[Show abstract][Hide abstract] ABSTRACT: The transport of air masses originating from the Asian monsoon anticyclone into the extratropical upper troposphere and lower stratosphere (Ex-UTLS) above potential temperatures Θ = 380 K was identified during the HALO aircraft mission TACTS in August and September 2012. In-situ measurements of CO, O3 and N2O during TACTS Flight 2 on the 30 August 2012 show the irreversible mixing of aged with younger (originating from the troposphere) stratospheric air masses within the Ex-UTLS. Backward trajectories calculated with the trajetory module of the CLaMS model indicate that these tropospherically affected air masses originate from the Asian monsoon anticyclone. From the monsoon circulation region these air masses are quasi-isentropically transported above Θ = 380 K into the Ex-UTLS where they subsequently mix with stratospheric air masses. The overall trace gas distribution measured during TACTS shows that this transport pathway has a significant impact on the Ex-UTLS during boreal summer and autumn. This leads to an intensification of the tropospheric influence on the Ex-UTLS with ΔΘ > 30 K (relative to the tropopause) within three weeks during the TACTS mission. In the same time period a weakening of the tropospheric influence on the lowermost stratosphere (LMS) is determined. Therefore, the study shows that the transport of air masses originating from the Asian summer monsoon region within the lower stratosphere above Θ = 380 K is of major importance for the change of the chemical composition of the Ex-UTLS from summer to autumn.
Full-text · Article · Dec 2015 · Atmospheric Chemistry and Physics
[Show abstract][Hide abstract] ABSTRACT: The radiative role of ice clouds in the atmosphere is known to be important, but uncertainties remain concerning the magnitude and net effects. However, through measurements of the microphysical properties of cirrus clouds, we can better characterize them, which can ultimately allow for their radiative properties to be more accurately ascertained. It has recently been proposed that there are two types of cirrus clouds – in situ and liquid origin. In this study, we present observational evidence to show that two distinct types of cirrus do exist. Airborne, in situ measurements of cloud ice water content (IWC), ice crystal concentration (Nice), and ice crystal size from the 2014 ML-CIRRUS campaign provide cloud samples that have been divided according to their origin type. The key features that set liquid origin cirrus apart from the in situ origin cirrus are a higher frequency of high IWC (> 100 ppmv), higher Nice values, and larger ice crystals. A vertical distribution of Nice shows that the in situ origin cirrus clouds exhibit a median value of around 0.1 cm−3, while the liquid origin concentrations are slightly, but notably higher. The median sizes of the crystals contributing the most mass are less than 200 μm for in situ origin cirrus, with some of the largest crystals reaching 550 μm in size. The liquid origin cirrus, on the other hand, were observed to have median diameters greater than 200 μm, and crystals that were up to 750 μm. An examination of these characteristics in relation to each other and their relationship to temperature provides strong evidence that these differences arise from the dynamics and conditions in which the ice crystals formed. Additionally, the existence of these two groups in cirrus cloud populations may explain why a bimodal distribution in the IWC-temperature relationship has been observed. We hypothesize that the low IWC mode is the result of in situ origin cirrus and the high IWC mode is the result of liquid origin cirrus.
No preview · Article · Dec 2015 · Atmospheric Chemistry and Physics
[Show abstract][Hide abstract] ABSTRACT: The microphysical and radiative properties of cirrus clouds continue to be beyond understanding and thus still represent one of the largest uncertainties in the prediction of the Earth's climate (IPCC, 2013). Our study aims to provide a guide to cirrus microphysics, which is compiled from an extensive set of model simulations, covering the broad range of atmospheric conditions for cirrus formation and evolution. The model results are portrayed in the same parameter space as field measurements, i.e. in the Ice Water Content-Temperature (IWC-T) parameter space. We validate this cirrus analysis approach by evaluating cirrus data sets from seventeen aircraft campaigns, conducted in the last fifteen years, spending about 94 h in cirrus over Europe, Australia, Brazil as well as Southern and Northern America. Altogether, the approach of this study is to track cirrus IWC development with temperature by means of model simulations, compare with observations and then assign, to a certain degree, cirrus microphysics to the observations. Indeed, the field observations show characteristics expected from the simulated cirrus guide. For example, high/low IWCs are found together with high/low ice crystal concentrations Nice. An important finding from our study is the classification of two types of cirrus with differing formation mechanisms and microphysical properties: the first cirrus type is rather thin with lower IWCs and forms directly as ice (in-situ origin cirrus). The second type consists predominantly of thick cirrus originating from mixed phase clouds (i.e. via freezing of liquid droplets – liquid origin cirrus), which are completely glaciated while lifting to the cirrus formation temperature region (< 235 K). In the European field campaigns, in-situ origin cirrus occur frequently at slow updrafts in low and high pressure systems, but also in conjunction with faster updrafts. Also, liquid origin cirrus mostly related to warm conveyor belts are found. In the US and tropical campaigns, thick liquid origin cirrus which are formed in large convective systems are detected more frequently.
