Vertical transport and photochemistry in the terrestrial mesosphere and lower thermosphere (50–120 km)

The coupled effects of kinetics, solar cycle flux variations and vertical transport on the distribution of long-lived hydrogen-carbon-oxygen compounds in the terrestrial mesosphere and lower thermosphere are studied using a one-dimensional aeronomy model. The calculations account for the important chemical reactions and use rocket measurements of the solar flux at solar minimum and maximum. Photodissociation rates appropriate for the mesosphere are determined with a spherical shell atmosphere formalism; detailed corrections for the O_2 Schumann-Runge bands and the temperature dependence of the CO_2 cross sections are used. Then an eddy diffusion profile is derived which gives agreement with the Aladdin 74 mass spectral measurements of atomic O, O_2, CO_2, and Ar in the lower thermosphere and observations of the O_3 minimum at ∼80 km. The 115 GHz CO radio emission line computed for the CO mixing ratio profile predicted with the new eddy diffusion profile compares well with recent observations of W. J. Wilson. Differences between the calculated CO mixing ratio profile and previous theoretical and observational determinations are discussed. Our derived eddy diffusion profile has a sudden decrease at 92 km which is necessary to produce the atomic O peak at 98 km that appears in the Aladdin 74 measurements. This stagnant region apparently is a recurrent or persistent feature of the upper atmosphere since an atomic O peak around 98 km has been seen by different techniques in different seasons over several years. Slow eddy diffusion in the lower thermosphere through the homopause was also the conclusion of earlier Ar/N_2 rocket measurements studies. The analytic approach of this paper could be used in the future to monitor variations in middle atmosphere dynamics, if regularly conducted simultaneous observations of various groups of species were available

Seasonal variations of water vapor in the tropical lower stratosphere

Measurements of stratospheric water vapor by the Microwave Limb Sounder aboard the Upper Atmosphere Research Satellite show that in the tropical lower stratosphere, low‐frequency variations are closely related to the annual cycle in tropical tropopause temperatures. Tropical stratospheric air appears to retain information about the tropopause conditions it encountered for over a year as it rises through the stratosphere. We use a two‐dimensional Lagrangian model to relate MLS measurements to the temperature that tropical air parcels encountered when crossing the 100 hPa surface

Variations of tropical upper tropospheric clouds with sea surface temperature and implications for radiative effects

The variations of tropical upper tropospheric (UT) clouds with sea surface temperature (SST) are analyzed using effective cloud fraction from the Atmospheric Infrared Sounder (AIRS) on Aqua and ice water content (IWC) from the Microwave Limb Sounder (MLS) on Aura. The analyses are limited to UT clouds above 300 hPa. Our analyses do not suggest a negative correlation of tropical-mean UT cloud fraction with the cloud-weighted SST (CWT). Instead, both tropical-mean UT cloud fraction and IWC are found to increase with CWT, although their correlations with CWT are rather weak. The rate of increase of UT cloud fraction with CWT is comparable to that of precipitation, while the UT IWC and ice water path (IWP) increase more strongly with CWT. The radiative effect of UT clouds is investigated, and they are shown to provide a net warming at the top of the atmosphere. An increase of IWP with SST yields an increase of net warming that corresponds to a positive feedback, until the UT IWP exceeds a value about 50% greater than presently observed by MLS. Further increases of the UT IWP would favor the shortwave cooling effect, causing a negative feedback. Sensitivities of UT cloud forcing to the uncertainties in UT CFR and IWC measurements are discussed

The Evolution of the Stratopause During the 2006 Major Warming: Satellite Data and Assimilated Meteorological Analyses

Microwave Limb Sounder and Sounding of the Atmosphere with Broadband Emission Radiometry data show the polar stratopause, usually higher than and separated from that at midlatitudes, dropping from <55-60 to near 30 km, and cooling dramatically in January 2006 during a major stratospheric sudden warming (SSW). After a nearly isothermal period, a cool stratopause reforms near 75 km in early February, then drops to <55 km and warms. The stratopause is separated in longitude as well as latitude, with lowest temperatures in the transition regions between higher and lower stratopauses. Operational assimilated meteorological analyses, which are not constrained by data at stratopause altitude, do not capture a secondary temperature maximum that overlies the stratopause or the very high stratopause that reforms after the SSW; they underestimate the stratopause altitude variation during the SSW. High-quality daily satellite temperature measurements are invaluable in improving our understanding of stratopause evolution and its representation in models and assimilation systems

