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Anthropogenic emissions of trichloromethane (CHCl3, chloroform) in 1990 have been estimated with a variety of methods specific to the source category. The largest source category for CHCl3 was found to be pulp and paper manufacturing, responsible for an estimated 30±8 Gg yr−1 reactive chlorine in the form of CHCl3. Water treatment of various types was estimated to contribute another 19±12 Gg. Manufacturing facilities of products other than pulp or paper and other relatively minor sources were estimated to emit an additional 13±5 Gg yr−1, for a total of 62±25 Gg yr−1 reactive chlorine in the form of CHCl3. The global flux of chlorodifluoromethane (HCFC-22) is well characterized from industrial and regulatory data to have been 195 Gg in 1990, equivalent to 80±0.6 Gg yr−1 as active chlorine. The fluxes of reactive chlorine from CHCl3 and HCFC-22, distributed globally in a 1° latitude times 1° longitude grid, revealed areas highest in emissions.

Wood smoke in the atmosphere often accounts for 20–30% of the ambient fine-particle concentrations. In communities where wood is burned for home heating, wood smoke can at times contribute the majority of the atmospheric fine-particle burden. Chemical mass balance receptor models that use organic compounds as tracers can be used to determine the contributions of different emission sources, including wood smoke, to atmospheric fine-particle samples. In order for organic chemical tracer techniques to be applied to communities across the United States, differences in wood smoke composition that arise from differences in the type of wood burned in various regions must be understood. A continental-scale accounting of particulate organic compound emissions from residential wood combustion has been constructed which helps to quantify the regional differences in wood smoke composition that exist between different parts of the United States. Data from a series of source tests conducted on 22 North American wood species have been used to assemble a national inventory of emissions for more than 250 individual organic compounds that are released from wood combustion in fireplaces and wood stoves in the United States. The emission rates of important wood smoke markers, such as levoglucosan, certain substituted syringols and guaiacols, and phytosterols vary greatly with wood type and combustor type. These differences at the level of individual wood type and combustion conditions translate into regional differences in the aggregate composition of ambient wood smoke. By weighting the source test results in proportion to the availability of firewood from specific tree species and the quantities of wood burned in each locale, it is possible to investigate systematic differences that exist between wood smokes from different regions of North America. The relative abundance of 10 major wood smoke components averaged over the emissions inventory in different regions of the United States is computed and then used to illustrate the extent to which wood smoke composition differs from region to region in North America.

Author(s): Rutz, JJ; Shields, CA; Lora, JM; Payne, AE; Guan, B; Ullrich, P; O’Brien, T; Leung, LR; Ralph, FM; Wehner, M; Brands, S; Collow, A; Goldenson, N; Gorodetskaya, I; Griffith, H; Kashinath, K; Kawzenuk, B; Krishnan, H; Kurlin, V; Lavers, D; Magnusdottir, G; Mahoney, K; McClenny, E; Muszynski, G; Nguyen, PD; Prabhat, M; Qian, Y; Ramos, AM; Sarangi, C; Sellars, S; Shulgina, T; Tome, R; Waliser, D; Walton, D; Wick, G; Wilson, AM; Viale, M | Abstract: Atmospheric rivers (ARs) are now widely known for their association with high-impact weather events and long-term water supply in many regions. Researchers within the scientific community have developed numerous methods to identify and track of ARs—a necessary step for analyses on gridded data sets, and objective attribution of impacts to ARs. These different methods have been developed to answer specific research questions and hence use different criteria (e.g., geometry, threshold values of key variables, and time dependence). Furthermore, these methods are often employed using different reanalysis data sets, time periods, and regions of interest. The goal of the Atmospheric River Tracking Method Intercomparison Project (ARTMIP) is to understand and quantify uncertainties in AR science that arise due to differences in these methods. This paper presents results for key AR-related metrics based on 20+ different AR identification and tracking methods applied to Modern-Era Retrospective Analysis for Research and Applications Version 2 reanalysis data from January 1980 through June 2017. We show that AR frequency, duration, and seasonality exhibit a wide range of results, while the meridional distribution of these metrics along selected coastal (but not interior) transects are quite similar across methods. Furthermore, methods are grouped into criteria-based clusters, within which the range of results is reduced. AR case studies and an evaluation of individual method deviation from an all-method mean highlight advantages/disadvantages of certain approaches. For example, methods with less (more) restrictive criteria identify more (less) ARs and AR-related impacts. Finally, this paper concludes with a discussion and recommendations for those conducting AR-related research to consider.

Terrestrial gamma ray flashes (TGFs) are sub‐millisecond bursts of high energetic gamma radiation associated with intracloud flashes in thunderstorms. In this paper we use the simultaneity of lightning detections by World Wide Lightning Location Network to find TGFs in the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) data that are too faint to be identified by standard search algorithms. A similar approach has been used in an earlier paper, but here we expand the data set to include all years of RHESSI + World Wide Lightning Location Network data and show that there is a population of observationally weak TGFs all the way down to 0.22 of the RHESSI detection threshold (three counts in the detector). One should note that the majority of these are “normal” TGFs that are produced further away from the subsatellite point (and experience a 1/r 2 effect) or produced at higher latitudes with a lower tropoause and thus experience increased atmospheric attenuation. This supports the idea that the TGF production rate is higher than currently reported. We also show that compared to lightning flashes, TGFs are more partial to ocean and coastal regions than over land., Key Points We show that there is a fluence distribution of TGFs down to 0.22 of the detection threshold, and we do not see a cutoff in the RHESSI TGF distributionWe find that there are six times as many TGFs inside of RHESSI's field of view, than can be identified by current search algorithmsWe find that observationally weak TGFs largely originate at larger radial distances and higher latitudes from the subsatellite point

In this paper we report the first close, high-resolution observations of downward-directed terrestrial gamma-ray flashes (TGFs) detected by the large-area Telescope Array cosmic ray observatory, obtained in conjunction with broadband VHF interferometer and fast electric field change measurements of the parent discharge. The results show that the TGFs occur during strong initial breakdown pulses (IBPs) in the first few milliseconds of negative cloud-to-ground and low-altitude intracloud flashes, and that the IBPs are produced by a newly-identified streamer-based discharge process called fast negative breakdown. The observations indicate the relativistic runaway electron avalanches (RREAs) responsible for producing the TGFs are initiated by embedded spark-like transient conducting events (TCEs) within the fast streamer system, and potentially also by individual fast streamers themselves. The TCEs are inferred to be the cause of impulsive sub-pulses that are characteristic features of classic IBP sferics. Additional development of the avalanches would be facilitated by the enhanced electric field ahead of the advancing front of the fast negative breakdown. In addition to showing the nature of IBPs and their enigmatic sub-pulses, the observations also provide a possible explanation for the unsolved question of how the streamer to leader transition occurs during the initial negative breakdown, namely as a result of strong currents flowing in the final stage of successive IBPs, extending backward through both the IBP itself and the negative streamer breakdown preceding the IBP., Comment: Typo fixed and reference added. Manuscript is 36 pages. Supplemental Information is 42 pages. This paper is to be published in the Journal of Geophysical Research: Atmospheres. Online data repository: Open Science Framework DOI: 10.17605/OSF.IO/Z3XDA