Full-text · Article · Nov 2015 · Atmospheric Chemistry and Physics
[Show abstract][Hide abstract] ABSTRACT: The SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY) aboard the Envisat satellite provided measurements from August 2002 until April 2012. SCIAMACHY measured the scattered or direct sunlight using different observation geometries. The limb viewing geometry allows the retrieval of water vapour at about 10 to 25 km height from the near infrared spectral range (1353–1410 nm). These data cover the Upper Troposphere and Lower Stratosphere (UTLS), a region in the atmosphere, which is of special interest for a variety of dynamical and chemical processes as well as for the radiative forcing. Here, the latest data version of water vapour (V3.01) from SCIAMACHY limb measurements is presented and validated by comparisons with data sets from other satellite and in situ measurements. Considering retrieval tests and the results of these comparisons, the V3.01 data is reliable from about 11 to 23 km and the best results are found in the middle of the profiles between about 14 and 20 km. Above 20 km in the extra tropics V3.01 is drier than all other data sets. Additionally, for altitudes above about 19 km the vertical resolution of the retrieved profile is not sufficient to resolve signals with a short vertical structure like the tape recorder. Below 14 km SCIAMACHY water vapour V3.01 is wetter than most collocated data sets, but the high variability of water vapour in the troposphere complicates the comparison. For 14 to 20 km height, the expected errors from the retrieval and simulations and the mean differences to collocated data sets are usually smaller than 10 % when the resolution of the SCIAMACHY data is taken into account. In general, the temporal changes agree well with collocated data sets except for the Northern Hemisphere extratropical stratosphere, where larger differences are observed. This indicates a possible drift in V3.01 most probably caused by the incomplete treatment of volcanic aerosols in the retrieval. In all other regions a good temporal stability is shown. In the tropical stratosphere an increase in water vapour is found between 2002 and 2012, which is in agreement with other satellite data sets for overlapping time periods.
[Show abstract][Hide abstract] ABSTRACT: We report first chemistry mode retrieval results from the new airborne limb-imaging infrared FTS (Fourier transform spectrometer) GLORIA (Gimballed Limb Observer for Radiance Imaging of the Atmosphere) and comparisons with observations by the conventional airborne limb-scanning infrared FTS MIPAS-STR (Michelson Interferometer for Passive Atmospheric Sounding – STRatospheric aircraft). For GLORIA, the flights aboard the high-altitude research aircraft M55 Geophysica during the ESSenCe campaign (ESa Sounder Campaign 2011) were the very first in field deployment after several years of development. The simultaneous observations of GLORIA and MIPAS-STR during the flight on 16 December 2011 inside the polar vortex and under conditions of optically partially transparent polar stratospheric clouds (PSCs) provided us the first opportunity to compare the observations by two different infrared FTS generations directly. We validate the GLORIA results with MIPAS-STR based on the lower vertical resolution of MIPAS-STR and compare the vertical resolutions of the instruments derived from their averaging kernels. The retrieval results of temperature, HNO3, O3, H2O, CFC-11 and CFC-12 show reasonable agreement of GLORIA with MIPAS-STR and collocated in situ observations. For the horizontally binned hyperspectral limb images, the GLORIA sampling outnumbered the horizontal cross-track sampling of MIPAS-STR by up to 1 order of magnitude. Depending on the target parameter, typical vertical resolutions of 0.5 to 2.0 km were obtained for GLORIA and are typically a factor of 2 to 4 better compared to MIPAS-STR. While the improvement of the performance, characterization and data processing of GLORIA are the subject of ongoing work, the presented first results already demonstrate the considerable gain in sampling and vertical resolution achieved with GLORIA.
