New investigations on homogeneous ice nucleation: the effects of water activity and water saturation formulations

Laboratory measurements at the AIDA cloud chamber and airborne in situ observations suggest that the homogeneous freezing thresholds at low temperatures are possibly higher than expected from the so-called “Koop line”. This finding is of importance, because the ice onset relative humidity affects the cirrus cloud coverage and, at the very low temperatures of the tropical tropopause layer, together with the number of ice crystals also the transport of water vapor into the stratosphere. Both the appearance of cirrus clouds and the amount of stratospheric water feed back to the radiative budget of the atmosphere. In order to explore the enhanced ice onset humidities, we re-examine the entire homogeneous ice nucleation process, ice onset, and nucleated crystal numbers, by means of a two-moment microphysics scheme embedded in the trajectory-based model (CLaMS-Ice) as follows: the well-understood and described theoretical framework of homogeneous ice nucleation includes certain formulations of the water activity of the freezing aerosol particles and the saturation vapor pressure of water with respect to liquid water. However, different formulations are available for both parameters. Here, we present extensive sensitivity simulations testing the influence of three different formulations for the water activity and four for the water saturation on homogeneous ice nucleation. We found that the number of nucleated ice crystals is almost independent of these formulations but is instead sensitive to the size distribution of the freezing aerosol particles. The ice onset humidities, also depending on the particle size, are however significantly affected by the choices of the water activity and water saturation, in particular at cold temperatures ≲205 K. From the CLaMS-Ice sensitivity simulations, we here provide combinations of water saturation and water activity formulations suitable to reproduce the new, enhanced freezing line.

Stable water isotope signals in tropical ice clouds in the West African monsoon simulated with a regional convection-permitting model

Tropical ice clouds have an important influence on the Earth’s radiative balance. They often form as a result of tropical deep convection, which strongly affects the water budget of the tropical tropopause layer. Ice cloud formation involves complex interactions on various scales, which are not fully understood yet and lead to large uncertainties in climate predictions. In this study, we investigate the formation of tropical ice clouds related to deep convection in the West African monsoon, using stable water isotopes as tracers of moist atmospheric processes. We perform simulations using the regional isotope-enabled model COSMOiso with different resolutions and treatments of convection for the period of June–July 2016. First, we evaluate the ability of our simulations to represent the isotopic composition of monthly precipitation through comparison with GNIP observations, and the precipitation characteristics related to the monsoon evolution and convective storms based on insights from the DACCIWA field campaign in 2016. Next, a case study of a mesoscale convective system (MCS) explores the isotope signatures of tropical deep convection in atmospheric water vapour and ice. Convective updrafts within the MCS inject enriched ice into the upper troposphere leading to depletion of vapour within these updrafts due to the preferential condensation and deposition of heavy isotopes. Water vapour in downdrafts within the same MCS are enriched by non-fractionating sublimation of ice. In contrast to ice within the MCS core regions, ice in widespread cirrus shields is isotopically in approximate equilibrium with the ambient vapour, which is consistent with in situ formation of ice. These findings from the case study are supported by a statistical evaluation of isotope signals in the West African monsoon ice clouds. The following five key processes related to tropical ice clouds can be distinguished based on their characteristic isotope signatures: (1) convective lofting of enriched ice into the upper troposphere, (2) cirrus clouds that form in situ from ambient vapour under equilibrium fractionation, (3) sedimentation and sublimation of ice in the mixed-phase cloud layer in the vicinity of convective systems and underneath cirrus shields, (4) sublimation of ice in convective downdrafts that enriches the environmental vapour, and (5) the freezing of liquid water in the mixed-phase cloud layer at the base of convective updrafts. Importantly, the results show that convective systems strongly modulate the humidity budget and the isotopic composition of the lower tropical tropopause layer. They contribute to about 40 % of the total water and 60 % of HDO in the 175–125 hPa layer in the African monsoon region according to estimates based on our model simulations. Overall, this study demonstrates that isotopes can serve as useful tracers to disentangle the role of different processes in the Earth’s water cycle, including convective transport, the formation of ice clouds, and their impact on the tropical tropopause layer.

