Vivienne H. Payne

Development and recent evaluation of the MT_CKD model of continuum absorption

Water vapour continuum absorption is an important contributor to the Earths radiative cooling and energy balance. Here, we describe the development and status of the MT_CKD (MlawerTobinCloughKneizysDavies) water vapour continuum absorption model. The perspective adopted in developing the MT_CKD model has been to constrain the model so that it is consistent with quality analyses of spectral atmospheric and laboratory measurements of the foreign and self continuum. For field measurements, only cases for which the characterization of the atmospheric state has been highly scrutinized have been used. Continuum coefficients in spectral regions that have not been subject to compelling analyses are determined by a mathematical formulation of the spectral shape associated with each water vapour monomer line. This formulation, which is based on continuum values in spectral regions in which the coefficients are well constrained by measurements, is applied consistently to all water vapour monomer lines from the microwave to the visible. The results are summed-up (separately for the foreign and self) to obtain continuum coefficients from 0 to 20 000 cm−1. For each water vapour line, the MT_CKD line shape formulation consists of two components: exponentially decaying far wings of the line plus a contribution from a water vapour molecule undergoing a weak interaction with a second molecule. In the MT_CKD model, the first component is the primary agent for the continuum between water vapour bands, while the second component is responsible for the majority of the continuum within water vapour bands. The MT_CKD model should be regarded as a semi-empirical model with strong constraints provided by the known physics. Keeping the MT_CKD continuum consistent with current observational studies necessitates periodic updates to the water vapour continuum coefficients. In addition to providing details on the MT_CKD line shape formulation, we describe the most recent update to the model, MT_CKD_2.5, which is based on an analysis of satellite- and ground-based observations from 2385 to 2600 cm−1 (approx. 4 μm).

Water Vapor Continuum Absorption in the Microwave

The accurate modeling of continuum absorption is crucial for the so-called window regions of the spectrum, the relatively transparent regions between lines. The window regions in the microwave are of critical importance for Earth remote sensing and data assimilation. Presented in this paper is an evaluation of the widely used Mlawer, Tobin, Clough, Kneizys, and Davis (MT_CKD) water vapor continuum model in the microwave region, performed using measurements from ground-based radiometers operated by the Department of Energys Atmospheric Radiation Measurement Program at sites in Oklahoma, USA, and the Black Forest, Germany. The radiometers used were the Radiometrics 23.8/31.4-GHz microwave radiometers (MWRs), the Radiometer Physics GmbH 90/150-GHz MWR at high frequencies (MWRHF), and the Radiometrics 183 GHz G-band vapor radiometer profiler (GVRP). Radiometer measurements were compared with brightness temperatures calculated using radiosonde temperature and humidity profiles input to the monochromatic radiative transfer model (MonoRTM), which uses the MT_CKD continuum model. Measurements at 23.8 GHz were used to correct for biases in the total precipitable water vapor (PWV) from the radiosondes. The long-term 31.4 GHz data set, with a range of PWV values spanning from 0.15 to 5 cm, allowed the separation of uncertainties in the self- and foreign-broadened components of the water vapor continuum. The MT_CKD model has been updated in the microwave region to provide improved agreement with the measurements. MonoRTM has been updated accordingly. The results for the different instruments and frequencies were consistent, providing high confidence in the continuum updates. The estimated uncertainties on the updated continuum coefficients in MT_CKD are 4% on the foreign-broadened water vapor continuum and 4% on the self-broadened water vapor continuum.

Air-Broadened Half-Widths of the 22- and 183-GHz Water-Vapor Lines

Air-broadened half-widths of the 22- and 183-GHz water-vapor lines and associated uncertainties have been determined using comparisons between ground-based radiometric measurements from Atmospheric Radiation Measurement sites in Oklahoma and Alaska, and MonoRTM, a radiative transfer model. Values of the widths obtained using the measurements are 0.0900 cm-1/atm with 1.6% uncertainty for the 22-GHz line and 0.0992 cm-1/atm with 2.4% uncertainty for the 183-GHz line. Also presented are spectroscopic parameters for these lines from new calculations performed using the complex implementation of the Robert-Bonamy theory (CRB). The CRB values of the air-broadened widths are 0.0913 cm-1/atm with 3% uncertainty and a temperature exponent of 0.755 for the 22-GHz line and 0.0997 cm-1/atm with 3% uncertainty and a temperature exponent of 0.769 for the 183-GHz line. The values for the air-broadened half-widths derived from the measurement/model comparisons show good agreement with the new CRB calculations. For future versions of MonoRTM, width values of 0.0900 and 0.0997 cm-1/atm are to be adopted with temperature dependences of 0.76 and 0.77 for the 22- and 183-GHz lines, respectively.