No preview · Article · Jun 2015 · Atmospheric Measurement Techniques
[Show abstract][Hide abstract] ABSTRACT: The Gimballed Limb Observer for Radiance Imaging of the Atmosphere (GLORIA) is an airborne infrared limb imager combining a two-dimensional infrared detector with a Fourier transform spectrometer. It was operated aboard the new German Gulfstream G550 High Altitude LOng Range (HALO) research aircraft during the Transport And Composition in the upper Troposphere/lowermost Stratosphere (TACTS) and Earth System Model Validation (ESMVAL) campaigns in summer 2012. This paper describes the retrieval of temperature and trace gas (H2O, O3, HNO3) volume mixing ratios from GLORIA dynamics mode spectra that are spectrally sampled every 0.625 cm−1. A total of 26 integrated spectral windows are employed in a joint fit to retrieve seven targets using consecutively a fast and an accurate tabulated radiative transfer model. Typical diagnostic quantities are provided including effects of uncertainties in the calibration and horizontal resolution along the line of sight. Simultaneous in situ observations by the Basic Halo Measurement and Sensor System (BAHAMAS), the Fast In-situ Stratospheric Hygrometer (FISH), an ozone detector named Fairo, and the Atmospheric chemical Ionization Mass Spectrometer (AIMS) allow a validation of retrieved values for three flights in the upper troposphere/lowermost stratosphere region spanning polar and sub-tropical latitudes. A high correlation is achieved between the remote sensing and the in situ trace gas data, and discrepancies can to a large extent be attributed to differences in the probed air masses caused by different sampling characteristics of the instruments. This 1-D processing of GLORIA dynamics mode spectra provides the basis for future tomographic inversions from circular and linear flight paths to better understand selected dynamical processes of the upper troposphere and lowermost stratosphere.
No preview · Article · Jun 2015 · Atmospheric Measurement Techniques
[Show abstract][Hide abstract] ABSTRACT: he Fast In-situ Stratospheric Hygrometer (FISH) is an airborne Lyman-α photofragment fluorescence hygrometer for accurate and precise measurement of total water mixing ratios (WMR) (gas phase + evaporated ice) in the upper troposphere and lower stratosphere (UT/LS) since almost two decades. Here, we present a comprehensive review of the measurement technique, calibration procedure, accuracy and reliability of FISH. A crucial part for the FISH measurement quality is the regular calibration to a water vapor reference, namely the commercial frostpoint hygrometer DP30. In the frame of this work this frostpoint hygrometer is compared to German and British traceable metrological water standards and its accuracy is found to be 2–4%. Overall, in the range from 4–1000 ppmv, the total accuracy of FISH was found to be 6–8% as stated also in previous publications. For lower mixing ratios down to 1 ppmv, the uncertainty reaches a lower limit of 0.3 ppmv. For specific, non-atmospheric conditions, as set in experiments at the AIDA chamber – namely mixing ratios below 10 and above 100 ppmv in combination with high and low pressure conditions – the need to apply a modified FISH calibration evaluation has been identified. The new evaluation improves the agreement of FISH with other hygrometers to ± 10% accuracy in the respective mixing ratio ranges. Further, a quality check procedure for high total water measurements in cirrus clouds at high pressures (400–500 hPa) is introduced. The performance of FISH in the field is assessed by reviewing intercomparisons of FISH water vapor data with other in-situ and remote sensing hygrometers over the last two decades. We find that the agreement of FISH with the other hygrometers has improved over that time span from overall up to ±30% or more to about ±5–20% @ < 10 ppmv and to ±0–15% @ > 10 ppmv.
As presented here, the robust and continuous calibration and operation procedures of the FISH instrument over the last two decades, establish the position of FISH as one of the core instruments for in-situ observations of water vapor in the UT/LS.
Full-text · Article · Mar 2015 · Atmospheric Chemistry and Physics
[Show abstract][Hide abstract] ABSTRACT: In situ observational data on the relative humidity (RH) in the upper troposphere and lowermost stratosphere (UT/LS), or tropopause region, collected aboard civil passenger aircraft in the MOZAIC (Measurements of OZone, water vapour, carbon monoxide and nitrogen oxides by in-service AIrbus airCraft) programme were reanalysed for the period 2000 to 2009. Previous analyses of probability distribution functions (PDFs) of upper troposphere humidity (UTH) data from MOZAIC observations from year 2000 and later indicated a bias of UTH data towards higher RH values compared to data of the period 1994 to 1999. As a result, the PDF of UTH data show a substantial fraction of observations above 100% relative humidity with respect to liquid water. Such supersaturations, however, do not occur in the atmosphere because there is always a sufficient number of condensation nuclei available, that trigger condensation as soon as liquid saturation is slightly exceeded. An in-depth reanalysis of the data set identified a coding error in the calibration procedure from year 2000 on. The error did not affect earlier data from 1994 to 1999. The full data set for 2000–2009 was reanalysed applying the corrected calibration procedure. Applied correction schemes and a revised error analysis are presented along with the reanalysed PDF of relative humidity with respect to liquid water (RHliquid) and ice (RHice).