The intercalibration between a broadband and a hyperspectral satellite Earth observation system requires the convolution of the hyperspectral data with the spectral response functions (SRFs) of the corresponding broadband channels. There are two potential issues associated with the convolution procedure. First, the finite resolution of a hyperspectral spectrum, that is, the deviation from the highly accurate line-by-line monochromatic radiances, will contribute to convolution errors. The magnitude of the errors depends on the spectral resolution and the SRF shape of the hyperspectral instrument. This type of the convolution error has not been well recognized, and there is a lack of corresponding discussion in most published papers. Although it is small as compared with the instrument accuracy of existing hyperspectral sounders, the error is deemed to be signicant when it is compared with the stringent calibration requirement imposed by future climate missions like the Climate Absolute Radiance and Refractivity Observatory (CLARREO). Second, some broadband channels are insufficiently covered by the hyperspectral data, causing spectral gaps that lead to convolution errors. Although several methods have been developed to fill the spectral gaps and hence compensate for the second type of convolution error, the correction accuracy may still need improvement especially when a large spectral gap needs to be lled. This paper presents a methodology to accurately quantify and compensate for both types of convolution errors. This methodology utilizes the available hyperspectral information to correct the scene-dependent convolution errors due to either the limited spectral resolution or spectral gaps. We use simulations to characterize the intercalibration errors between the Moderate resolution Imaging Spectroradiometer (MODIS) and current operational infrared sounders. We demonstrate that convolution errors can be effectively removed to meet the highly accurate intersatellite calibration requirement proposed by the Climate Absolute Radiance and Refractivity Observatory. Our methodology is also validated using real satellite data for the intercalibration between Aqua MODIS and Aqua Atmospheric Infrared Sounders (AIRS). Our study demonstrates that the accurate characterization and correction for the convolution errors greatly reduces the scene-dependent and spectrally dependent errors, being critical to the consistency check between Infrared Atmospheric Sounding Interferometer (IASI) and AIRS using the double-difference method. The convolution correction also facilitates the evaluation for other intercalibration errors (e.g., the drift of MODIS SRFs). Our derived SRF shift values from MODIS-AIRS (after convolution error corrections) and from MODIS-IASI intercalibration are consistent with each other. We further extend the methodology to study the calibration of a broadband channel which is either completely or largely uncovered bya hyperspectral measurement.The large spectral gap-filling methodology is validated by demonstrating the accurate prediction of the MODIS radiance of band 29 using the Cross-track Infrared Sounder spectra, with the real IASI spectral data being used as the reference.

A companion paper by Fritts et al. [2017a] employed an anelastic numerical model to explore the dynamics of gravity waves (GWs) encountering a mesospheric inversion layer (MIL) having a moderate static stability enhancement and a layer of weaker static stability above. That study revealed that MIL responses, including GW transmission, reflection, and instabilities, are sensitive functions of GW parameters. This paper expands on two of the Fritts et al. [2017a] simulations to examine GW instability dynamics and turbulence in the MIL, forcing of the mean wind and stability environments by GW, instability, and turbulence fluxes, and associated heat and momentum transports. These direct numerical simulations resolve turbulence inertial-range scales and yield the following results: GW breaking and turbulence in the MIL occur below where they would otherwise due to enhancements of GW amplitudes and shears in the MIL, 2D GW and instability heat and momentum fluxes are ~20-30 times larger than 3D instability and turbulence fluxes, mean fields are driven largely by 2D GW and instability dynamics rather than 3D instabilities and turbulence, 2D and 3D heat fluxes in regions of strong turbulence yield small departures from initial T(z) and N2(z) profiles, hence do not yield nearly adiabatic “mixed” layers, and our MIL results are consistent with the relation between the turbulent vertical velocity variance and energy dissipation rate proposed by Weinstock [1981] for the limited intervals evaluated.

Single-column model simulations of mixed-phase altocumulus clouds were shown to have a strong vertical-resolution sensitivity in Part I of this paper. Coarse resolution models were unable to simulate the long-lived supercooled-liquid-layer at cloud top, typically only 200-m thick. In this paper, the sensitivity to vertical resolution is investigated using idealized simulations. Vertical gradients of ice water mixing ratio and temperature near cloud top are found to be inadequately represented at coarse-resolution. The vertical discretization using grid-box mean values, rather than the full vertical profile, leads to biased calculations of mixed-phase microphysical process rates and affects the diagnosis of thin liquid-water layers. As a result the liquid-water layer becomes quickly glaciated and altocumulus cloud lifetime is underestimated. Similar impacts are expected for mixed-phase boundary layer clouds commonly observed at high latitudes. A novel parameterization is introduced that accounts for the vertical gradients of ice water mixing ratio and temperature in the microphysics calculations and the diagnosis of liquid near cloud top. It substantially improves the representation of altocumulus layers in coarse vertical-resolution single-column model simulations and reduces the bias identified in Part I. The new parameterization removes the large underestimate in supercooled water content caused by the resolution sensitivity for temperatures warmer than −30∘C. Given the radiative importance of mixed-phase altocumulus clouds, their underestimation by numerical weather prediction models and their potential to act as a negative climate feedback, there is a need to re-evaluate the global climate sensitivity by implementing the findings in these two papers in a climate model.

In this paper the contribution of turbulence into the electrification of thunderstorms, snowstorms, and dust storms is investigated for the first time. A model of large-scale electric field generation in a weakly conducting medium, containing two types of particles charging by collisions, is used. Thunderstorm and snowstorm electrification are considered in detail in this paper; dust storm electrification is also considered, despite being substantially different from the two other cases, to demonstrate the universality of the proposed method. A comparison of the results with the experimental data for thunderstorms, blizzards, and dust storms is carried out. It is found that the situation is notably different for inductive and noninductive charge separations. For inductive charge separation there is a range of thunderstorm and snowstorm parameters (conductivity and the particle radii being the most important factors) for which the electric field grows exponentially with time. This effect can make the inductive mechanism dominant near the breakdown field in turbulent zones of thunderclouds. For noninductive (or triboelectric) charge separation caused by intense velocity fluctuations, the electric field strength grows only linearly with time. The most substantial effect of turbulence on noninductive charging is expected to occur in snowstorms and dust storms, whereas noninductive turbulent charging has a little impact on the thunderstorm electrification.

Savanna fires contribute significantly to global aerosol loading and hence to the earth's radiative budget. Modelling of the climatic impact of these aerosols is made difficult due to a lack of knowledge of their size distribution. Australia is the third largest source of global carbon emissions from biomass burning, with emissions dominated by tropical savanna fires. Despite this, only a few previous studies have reported emission factors of trace gases from this important ecosystem and there are no previous published emission factors for the aerosol properties reported here for Australian savanna fires. In June 2014, the SAFIRED campaign (Savanna Fires in the Early Dry season) took place in the northern territory of Australia, with the purpose of investigating emissions and aging of aerosols from Australian savanna fires. This paper presents observed enhancement ratios and inferred emission factors of trace gases (CO2, CO, CH4, N2O and gaseous elemental mercury), particles over different size modes (Aitken and accumulation) and speciated aerosols components (organics, sulfate, nitrate, ammonium and chloride). Nine smoke events were identified from the data using large enhancements in CO and/or aerosol data to indicate biomass burning event. The results reported in this paper include the first emission factors for Aitken and accumulation mode aerosols from savanna fires, providing useful size information to enable better modelling of the climatic impact of this important source of global aerosols.

During recent decades, the use of probabilistic forecasting methods has increased markedly. However, these predictions still need improvement in uncertainty quantification and predictability analysis. For this reason, the main aim of this paper is to develop tools for quantifying uncertainty and predictability of wind speed over the Iberian Peninsula. To achieve this goal, several spread indexes extracted from an ensemble prediction system are defined in this paper. Subsequently, these indexes were evaluated with the aim of selecting the most appropriate for the characterization of uncertainty associated to the forecasting. Selection is based on comparison of the average magnitude of ensemble spread (ES) and mean absolute percentage error (MAPE). MAPE is estimated by comparing the ensemble mean with wind speed values from different databases. Later, correlation between MAPE and ES was evaluated. Furthermore, probability distribution functions (PDFs) of spread indexes are analyzed to select the index with greater similarity to MAPE PDFs. Then, the spread index selected as optimal is used to carry out a spatiotemporal analysis of model uncertainty in wind forecasting. The results indicate that mountainous regions and the Mediterranean coast are characterized by strong uncertainty, and the spread increases more rapidly in areas affected by strong winds. Finally, a predictability index is proposed for obtaining a tool capable of providing information on whether the predictability is higher or lower than average. The applications developed may be useful in the forecasting of wind potential several days in advance, with substantial importance for estimating wind energy production.