Comparison of Ground-Based Millimeter-Wave Observations and Simulations in the Arctic Winter

During the Radiative Heating in Underexplored Bands Campaign (RHUBC), held in February-March 2007, three millimeter-wave radiometers were operated at the Atmospheric Radiation Measurement Programs site in Barrow, Alaska. These radiometers contain several channels located around the strong 183.31-GHz water vapor line, which is crucial for ground-based water-vapor measurements in very dry conditions, typical of the Arctic. Simultaneous radiosonde observations were carried out during conditions with very low integrated-water-vapor (IWV) content (< 2 mm). Observations from the three instruments are compared, accounting for their different design characteristics. The overall agreement during RHUBC among the three instruments and between instruments and forward model is discussed quantitatively. In general, the instrument cross-validation performed for sets of channel pairs showed agreement within the total expected uncertainty. The consistency between instruments allows the determination of the IWV to within around 2% for these dry conditions. Comparisons between these data sets and forward-model simulations using radiosondes as input show spectral features in the brightness-temperature residuals, indicating some degree of inconsistency between the instruments and the forward model. The most likely cause of forward-model error is systematic errors in the radiosonde humidity profiles.

Effect of the Oxygen Line-Parameter Modeling on Temperature and Humidity Retrievals From Ground-Based Microwave Radiometers

The Atmospheric Radiation Measurement (ARM) Program maintains a suite of instruments in various locations to provide continuous monitoring of atmospheric parameters. Temperature and humidity retrievals are two of the key parameters used by the climate-modeling community. Accuracy in the spectroscopy adopted by the various radiative transfer models is crucial for obtaining accurate retrievals. While the accuracy of the spectroscopic parameters used for water-vapor retrievals is satisfactory, temperature retrievals continue to be affected by uncertainties in oxygen line parameters leading to discrepancies between the modeled and observed brightness temperatures. In this paper, we compare the model calculations in the oxygen-band channels with the measurements collected by the ARM-operated 12-channel Microwave Radiometer Profiler (MWRP). The dataset used spans a wide range of atmospheric temperature conditions, with ground temperatures varying between -40degC and +20degC. Model calculations are performed by using line parameters from the high-resolution transmission molecular-absorption (HITRAN) database and from a set of newly published parameters. Our comparison shows that the newly published parameters agree more closely with the MWRP measurements and confirms the need to update the HITRAN database for the oxygen lines. We show the effect of line parameters on the retrievals of temperature, water vapor, and liquid water, and show that improved oxygen absorption is essential to reduce the clear-sky bias in the liquid-water path retrievals.

Ozone export from East Asia: The role of PAN

Peroxyacetyl nitrate (PAN) is an important ozone (O3) precursor. The lifetime of PAN is approximately 1month in the free troposphere, and this allows O3 production to occur in pollution plumes at intercontinental distances from its source. In this study we use the Goddard Earth Observing System (GEOS)-Chem global chemical transport model, new satellite measurements of PAN from the Aura Tropospheric Emission Spectrometer (TES), and data from the Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) field campaign over North America, to study the role of natural and anthropogenic Asian emissions on free tropospheric (900–400 hPa) PAN distributions and subsequent O3 production. Using the ARCTAS data with GEOS-Chem, we show that while GEOS-Chem is unbiased with respect to the aircraft data, TES version 7 PAN data are biased high for regions with surface temperatures colder than 285 K. However, GEOS-Chem and TES measurements provide a consistent representation (within 15% difference) of PAN abundance over East Asia. Because of the good agreement between model and observations, we use the GEOS-Chem model to evaluate the sources of PAN precursors and the effect of free tropospheric PAN on the export of O3 from Asia to North America. The GEOS-Chem model results show that the largest contributors to free tropospheric PAN over Asia and the northern Pacific are anthropogenic and soil NOx emissions. Biomass burning emissions have important contributions to free tropospheric PAN over northern Pacific (25% in April), while the contribution from lightning over northern Pacific is significant in July (40%). Strong springtime transport in April results in more export of free tropospheric PAN and O3 from East Asian emissions. This free tropospheric PAN contributes about 35% to the abundance of free tropospheric O3 over western North America in spring and 25% in summer.

Impact of intercontinental pollution transport on North American ozone air pollution: an HTAP phase 2 multi-model study

