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  • 1
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    Observational constraints on the efficiency of dehydration mechanisms in the tropical tropopause layer (2016)
    Wiley
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    Publication Date: 2016-03-07
    Description: The efficiency of dehydration in the tropical tropopause layer (TTL) determines how closely water vapor will be reduced to the lowest saturation mixing ratio encountered along a trajectory to the stratosphere, thereby strongly influencing stratospheric humidity. The NASA ATTREX experiment provided an unprecedented number and quality of in situ observations to constrain the key mechanisms controlling this dehydration. Statistical analyses of the ATTREX data show that nucleation, growth, and sedimentation each result in TTL dehydration becoming increasingly inefficient at temperatures below 200 K. Because of these inefficiencies, models that ignore these mechanisms likely overestimate water vapor at the stratospheric entry point by ~10-20% at the lowest temperatures.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 2
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    Impact of radiative heating, wind shear, temperature variability, and microphysical processes on the structure and evolution of thin cirrus in the tropical tropopause layer (2011)
    Wiley
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    Publication Date: 2011-06-29
    Description: Thin cirrus that frequently form in the tropical tropopause layer (TTL) are important for vertical transport through the TTL, regulation of stratospheric humidity, and the Earth's radiation budget. Here, we use a three-dimensional cloud-resolving model to investigate the impact of circulations driven by TTL cirrus radiative heating on the cloud evolution. We use observations of TTL environmental conditions (thermal stability and wind shear) and TTL cirrus microphysical properties (ice crystal sizes and concentrations) to constrain the simulations. We show that with ice crystal sizes consistent with available observations (effective radii ≥ 12 μm), typical thermal stability, and moderate wind shear, the ice cloud sediments to lower levels before radiative heating can drive a circulation to maintain the cloud and before small-scale convection builds up. In this case, the cloud lifetime is controlled by sedimentation of ice crystals into subsaturated air below the initial cloud level, followed by sublimation. Strong wind shear (〉10 m s−1 km−1) tends to hasten the cloud dissipation. With relatively weak thermal stability, small-scale convection builds up rapidly, resulting in mixing at cloud top and extension of the cloud lifetime. We also consider the impact of synoptic-scale and mesoscale temperature variability on cloud lifetime. Using TTL trajectories with small-scale wave temperature perturbations superimposed, we show that TTL cirrus will often dissipate within 12–24 h simply as a result of background temperature variability.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 3
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    Investigation of the transport processes controlling the geographic distribution of carbon monoxide at the tropical tropopause (2015)
    Wiley
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    Publication Date: 2015-02-03
    Description: Convectively-influenced trajectory calculations are used to investigate the impact of different Tropical Tropopause Layer (TTL) transport pathways for establishing the distribution of carbon monoxide (CO) at 100 hPa as observed by the Microwave Limb Sounder (MLS) on board the Aura satellite. Carbon monoxide is a useful tracer for investigating TTL transport and convective influence because the CO lifetime (⊔1–2 months) is comparable to the time required for slow ascent through the TTL. MERRA horizontal winds are used for the diabatic trajectories, and offline calculations of TTL radiative heating are used to determine the vertical motion field. The locations and times of convective influence events along the trajectories are determined from 3-hourly, geostationary satellite measurements of convective clouds. The trajectory model reproduces most of the prominent features in the 100-hPa CO geographic distribution indicated by the MLS observations for the winter and summer 2007 periods simulated. CO concentrations and tendencies simulated with the Whole Atmosphere Climate Chemistry Model (WACCM) are used to specify boundary-layer concentrations for convective influence and CO loss rates resulting from reaction with OH. The broad maximum in CO concentration over the Pacific during Boreal winter is primarily a result of the strong radiative heating (corresponding to upward vertical motion) associated with the abundant TTL cirrus in this region. Convection over the Pacific brings clean maritime air to the tropopause region and actually decreases the 100-hPa CO. The relative abundance of CO over the continental convective regions during wintertime is sensitive to small variations in convective cloud-top height. Both the simulated and the observed summertime 100-hPa CO distributions are dominated by the maximum co-located with the upper-level anticyclone forced by the Asian monsoon convection. Sensitivity tests indicate that the summertime Asian monsoon anticyclone 100-hPa CO maximum is dominated by extreme convective systems with detrainment of polluted air above about 360–365 K potential temperature. This result stems directly from the fact that the heating rates are negative (downward motion) below 360–365 K during summertime through most of the tropics; therefore, air detrained from convection at lower levels will generally just sink back down into the middle troposphere. We find that most of the CO feeding into the Asian monsoon anticyclone comes from convection over the Tibetan Plateau and India, with relatively minor contributions from southeast Asia and eastern China.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 4