The primary goal of this paper is to introduce two new surface reflectivity climatologies. The two databases contain the Lambertian-equivalent reflectivity (LER) of the Earth's surface, and they are meant to support satellite retrieval of trace gases and of cloud and aerosol information. The surface LER databases are derived from the Global Ozone Monitoring Experiment (GOME)-2 and Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY) instruments and can be considered as improved and extended descendants of earlier surface LER climatologies based on the Total Ozone Mapping Spectrometer (TOMS), GOME-1, and Ozone Monitoring Instrument (OMI) instruments. The GOME-2 surface LER database consists of 21 wavelength bands that span the wavelength range from 335 to 772 nm. The SCIAMACHY surface LER database covers the wavelength range between 335 and 1670 nm in 29 wavelength bands. The two databases are made for each month of the year, and their spatial resolution is 1° × 1°. In this paper we present the methods that are used to derive the surface LER; we analyze the spatial and temporal behavior of the surface LER fields and study the amount of residual cloud contamination in the databases. For several surface types we analyze the spectral surface albedo and the seasonal variation. When compared to the existing surface LER databases, both databases are found to perform well. As an example of possible application of the databases we study the performance of the Fast Retrieval Scheme for Clouds from the Oxygen A-band (FRESCO) cloud information retrieval when it is equipped with the new surface albedo databases. We find considerable improvements. The databases introduced here can not only improve retrievals from GOME-2 and SCIAMACHY but also support those from other instruments, such as TROPOspheric Monitoring Instrument (TROPOMI), to be launched in 2017.

It has been discussed that lightning flashes emit high-energy electrons, positrons, photons, and neutrons with single energies of several tens of MeV. In the first part of this paper we study the absorption of neutron beams in the atmosphere. We initiate neutron beams of initial energies of 350 keV, 10 MeV, and 20 MeV at source altitudes of 4 km and 16 km upward and downward and see that in all these cases neutrons reach ground altitudes and that the cross-section areas extend to several km2. We estimate that for terrestrial gamma-ray flashes approximately between 10 and 2000 neutrons per ms and m2 are possibly detectable at ground, at 6 km, or at 500 km altitude. In the second part of the paper we discuss a feedback model involving the generation and motion of electrons, positrons, neutrons, protons, and photons close to the vicinity of lightning leaders. In contrast to other feedback models, we do not consider large-scale thundercloud fields but enhanced fields of lightning leaders. We launch different photon and electron beams upward at 4 km altitude. We present the spatial and energy distribution of leptons, hadrons, and photons after different times and see that leptons, hadrons, and photons with energies of at least 40 MeV are produced. Because of their high rest mass hadrons are measurable on a longer time scale than leptons and photons. The feedback mechanism together with the field enhancement by lightning leaders yields particle energies even above 40 MeV measurable at satellite altitudes.

Estimating the impact of radiation transport through cloud sides on the global energy budget is hampered by the lack of a fast radiation scheme suitable for use in global atmospheric models that can represent these effects in both the shortwave and longwave. This two-part paper describes the development of such a scheme, which we refer to as the Speedy Algorithm for Radiative Transfer through Cloud Sides (SPARTACUS). The principle of the method is to add extra terms to the two-stream equations to represent lateral transport between clear and cloudy regions, which vary in proportion to the length of cloud edge as a function of height. The present paper describes a robust and accurate method for solving the coupled system of equations in both the shortwave and longwave in terms of matrix exponentials. This solver has been coupled to a correlated-k model for gas absorption. We then confirm the accuracy of SPARTACUS by performing broadband comparisons with fully 3-D radiation calculations by the Monte Carlo model “MYSTIC” for a cumulus cloud field, examining particularly the percentage change in cloud radiative effect (CRE) when 3-D effects are introduced. In the shortwave, SPARTACUS correctly captures this change to CRE, which varies with solar zenith angle between −25% and +120%. In the longwave, SPARTACUS captures well the increase in radiative cooling of the cloud, although it is only able to correctly simulate the 30% increase in surface CRE (around 4 W m−2) if an approximate correction is made for cloud clustering.

Current weather and climate models neglect 3-D radiative transfer through cloud sides, which can change the cloud radiative effect (CRE) significantly. This two-part paper describes the development of the SPeedy Algorithm for Radiative TrAnsfer through CloUd Sides (SPARTACUS) to capture these effects efficiently in a two-stream radiation scheme for use in global models. The present paper concerns the longwave spectral region, where not much work has been done previously, although the limited previous work has suggested that radiative transfer through cloud sides increases the longwave surface CRE of shallow cumulus by around 30%. To assist the development of a longwave capability for SPARTACUS, we use a reference case of an isolated, isothermal, optically thick, cubic cloud in vacuum, for which 3-D effects increase CRE by exactly 200%. It is shown that for any cloud shape, the 3-D effect can be represented in SPARTACUS provided that correct account is made for (1) the effective zenith angle of diffuse radiation emitted from a cloud, (2) the spatial distribution of fluxes in the cloud, (3) cloud clustering that enhances the interception of emitted radiation by neighboring clouds, and (4) radiative smoothing leading to the effective cloud edge length being less than the measured value. We find empirically that the circumference of an ellipse fitted to a horizontal cross section through a cumulus cloud provides a good estimate of the radiatively effective cloud edge length, which provides some guidance to how cloud observations could be analyzed to extract their most important properties for radiation.

While satellites are a proven resource for detecting and tracking volcanic ash and dust clouds, existing algorithms for automatically detecting volcanic ash and dust either exhibit poor overall skill or can only be applied to a limited number of sensors and/or geographic regions. As such, existing techniques are not optimized for use in real-time applications like volcanic eruption alerting and data assimilation. In an effort to significantly improve upon existing capabilities, the Spectrally Enhanced Cloud Objects (SECO) algorithm was developed. The SECO algorithm utilizes a combination of radiative transfer theory, a statistical model, and image processing techniques to identify volcanic ash and dust clouds in satellite imagery with a very low false alarm rate. This fully automated technique is globally applicable (day and night) and can be adapted to a wide range of low earth orbit and geostationary satellite sensors or even combinations of satellite sensors. The SECO algorithm consists of four primary components: conversion of satellite measurements into robust spectral metrics, application of a Bayesian method to estimate the probability that a given satellite pixel contains volcanic ash and/or dust, construction of cloud objects, and the selection of cloud objects deemed to have the physical attributes consistent with volcanic ash and/or dust clouds. The first two components of the SECO algorithm are described in this paper, while the final two components are described in a companion paper.

Using daily and hourly rain gauge records and Japanese 25 year reanalysis data over 30 years, this paper reveals two major circulation structures leading to heavy summer rainfall events in central North China (CNC), and further analyzes the effects of the circulations on these rainfall events. One circulation structure has an extensive upper tropospheric warm anomaly (UTWA) covering North China (NC). By strengthening the upper anticyclonic anomaly and lower southerly flows around NC, the UTWA plays a positive role in forming upper level divergence and lower level moisture convergence. As a result, the warm anomalous circulation has a solid relationship with large-scale, long-duration rainfall events with a diurnal peak around midnight to early morning. The other circulation structure has an upper tropospheric cold anomaly (UTCA) located in the upper stream of NC. Contributed to by the UTCA, a cold trough appears in the upper stream of NC and an unstable configuration with upper (lower) cold (warm) anomalies forms around CNC. Consequently, CNC is covered by strong instability and high convective energy, and the cold anomalous circulation is closely connected with local, short-duration rainfall events concentrated from late afternoon to early nighttime. The close connections between circulation structures and typical rainfall events are confirmed by two independent converse analysis processes: from circulations to rainfall characteristics, and from typical rainfall events to circulations. The results presented in this paper indicate that the upper tropospheric temperature has significant influences on heavy rainfall, and thus more attention should be paid to the upper tropospheric temperature in future analyses.

