Ice nucleation and dehydration in the Tropical Tropopause Layer

Abstract

Optically thin cirrus near the tropical tropopause regulate the humidity of air entering the stratosphere, which in turn has a strong influence on the Earth's radiation budget and climate. Recent highaltitude, unmanned aircraft measurements provide evidence for two distinct classes of cirrus formed in the tropical tropopause region: (i) vertically extensive cirrus with low ice number concentrations, low extinctions, and large supersaturations (up to ∼70%) with respect to ice; and (ii) vertically thin cirrus layers with much higher ice concentrations that effectively deplete the vapor in excess of saturation. The persistent supersaturation in the former class of cirrus is consistent with the long time-scales (several hours or longer) for quenching of vapor in excess of saturation given the low ice concentrations and cold tropical tropopause temperatures. The low-concentration clouds are likely formed on a background population of insoluble particles with concentrations less than 100 L −1 (often less than 20 L −1 ), whereas the high ice concentration layers (with concentrations up to 10,000 L −1 ) can only be produced by homogeneous freezing of an abundant population of aqueous aerosols. These measurements, along with past high-altitude aircraft measurements, indicate that the low-concentration cirrus occur frequently in the tropical tropopause region, whereas the high-concentration cirrus occur infrequently. The predominance of the low-concentration clouds means cirrus near the tropical tropopause may typically allow entry of air into the stratosphere with as much as ∼1.7 times the ice saturation mixing ratio. ATTREX | ice nucleiA lthough the stratosphere is extremely dry compared with the troposphere, stratospheric humidity plays crucial roles in both atmospheric chemistry and climate. As the primary source of hydroxyl (OH) radicals, H 2 O plays an important role in the regulation of stratospheric ozone. Small changes in stratospheric humidity can have significant influence on the Earth's radiation budget and climate (1, 2). Model simulations show that increases in stratospheric humidity enhance the rate of ozone destruction, cool the lower stratosphere, and warm the surface (3, 4).Air enters the stratospheric overworld (above 380 K potential temperature) almost exclusively via ascent across the tropical tropopause. This fact has motivated interest in understanding processes occurring in the transition layer between the tropical troposphere and stratosphere, a region of the atmosphere referred to as the Tropical Tropopause Layer (TTL). Physical processes occurring in this layer set the boundary condition for the composition and humidity of the stratosphere. It is well established that the aridity of the stratosphere is associated with "freeze-drying" of air as it passes through the cold TTL (5, 6). Deposition growth of ice crystals in optically thin, laminar TTL cirrus depletes vapor in excess of saturation, and the ice crystals sediment relative to the slowly ascending air. The common occurrence of these thin, ...

Impact of gravity waves on the motion and distribution of atmospheric ice particles

“…For instance, in the TTL, the clouds that 25 are characterized by RH i 100% generally have large numbers of ice crystals (Jensen et al, 2013;Rollins et al, 2016), which is consistent with the vapor-quenching explanation. Furthermore, even for low ice crystal number TTL cirrus, the assumption made here of negligible water consumption is unrealistic.…”

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

“…These observations turned the "supersaturation" into a "nucleation puzzle", which is supported by earlier measurements of low ice crystal numbers (McFarquhar et al, 2000;Thomas et al, 2002;Lawson et al, 2008). The "nucleation puzzle" is currently being intensely discussed and other nucleation pathways suppressing, modifying or replacing homogeneous freezing have been proposed (Kärcher and Koop, 2005;Gensch et al, 2008;Murray, 2008;Zobrist et al, 2008;Krämer et al, 2009;Spichtinger and Gierens, 2009c;Murray et al, 2010;Jensen et al, 2013). Most of the former approaches explaining the TTL ice nucleation are of a chemical or microphysical nature.…”

Effects of pre-existing ice crystals on cirrus clouds and comparison between different ice nucleation parameterizations with the Community Atmosphere Model (CAM5)

“…The balance of these two processes depends mainly on cirrus optical properties and thus on ice crystal number concentration (Haag, 2004;Kay et al, 2006;Fusina et al, 2007;Gettelman et al, 2012). Furthermore, the microphysical properties of cirrus clouds strongly influence the efficiency of dehydration at the tropical tropopause layer and modulate water vapor in the upper troposphere and lower stratosphere (Korolev and Isaac, 2006;Krämer et al, 2009;Jensen et al, 2013).…”

“…In recent years, significant progress has been made in both cirrus cloud measurements and cirrus cloud modeling (e.g., Heymsfield et al, 2005;Krämer et al, 2009;DeMott et al, 2011;Cziczo et al, 2013;Jensen et al, 2013; Diao et al, 2014;Barahona and Nenes, 2011;Jensen et al, 2012;Spichtinger and Krämer, 2013;Murphy, 2014). Ice crystals may form by both homogeneous freezing of soluble aerosol/droplet particles and heterogeneous ice nucleation on insoluble aerosol particles, called ice nuclei (IN;Pruppacher and Klett, 1997).…”