Ice nucleation and dehydration in the Tropical Tropopause Layer (original) (raw)
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The impact of subvisible cirrus clouds near the tropical tropopause on stratospheric water vapor
Geophysical Research Letters, 1998
The radiative impact of subvisible cirrus ice clouds at and just below the tropical tropopause has been studied using a zonally averaged interactive chemistryradiation-dynamics model. Model runs have been performed with and without the inclusion of the radiative heating of these thin ice clouds, and with and without sedimentation. Near-infrared optical depths of 0.005-0.08 were computed for assumed log-normal size distributions of spherical particles having mode radii of 2-10 pm. Particles with 6 pm mode radii have computed scattering ratios of 3-15 at 603 nm, in good agreement with lidar observations. The increased radiative heating of these clouds, 0.1-0.2 K/day, results in temperature increases of 1-2 K and vertical velocity increases of 0.02-0.04 mm/s. As a consequence of the warmer tropopause, lower stratosphere water vapor increases by as much as i ppmv. The dehydration resulting from sedimentation was found to be a much smaller effect.
On the formation and persistence of subvisible cirrus clouds near the tropical tropopause
Journal of Geophysical Research: Atmospheres, 1996
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.,
Water Vapor, Clouds, and Saturation in the Tropical Tropopause Layer
J. Geophys. Res., 2019
The goal of this investigation is to understand the mechanism behind the observed high relative humidity with respect to ice (RHi) in the tropical region between~14 km (150 hPa) and the tropopause, often referred to as the tropical tropopause layer (TTL). As shown by satellite, aircraft, and balloon observations, high (>80%) RHi regions are widespread within the TTL. Regions with the highest RHi are colocated with extensive cirrus. During boreal winter, the TTL RHi is highest over the Tropical Western Pacific (TWP) with a weaker maximum over South America and Africa. In the winter, TTL temperatures are coldest and upward motion is the greatest in the TWP. It is this upward motion, driving humid air into the colder upper troposphere that produces the persistent high RHi and cirrus formation. Back trajectory calculations show that comparable adiabatic and diabatic processes contribute to this upward motion. We construct a bulk model of TWP TTL water vapor transport that includes cloud nucleation and ice microphysics that quantifies how upward motion drives the persistent high RHi in the TTL region. We find that atmospheric waves triggering cloud formation regulate the RHi and that convection dehydrates the TTL. Our forward domain-filling trajectory model is used to more precisely simulate the TTL spatial and vertical distribution of RHi. The observed RHi distribution is reproduced by the model, and we show that convection increases RHi below the base of the TTL with little impact on the RHi in the TTL region. Plain Language Summary Satellite, aircraft, and balloon observations show that the upper tropical tropospheric humidity is close to saturation. This high humidity is the result of the near-continuous upward movement of water vapor from the midtroposphere into the colder upper troposphere that results in extensive cirrus formation. Bulk and trajectory model simulations show how this process works and that convective injection of water into the tropical upper troposphere is relatively unimportant.
Aerosols that form subvisible cirrus at the tropical tropopause
Atmospheric Chemistry and Physics, 2010
The composition of residual particles from evaporated cirrus ice crystals near the tropical tropopause as well as unfrozen aerosols were measured with a single particle mass spectrometer. Subvisible cirrus residuals were predominantly composed of internal mixtures of neutralized sulfate with organic material and were chemically indistinguishable from unfrozen sulfate-organic aerosols. Ice residuals were also similar in size to unfrozen aerosol. Heterogeneous ice nuclei such as mineral dust were not enhanced in these subvisible cirrus residuals. Biomass burning particles were depleted in the residuals. Cloud probe measurements showing low cirrus ice crystal number concentrations were inconsistent with conventional homogeneous freezing. Recent laboratory studies provide heterogeneous nucleation scenarios that may explain tropopause level subvisible cirrus formation.