No preview · Article · Dec 2014 · Atmospheric Chemistry and Physics
[Show abstract][Hide abstract] ABSTRACT: Enhanced tropospheric trace gases such as CO, CH4 and H2O and reduced stratospheric O-3 were measured in situ in the lowermost stratosphere over northern Europe on 26 September 2012 during the TACTS aircraft campaign. The measurements indicate that these air masses clearly differ from the stratospheric background. The calculation of 40-day backward trajectories with the trajectory module of the CLaMS model shows that these air masses are affected by the Asian monsoon anticyclone. Some air masses originate from the boundary layer in Southeast Asia/West Pacific and are rapidly lifted (1-2 days) within a typhoon up to the outer edge of the Asian monsoon anticyclone. Afterwards, the air parcels are entrained by the anticyclonic circulation of the Asian monsoon. The subsequent long-range transport (8-14 days) of enhanced water vapour and pollutants to the lowermost stratosphere in northern Europe is driven by eastward transport of tropospheric air from the Asian monsoon anticyclone caused by an eddy shedding event. We found that the combination of rapid uplift by a typhoon and eastward eddy shedding from the Asian monsoon anticyclone is a novel fast transport pathway that may carry boundary emissions from Southeast Asia/West Pacific within approximately 5 weeks to the lowermost stratosphere in northern Europe.
Full-text · Article · Dec 2014 · Atmospheric Chemistry and Physics
[Show abstract][Hide abstract] ABSTRACT: In January 2010 and December 2011, synopticscale
polar stratospheric cloud (PSC) fields were probed during
seven flights of the high-altitude research aircraft M-
55 Geophysica within the RECONCILE (Reconciliation of
essential process parameters for an enhanced predictability
of Arctic stratospheric ozone loss and its climate interaction)
and the ESSenCe (ESSenCe: ESA Sounder Campaign)
projects. Particle size distributions in a diameter range between
0.46 and 40 μm were recorded by four different optical
in situ instruments. Three of these particle instruments
are based on the detection of forward-scattered light by single
particles. The fourth instrument is a grayscale optical array
imaging probe. Optical particle diameters of up to 35 μm
were detected with particle number densities and total particle
volumes exceeding previous Arctic measurements. Also,
gas-phase and particle-bound NOy was measured, as well
as water vapor concentrations.
No preview · Article · Oct 2014 · ATMOSPHERIC CHEMISTRY AND PHYSICS
[Show abstract][Hide abstract] ABSTRACT: The AquaVIT-1 Intercomparison of Atmospheric Water Vapor Measurement Techniques was conducted at the aerosol and cloud simulation chamber AIDA at the Karlsruhe Institute of Technology, Germany, in October 2007. The overall objective was to intercompare state-of-the-art and prototype atmospheric hygrometers with each other and with independent humidity standards under controlled conditions. This activity was conducted as a blind intercomparison with coordination by selected referees. The effort was motivated by persistent discrepancies found in atmospheric measurements involving multiple instruments operating on research aircraft and balloon platforms, particularly in the upper troposphere and lower stratosphere where water vapor reaches its lowest atmospheric values (less than 10 ppm). With the AIDA chamber volume of 84 m3, multiple instruments analyzed air with a common water vapor mixing ratio, either by extracting air into instrument flow systems, locating instruments inside the chamber, or sampling the chamber volume optically. The intercomparison was successfully conducted over 10 days during which pressure, temperature, and mixing ratio were systematically varied (50 to 500 hPa, 185 to 243 K, and 0.3 to 152 ppm). In the absence of an accepted reference instrument, the reference value was taken to be the ensemble mean of a core subset of the measurements. For these core instruments, the agreement between 10 and 150 ppm of water vapor is considered good with variation about the reference value of about ±10% (±1σ). In the region of most interest between 1 and 10 ppm, the core subset agreement is fair with variation about the reference value of ±20% (±1σ). The upper limit of precision was also derived for each instrument from the reported data. These results indicate that the core instruments, in general, have intrinsic skill to determine unknown water vapor mixing ratios with an accuracy of at least ±20%. The implication for atmospheric measurements is that the substantially larger differences observed during in-flight intercomparisons stem from other factors associated with the moving platforms or the non-laboratory environment. The success of AquaVIT-1 provides a template for future intercomparison efforts with water vapor or other species that are focused on improving the analytical quality of atmospheric measurements on moving platforms.