Sensitivity of Tropical Tropopause Layer Cirrus Prediction in GRAPES Global Forecast System

A warm bias with a maximum value of over 4 K in the tropical tropopause layer (TTL) is detected in day-5 operational forecasts of the Global/Regional Assimilation and Prediction System (GRAPES) for global medium-range numerical weather prediction (GRAPES_GFS). In this study, the predicted temperature changes caused by different processes are examined, and the predicted cloud fractions are compared with the European Centre for Medium-Range Weather Forecasts ERA5 reanalysis data. It is found that the overprediction of the TTL cirrus fraction contributes to the warm bias due to cloud-radiative heating. The interactions among the ice nucleation, deposition/sublimation, and the large-scale condensation together determine the results of the TTL ice crystal content prediction. Moreover, a range of sensitivity experiments show that the TTL ice crystal content prediction is sensitive to the threshold relative humidity over ice (RHi) in the ice nucleation process. Then the uncertainties of the formulas for saturation vapor pressure over ice at very low temperatures are discussed. The RHi calculated based on the Magnus–Tetens formula is up to 10% higher than that based on the Goff–Gratch formula. As the Goff–Gratch formula is applicable over a broader range of 184–273 K, it is more suitable for the cold TTL. When the Goff–Gratch formula rather than the Magnus–Tetens formula is used in the microphysics scheme, the TTL cirrus forecasts are improved greatly, and the warm bias disappears completely. After investigating the interplay of the dynamical, microphysical, and radiative processes, we find a positive feedback mechanism that exacerbates the TTL cirrus prediction error.

Impacts of tropical cyclones on the thermodynamic conditions in the tropical tropopause layer observed by A-Train satellites

The tropical tropopause layer (TTL) is the transition layer between the troposphere and the stratosphere. Tropical cyclones may impact the TTL by perturbing the vertical distributions of cloud, temperature, and water vapor. This study combines several A-Train instruments, including radar from CloudSat, lidar from the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellite, and the Atmospheric Infrared Sounder (AIRS) on the Aqua satellite, to detect signatures of cyclone impacts on the distribution patterns of cloud, water vapor, temperature, and radiation by compositing these thermodynamic fields relative to the cyclone center location. Based on the CloudSat 2B-CLDCLASS-LIDAR product, this study finds that tropical cyclone events considerably increase the occurrence frequencies of TTL clouds, in the form of cirrus clouds above a clear troposphere. The amount of TTL cloud ice, however, is found to be mostly contributed by overshooting deep convection that penetrates the base of the TTL at 16 km. To overcome the lack of temperature and water vapor products in cloudy conditions, this study implements a synergistic method that retrieves temperature, water vapor, ice water content, and effective radius simultaneously by incorporating observations from AIRS, CloudSat, and CALIPSO. Using the synergistic method, we find a vertically oscillating pattern of temperature anomalies above tropical cyclones, with warming beneath the cloud top (around 16 km) and cooling above. Based on water vapor profiles retrieved by the synergistic method, we find that the layer integrated water vapor (LIWV) above 16 km is higher above tropical cyclones, especially above overshooting deep convective clouds, compared to climatological values. Moreover, we find that the longwave and net radiative cooling effect of clouds prevails within 1000 km of tropical cyclone centers. The radiative heating effects of clouds from the CloudSat 2B-FLXHR-LIDAR product are well differentiated by the collocated brightness temperature of an infrared window channel from the collocated AIRS L1B product. By performing instantaneous radiative heating rate calculations, we further find that TTL hydration is usually associated with radiative cooling of the TTL, which inhibits the diabatic ascent of moist air across isentropic surfaces to the stratosphere. Therefore, the radiative balance of the TTL under the impact of the cyclone does not favor the maintenance of moist anomalies in the TTL or transporting water vertically to the stratosphere.