The land-atmosphere system is characterized by pronounced land surface heterogeneity and vigorous atmospheric turbulence both covering a wide range of scales. The multiscale surface heterogeneities and multiscale turbulent eddies interact nonlinearly with each other. Understanding these multiscale processes quantitatively is essential to the subgrid parameterizations for weather and climate models. In this paper, we propose a method for surface heterogeneity quantification and turbulence structure identification. The first part of the method is an orthogonal transform in the probability density function (PDF) domain, in contrast to the orthogonal wavelet transforms which are performed in the physical space. As the basis of the whole method, the orthogonal PDF transform (OPT) is used to asymptotically reconstruct the original signals by representing the signal values with multilevel approximations. The “patch” idea is then applied to these reconstructed fields in order to recognize areas at the land surface or in turbulent flows that are of the same characteristics. A patch here is a connected area with the same approximation. For each recognized patch, a length scale is then defined to build the energy spectrum. The OPT and related energy spectrum analysis, as a whole referred to as the orthogonal PDF decomposition (OPD), is applied to two-dimensional heterogeneous land surfaces and atmospheric turbulence fields for test. The results show that compared to the wavelet transforms, the OPD can reconstruct the original signal more effectively, and accordingly, its energy spectrum represents the signal's multiscale variation more accurately. The method we propose in this paper is of general nature and therefore can be of interest for problems of multiscale process description in other geophysical disciplines.

This paper investigates the influence of corona on currents and electromagnetic fields predicted by a return-stroke model that represents the lightning channel as a nonuniform transmission line with time-varying (nonlinear) resistance. The corona model used in this paper allows the calculation of corona currents as a function of the radial electric field in the vicinity of the channel. A parametric study is presented to investigate the influence of corona parameters, such as the breakdown electric field and the critical electric field for the stable propagation of streamers, on predicted currents and electromagnetic fields. The results show that, regardless of the assumed corona parameters, the incorporation of corona into the nonuniform and nonlinear transmission line model under investigation modifies the model predictions so that they consistently reproduce most of the typical features of experimentally observed lightning electromagnetic fields and return-stroke speed profiles. In particular, it is shown that the proposed model leads to close vertical electric fields presenting waveforms, amplitudes, and decay with distance in good agreement with dart leader electric field changes measured in triggered lightning experiments. A comparison with popular engineering return-stroke models further confirms the model's ability to predict consistent electric field waveforms in the close vicinity of the channel. Some differences observed in the field amplitudes calculated with the different models can be related to the fact that current distortion, while present in the proposed model, is ultimately neglected in the considered engineering return-stroke models.

The Visible Infrared Imaging Radiometer Suite (VIIRS) sensor aboard the Suomi National Polar-orbiting Partnership spacecraft has successfully operated since its launch in October 2011. The VIIRS collects data in 22 spectral bands that are calibrated by a set of onboard calibrators (OBC). In addition, lunar observations are made to independently track VIIRS long-term calibration stability for the reflective solar bands (RSB). This paper provides an overview of VIIRS OBC functions as well as its on-orbit operation and calibration activities. It also describes sensor calibration methodologies and demonstrates VIIRS on-orbit performance from launch to present. Results reported in this paper include on-orbit changes in sensor spectral band responses, detector noise characterization, and key calibration parameters. Issues identified and their potential impacts on sensor calibration are also discussed. Since launch, the VIIRS instrument nominal operation temperature has been stable to within ±1.0 K. The cold focal plane temperatures have been well controlled, with variations of less than 20 mK over a period of 1.5 years. In general, changes in thermal emissive bands (TEB) detector responses have been less than 0.5%. Despite large response degradation in several near-infrared and short-wave infrared bands and large SD degradation at short visible wavelengths, the VIIRS sensor and OBC overall performance has been excellent postlaunch. The degradation caused by the telescope mirror coating contamination has been modeled and its impact addressed through the use of modulated relative spectral response in the improved calibration and the current sensor data record data production. Based on current instrument characteristics and performance, it is expected that the VIIRS calibration will continue to meet its design requirements, including RSB detector signal to noise ratio and TEB detector noise equivalent temperature difference, throughout its 7 year design lifetime.

In his comment, Laprise raises several points that we agree merit consideration. His primary critique is that our study [Racherla et al., 2012] tested the ability of the WRF regional climate model to reproduce historical temperature and precipitation change relative to the driving global climate model (GCM) using only a single simulation rather than an ensemble. He asserts that the observed changes are smaller than the internal variability in the climate system (i.e., not statistically significant) and that thus a single simulation should not necessarily be able to capture the observations. Laprise points out that the statistical signal is reduced for a multi-decadal trend such as the one we analyzed in comparison with mean climatology and cites two studies showing that for particular climate parameters it can take any years for a signal to be discerned over internal variability. He states that The results of theexperiment as designed were strongly influenced by the presence of internal variability and sampling errors,which masked the rather small climate changes that may have occurred as a consequence of changes inforcing during the period considered. While Laprise discusses statistics in general terms at some length, for the actual climate trends examined in our study, he offers no evidence that the forced signal was smallcompared with internal variability. The two studies he cites [de Ela et al., 2013; Maraun, 2013] do not provide convincing evidence as they concern climate variables averaged over different times and areas. One in fact examines extreme precipitation events, which by definition are rare and thus have a lower significance level. We accept the general point that it is important to consider internal variability, and as noted in our paper we agree that an ensemble of simulations is in principle an optimal, though computationally expensive, approach. While we did not present the statistical significance of the observations in our original paper, we have now evaluated those for the regional temperature trends used in our study to evaluate the added value of WRF and thus can analyze data as to the magnitude of the trends with respect to internal variability.

From June through September, the intraseasonal variability of the Asian summer monsoon is dominated by the so-called “monsoon intraseasonal oscillation (MISO).” This paper provides a comprehensive description of the MISO based on outgoing longwave radiation (OLR) data. The MISO is characterized by alternating active periods, in which the primary rain area of the Asian summer monsoon that stretches from the northern Arabian Sea east southeastward almost all the way to the northwest Pacific Intertropical Convergence Zone is relatively intense, and break periods, in which the heaviest rainfall shifts from south Asia to the central and eastern equatorial Indian Ocean. The MISO is attended by well-defined but weak sea surface temperature (SST) perturbations whose phase is indicative of a negative feedback upon the atmospheric perturbations. Meridional profile of variables on the various regression maps shown in this paper averaged along a set of tilted axes parallel to the west-northwest to east-southeast (WNW-ESE) sloping lines in empirical orthogonal function 1 of OLR have been made, and it is found that the strongest westerly 850 hPa wind anomalies are located two grid points (5° of latitude) to the south of the reference latitude. At the 150 hPa level, the meridional profile of divergence is closely aligned with the OLR profile. SST profile is lowest at approximately 2.5° of latitude to the south of the minimum OLR and 2.5° to the north of the strongest westerly 850 hPa wind anomalies. The sea level pressure profiles and the midlower tropospheric geopotential height profiles are almost in phase. It is observed that in most years, there are two–three bands of intensified and suppressed rainfall that cross the reference line from south to north (northward propagating) at the interval of 30–60 days over South Asia. The degree of correspondence between the MISO and active and break spells of the Indian summer monsoon rainfall is also documented.

