David Stuart Sayres

Process-evaluation of tropospheric humidity simulated by general circulation models using water vapor isotopologues: 1. Comparison between models and observations

The goal of this study is to determine how H2O and HDO measurements in water vapor can be used to detect and diagnose biases in the representation of processes controlling tropospheric humidity in atmospheric general circulation models (GCMs). We analyze a large number of isotopic data sets (four satellite, sixteen ground-based remote-sensing, five surface in situ and three aircraft data sets) that are sensitive to different altitudes throughout the free troposphere. Despite significant differences between data sets, we identify some observed HDO/H2O characteristics that are robust across data sets and that can be used to evaluate models. We evaluate the isotopic GCM LMDZ, accounting for the effects of spatiotemporal sampling and instrument sensitivity. We find that LMDZ reproduces the spatial patterns in the lower and mid troposphere remarkably well. However, it underestimates the amplitude of seasonal variations in isotopic composition at all levels in the subtropics and in midlatitudes, and this bias is consistent across all data sets. LMDZ also underestimates the observed meridional isotopic gradient and the contrast between dry and convective tropical regions compared to satellite data sets. Comparison with six other isotope-enabled GCMs from the SWING2 project shows that biases exhibited by LMDZ are common to all models. The SWING2 GCMs show a very large spread in isotopic behavior that is not obviously related to that of humidity, suggesting water vapor isotopic measurements could be used to expose model shortcomings. In a companion paper, the isotopic differences between models are interpreted in terms of biases in the representation of processes controlling humidity. Copyright

UV Dosage Levels in Summer: Increased Risk of Ozone Loss from Convectively Injected Water Vapor

Water In, Ozone Out The danger of stratospheric ozone loss burst into public awareness in the 1980s, when the Antarctic ozone hole was discovered and described. Since then, the specter of ozone depletion in other locations, notably the Arctic, has been identified. Ozone loss is not confined to high latitudes, however, nor is it only the result of the addition of anthropogenic compounds containing chlorine and bromine in the stratosphere, as Anderson et al. (p. 835, published online 26 July; see the Perspective by Ravishankara) now demonstrate. Data from the atmosphere above the continental United States revealed that convective injection of water vapor into the stratosphere affects the free radical chemistry involving the (mostly anthropogenic) chlorine and bromine, thus accelerating ozone loss. This process could become important in the stratospheric ozone budget if the frequency and intensity of these water-injection events, which are most common in the summer, increase as a result of global warming. Convective injection of water vapor into the stratosphere increases the rate of ozone destruction there. The observed presence of water vapor convectively injected deep into the stratosphere over the United States can fundamentally change the catalytic chlorine/bromine free-radical chemistry of the lower stratosphere by shifting total available inorganic chlorine into the catalytically active free-radical form, ClO. This chemical shift markedly affects total ozone loss rates and makes the catalytic system extraordinarily sensitive to convective injection into the mid-latitude lower stratosphere in summer. Were the intensity and frequency of convective injection to increase as a result of climate forcing by the continued addition of CO2 and CH4 to the atmosphere, increased risk of ozone loss and associated increases in ultraviolet dosage would follow.

Nitric Acid Uptake on Subtropical Cirrus Cloud Particles

The redistribution of HNO 3 via uptake and sedimentation by cirrus cloud particles is considered an important term in the upper tropospheric budget of reactive nitrogen. Numerous cirrus cloud encounters by the NASA WB-57F high-altitude research aircraft during the Cirrus Regional Study of Tropical Anvils and Cirrus Layers-Florida Area Cirrus Experiment (CRYSTAL-FACE) were accompanied by the observation of condensed-phase HNO 3 with the NOAA chemical ionization mass spectrometer. The instrument measures HNO 3 with two independent channels of detection connected to separate forward and downward facing inlets that allow a determination of the amount of HNO 3 condensed on ice particles. Subtropical cirrus clouds, as indicated by the presence of ice particles, were observed coincident with condensed-phase HNO 3 at temperatures of 197-224 K and pressures of 122-224 hPa. Maximum levels of condensed-phase HNO 3 approached the gas-phase equivalent of 0.8 ppbv. Ice particle surface coverages as high as 1.4 x 10 14 molecules cm -2 were observed. A dissociative Langmuir adsorption model, when using an empirically derived HNO 3 adsorption enthalpy of -11.0 kcal mol -1 , electively describes the observed molecular coverages to within a factor of 5. The percentage of total HNO 3 in the condensed phase ranged from near zero to 100% in the observed cirrus clouds. With volume-weighted mean particle diameters up to 700 μm and particle fall velocities up to 10 m s -1 , some observed clouds have significant potential to redistribute HNO 3 in the upper troposphere.