Pulsed Feedback Defers Cellular Differentiation

Environmental signals induce diverse cellular differentiation programs. In certain systems, cells defer differentiation for extended time periods after the signal appears, proliferating through multiple rounds of cell division before committing to a new fate. How can cells set a deferral time much longer than the cell cycle? Here we study Bacillus subtilis cells that respond to sudden nutrient limitation with multiple rounds of growth and division before differentiating into spores. A well-characterized genetic circuit controls the concentration and phosphorylation of the master regulator Spo0A, which rises to a critical concentration to initiate sporulation. However, it remains unclear how this circuit enables cells to defer sporulation for multiple cell cycles. Using quantitative time-lapse fluorescence microscopy of Spo0A dynamics in individual cells, we observed pulses of Spo0A phosphorylation at a characteristic cell cycle phase. Pulse amplitudes grew systematically and cell-autonomously over multiple cell cycles leading up to sporulation. This pulse growth required a key positive feedback loop involving the sporulation kinases, without which the deferral of sporulation became ultrasensitive to kinase expression. Thus, deferral is controlled by a pulsed positive feedback loop in which kinase expression is activated by pulses of Spo0A phosphorylation. This pulsed positive feedback architecture provides a more robust mechanism for setting deferral times than constitutive kinase expression. Finally, using mathematical modeling, we show how pulsing and time delays together enable “polyphasic” positive feedback, in which different parts of a feedback loop are active at different times. Polyphasic feedback can enable more accurate tuning of long deferral times. Together, these results suggest that Bacillus subtilis uses a pulsed positive feedback loop to implement a “timer” that operates over timescales much longer than a cell cycle

An atmospheric tape recorder: the imprint of tropical tropopause temperatures on stratospheric water vapor

We describe observations of tropical stratospheric water vapor q that show clear evidence of large‐scale upward advection of the signal from annual fluctuations in the effective “entry mixing ratio” qE of air entering the tropical stratosphere. In other words, air is “marked,” on emergence above the highest cloud tops, like a signal recorded on an upward moving magnetic tape. We define qE as the mean water vapor mixing ratio, at the tropical tropopause, of air that will subsequently rise and enter the stratospheric “overworld” at about 400 K. The observations show a systematic phase lag, increasing with altitude, between the annual cycle in qE and the annual cycle in q at higher altitudes. The observed phase lag agrees with the phase lag calculated assuming advection by the transformed Eulerian‐mean vertical velocity of a qE crudely estimated from 100‐hPa temperatures, which we use as a convenient proxy for tropopause temperatures. The phase agreement confirms the overall robustness of the calculation and strongly supports the tape recorder hypothesis. Establishing a quantitative link between qE and observed tropopause temperatures, however, proves difficult because the process of marking the tape depends subtly on both small‐ and large‐scale processes. The tape speed, or large‐scale upward advection speed, has a substantial annual variation and a smaller variation due to the quasi‐biennial oscillation, which delays or accelerates the arrival of the signal by a month or two in the middle stratosphere. As the tape moves upward, the signal is attenuated with an e‐folding time of about 7 to 9 months between 100 and 50 hPa and about 15 to 18 months between 50 and 20 hPa, constraining possible orders of magnitude both of vertical diffusion Kz and of rates of mixing in from the extratropics. For instance, if there were no mixing in, then Kz would be in the range 0.03–0.09 m2 s−1; this is an upper bound on Kz

Solar Occultation Satellite Data and Derived Meteorological Products: Sampling Issues and Comparisons with Aura MLS

Derived Meteorological Products (DMPs, including potential temperature (theta), potential vorticity, equivalent latitude (EqL), horizontal winds and tropopause locations) have been produced for the locations and times of measurements by several solar occultation (SO) instruments and the Aura Microwave Limb Sounder (MLS). DMPs are calculated from several meteorological analyses for the Atmospheric Chemistry Experiment-Fourier Transform Spectrometer, Stratospheric Aerosol and Gas Experiment II and III, Halogen Occultation Experiment, and Polar Ozone and Aerosol Measurement II and III SO instruments and MLS. Time-series comparisons of MLS version 1.5 and SO data using DMPs show good qualitative agreement in time evolution of O3, N2O, H20, CO, HNO3, HCl and temperature; quantitative agreement is good in most cases. EqL-coordinate comparisons of MLS version 2.2 and SO data show good quantitative agreement throughout the stratosphere for most of these species, with significant biases for a few species in localized regions. Comparisons in EqL coordinates of MLS and SO data, and of SO data with geographically coincident MLS data provide insight into where and how sampling effects are important in interpretation of the sparse SO data, thus assisting in fully utilizing the SO data in scientific studies and comparisons with other sparse datasets. The DMPs are valuable for scientific studies and to facilitate validation of non-coincident measurements