Enhanced upward motion through the troposphere over the tropical western Pacific and its implication to transport of trace gases from the troposphere to the stratosphere

The tropical western Pacific (TWP) is a preferential area of air uplifting from the surface to the upper troposphere. A significantly intensified upward motion through the troposphere over the TWP in the boreal wintertime (November to March of the next year) has been detected from 1958 to 2017 using the reanalysis datasets. Model simulations using the Whole Atmosphere Community Climate Model, version 4 (WACCM4) suggest that warming global sea surface temperatures (SSTs), particularly TWP SSTs, play a dominant role in the intensification of the upward motion by strengthening the Pacific Walker circulation and enhancing the deep convection over the TWP. Using CO as a tropospheric tracer, numeric simulations show that more CO could be elevated to the tropical tropopause layer (TTL) by the enhanced upward motion over the TWP and subsequently into the stratosphere by the strengthened Brewer-Dobson (BD) circulation which is also mainly caused by global SST warming. This implies that more tropospheric trace gases and aerosols may enter the stratosphere through the TWP region and affect the stratospheric chemistry and climate.

In situ observation of new particle formation (NPF) in the tropical tropopause layer of the 2017 Asian monsoon anticyclone – Part 1: Summary of StratoClim results

During the monsoon season of the year 2017 the airborne StratoClim mission took place in Kathmandu, Nepal, with eight mission flights of the M-55 Geophysica in the upper troposphere–lower stratosphere (UTLS) of the Asian monsoon anticyclone (AMA) over northern India, Nepal, and Bangladesh. More than 100 events of new particle formation (NPF) were observed. In total, more than 2 h of flight time was spent under NPF conditions as indicated by the abundant presence of nucleation-mode aerosols, i.e. with particle diameters dp smaller than 15 nm, which were detected in situ by means of condensation nuclei counting techniques. Mixing ratios of nucleation-mode particles (nnm) of up to ∼ 50 000 mg−1 were measured at heights of 15–16 km (θ ≈ 370 K). NPF was most frequently observed at ∼ 12–16 km altitude (θ ≈ 355–380 K) and mainly below the tropopause. Resulting nnm remained elevated (∼ 300–2000 mg−1) up to altitudes of ∼ 17.5 km (θ ≈ 400 K), while under NPF conditions the fraction (f) of sub-micrometre-sized non-volatile residues (dp > 10 nm) remained below 50 %. At ∼ 12–14 km (θ ≈ 355–365 K) the minimum of f (< 15 %) was found, and underneath, the median f generally remains below 25 %. The persistence of particles at nucleation-mode sizes is limited to a few hours, mainly due to coagulation, as demonstrated by a numerical simulation. The frequency of NPF events observed during StratoClim 2017 underlines the importance of the AMA as a source region for UTLS aerosols and for the formation and maintenance of the Asian tropopause aerosol layer (ATAL). The observed abundance of NPF-produced nucleation-mode particles within the AMA is not unambiguously attributable to (a) specific source regions in the boundary layer (according to backward trajectory analyses), or (b) the direct supply with precursor material by convective updraught (from correlations of NPF with carbon monoxide), or (c) the recent release of NPF-capable material from the convective outflow (according to air mass transport times in the tropical tropopause layer, TTL). Temperature anomalies with ΔT of 2 K (peak-to-peak amplitude), as observed at a horizontal wavelength of ∼ 70–100 km during a level flight of several hours, match with NPF detections and represent an additional mechanism for local increases in supersaturation of the NPF precursors. Effective precursor supply and widely distributed temperature anomalies within the AMA can explain the higher frequency of intense NPF observed during StratoClim 2017 than all previous NPF detections with COPAS (COndensation PArticle counting System) at TTL levels over Brazil, northern Australia, or West Africa.

Influence of Upper-Troposphere Stratification and Cloud-Radiation Interaction on Convective Overshoots in the Tropical Tropopause Layer

It is still debated whether radiative heating observed in the tropical tropopause layer (TTL) is balanced primarily by cooling from convective overshoots, as in an entrainment layer, or by adiabatic cooling from large-scale eddy-driven upwelling. In this study, three-dimensional cloud-resolving model simulations of radiative-convective equilibrium were carried out with three different cloud microphysics schemes and 1-km horizontal resolution. We demonstrate that overshooting cooling in the TTL can be strongly modulated by upper-troposphere stratification. Two of the schemes produce a hard-landing scenario in which convective overshoots reach the TTL with frequent large vertical velocity leading to strong overshooting cooling (~ −0.2 K day-1). The third scheme produces a soft-landing scenario in which convective overshoots rarely reach the TTL with large vertical velocity and produce little overshooting cooling (~ −0.03 K day-1). The difference between the two scenarios is attributed to changes in the upper-troposphere stratification related to different atmospheric cloud radiative effects (ACRE). The microphysics scheme that produces the soft-landing scenario has much stronger ACRE in the upper troposphere leading to a ~3K warmer and more stable layer which acts as a buffer zone to slow down the convective updrafts. The stratification mechanism suggests the possibility for the ozone variation or eddy-driven upwelling in the TTL to modulate convective overshoots. We further test the sensitivity of overshooting cooling to changes in model resolution by increasing the horizontal resolution to 100 m. The corresponding change of overshooting cooling is much smaller compared with the difference between the hard-landing and soft-landing scenarios.