eISSN 1867-8610 = AMTD
[Show abstract][Hide abstract] ABSTRACT: accurate measurements of water vapor at the low mixing ratios (< 10 ppm) encountered in the upper troposphere and lower stratosphere (UT/LS) has proven to be a significant analytical challenge evidenced by persistent disagreements between high-precision hygrometers. These disagreements have caused uncertainties in the description of the physical processes controlling dehydration of air in the tropical tropopause layer and entry of water into the stratosphere and have hindered validation of satellite water vapor retrievals. A 2011 airborne intercomparison of a large group of in situ hygrometers onboard the NASA WB-57F high-altitude research aircraft and balloons has provided an excellent opportunity to evaluate progress in the scientific community toward improved measurement agreement. In this work we intercompare the measurements from the Midlatitude Airborne Cirrus Properties Experiment (MACPEX) and discuss the quality of agreement. Differences between values reported by the instruments were reduced in comparison to some prior campaigns but were nonnegligible and on the order of 20% (0.8 ppm). Our analysis suggests that unrecognized errors in the quantification of instrumental background for some or all of the hygrometers are a likely cause. Until these errors are understood, differences at this level will continue to somewhat limit our understanding of cirrus microphysical processes and dehydration in the tropical tropopause layer.
[Show abstract][Hide abstract] ABSTRACT: We present high-resolution measurements of water vapour, aerosols and
clouds in the Arctic stratosphere in January and February 2010 carried
out by in-situ instrumentation on balloon-sondes and high-altitude
aircraft combined with satellite observations. The measurements provide
unparalleled evidence of dehydration and rehydration due to
gravitational settling of ice particles. An extreme cooling of the
Arctic stratospheric vortex during the second half of January 2010
resulted in a rare synoptic-scale outbreak of ice PSCs (polar
stratospheric clouds) detected remotely by the lidar aboard the CALIPSO
satellite. The widespread occurrence of ice clouds was followed by
sedimentation and consequent sublimation of ice particles, leading to
vertical redistribution of water inside the vortex. A sequence of
balloon and aircraft soundings with chilled mirror and Lyman-α
hygrometers (CFH, FISH, FLASH) and backscatter sondes (COBALD) conducted
in January 2010 within the LAPBIAT and RECONCILE campaigns captured
various phases of this phenomenon: ice formation, irreversible
dehydration and rehydration. Consistent observations of water vapour by
these independent measurement techniques show clear signatures of
irreversible dehydration of the vortex air by up to 1.6 ppmv in the
20-24 km altitude range and rehydration by up to 0.9 ppmv in a 1
km-thick layer below. Comparison with space-borne Aura MLS water vapour
observations allow the spatiotemporal evolution of dehydrated air masses
within the Arctic vortex to be derived and upscaled.
Preview · Article · Nov 2013 · Atmospheric Chemistry and Physics
[Show abstract][Hide abstract] ABSTRACT: Bei chemischen Analysatoren auf Basis optischer Absorptionsmessungen stellt sich häufig die Problematik, dass die optische Messstelle von der Lichtquelle räumlich getrennt werden muss, um Störeinflüsse auf diese zu minimieren, Platzrestriktionen zu umgehen, Gefahrenbereiche zu meiden und eine Gasanalyse ohne Probennahme zu ermöglichen. Für die hochspezifische Wasserdampfmessung auf dem neuen deutschen Forschungsflugzeug HALO wurde das Konzept der direkt fasergekoppelten White-Zelle auf den simultanen Einsatz zweier optischer Wellenlängen (1.4 µm und 2.6 µm) erweitert. Ziel ist dabei, trotz einer bis zu 1000 km/h schnellen Durchströmung der offenen Messzelle, Gastemperaturen von -70 °C bis +50 °C und einem Außendruck von 100-1000 hPa die absolute Bestimmung des Wasserdampfgehaltes der Luft (1 bis 30000 ppmv) mit hoher Zeitauflösung auf der Außenhaut zu ermöglichen. Hierfür werden beiden Wellenlängen über zwei single mode-Lichtwellenleiter in eine gemeinsame White-Zelle ohne weitere Transferoptiken eingekoppelt. Auf der Tagung werden wir den Aufbau und erste Messdaten zur Beurteilung der optischen Qualität sowie deren Eignung für den Nachweis weitere Spezies wie z.B. CO2.diskutieren.