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    Physical processes controlling ice concentrations in cold cirrus near the tropical tropopause (2012)
    Wiley
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    Publication Date: 2012-06-05
    Description: Previous ice nucleation calculations have suggested that in the presence of wave-driven temperature perturbations typical of the tropical tropopause layer (TTL), homogeneous freezing should produce ice concentrations well in excess of measured values. A statistical ice cloud parameterization that includes effects of sedimentation was recently used to show that if the wave amplitudes are not too large, a quasi steady state may be established wherein loss of ice crystals by sedimentation is balanced by nucleation of new ice crystals, and the resulting cloud ice concentrations agree well with observations. Here, we use numerical models to further evaluate the evolution of ice concentrations in TTL cirrus, including a range of cloud physical processes (homogeneous and heterogeneous ice nucleation, sedimentation, and radiatively driven dynamics). We use a one-dimensional microphysical model with bin microphysics to show that as a result of gravitational size sorting, the mean ice concentrations over the life cycle of the clouds are considerably smaller than the peak ice concentrations produced by ice nucleation events. However, the mean ice concentrations predicted here are considerably higher than either those reported based on the statistical model or those indicated by the observations. With the baseline wave amplitudes, ice crystals nucleated heterogeneously do not quench rising supersaturation in cooling air parcels and prevent homogeneous nucleation that produces high ice concentrations. We also use a three-dimensional cloud resolving model to show that radiatively driven internal circulations and entrainment do slowly shift the ice concentrations toward lower values, but the time required to dilute ice concentrations produced by homogeneous freezing to values comparable to measured ice concentrations is of the order of 12–24 h, which may be longer than typical TTL cirrus lifetimes.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 5
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    Evaluation of UT/LS hygrometer accuracy by intercomparison during the NASA MACPEX mission (2014)
    Wiley
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    Publication Date: 2014-01-14
    Description: [1]  Acquiring accurate measurements of water vapor at the low mixing ratios (〈 10 ppm) encountered in the upper troposphere and lower stratosphere 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 towards improved measurement agreement. In this work we intercompare the measurements from the Mid-latitude 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 non-negligible 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.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 6
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    Improved cirrus simulations in a GCM using CARMA sectional microphysics (2013)
    Wiley
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    Publication Date: 2013-10-08
    Description: [1]  We have developed a new cirrus model incorporating sectional ice microphysics from the Community Aerosol and Radiation Model for Atmospheres (CARMA) in the latest version of NCAR's Community Atmosphere Model (CAM5). Comparisons with DARDAR and 2C-ICE show that CAM5/CARMA improves cloud fraction, ice water content, and ice water path compared to the standard CAM5. Prognostic snow in CAM5/CARMA increases overall ice mass and results in a melting layer at ~4 km in the tropics that is largely absent in CAM5. Subgrid scale supersaturation following Wilson and Ballard (1999) improves ice mass and relative humidity. Increased middle and upper tropospheric condensate in CAM5/CARMA requires a reduction in low-level cloud for energy balance, resulting in a 3.1 W m -2 improvement in shortwave cloud forcing and a 3.8 W m -2 improvement in downwelling shortwave flux at the surface compared to CAM5 and CERES. Total and clear sky longwave upwelling flux at the top are improved in CAM5/CARMA by 1.0 and 2.6 W m -2 respectively. CAM has a 2–3 K cold bias at the tropical tropopause mostly from the prescribed ozone file. Correction of the prescribed ozone or nudging the CAM5/CARMA model to GEOS5-DAS meteorology yields tropical tropopause temperatures and water vapor that agree with COSMIC and MLS. CAM5 relative humidity appears to be too large resulting in a +1.5 ppmv water vapor bias at the tropical tropopause when using GEOS5-DAS meteorology. In CAM5/CARMA, 75% of the cloud ice mass originates from ice particles detrained from convection compared to 25% from in situ nucleation.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 7
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    PI(4)P pools in PM and Golgi maintain PM PI(4,5)P2 [Cell Biology] (2014)
    National Academy of Sciences
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    Publication Date: 2014-06-04
    Description: Plasma membrane (PM) phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2] regulates the activity of many ion channels and other membrane-associated proteins. To determine precursor sources of the PM PI(4,5)P2 pool in tsA-201 cells, we monitored KCNQ2/3 channel currents and translocation of PHPLCδ1 domains as real-time indicators of PM PI(4,5)P2, and translocation of PHOSH2×2, and...