Dehydration potential of ultrathin clouds at the tropical tropopause
Geophysical Research Letters, 2003
1] We report on the first simultaneous in situ and remote measurements of subvisible cirrus in the uppermost tropical troposphere. The observed cirrus, called UTTCs (ultrathin tropical tropopause clouds), are the geometrically (200-300 m) and optically (t % 10 À4 ) thinnest large-scale clouds ever sampled (%10 5 km 2 ). UTTCs consist of only a few ice particles per liter with mean radius %5 mm, containing only 1 -5 % of the total water. Yet, brief adiabatic cooling events only 1 -2 K below mean ambient temperature destabilize UTTCs, leading to large sedimenting particles (r % 25 mm). Due to their extreme altitude above 17 km and low particle number density, UTTCs may efficiently dehydrate air during its last encounter with the ice phase before entering the stratosphere.
Journal of Geophysical Research, 2007
In the framework of the European Project STAR the Mobile Aerosol Raman Lidar (MARL) of the Alfred Wegener Institute (AWI) was operated in Paramaribo, Suriname (5.8°N, 55.2°W), and carried out extensive observations of tropical cirrus clouds during the local dry season from 28 September 2004 to 16 November 2004. The coverage with ice clouds was very high with 81% in the upper troposphere (above 12 km). The frequency of occurrence of subvisual clouds was found to be clearly enhanced compared to similar observations performed with the same instrument at a station in the midlatitudes. The extinction-to-backscatter ratio of thin tropical cirrus is with 26 ± 7 sr significantly higher than that of midlatitude cirrus (16 ± 9 sr). Subvisual cirrus clouds often occur in the tropical tropopause layer (TTL) above an upper tropospheric inversion. Our observations show that the ice-forming ability of the TTL is very high. The transport of air in this layer was investigated by means of a newly developed trajectory model. We found that the occurrence of clouds is highly correlated with the temperature and humidity history of the corresponding air parcel. Air that experienced a temperature minimum before the measurement took place was generally cloud free, while air that was at its temperature minimum during the observation and thus was saturated contained ice. We also detected extremely thin cloud layers slightly above the temperature minimum in subsaturated air. The solid particles of such clouds are likely to consist of nitric acid trihydrate (NAT) rather than ice.
A case study of formation and maintenance of a lower stratospheric cirrus cloud over the tropics
A rare occurrence of stratospheric cirrus at 18.6 km height persisting for about 5 days during 3-7 March 2014 is inferred from the ground-based Mie lidar observations over Gadanki (13.5 • N, 79.2 • E) and spaceborne observations. Due to the vertical transport by large updrafts on 3 March in the troposphere, triggered by a potential vorticity intrusion, the water vapour mixing ratio shows an increase around the height of 18.6 km. Relative humidity with respect to ice is ∼ 150 %, indicating that the cirrus cloud may be formed though homogeneous nucleation of sulfuric acid. The cirrus cloud persists due to the cold anomaly associated with the presence of a 4-day wave.
Cloud Formation, Convection, and Stratospheric Dehydration
Using the Modern Era Retrospective-Analysis for Research and Applications (MERRA) reanalysis winds, temperatures and anvil cloud ice, we use our domain-filling, forward trajectory model combined with a new cloud module to show that convective transport of saturated air and ice to altitudes below the tropopause has a significant impact on stratospheric water vapor and upper tropospheric clouds. We find that including cloud microphysical processes (rather than assuming that parcel water vapor never exceeds saturation) increases the lower-stratospheric average H2O by 10-20%. Our model computed cloud fraction shows reasonably good agreement with tropical upper troposphere (TUT) cloud frequency observed by the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) instrument in boreal winter with poorer agreement in summer. Our results suggest that over 40% of TUT cirrus is due to convection, and it is the saturated air from convection rather than injected cloud ice that primarily con...
Aircraft observations of thin cirrus clouds near the tropical tropopause
Journal of Geophysical Research: Atmospheres, 2001
This work describes aircraft-based lidar observations of thin cirrus clouds at the tropical tropopause in the central Pacific obtained during the Tropical Ozone Transport Experiment/Vortex Ozone Transport Experiment (TOTE/VOTE) in December 1995 and February 1996. Thin cirrus clouds were found at the tropopause on each of the four flights which penetrated within 15 degrees of the equator at 200-210 east longitude. The altitudes of these clouds exceeded 18 km at times. The cirrus