[1] The Cross-track Infrared Sounder (CrIS) is a spaceborne Fourier transform spectrometer (FTS) that was launched into orbit on 28 October 2011 onboard the Suomi National Polar-orbiting Partnership satellite. CrIS is a sophisticated sounding sensor that accurately measures upwelling infrared radiance at high spectral resolution. Data obtained from this sensor are used for atmospheric profiles retrieval and assimilation by numerical weather prediction models. Optimum vertical sounding resolution is achieved with high spectral resolution and multiple spectral channels; however, this can lead to increased noise. The CrIS instrument is designed to overcome this problem. Noise Equivalent Differential Radiance (NEdN) is one of the key parameters of the Sensor Data Record product. The CrIS on-orbit NEdN surpasses mission requirements with margin and has comparable or better performance when compared to heritage hyperspectral sensors currently on orbit. This paper describes CrIS noise performance through the characterization of the sensor's NEdN and compares it to calibration data obtained during ground test. In addition, since FTS sensors can be affected by vibration that leads to spectrally correlated noise on top of the random noise inherent to infrared detectors, this paper also characterizes the CrIS NEdN with respect to the correlated noise contribution to the total NEdN. Lastly, the noise estimated from the imaginary part of the complex FTS spectra is extremely useful to assess and monitor in-flight FTS sensor health. Preliminary results on the imaginary spectra noise analysis are also presented.

[1] In our recent paper, the sensitivity of infrasound to the upper atmosphere is investigated using impulsive signals from the Tungurahua volcano in Ecuador. We reported on the coherent variability of thermospheric travel times, with periods equal to those of the tidal harmonics. Moreover, it was shown that the error in predicted thermospheric travel time is in accord with typical uncertainties in the upper atmospheric wind speed models. Given the observed response of the infrasound celerities to upper atmospheric tidal variability, it was suggested that infrasound observations may be used to reduce uncertainty in the knowledge of the atmospheric specifications in the upper atmosphere. In this paper, we discuss the estimation of upper atmospheric wind model updates from the infrasound data described in the aforementioned paper. The parameterization of the model space by empirical orthogonal functions is described; it is found that the wind model in the upper mesosphere and lower thermosphere can be described by a four-parameter model. Due to the small dimensionality of the model space, a grid search method can be used to solve the inverse problem. A Bayesian method is used to assess the uncertainty in the inverse solution given the a priori uncertainty in the data and model spaces and the nonlinearity of the inverse problem at hand. We believe that this is the first study in which such methods are applied to real infrasound data, allowing for a rigorous analysis of this inverse problem. It is found that the complexity of the a posteriori model distribution increases for a larger dimensional model space and larger uncertainties in the data. A case study is presented in which the nonlinear propagation from source to receiver is simulated using an updated wind model and nonlinear ray theory. As nonlinear propagation effects further constrain the propagation path, this is a way to check the physical self-consistency of the travel time inversion approach. We obtain excellent agreement between the simulated and observed waveforms.

[1] Recent progress in observation and modeling studies indicates that the Tibetan Plateau (TP) has been experiencing solar dimming coincident with changes of surface and atmospheric conditions since the early 1980s. However, changes and variability of the surface radiation budget over the entire Tibetan Plateau have rarely been analyzed in association with meteorological observations. Taking into consideration the limitations in ground measurements, reanalysis products, and remote sensing products, this paper applies the fused data (described in Part 1 of this two-paper series) to analyze the seasonal and annual spatial pattern and temporal variation in the surface radiation budget (SRB) over the TP from 1984 to 2007. The climatology and interannual variability of five SRB components—downward shortwave irradiance, albedo, downward longwave flux, upward longwave flux, and net all-wave radiation—are presented and analyzed in conjunction with atmospheric (cloud cover, water vapor) and surface (temperature, snow cover, normalized difference vegetation index) conditions over the Tibetan Plateau. Over the entire Tibetan Plateau, regardless of the increase of downward longwave radiation that counteracts the increase of upward longwave radiation, the interaction of solar dimming with changes of surface albedo has dominated the marked decrease of all-wave net radiation since the mid-1980s. This result indicates that the weakening and strengthening of the relationship between the components of SRB and the correlated variables of atmospheric or surface conditions exhibit a seasonal dependency over the TP. Moreover, most peaks and drops in the SRB anomalies are consistent to variations in two dominant variables.

[1] This paper explores the use of a parametric geostatistical model for combining rainfall characteristics derived from rain gauge data with the same characteristics derived from remote-sensed data sets. Hypotheses can then be tested about which predictors significantly increase precision of an estimated characteristic. Although applicable wherever ground-level data and remote-sensed data are to be combined, the statistical procedure set out in the paper is developed for two examples of rainfall characteristics: (i) G, the mean annual rainfall at an ungauged site, conditional on knowledge of two predictor variables T (the mean annual rainfall calculated from the TRMM 3B42 data set for 1998–2009), and C (mean annual rainfall derived from the CMORPH data set for 2003–2009); (ii) the mean annual maximum 1 day rainfall H, interpolated using the same modeling procedure with predictor variables T and C derived from annual maximum 1 day rainfalls in the same remote-sensed data sets. Prediction errors showed no bias, skewness of distribution, or spatial heterogeneity. The model's generality means that it could be used with any predictors other than T and C, possibly derived from other satellite data sets or radar. Provided that predictor variables are correlated with the variable to be predicted, it is not necessary for the model relating them to be fitted using data from identical periods nor for the grid spacing of T and C to be identical. Model performance was evaluated by using a “leave-one-site-out” procedure, which showed that the root mean square error (RMSE) of model predictions at omitted sites was smaller than RMSEs obtained from five other well-known spatial predictors.

[1] This paper aims to test the robustness of the detection and attribution of anthropogenic climate change using four different empirical models that were previously developed to explain the observed global mean temperature changes over the last few decades. These studies postulated that the main drivers of these changes included not only the usual natural forcings, such as solar and volcanic, and anthropogenic forcings, such as greenhouse gases and sulfates, but also other known Earth system oscillations such as El NiQ no Southern Oscillation (ENSO) or the Atlantic Multidecadal Oscillation (AMO). In this paper, we consider these signals, or forced responses, and test whether or not the anthropogenic signal can be robustly detected under different assumptions for the internal variability of the climate system. We assume that the internal variability of the global mean surface temperature can be described by simple stochastic models that explore a wide range of plausible temporal autocorrelations, ranging from short memory processes exemplified by an AR(1) model to long memory processes, represented by a fractional differenced model. In all instances, we conclude that human-induced changes to atmospheric gas composition is affecting global mean surface temperature changes.

Volcanic aerosols change the atmospheric composition and thereby affect weather and climate. Aerosol dynamic processes such as nucleation, condensation, and coagulation modify the shape, size, and mass of aerosol particles, which influence their atmospheric lifetime and radiative properties. Nevertheless, most models omit these processes for ash particles. In this work, we explore the ash aerosol aging and sulfate production during the first 4 days following the 2021 La Soufrière (St. Vincent) eruption with the ICON-ART model (ICOsahedral Nonhydrostatic model with Aerosol and Reactive Trace gases). Online coupling of ICON-ART with a one-dimensional volcanic plume model calculates volcanic emission, which makes it possible to resolve the different eruption phases of the noncontinuous La Soufrière eruption. We compared our simulated aerosol distribution and composition with observations from the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) instrument, the Multiangle Imaging SpectroRadiometer (MISR) Research Aerosol (RA) Algorithm, and the Barbados Cloud Observatory (BCO). We show that online coupling is essential to adequately model the emissions and plume development close to the volcano. The modeled aerosol aging is in very good agreement with observations from MISR near the emission source and with CALIOP at larger distances. Furthermore, particle aging occurs faster in the troposphere than in the stratosphere due to the availability of water vapor and OH, but a layer of coated ash appears at the plume top due to faster oxidation of SO$_2$ and lofting by aerosol-radiation interaction. This paper gives the first direct comparison of aerosol aging in volcanic eruption plumes between simulations and observations.

