Multifilter Rotating Shadowband Radiometer Research Articles

Retrieval of precipitable water vapor using MFRSR and comparison with other multisensors over the semi-arid area of northwest China

Precipitable water vapor (PWV) was retrieved using direct solar irradiance at 938nm measured by a multifilter rotating shadowband radiometer (MFRSR) at the Semi-Arid Climate and Environment Observatory of Lanzhou University (SACOL) located in the semi-arid area of northwest China from August 2007 to June 2010. Measurement also occurred at Zhangye, China, at the Atmosphere Radiation Measurements (ARM) Program's Ancillary Facility during the dust period from April to June 2008. The line-by-line radiative transfer model (LBLRTM) code combined with the HITRAN 2004 spectral database is used to model the water vapor spectral transmittance throughout the 938-nm spectral response of MFRSR in the retrieval algorithm. Gaussian fitting is proposed to determine the daily calibration constant at the top of atmosphere for a long-term series under an obvious annual change in solar radiation.PWV retrieved by MFRSR over SACOL shows that 90% of PWV values are smaller than 1.52cm, and PWV distribution has a seasonal variation, with maximum in summer and minimum in winter. The comparisons between MFRSR and other measurements show a better agreement between MFRSR and sunphotometer (AERONET's Cimel) PWV retrievals with relative bias of 2.9% and RMS difference of 9.1% than between MFRSR and microwave radiometer (MWR) with relative bias of 10% and RMS difference of 23% over SACOL, and an excellent agreement between MFRSR and sunphotometer with relative bias of 0.56% and RMS difference of 6.1% over Zhangye.To verify satellite PWV products over the semi-arid area of northwest China, the comparisons of PWV from MODIS and AIRS with MFRSR suggest that the agreement between satellite and MFRSR PWV retrievals is not as good as that between MFRSR and other ground-based instruments. MODIS appears to slightly underestimate PWV in a dry atmosphere but overestimate PWV in a moist atmosphere against MFRSR. A method is proposed to correct MODIS PWV products. AIRS PWV products relative to MFRSR show a systematic underestimation.

Contributions of dust and biomass burning to aerosols at a Colorado mountain-top site

Abstract. Visible Multifilter Rotating Shadowband Radiometer (vis-MFRSR) data were collected at Storm Peak Laboratory (SPL), a mountain-top facility in northwest Colorado, from 1999 to 2011 and in 2013. From 2011 to 2014, in situ measurements of aerosol light scattering were also obtained. Using these data sets together, the seasonal impact of dust and biomass burning is considered for the western USA. Analysis indicates that the median contributions to spring and summer aerosol optical depth (AOD) from dust and biomass-burning aerosols across the data set are comparable. The mean AOD is slightly greater in the summer, with significantly more frequent and short-duration high AOD measurements due to biomass-burning episodes than in the spring. The Ångström exponent showed a significant increase in the summer for both the in situ and vis-MFRSR data, suggesting an increase in combustion aerosols. Spring dust events are less distinguishable in the in situ data than the column measurement, suggesting that a significant amount of dust may be found above the elevation of SPL, 3220 m a.s.l. Twenty-two known case studies of intercontinental dust, regional dust, and biomass-burning events were investigated. These events were found to follow a similar pattern, in both aerosol loading and Ångström exponent, as the seasonal mean signal in both the vis-MFRSR and ground-based nephelometer. This data set highlights the wide-scale implications of a warmer, drier climate on visibility in the western USA.

Total ozone column measurements using an ultraviolet multi-filter radiometer

We have developed and used a method to retrieve total ozone column (TOC), from Ultraviolet Multi-filter Rotating Shadowband Radiometer (UVMFR) measurements in combination with radiative transfer model calculations. Look-up tables of ratios of the direct solar irradiance at (DI) 305 and 325nm in terms of TOC, solar zenith angle, and aerosol optical depth (AOD) have been constructed and compared with TOC retrievals estimated directly from UVMFR irradiance measurements. Sensitivity analysis of the influence of AOD on the calculated TOC has been investigated and found to be 1 Dobson unit per 0.1 change in AOD. We also examined the impact of ozone effective temperature on the TOC retrieval and found that it leads to a 0.9% change in TOC per K. UVMFR direct irradiance measurements in Athens, Greece, during the period July 2009–May 2014 were used to create a time series of high-temporal-frequency measurements (1 min for cloudless conditions) of TOC, which facilitated an analysis of the diurnal variation of TOC. Comparison of the TOC retrievals from the UVMFR with co-located and synchronous daily TOC retrievals from a Brewer MKIV spectrophotometer showed very good agreement (correlation coefficient 0.98). Daily TOC retrievals from the UVMFR were within ±3% compared with the ones measured by the Ozone Monitoring Instrument overpasses on board the Aura satellite.