Convective Transport Explains “Missing” Ice near the Tropical Tropopause

Spaceborne lidar shows that more ice than expected is leaving the tropical tropopause layer in the atmosphere.

Impacts of tropical cyclones on the thermodynamic conditions in the tropical tropopause layer observed by A-train satellites

The tropical tropopause layer (TTL) is the transition layer between the troposphere and the stratosphere. Tropical cyclones may impact the TTL by perturbing the vertical distributions of cloud, temperature, and water vapor, although this impact is poorly quantified due to the lack of collocated data. To address this problem, we implement a synergistic retrieval approach to obtain the thermodynamic profiles and ice water content above thick high-level clouds using the A-Train satellite measurements that pass over the tropical cyclones. This study detects the signature of cyclone impact on the distribution patterns of cloud, water vapor, temperature, and radiation by compositing these thermodynamic fields with respect to cyclone center locations. It is found that tropical cyclone events considerably increase the occurrence of TTL clouds, in the form of cirrus clouds above a clear troposphere. The amount of TTL cloud ice, however, is found to be mostly contributed by overshooting deep convections that penetrate the bottom of TTL. Using the synergistic retrieval method, we find a vertically oscillating pattern of temperature anomalies above tropical cyclones, with warming beneath the cloud top (around 16 km) and cooling above. The atmospheric column above 16 km is generally hydrated by overshooting convections, although dehydration is detected above non-overshooting TTL clouds. Above overshooting deep convections, the column-integrated water vapor is found to be on average 40 % higher than the climatology. Moreover, the TTL above tropical cyclones is cooled due to longwave radiative cooling. The radiative heating rates above cyclones are well differentiated by the brightness temperature of a satellite infrared channel in the window band. Using radiative calculations, it is found that TTL hydration is usually associated with radiative cooling of the TTL, which inhibits the diabatic ascent of moist air. The radiative balance of the TTL under the impact of the cyclone, therefore, is not in favor of maintaining the moist anomalies in the TTL or transporting water vertically to the stratosphere.

New investigations on homogeneous ice nucleation: the effects of water activity and water saturation formulations

Laboratory measurements at the AIDA cloud chamber and airborne in-situ observations suggest that the homogeneous freezing thresholds at low temperatures are possibly higher than expected from the so-called “Koop-line”. This finding is of importance, because the ice onset relative humidity affects the cirrus cloud coverage and, at the very low temperatures of the tropical tropopause layer, together with the number of ice crystals also the transport of water vapor into the stratosphere. Both, the appearance of cirrus clouds and the amount of stratospheric water feed back to the radiative budget of the atmosphere. In order to explore the enhanced ice onset humidities, we re-examine the entire homogeneous ice nucleation process, ice onset and nucleated crystal numbers, by means of a two-moment microphysics scheme embedded in the trajectory based model (CLaMS-Ice) as follows: the well-understood and described theoretical framework of homogeneous ice nucleation yet includes certain formulations of the water activity of the freezing aerosol particles and the saturation vapor pressure of water with respect to liquid water. However, different formulations are available for both parameters. Here, we present extensive sensitivity simulations testing the influence of three different formulations for the water activity and four for the water saturation on homogeneous ice nucleation. We found that the number of nucleated ice crystals is almost independent of these formulations but is instead sensitive to the size distribution of the freezing aerosol particles. The ice onset humidities, also depending on the particle size, are however significantly affected by the choices of the water activity and water saturation, in particular at cold temperatures 205 K. From the CLaMS-Ice sensitivity simulations, we here provide combinations of water saturation and water activity formulations suitable to reproduce the new, enhanced freezing line.