    Print ISSN: 0027-8424
    Electronic ISSN: 1091-6490
    Topics: Biology , Medicine , Natural Sciences in General
    Published by National Academy of Sciences
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  • 8
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    Role of the stratospheric polar freezing belt in denitrification (2001)
    American Association for the Advancement of Science (AAAS)
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    Publication Date: 2001-04-03
    Description: Homogeneous freezing of nitric acid hydrate particles can produce a polar freezing belt in either hemisphere that can cause denitrification. Computed denitrification profiles for one Antarctic and two Arctic cold winters are presented. The vertical range over which denitrification occurs is normally quite deep in the Antarctic but limited in the Arctic. A 4 kelvin decrease in the temperature of the Arctic stratosphere due to anthropogenic and/or natural effects can trigger the occurrence of widespread severe denitrification. Ozone loss is amplified in a denitrified stratosphere, so the effects of falling temperatures in promoting denitrification must be considered in assessment studies of ozone recovery trends.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Tabazadeh, A -- Jensen, E J -- Toon, O B -- Drdla, K -- Schoeberl, M R -- New York, N.Y. -- Science. 2001 Mar 30;291(5513):2591-4.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉NASA Ames Research Center, Earth Science Division, MS:245-4 Moffett Field, CA 94035, USA. atabazadeh@mail.arc.nasa.gov〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/11283368" target="_blank"〉PubMed〈/a〉
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Computer Science , Medicine , Natural Sciences in General , Physics
    Published by American Association for the Advancement of Science (AAAS)
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  • 9
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    Clarifying the dominant sources and mechanisms of cirrus cloud formation (2013)
    American Association for the Advancement of Science (AAAS)
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    Publication Date: 2013-05-11
    Description: Formation of cirrus clouds depends on the availability of ice nuclei to begin condensation of atmospheric water vapor. Although it is known that only a small fraction of atmospheric aerosols are efficient ice nuclei, the critical ingredients that make those aerosols so effective have not been established. We have determined in situ the composition of the residual particles within cirrus crystals after the ice was sublimated. Our results demonstrate that mineral dust and metallic particles are the dominant source of residual particles, whereas sulfate and organic particles are underrepresented, and elemental carbon and biological materials are essentially absent. Further, composition analysis combined with relative humidity measurements suggests that heterogeneous freezing was the dominant formation mechanism of these clouds.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Cziczo, Daniel J -- Froyd, Karl D -- Hoose, Corinna -- Jensen, Eric J -- Diao, Minghui -- Zondlo, Mark A -- Smith, Jessica B -- Twohy, Cynthia H -- Murphy, Daniel M -- New York, N.Y. -- Science. 2013 Jun 14;340(6138):1320-4. doi: 10.1126/science.1234145. Epub 2013 May 9.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA. djcziczo@mit.edu〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/23661645" target="_blank"〉PubMed〈/a〉
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Computer Science , Medicine , Natural Sciences in General , Physics
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  • 10
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    Evidence for the predominance of mid-tropospheric aerosols as subtropical anvil cloud nuclei (2004)
    American Association for the Advancement of Science (AAAS)
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    Publication Date: 2004-05-01
    Description: NASA's recent Cirrus Regional Study of Tropical Anvils and Cirrus Layers-Florida Area Cirrus Experiment focused on anvil cirrus clouds, an important but poorly understood element of our climate system. The data obtained included the first comprehensive measurements of aerosols and cloud particles throughout the atmospheric column during the evolution of multiple deep convective storm systems. Coupling these new measurements with detailed cloud simulations that resolve the size distributions of aerosols and cloud particles, we found several lines of evidence indicating that most anvil crystals form on mid-tropospheric rather than boundary-layer aerosols. This result defies conventional wisdom and suggests that distant pollution sources may have a greater effect on anvil clouds than do local sources.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Fridlind, Ann M -- Ackerman, Andrew S -- Jensen, Eric J -- Heymsfield, Andrew J -- Poellot, Michael R -- Stevens, David E -- Wang, Donghai -- Miloshevich, Larry M -- Baumgardner, Darrel -- Lawson, R Paul -- Wilson, James C -- Flagan, Richard C -- Seinfeld, John H -- Jonsson, Haflidi H -- VanReken, Timothy M -- Varutbangkul, Varuntida -- Rissman, Tracey A -- New York, N.Y. -- Science. 2004 Apr 30;304(5671):718-22.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉National Aeronautics and Space Administration (NASA) Ames Research Center, Moffett Field, CA 94035, USA. ann.fridlind@nasa.gov〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/15118158" target="_blank"〉PubMed〈/a〉
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Computer Science , Medicine , Natural Sciences in General , Physics
    Published by American Association for the Advancement of Science (AAAS)
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