A new cavity based absorption instrument for detection of water isotopologues in the upper troposphere and lower stratosphere

We describe here the Harvard integrated cavity output spectroscopy (ICOS) isotope instrument, a mid-IR infrared spectrometer using ICOS to make in situ measurements of the primary isotopologues of water vapor (H(2)O, HDO, and H(2) (18)O) in the upper troposphere and lower stratosphere (UTLS). The long path length provided by ICOS provides the sensitivity and accuracy necessary to measure these or other trace atmospheric species at concentrations in the ppbv range. The Harvard ICOS isotope instrument has been integrated onto NASAs WB-57 high-altitude research aircraft and to date has flown successfully in four field campaigns from winter 2004-2005 to the present. Off-axis alignment and a fully passive cavity ensure maximum robustness against the vibrationally hostile aircraft environment. The very simple instrument design permitted by off-axis ICOS is also helpful in minimizing contamination necessary for accurate measurements in the dry UTLS region. The instrument is calibrated in the laboratory via two separate water addition systems and crosscalibrated against other instruments. Calibrations have established an accuracy of 5% for all species. The instrument has demonstrated measurement precision of 0.14 ppmv, 0.10 ppbv, and 0.16 ppbv in 4 s averages for H(2)O, HDO, and H(2) (18)O, respectively. At a water vapor mixing ratio of 5 ppmv the isotopologue ratio precision is 50[per thousand] and 30[per thousand] for deltaD and delta(18)O, respectively.

Validation of the Harvard Lyman-α in situ water vapor instrument: Implications for the mechanisms that control stratospheric water vapor

Building on previously published details of the laboratory calibrations of the Harvard Lyman-α photofragment fluorescence hygrometer (HWV) on the NASA ER-2 and WB-57 aircraft, we describe here the validation process for HWV, which includes laboratory calibrations and intercomparisons with other Harvard water vapor instruments at water vapor mixing ratios from 0 to 10 ppmv, followed by in-flight intercomparisons with the same Harvard hygrometers. The observed agreement exhibited in the laboratory and during intercomparisons helps corroborate the accuracy of HWV. In light of the validated accuracy of HWV, we present and evaluate a series of intercomparisons with satellite and balloon borne water vapor instruments made from the upper troposphere to the lower stratosphere in the tropics and midlatitudes. Whether on the NASA ER-2 or WB-57 aircraft, HWV has consistently measured about 1–1.5 ppmv higher than the balloon-borne NOAA/ESRL/GMD frost point hygrometer (CMDL), the NOAA Cryogenic Frost point Hygrometer (CFH), and the Microwave Limb Sounder (MLS) on the Aura satellite in regions of the atmosphere where water vapor is <10 ppmv. Comparisons in the tropics with the Halogen Occultation Experiment (HALOE) on the Upper Atmosphere Research Satellite show large variable differences near the tropopause that converge to ~10% above 460 K, with HWV higher. Results we show from the Aqua Validation and Intercomparison Experiment (AquaVIT) at the AIDA chamber in Karlsruhe do not reflect the observed in-flight differences. We illustrate that the interpretation of the results of comparisons between modeled and measured representations of the seasonal cycle of water entering the lower tropical stratosphere is dictated by which data set is used.

Evaluation of UT/LS hygrometer accuracy by intercomparison during the NASA MACPEX mission

Acquiring 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.

Measurements of the Total Water Content of Cirrus Clouds. Part I: Instrument Details and Calibration

Abstract This paper describes an instrument designed to measure the sum of gas phase and solid phase water, or total water, in cirrus clouds, and to be mounted in a pallet in the underbelly of the NASA WB-57 research aircraft. The ice water content of cirrus is determined by subtracting water vapor measured simultaneously by the Harvard water vapor instrument on the aircraft. The total water instrument uses an isokinetic inlet to maintain ambient particle concentrations as air enters the instrument duct, a 600-W heater mounted directly in the flow to evaporate the ice particles, and a Lyman-α photofragment fluorescence technique for detection of the total water content of the ambient air. Isokinetic flow is achieved with an actively controlled roots pump by referencing aircraft pressure, temperature, and true airspeed, together with instrument flow velocity, temperature, and pressure. Laboratory calibrations that utilize a water vapor addition system that adds air with a specific humidity tied to the vapo...