The recent update on the US National Ambient Air Quality Standards (NAAQS) of the ground-level ozone (O3/ can benefit from a better understanding of its source contributions in different US regions during recent years. In the Hemispheric Transport of Air Pollution experiment phase 1 (HTAP1), various global models were used to determine the O3 source–receptor (SR) relationships among three continents in the Northern Hemisphere in 2001. In support of the HTAP phase 2 (HTAP2) experiment that studies more recent years and involves higher-resolution global models and regional models’ participation, we conduct a number of regional-scale Sulfur Transport and dEposition Model (STEM) air quality base and sensitivity simulations over North America during May–June 2010. STEM’s top and lateral chemical boundary conditions were downscaled from three global chemical transport models’ (i.e., GEOS-Chem, RAQMS, and ECMWF C-IFS) base and sensitivity simulations in which the East Asian (EAS) anthropogenic emissions were reduced by 20 %. The mean differences between STEM surface O3 sensitivities to the emission changes and its corresponding boundary condition model’s are smaller than those among its boundary condition models, in terms of the regional/period-mean (<10 %) and the spatial distributions. An additional STEM simulation was performed in which the boundary conditions were downscaled from a RAQMS (Realtime Air Quality Modeling System) simulation without EAS anthropogenic emissions. The scalability of O3 sensitivities to the size of the emission perturbation is spatially varying, and the full (i.e., based on a 100% emission reduction) source contribution obtained from linearly scaling the North American mean O3 sensitivities to a 20% reduction in the EAS anthropogenic emissions may be underestimated by at least 10 %. The three boundary condition models’ mean O3 sensitivities to the 20% EAS emission perturbations are ~8% (May–June 2010)/~11% (2010 annual) lower than those estimated by eight global models, and the multi-model ensemble estimates are higher than the HTAP1 reported 2001 conditions. GEOS-Chem sensitivities indicate that the EAS anthropogenic NOx emissions matter more than the other EAS O3 precursors to the North American O3, qualitatively consistent with previous adjoint sensitivity calculations. In addition to the analyses on large spatial–temporal scales relative to the HTAP1, we also show results on subcontinental and event scales that are more relevant to the US air quality management. The EAS pollution impacts are weaker during observed O3 exceedances than on all days in most US regions except over some high-terrain western US rural/remote areas. Satellite O3 (TES, JPL–IASI, and AIRS) and carbon monoxide (TES and AIRS) products, along with surface measurements and model calculations, show that during certain episodes stratospheric O3 intrusions and the transported EAS pollution influenced O3 in the western and the eastern US differently. Free-running (i.e., without chemical data assimilation) global models underpredicted the transported background O3 during these episodes, posing difficulties for STEM to accurately simulate the surface O3 and its source contribution. Although we effectively improved the modeled O3 by incorporating satellite O3 (OMI and MLS) and evaluated the quality of the HTAP2 emission inventory with the Royal Netherlands Meteorological Institute–Ozone Monitoring Instrument (KNMI–OMI) nitrogen dioxide, using observations to evaluate and improve O3 source attribution still remains to be further explored.

Intercalibration of the GPM Microwave Radiometer Constellation

AbstractThe Global Precipitation Measurement (GPM) mission is a constellation-based satellite mission designed to unify and advance precipitation measurements using both research and operational microwave sensors. This requires consistency in the input brightness temperatures (Tb), which is accomplished by intercalibrating the constellation radiometers using the GPM Microwave Imager (GMI) as the calibration reference. The first step in intercalibrating the sensors involves prescreening the sensor Tb to identify and correct for calibration biases across the scan or along the orbit path. Next, multiple techniques developed by teams within the GPM Intersatellite Calibration Working Group (XCAL) are used to adjust the calibrations of the constellation radiometers to be consistent with GMI. Comparing results from multiple approaches helps identify flaws or limitations of a given technique, increase confidence in the results, and provide a measure of the residual uncertainty. The original calibration difference...

TES observations of the interannual variability of PAN over Northern Eurasia and the relationship to springtime fires

Peroxyacetyl nitrate (PAN) plays an important role in atmospheric chemistry through its impact on remote oxidant and nitrogen budgets. PAN is formed rapidly in boreal fire plumes through the oxidation of short-lived volatile organic compounds in the presence of nitrogen oxide radicals. Here we present new satellite observations of PAN from the Tropospheric Emission Spectrometer (TES) over northern Eurasia for April 2006–2010. We observe large interannual variability in TES PAN observations, and we show that fires are one source of this variability using (1) Moderate Resolution Imaging Spectroradiometer Mean Fire Radiative Power observations and (2) Hybrid Single-Particle Lagrangian Integrated Trajectory backward trajectories. We also show that cold springtime temperatures and enhanced vertical mixing in the lower free troposphere over northeastern Eurasia likely played a role in the detection of PAN from TES in April 2006 in this region.

An Assessment of SAPHIR Calibration Using Quality Tropical Soundings

AbstractThe Sondeur Atmospherique du Profil d’Humidite Intertropicale par Radiometrie (SAPHIR) instrument on board the Megha-Tropiques (MT) platform is a cross-track, multichannel microwave humidity sounder with six channels near the 183.31-GHz water vapor absorption line, a maximum scan angle of 42.96° (resulting in a maximum incidence angle of 50.7°), a 1700-km-wide swath, and a footprint resolution of 10 km at nadir. SAPHIR L1A2 brightness temperature (BT) observations have been compared to BTs simulated by the radiative transfer model (RTM) Radiative Transfer for the Television and Infrared Observation Satellite (TIROS) Operational Vertical Sounder (RTTOV-10), using in situ measurements from radiosondes as input. Selected radiosonde humidity observations from the Cooperative Indian Ocean Experiment on Intraseasonal Variability in the Year (CINDY)–Dynamics of the Madden–Julian Oscillation (DYNAMO) campaign (September 2011–March 2012) were spatiotemporally collocated with MT overpasses. Although several...