The Mesoscale Heavy Rainfall Observing System (MHROS), supported by the Institute of Heavy Rain (IHR), Chinese Meteorology Administration, is one of the major systems to observe mesoscale convective systems (MCSs) over the middle region of the Yangtze River in China. The IHR MHROS consists of mobile C-POL and X-POL precipitation radars, millimeter wavelength cloud radar (MMCR), fixed S-band precipitation radars, GPS network, microwave radiometers, radio soundings, wind profiler radars and disdrometers. The atmospheric variables observed or retrieved by these instruments include the profiles of atmospheric temperature, moisture, wind speed and direction, vertical structures of MCS clouds and precipitation, atmospheric water vapor, and cloud liquid water. These quality controlled observations and retrievals have been used in mesoscale numerical weather prediction to improve the accuracy of weather forecasting and MCS research since 2007. These long-term observations have provided the most comprehensive datasets for researchers to investigate the formation-dissipation processes of MCSs and for modelers to improve their simulations of MCSs. As the first paper of a series, we briefly introduce the IHR MHROS and describe the specifications of its major instruments. Then, we provide an integrative analysis of the IHR MHROS observations for a heavy rain case on 3–5 July 2014 as well as the application of IHR MHROS observations in improving the model simulations. In a series of papers, we will tentatively answer several key scientific questions related to the MCS and Meiyu frontal systems over the middle region of the Yangtze River using the IHR MHROS observations.

The present work examines downwelling radiance spectra measured at the ground during 2013 by a Far Infrared Fourier Transform Spectrometer at Dome C, Antarctica. A tropospheric backscatter and depolarization lidar is also deployed at same site, and a radiosonde system is routinely operative. The measurements allow characterization of the water vapor and clouds infrared properties in Antarctica under all sky conditions. In this paper we specifically discuss cloud detection and the analysis in clear sky condition, required for the discussion of the results obtained in cloudy conditions. First, the paper discusses the procedures adopted for the quality control of spectra acquired automatically. Then it describes the classification procedure used to discriminate spectra measured in clear sky from cloudy conditions. Finally a selection is performed and 66 clear cases, spanning the whole year, are compared to simulations. The computation of layer molecular optical depth is performed with line-by-line techniques and a convolution to simulate the Radiation Explorer in the Far InfraRed-Prototype for Applications and Development (REFIR-PAD) measurements; the downwelling radiance for selected clear cases is computed with a state-of-the-art adding-doubling code. The mean difference over all selected cases between simulated and measured radiance is within experimental error for all the selected microwindows except for the negative residuals found for all microwindows in the range 200 to 400 cm−1, with largest values around 295.1 cm−1. The paper discusses possible reasons for the discrepancy and identifies the incorrect magnitude of the water vapor total absorption coefficient as the cause of such large negative radiance bias below 400 cm−1.

On the basis of the evaluation of recent Greenland precipitation studies, some of the deficiencies in the modeled precipitation are probably related to the topographic data employed in modeling. In this paper the modern digital elevation data of Ekholm [1996] is used. If the horizontal pressure gradient force in σ coordinates is separated into its irrotational and rotational parts, which are expressed by the equivalent geopotential and geo-stream-function, respectively, the topographic effect on the precipitation can be accurately modeled. The equivalent geopotential and geo-stream-function are implemented in a fully consistent manner in the generalized ω-equation in this paper. A simplified large-scale condensation without evaporation of condensate is also used. These improvements are combined to yield an improved dynamic method. Two aspects of the precipitation distribution are refined by the improved dynamic method. One is the 10 cm yr−1 contour near Summit, Greenland, and the other is a relative large precipitation area centered near the point (70°N, 49°W). Extensive comparisons are made between the retrieved precipitation and the observed annual accumulation time series from 11 ice core sites on the ice sheet. The modeled precipitation from the original method must use sealers to have a high degree of interannual correspondence between the measured accumulation and the retrieved precipitation, but the retrieved precipitation from the improved method increases at all ice core sites and a good correspondence is obtained without any sealer being required. The spatial average of multiyear mean error ( e¯j) is 11.5 cm yr−1 for the modeled precipitation from the improved method, while that for P from ERA-15 is 14.5 cm yr−1. The total mean error (eM) is 3.0 cm yr−1 for the improved method, while eM for the P from ERA-15 is 4.0 cm yr−1. These two errors show that the precipitation modeled by the improved method is better than the P from ERA-15. Thus the distribution of precipitation over the 11 sites retrieved by the improved dynamic method is considerably refined. Large downward trends in annual amounts are present in the precipitation retrieved by the improved dynamic method for all of Greenland and its southern and central west coastal regions. The modeled precipitation from the improved dynamic method and observed accumulation from ice cores are all in agreement with the Thomas et al. [1999] result that the southern Greenland ice sheet above 2000 m is approximately in balance. It also shows that local thickening and thinning areas of the ice sheet derived by airborne laser altimetry from 1993 to 1999 over the entire Greenland above 2000 m [Krabill et al., 2000] are approximately consistent with precipitation change retrieved by the improved dynamic method.

The formation of polar stratospheric clouds (PSCs) is sometimes attributed to cooling induced by mountain waves. Some examples of PSCs explained by this mechanism are found in the literature. Other studies show that the cooling producing PSCs is of synoptic scale. In this paper we use data from Polar Ozone Aerosol Measurement (POAM) II and from TIROS Operational Vertical Sounder (TOVS) showing coincident occurrences of PSCs and ozone miniholes over the sea and land. Using European Centre for Medium-Range Weather Forecasts (ECMWF) analyses, we show that when both PSCs and localized ozone minima appear, they are associated primarily with anticyclonic potential vorticity anomalies near the tropopause. The flow anomaly penetrates upward and downward, inducing an upward displacement of isentropic surfaces above and a downward displacement below, the upward and downward penetration being consistent with the deformation scale. These flow anomalies result in synoptic-scale quasi-adiabatic uplift through the lower stratosphere. The adiabatic cooling of the air masses creates the conditions for PSC formation. Coincidentally, the ozone partial pressure decreases, and the localized ozone minimum appears. Our purpose in this paper is to show that the primary PSC formation mechanism in the Arctic is the same as for minihole formation: synoptic-scale dynamics. We show three multiday sequences of PSCs and localized ozone minima. We reveal the robustness of the PSC/dynamics link by showing multiyear, monthly statistics of POAM II PSC sighting fraction compared with PSC formation temperature and isentropic geopotential.

In this paper, a novel lidar/radar method for simultaneously determining cloud particle effective size profiles (for water and ice clouds) and the lidar attenuation profile is described. Simulations and application to real data show that this procedure can be quite robust even in cases where significant lidar attenuation is present. In addition, the concept of a suitable lidar/radar effective particle size is introduced, and the determination of water contents for both water and ice clouds using this effective size together with the radar reflectivity profile is discussed. This paper concludes by presenting examples of effective size profiles and water content profiles inferred from measurements made during the Dutch Clouds and Radiation (CLARA) campaign. In a companion paper, it is demonstrated that the results of the inversion procedure compare favorably with infrared radiometer measurements as well as with in-situ measurement results.

This paper introduces a method that can incorporate different information into the lidar retrieval problem as an attempt to address the backscatter-to-extinction ambiguity that plagues the usefulness of lidar backscattering measurements. The approach, suited for application to spaceborne lidar data, inverts the lidar equation via an optimal estimation method. This method is illustrated using three examples drawn from LITE data. Retrievals using only lidar backscatter as input were compared to retrievals performed using an iterative solution to the lidar inversion with the same input. The two methods produced essentially identical results. The new method, however, offers a number of advantages compared to other methods, including (1) the ability to incorporate different kinds of information under a common retrieval philosophy. This feature is illustrated with the formal introduction of optical depth into the lidar inversion. In this paper, optical depth information, derived from the ldar transmission estimates, is combined with backscatter measurements making it possible to retrieve the backscatter-to-extinction ratio in addition to extinction profiles given certain caveats noted in the paper. (2) The method provides a number of ways for evaluating the quality of the retrieval. Notably, the retrieval approach predicts full error diagnostics identifying sources of error due to measurement uncertainty (instrument noise and calibration uncertainty), model error (containing all the assumptions built into the lidar equation and its parameters), as well as a priori error due to the influence of compiled databases on lidar backscatter of aerosol and cloud. When no optical depth information is available, the retrieval errors are largely dominated by the (large) uncertainty attached to the backscatter-to-extinction coefficient k. Under these circumstances the retrievals are only meaningful to the extent that k and its related uncertainty is known. When optical depth is introduced as a form of measurement, the error contributions shift to the extent that retrieval errors become dominated by the measurement error attached to the optical depth itself.

