Convective hydration in the tropical tropopause layer during the StratoClim aircraft campaign: pathway of an observed hydration patch

Abstract

Abstract. The source and pathway of the hydration patch in the TTL (tropical tropopause layer) that was measured during the Stratospheric and upper tropospheric processes for better climate predictions (StratoClim) field campaign during the Asian summer monsoon in 2017 and its connection to convective overshoots are investigated. During flight no. 7, two remarkable layers are measured in the TTL, namely (1) the moist layer (ML) with a water vapour content of 4.8–5.7 ppmv in altitudes of 18–19 km in the lower stratosphere and (2) the ice layer (IL) with ice content up to 1.9 eq. ppmv (equivalent parts per million by volume) in altitudes of 17–18 km in the upper troposphere at around 06:30 UTC on 8 August to the south of Kathmandu (Nepal). A Meso-NH convection-permitting simulation succeeds in reproducing the characteristics of the ML and IL. Through analysis, we show that the ML and IL are generated by convective overshoots that occurred over the Sichuan Basin about 1.5 d before. Overshooting clouds develop at altitudes up to 19 km, hydrating the lower stratosphere of up to 20 km with 6401 t of water vapour by a strong-to-moderate mixing of the updraughts with the stratospheric air. A few hours after the initial overshooting phase, a hydration patch is generated, and a large amount of water vapour (above 18 ppmv) remains at even higher altitudes up to 20.5 km while the anvil cloud top descends to 18.5 km. At the same time, a great part of the hydrometeors falls shortly, and the water vapour concentration in the ML and IL decreases due to turbulent diffusion by mixing with the tropospheric air, ice nucleation, and water vapour deposition. As the hydration patch continues to travel toward the south of Kathmandu, tropospheric tracer concentration increases up to ∼30 % and 70 % in the ML and IL, respectively. The air mass in the layers becomes gradually diffused, and it has less and less water vapour and ice content by mixing with the dry tropospheric air.

Modelling the effect of condensed-phase diffusion on the homogeneous nucleation of ice in ultra-viscous particles

“…The molecular weight (136.2 g mol −1 ) and density (1260 kg m −3 ) of α-pinene SOA were kept constant throughout the study. A number of studies have aimed to quantify the effect of aerosol viscosity on equilibration timescales through αpinene SOA by estimating coefficients of water diffusion (Renbaum-Wolff et al, 2013;Price et al, 2015;Lienhard et al, 2015). The water diffusion coefficients for α-pinene SOA from Lienhard et al (2015) were specifically used in this study because the parameterisation is valid down to the low temperatures observed high in the troposphere and covers the large temperature range appropriate for our experiment as indicated by the initial conditions in Table 1.…”

“…A number of studies have aimed to quantify the effect of aerosol viscosity on equilibration timescales through αpinene SOA by estimating coefficients of water diffusion (Renbaum-Wolff et al, 2013;Price et al, 2015;Lienhard et al, 2015). The water diffusion coefficients for α-pinene SOA from Lienhard et al (2015) were specifically used in this study because the parameterisation is valid down to the low temperatures observed high in the troposphere and covers the large temperature range appropriate for our experiment as indicated by the initial conditions in Table 1. Figure 3 shows the diffusion coefficient as a function of temperature and water activity; highlighting how temperature conditions for homogeneous freezing, shown by the grey shaded region, interact with the published α-pinene SOA diffusion coefficients.…”

“…A combination of laboratory studies (Lienhard et al, 2015;Price et al, 2015) and computational models (Zobrist et al, 2011;Shiraiwa et al, 2013;O'Meara et al, 2016) have used equilibration times though viscous secondary organic aerosol particles, produced from the oxidation products of αpinene, to extract diffusion coefficients at low temperatures through inverse modelling. Diffusion coefficients were then used in single particle simulations to interpret how equilibration timescales could interact with microphysical cloud properties and modes of ice formation (Lienhard et al, 2015). At low temperatures, less than 195 K, the viscosity of α-pinene SOA particles is such that homogeneous ice nucleation may be inhibited due to restricted particle growth (Lienhard et al, 2015).…”

“…Diffusion coefficients were then used in single particle simulations to interpret how equilibration timescales could interact with microphysical cloud properties and modes of ice formation (Lienhard et al, 2015). At low temperatures, less than 195 K, the viscosity of α-pinene SOA particles is such that homogeneous ice nucleation may be inhibited due to restricted particle growth (Lienhard et al, 2015). However, at temperatures greater than 200 K and low cooling rates, less than 1 K s −1 , α-pinene SOA particles are expected to be in equilibrium with atmospheric conditions and to freeze homogeneously.…”

“…The task of this study is to develop a simple cloud parcel model, that takes into account water transport through particle bulk using an aerosol diffusion framework (Fowler et al, 2018) to better understand the effect of aerosol viscosity on ice nucleation. To appreciate the possible atmospheric implications of aerosol viscosity on ice nucleation, atmospherically relevant diffusion coefficients that depend on both temperature and water content should be used to ensure that evolving particle growth and morphology is taken into account (Lienhard et al, 2015;O'Meara et al, 2016). The overall aim of this study is to better understand the underlying physical mechanisms of ice formation in low-temperature clouds and consider whether by including aerosol viscosity effects, our model is better able to simulate their formation high in the troposphere.…”