[Show abstract][Hide abstract] ABSTRACT: The MACPEX mission permitted observation of aerosol size distributions in the 4 to 1000 nm diameter range, cloud particles and water vapor in and around clouds in the mid-latitude upper troposphere. The NMASS consists of 5 condensation particle counters (cpcs) operating in parallel. The 5 cpcs have lower detection limits of approximately 4 nm, 8 nm, 16 nm, 32 nm and 50 nm. The FCAS measures the optical size of particles in the 100 nm to 1000 nm range. The data from these instruments are combined to provide size distributions from 4 to 1000 nm. Size distributions that show a local maximum in the smallest size range are evidence for recent new particle formation since the lifetime of particles in this size range is short due to coagulation.
Size distributions showing evidence of new particle formation were observed inside and near clouds in the altitude range from 10 to 14 km. The cloud particles in these high clouds are expected to be ice. Care was taken to avoid interpreting shattering of ice on the aerosol inlets as new particles. The size distributions showing new particle formation are contrasted with size distributions that do not show new particle formation in and out of the clouds. Temperature, relative humidity and trace gas abundances in air parcels exhibiting new particle formation are contrasted with those in air parcels not showing new particle formation.
[Show abstract][Hide abstract] ABSTRACT: Ice clouds are known to be major contributors to radiative forcing in
the Earth's atmosphere, yet describing their microphysical properties in
climate models remains challenging. Among these properties, the ice
water content (IWC) of cirrus clouds is of particular interest both
because it is measurable and because it can be directly related to a
number of other radiatively important variables such as extinction and
effective radius. This study expands upon the work of Schiller et al.
(2008), extending a climatology of IWC by combining datasets from
several European and US airborne campaigns and ground-based lidar
measurements over Jülich, Germany. The relationship between IWC and
temperature is further investigated using the new merged dataset and
probability distribution functions (PDFs). A PDF-based formulation
allows for representation of not only the mean values of IWC, but also
the variability of IWC within a temperature band. The IWC-PDFs are found
to be bimodal over the whole cirrus temperature range, which might be
attributed to different cirrus formation mechanisms such as
heterogeneous and homogeneous freezing. The PDFs of IWC are further
compared to distributions of cirrus ice crystal number and mass mean
radius, which show that the general relationship between IWC and
temperature appears to be influenced much more by particle number than
by particle size.
Full-text · Article · Nov 2012 · Atmospheric Chemistry and Physics
[Show abstract][Hide abstract] ABSTRACT: The occurrence of high, persistent ice supersaturation inside and
outside cold cirrus in the tropical tropopause layer (TTL) remains an
enigma that is intensely debated as the "ice supersaturation puzzle".
However, it was recently confirmed that observed supersaturations are
consistent with very low ice crystal concentrations, which is
incompatible with the idea that homogeneous freezing is the major method
of ice formation in the TTL. Thus, the tropical tropopause "ice
supersaturation puzzle" has become an "ice nucleation puzzle". To
explain the low ice crystal concentrations, a number of mainly
heterogeneous freezing methods have been proposed. Here, we reproduce in
situ measurements of frequencies of occurrence of ice crystal
concentrations by extensive model simulations, driven by the special
dynamic conditions in the TTL, namely the superposition of slow
large-scale updraughts with high-frequency short waves. From the
simulations, it follows that the full range of observed ice crystal
concentrations can be explained when the model results of the scenarios
are mixed for both heterogeneous/homogeneous and pure homogeneous ice
formation occurring in very slow (<1 cm s-1) and faster
(>1 cm s-1) large-scale updraughts. This statistical
analysis shows that about 80% of TTL cirrus can be explained by
"classical" homogeneous ice nucleation, while the remaining 20% stem
from heterogeneous and homogeneous freezing occurring within the same
environment. The mechanism limiting ice crystal production via
homogeneous freezing in an environment full of gravity waves is the
shortness of the gravity waves, which stalls freezing events before a
higher ice crystal concentration can be formed.
Preview · Article · Oct 2012 · Atmospheric Chemistry and Physics