The Program for Research on Oxidants: Photochemistry, Emissions, and Transport (PROPHET) summer 1998 measurements intensive was conducted from a tower above a forested site in northern Michigan. This paper provides a brief overview of the meteorological conditions and establishes that the study period was moderately drier and warmer than the climatological mean. The paper also identifies and chemically characterizes the major air mass types influencing the site. Meteorological analyses and back trajectories establish that air mass origin oscillated between relatively clean Canadian regions (except for periods influenced by Canadian forest fires) and regions of greater anthropogenic emissions in the contiguous United States. Higher mixing ratios of ozone, CO, NOx, and NOz were generally associated with southerly transport, which occurred 24% of the time. Lower mixing ratios were observed under northerly transport, which occurred 44% of the time. The dominance of northerly transport was due to a stronger than normal Hudson Bay low. The remaining 32% of the time was occupied by transitional periods between distinct air masses. A positive slope exists between ozone and CO when photochemically aged air masses are selected. However, no meaningful relationship between ozone and CO was observed when northerly and southerly transport periods were considered separately. Comparison to other summertime rural locations suggests that rapid and frequent transitions between air masses of contrasting source regions play an important role in maintaining the 0.3 O3/CO slope commonly observed in eastern North America.

This paper investigates the expected capabilities of the new generation of infrared satellite sounders for detecting CO2. A general circulation model is used to simulate realistic CO2 fields and to define the needed accuracy of CO2 observations in order to be useful in constraining surface sources and sinks of CO2, which will be described in more detail in a future paper. Optimal estimation retrieval theory is then used to determine the possible accuracy of the satellite measurements and to define the retrieval characteristics. A discussion of several factors that affect the retrievals is also included. We conclude that tropospheric column retrievals of CO2 are possible with an accuracy of better than 1 ppmv on a monthly mean basis. Several factors, like thin cirrus clouds and radiative transfer modeling errors, will degrade these results if not carefully accounted for. The possibility of extensive time and spatial averaging of the satellite observations will overcome some of these problems.

The subject of the paper is to discuss and quantify deviations from reciprocity of the bidirectional reflectance distribution function (BRDF), i.e., the difference of BRDF obtained when inverting illumination and viewing directions. Directional reciprocity is not valid in general, because when the illumination beam has a spatial extension larger than the viewed area (as is most often the case for BRDF measurements), some of the scatterers building up the observed radiance are located at different places in reciprocal measurements. The physical systems under consideration in the two experiments are different, hence the breakdown of reciprocity. The paper develops a theory aiming at a quantitative estimation of deviations from directional reciprocity due to this factor. The theory is based on integral forms of the radiative transfer equation in a horizontal slab of heterogeneous absorbing and scattering media. The observed scene radiance is expanded in a series of scattering orders. Integral expressions of the single- and multiple-scattering radiance are derived and put in a form suitable for the analysis of the reciprocity problem. The first-order expression leads to an estimate of the order of magnitude e of the relative deviations from reciprocity, , where D is the size of the viewed area (pixel size for imaging sensors), h is the vertical photon mean free path, δQ/Q is a measure of the scene heterogeneity, and θi, θv, and ϕ are the illumination and view zenith angles and the relative azimuth between illumination and view directions. It is argued that this order of magnitude should remain approximately valid if all orders of scattering are taken into account. A discussion of practical applications in various fields, laboratory optical reflectometry, Earth radiation budget monitoring, and terrestrial surfaces remote sensing is given.

The U.S. Department of Energy funded the development of the multifilter rotating shadowband radiometer (MFRSR) as part of the Atmospheric Radiation Measurement (ARM) program. This seven-channel radiometer began operation at the first ARM site in 1992 and at the Department of Energy Quantitative Links (QL) sites in the fall of 1991; three of the QL sites continue to operate, although this program was discontinued after 1995. This paper describes the use of the MFRSR in acquiring aerosol optical depth data, including the in-field calibration procedure and a partial validation of this process. Multiyear measurements of aerosol optical depth from three of the sites indicate similar phasing of seasonal and interannual changes, but with notable differences in the magnitude of the aerosol optical depth. Published papers that use these aerosol data are highlighted, and public access to these and future data sets for scientific studies are explained.

A dramatic front of airglow radiance and wave structure reported by Taylor et al. 1995 was attributed, in a previous paper of ours Dewan and Picard, 1998, to a manifestation of an internal undular bore in the mesosphere. In the current paper we address the question of what physical process could be responsible for generating such a bore at that altitude. While it is relatively easy to find sources of internal tropospheric bores, including the dramatic "morning glory," the same cannot be said for mesospheric bores. It will be argued here that a likely candidate for the generator of such bores is the interaction of gravity waves with the mean flow at a critical layer. This interaction could take place within an already existing inversion layer, the latter playing the role of the "channel" in which the bore propagates. As Huang et al 1998 have shown, a similar wave/critical-level interaction may be responsible for the inversion layer in question. Hence we are proposing that the physical process producing the channel is the same as the one responsible for subsequently generating the bore.

In this paper, we present numerical simulations of the radiance and the degree of linear polarization of light reflected by the terrestrial atmosphere in the O2 A absorption band, around 760 nm. Since the O2 A band is often used to derive cloud parameters, we included clouds in our model atmosphere. The simulations show that the polarization of the reflected light changes across the O2 A band, and that this change depends strongly on the spectral resolution of the instrument. The polarization of reflected light induces errors in radiances derived from observations by polarization sensitive instruments. For the Global Ozone Monitoring Experiment (GOME) satellite instrument, which measures radiances with about 0.4-nm spectral resolution in the O2 A band, broadband polarization measurements are used to correct the narrowband radiance observations for the instrument's polarization sensitivity. Although such correction schemes significantly improve the accuracy of derived radiances in the continuum, they do not account for changes of the polarization in narrow absorption bands, such as the O2 A band. The main purpose of this paper is to investigate for cloudy atmospheres the errors in the derived radiances due to polarization changes across the O2 A band, both for a polarization sensitive instrument with a high spectral resolution and for a GOME-like resolution. If no correction scheme is used, it is found that for nadir viewing directions, the maximum errors in the absorption band can increase by up to about 20% with decreasing width of the spectral response function when the instrument's sensitivity for radiation polarized perpendicularly to the principal plane is twice as large as that for radiation polarized parallel to this plane. If, in this case, a correction scheme based on the broadband value of the polarization is used, the radiance errors can still be up to 18% with a high spectral resolution and of the order of a few percent with a GOME-like resolution.

Land surface albedo constitutes a critical climatic variable, since it largely controls the actual amount of solar energy available to the Earth system. The purpose of this paper is to establish a theory for the exploitation of space observations to solve the atmosphere/surface radiation transfer problem on an operational basis and to generate surface albedo, aerosol load, and possibly land cover change products. Surface albedo is rather variable in space and time and depends both on the structure and on the radiative characteristics of the surface, as well as on the angular and spectral distribution of radiation at the bottom of the atmosphere. Weather and climate models often use preset distributions or simple parameterizations of this environment variable, even though such approaches do not accurately account for the actual effect of the underlying surface. From a mathematical point of view, the determination of the surface albedo corresponds to the estimation of a boundary condition for the radiation transfer problem in the coupled surface-atmosphere system. A relatively large database of 10 years or more of Meteosat data has been accumulated by EUMETSAT. These data, collected at half-hour intervals over the entire Earth disk visible from longitude 0°, constitute a unique resource to describe the anisotropy of the coupled surface-atmosphere system and provide the opportunity to document changes in surface albedo which may have occurred in these regions over that period. In addition, since the coupled surface-atmosphere radiation transfer problem must be solved, the proposed procedure also yields an estimate of the spatial and temporal distribution of aerosols. The proposed inversion procedure yields a characterization of surface radiative properties that may also be used to document and monitor land surface dynamics over the portion of the globe observed by Meteosat. Results from preliminary applications and an error budget analysis are discussed in a companion paper [Pinty et al., this issue].

