In the tropics, their formation is mainly triggered by deep convection (Jensen et al, 1996) while over the northern middle latitudes, they are associated to the existence of large ice supersaturated regions in the upper troposphere (Gierens et al, 2000). Cirrus cloud formation depends strongly on the existence of small particles that provide the nucleus for ice crystals (Jensen and Toon, 1997).…”