A new photolysis laser-induced fluorescence instrument for the detection of H2O and HDO in the lower stratosphere

We present a new instrument, Hoxotope, for the in situ measurement of H(2)O and its heavy deuterium isotopologue (HDO) in the upper troposphere and lower stratosphere aboard the NASA WB-57. Sensitive measurements of deltaD are accomplished through the vacuum UV photolysis of water followed by laser-induced fluorescence detection of the resultant OH and OD photofragments. The photolysis laser-induced fluorescence technique can obtain S/N>20 for 1 ppbv HDO and S/N>30 for 5 ppmv H(2)O for 10 s data, providing the sensitivity required for deltaD measurements in the tropopause region. The technique responds rapidly to changing water concentrations due to its inherently small sampling volume, augmented by steps taken to minimize water uptake on instrument plumbing. Data from the summer 2005 Aura Validation Experiment Water Isotope Intercomparison Flights (AVE-WIIF) out of Houston, TX show agreement for H(2)O between Hoxotope and the Harvard water vapor instrument and for HDO between Hoxotope and the Harvard ICOS water isotope instrument, to within stated instrument uncertainties. The successful intercomparison validates Hoxotope as a credible source of deltaD data in the upper troposphere and lower stratosphere.

Measurements of the Total Water Content of Cirrus Clouds. Part II: Instrument Performance and Validation

Abstract This paper describes the performance and in-flight validation of an instrument mounted in a pallet on the NASA WB-57 research aircraft that measures the sum of gas phase and solid phase water, or total water, in cirrus clouds. Using a heated isokinetic inlet and a Lyman-α photofragment fluorescence technique for detection, measurements of total water have been made over three orders of magnitude. During the Cirrus Regional Study of Tropical Anvils and Cirrus Layers Florida Area Cirrus Experiment (CRYSTAL FACE), the instrument operated at duct temperatures sufficiently warms to completely evaporate particles up to 150-μm diameter. Laboratory calibrations, in-flight diagnostics, intercomparison with water vapor measured by absorption in flight, and intercomparisons in clear air with the Harvard water vapor instrument validate the detection sensitivity of the instrument and illustrate the minimal hysteresis from instrument surface contamination. The Harvard total water and water vapor instruments to...

A new direct absorption tunable diode laser spectrometer for high precision measurement of water vapor in the upper troposphere and lower stratosphere

We present a new instrument for the measurement of water vapor in the upper troposphere and lower stratosphere (UT∕LS), the Harvard Herriott Hygrometer (HHH). HHH employs a tunable diode near-IR laser to measure water vapor via direct absorption in a Herriott cell. The direct absorption technique provides a direct link between the depth of the observed absorption line and the measured water vapor concentration, which is calculated based on spectroscopic parameters in the HITRAN database. While several other tunable diode laser (TDL) instruments have been used to measure water vapor in the UT∕LS, HHH is set apart by its use of an optical cell an order of magnitude smaller than those of other direct absorption TDLs in operation, allowing for a more compact, lightweight instrument. HHH is also unique in its integration into a common duct with the Harvard Lyman-α hygrometer, an independent photo-fragment fluorescence instrument which has been thoroughly validated over 19 years of flight measurements. The instrument was flown for the first time in the Mid-latitude Airborne Cirrus Properties Experiment (MACPEX) on NASAs WB-57 aircraft in spring, 2011, during which it demonstrated in-flight precision of 0.1 ppmv (1 s) with 1-sigma uncertainty of 5% ± 0.7 ppmv. Since the campaign, changes to the instrument have lead to improved accuracy of 5% ± 0.2 ppmv as demonstrated in the laboratory. During MACPEX, HHH successfully measured water vapor at concentrations from 3.5 to 600 ppmv in the upper troposphere and lower stratosphere. HHH and Lyman-α, measuring independently but under the same sampling conditions, agreed on average to within 1% at water vapor mixing ratios above 20 ppmv and to within 0.3 ppmv at lower mixing ratios. HHH also agreed with a number of other in situ water vapor instruments on the WB-57 to within their stated uncertainties, and to within 0.7 ppmv at low water. This agreement constitutes a significant improvement over past in situ comparisons, in which differences of 1.5-2 ppmv were routinely observed, and demonstrates that the accuracy of HHH is consistent with other instruments which use a range of detection methods and sampling techniques.