Convective outflow of South Asian pollution: A global CTM simulation compared with EOS MLS observations

A global 3-D chemical transport model is used to analyze observations of carbon monoxide (CO) and upper tropospheric clouds from the EOS Microwave Limb Sounder (MLS). MLS observations during 25 August–6 September 2004 reveal elevated CO and dense high clouds in the upper troposphere over the Tibetan plateau and southwest China, collocating with the upper level Tibetan anticyclone. Model simulations indicate the transport of boundary layer pollution by Asian summer monsoon (ASM) convection and orographic lifting to the upper troposphere over South Asia, where simulated distributions of CO resemble MLS observations. Model results also show elevated aerosols in the anticyclone region. Analysis of model simulated CO and aerosols indicate that the Tibetan anticyclone could ‘trap’ anthropogenic emissions lifted from northeast India and southwest China. These aerosols may be responsible for the formation of some of the dense high clouds.Engineering and Applied Science

Multi-messenger observations of a binary neutron star merger

On 2017 August 17 a binary neutron star coalescence candidate (later designated GW170817) with merger time 12:41:04 UTC was observed through gravitational waves by the Advanced LIGO and Advanced Virgo detectors. The Fermi Gamma-ray Burst Monitor independently detected a gamma-ray burst (GRB 170817A) with a time delay of ~1.7 s with respect to the merger time. From the gravitational-wave signal, the source was initially localized to a sky region of 31 deg2 at a luminosity distance of 40+8-8 Mpc and with component masses consistent with neutron stars. The component masses were later measured to be in the range 0.86 to 2.26 Mo. An extensive observing campaign was launched across the electromagnetic spectrum leading to the discovery of a bright optical transient (SSS17a, now with the IAU identification of AT 2017gfo) in NGC 4993 (at ~40 Mpc) less than 11 hours after the merger by the One- Meter, Two Hemisphere (1M2H) team using the 1 m Swope Telescope. The optical transient was independently detected by multiple teams within an hour. Subsequent observations targeted the object and its environment. Early ultraviolet observations revealed a blue transient that faded within 48 hours. Optical and infrared observations showed a redward evolution over ~10 days. Following early non-detections, X-ray and radio emission were discovered at the transient’s position ~9 and ~16 days, respectively, after the merger. Both the X-ray and radio emission likely arise from a physical process that is distinct from the one that generates the UV/optical/near-infrared emission. No ultra-high-energy gamma-rays and no neutrino candidates consistent with the source were found in follow-up searches. These observations support the hypothesis that GW170817 was produced by the merger of two neutron stars in NGC4993 followed by a short gamma-ray burst (GRB 170817A) and a kilonova/macronova powered by the radioactive decay of r-process nuclei synthesized in the ejecta

Multi-messenger Observations of a Binary Neutron Star Merger

On 2017 August 17 a binary neutron star coalescence candidate (later designated GW170817) with merger time 12:41:04 UTC was observed through gravitational waves by the Advanced LIGO and Advanced Virgo detectors. The Fermi Gamma-ray Burst Monitor independently detected a gamma-ray burst (GRB 170817A) with a time delay of ∼ 1.7 {{s}} with respect to the merger time. From the gravitational-wave signal, the source was initially localized to a sky region of 31 deg2 at a luminosity distance of {40}-8+8 Mpc and with component masses consistent with neutron stars. The component masses were later measured to be in the range 0.86 to 2.26 {M}ȯ . An extensive observing campaign was launched across the electromagnetic spectrum leading to the discovery of a bright optical transient (SSS17a, now with the IAU identification of AT 2017gfo) in NGC 4993 (at ∼ 40 {{Mpc}}) less than 11 hours after the merger by the One-Meter, Two Hemisphere (1M2H) team using the 1 m Swope Telescope. The optical transient was independently detected by multiple teams within an hour. Subsequent observations targeted the object and its environment. Early ultraviolet observations revealed a blue transient that faded within 48 hours. Optical and infrared observations showed a redward evolution over ∼10 days. Following early non-detections, X-ray and radio emission were discovered at the transient’s position ∼ 9 and ∼ 16 days, respectively, after the merger. Both the X-ray and radio emission likely arise from a physical process that is distinct from the one that generates the UV/optical/near-infrared emission. No ultra-high-energy gamma-rays and no neutrino candidates consistent with the source were found in follow-up searches. These observations support the hypothesis that GW170817 was produced by the merger of two neutron stars in NGC 4993 followed by a short gamma-ray burst (GRB 170817A) and a kilonova/macronova powered by the radioactive decay of r-process nuclei synthesized in the ejecta.</p