A simple parametric relationship is established between factors describing the shape of cloud condensation nuclei (CCN) activation spectra and observable properties of the aerosol population they grow on (size distribution and solubility). This is done independently for maritime and continental aerosol types because of their very different characteristics. The data used for the multiple statistical adjustments in the procedure described in this paper are generated by running a numerical model of aerosol growth coupled to a simple cloud droplet activation scheme. Each aerosol population (maritime and continental) is assumed to be of homogeneous chemical composition, lognormally distributed and with variable solubility. The parameterization is then evaluated using a large set of aerosol populations with randomized properties. Finally, the study presents a preliminary analysis of the most important aerosol properties that influence the shape of the CCN spectra. An idealized scenario of a clean maritime boundary layer cloud perturbed by anthropogenic emissions (such as the ship track problem) illustrates the capability of the parameterization to selectively increase the cloud droplet concentration in a partially polluted cloud. The calibration results presented in this paper are not meant to be the definitive activation spectra produced by any lognormally distributed aerosols. These results are indeed a step toward an objective initialization of CCN spectra and hence toward the computation of cloud droplet concentrations based on measurable multimodal aerosol features, as required by three-dimensional numerical models with a coupled interactive aerosol module.

This paper analyzes data obtained on July 3 and 18, 1995, during the summer 1995 Southern Oxidant Study (SOS) field campaign. In a previous paper (Nunnermacker et al., 1998) we analyzed measurements of key species that contribute to formation of O3 in the Nashville urban plume and presented a semiquantitative picture of O3 production in the plume from the point of emission to locations where no net O3 was being formed. In this paper we use a box model constrained by observed concentrations of stable species to obtain a detailed mechanistic description of the instantaneous processing of urban emissions at various times in the chemical evolution of the urban plume. Instantaneous ozone production rates and efficiencies with respect to NOx and to primary radical production are examined. At high NOx concentrations in the fresh urban plume the O3 production rate was found to be directly proportional to the hydrocarbon to NOx reactivity ratio. At lower NOx concentrations, corresponding to the mature urban plume and the background atmosphere, the O3 production rate was found to be directly proportional to the NOx concentration and independent of the hydrocarbon reactivity. NOx was found to be most efficiently used for ozone production at low NOx concentrations. In contrast, the efficiency with which the system uses primary radicals was found to be very low at low NOx concentrations and to peak at a NOx concentration of approximately 4 ppbv. A sensitivity study of the instantaneous O3 production rates to changes in NOx or hydrocarbon concentrations showed that the instantaneous 03 production rate at the center of the urban plume, when half of the urban NOx emissions had been processed, is hydrocarbon sensitive. However, 03 production becomes NOx sensitive as the plume matures.

The Lidar Atmospheric Sensing Experiment (LASE) was operated autonomously from the NASA high-altitude ER-2 aircraft on nine flights during July 10–26, 1996, as part of the Tropospheric Aerosol Radiative Forcing Observational Experiment (TARFOX). LASE measured high-resolution profiles of water vapor and aerosols in regions of urban haze plumes over the U.S. eastern seaboard. Real-time LASE aerosol measurements were used to guide the in situ aircraft to sample haze layers. In this paper the vertical and horizontal distributions of aerosol backscatter measured by LASE are presented along with the temporal evolution of the haze layers. The aerosol backscatter data also identify the presence of gradients in the aerosol plumes, the presence of low-altitude clouds, and optically thin cirrus. This information is useful for many of the radiometeric observations made during TARFOX and can help explain observational differences among ground, airborne, and satellite observations. An iterative procedure is discussed which was used to invert lidar data to retrieve aerosol scattering ratios, extinction, and total optical depths from the LASE measurements. The sensitivity of these retrievals to assumed parameters is discussed and the results of retrievals are also compared to the well-known Bernoulli method. LASE water vapor measurements were made across the entire troposphere using a three “line pair” method to cover the range of water vapor mixing ratio from < 0.01 g/kg near the tropopause to ∼ 20 g/kg near the surface in a single aircraft pass over the experiment region. These measurements also show two-dimensional distributions of large spatial gradients in water vapor in the lower and upper troposphere. These observations are useful in the calculation of IR radiation fields and relative humidity fields, since relative humidity has a strong influence on the growth of aerosols and their scattering properties. Water vapor profiles, aerosol scattering ratios, aerosol extinction coefficients and aerosol optical depths were derived using the methodology presented in this paper from LASE measurements during TARFOX. These measurements are compared with other in situ and remote measurements during TARFOX in the companion papers [Ferrare et al., this issue (a, b)]

This first part of the paper is devoted to the validation of the theoretical framework developed in the companion paper [Coelho et al., this issue] against laboratory data. Although there are limited data suitable for a full-scale validation of the model, model-calculated surface to mass ratios of soot aggregates are found to be consistent with laboratory measurements. Then we use the framework to estimate errors which can be generated in the derivation of fractal aggregate properties from measurements of equivalent sizes. It is shown that the derivations of the aggregate mass and the surface area enhancement factor can be in error by at least an order of magnitude. The calculations are substantially improved when the fractal character of soot is accounted for. Since the gyration radius is a key parameter of the model, useful relationships are provided for converting aggregate equivalent sizes (mass transfer equivalent radius, hydrodynamic radius) into gyration radius. Finally, uptake of chemical species on atmospheric soot is investigated for the lower stratosphere and the boundary layer. The uptake is found to be mostly reaction limited, justifying the assumption of proportionality between the soot surface area and the uptake rate. However, the uptake occurs in the transition regime for the relatively compact aggregates found in urban areas.

This paper discusses participate concentration and size distribution data gathered using the University of Missouri-Rolla Mobile Aerosol Sampling System (UMR-MASS), and used to investigate the southern extent of the eastern end of the North Atlantic Flight Corridor (NAFC) during project Pollution From Aircraft Emissions in the North Atlantic Flight Corridor/Subsonic Assessment (SASS) Ozone and Nitrogen Oxide Experiment (POLINAT 2/SONEX) from September 19 to October 23, 1997. The analysis presented in this paper focuses on “the corridor effect,” or enhancement of pollutants by jet aircraft combustion events. To investigate the phenomena, both vertical and horizontal profiles of the corridor, and regions immediately adjacent to the corridor, were performed. The profiles showed a time-dependent enhancement of particulates within the corridor, and a nonvolatile (with respect to thermal volatilization at 300°C) aerosol enhancement at corridor altitudes by a factor of 3.6. The southern extent of the North Atlantic Flight Corridor was established from a four flight average of the particulate data and yielded a boundary near 42.5°N during the study period. A size distribution analysis of the nonvolatile particulates revealed an enhancement in the

There are several theories of atmospheric gravity wave power spectral densities (PSDs) which have been published. These, in turn, have inspired numerous experimental tests. The spectra involved are in the class denoted “stochastic, red noise spectra.” This means that most of the power is at the low-frequency end and obeys a power law falloff in going to higher frequency. The present paper describes how some published experimental spectra are flawed by an artifact of spectral analysis which has not heretofore been recognized in the literature. It involves both an amplitude fluctuation enhancement and a coupling between spectral amplitude and slope, and it can be avoided only by stringent control of spectral leakage. Because of “trade-off” considerations every data set, depending on its length and signal characteristics, requires a different method of analysis. It is therefore required that PSD analysis programs must be adjusted and tested to fit each situation. For this purpose a simple method is described to simulate data of known general characteristics for test purposes (to avoid the pitfalls). Since the papers by Nastrom et al. [1997] and de la Torre et al. [1997] have the unfortunate artifact in their analyses, their conclusions regarding saturated gravity wave theories should be reexamined.