Wave like signatures in aerosol optical depth and associated radiative impacts over the central Himalayan region

Doppler Lidar and Multi-Filter Rotating Shadowband Radiometer (MFRSR) observations are utilized to show wave like signatures in aerosol optical depth (AOD) during daytime boundary layer evolution over the Himalayan region. Fourier analysis depicted 60–80min periods dominant during afternoon hours, implying that observed modulations could be plausible reason for the AOD forenoon–afternoon asymmetry which was previously reported. Inclusion of wave amplitude in diurnal variation of aerosol radiative forcing estimates showed ~40% additional warming in the atmosphere relative to mean AOD. The present observations emphasize the importance of wave induced variations in AOD and radiation budget over the site.

Empirical correction of multifilter rotating shadowband radiometer (MFRSR) aerosol optical depths for the aerosol forward scattering and development of a long-term integrated MFRSR-Cimel dataset at Lampedusa.

Aerosol optical properties have been measured on the island of Lampedusa (35.5°N, 12.6°E) with seven-band multifilter rotating shadowband radiometers (MFRSRs) and a CE 318 Cimel sunphotometer (part of the AERONET network) since 1999. Four different MFRSRs have operated since 1999. The Cimel sunphotometer has been operational for a short period in 2000 and in 2003-2006 and 2010-present. Simultaneous determinations of the aerosol optical depth (AOD) from the two instruments were compared over a period of almost 4 years at several wavelengths between 415 and 870 nm. This is the first long-term comparison at a site strongly influenced by desert dust and marine aerosols and characterized by frequent cases of elevated AOD. The datasets show a good agreement, with MFRSR underestimating the Cimel AOD in cases with low Ångström exponent; the underestimate decreases for increasing wavelength and increases with AOD. This underestimate is attributed to the effect of aerosol forward scattering on the relatively wide field of view of the MFRSR. An empirical correction of the MFRSR data was implemented. After correction, the mean bias (MB) between MFRSR and Cimel simultaneous AOD determinations is always smaller than 0.004, and the root mean square difference is ≤0.031 at all wavelengths. The MB between MFRSR and Cimel monthly averages (for months with at least 20 days with AOD determinations) is 0.0052. Thus, by combining the MFRSR and Cimel observations, an integrated long-term series is obtained, covering the period 1999-present, with almost continuous measurements since early 2002. The long-term data show a small (nonstatistically significant) decreasing trend over the period 2002-2013, in agreement with independent observations in the Mediterranean. The integrated Lampedusa dataset will be used for aerosol climatological studies and for verification of satellite observations and model analyses.

A New Cloud Screening Algorithm for Ground-Based Direct-Beam Solar Radiation

AbstractCloud screening of direct-beam solar radiation is an essential step for in situ calibration and atmospheric properties retrieval. The internal cloud screening module of a Langley analysis program [Langley Analyzer (LA)] used by the U.S. Department of Agriculture (USDA) UV-B Monitoring and Research Program (UVMRP) is used for screening the uncalibrated direct-beam measurements and for deriving Langley offset voltages for calibration of the UV version of the Multifilter Rotating Shadowband Radiometer (UV-MFRSR). The current LA cloud screening module utilizes data from extended clear-sky periods and tends to ignore shorter periods that typify periods of broken cloudiness, and as a result, fewer values are generated for sites with higher frequencies of cloudy days (cloudy sites). A new cloud screening algorithm is presented that calculates the total optical depth (TOD) difference between a target point and pairs of points, and identifies the target as cloudy if the mean TOD difference exceeds a certain threshold. The screening is an iterative process that finishes when no new cloudy points are found. The result at a typical clear/sunny site shows that values from partly cloudy days are consistent with those from cloud-free days, when the new method is employed. The new cloud screening algorithm picks up significantly more values at cloudy sites. The larger decrease of the annual mean value of at cloudy sites than at relatively clear sites suggests the potential for improving calibration accuracy at cloudy sites. The results also show that the new cloud screening method is capable of detecting clear points in short clear windows and in transitional regions.

Variability of the Daily-Mean Shortwave Cloud Radiative Forcing at the Surface at a Midlatitude Site in Southwestern Europe

Abstract The shortwave cloud radiative forcing is calculated from surface measurements taken in Évora from 2003 to 2010 with a multifilter rotating shadowband radiometer (MFRSR) and with an Eppley black and white pyranometer. A new approach to estimate the clear-sky irradiance based on radiative transfer calculations is also proposed. The daily-mean values of the cloud radiative forcing (absolute and normalized) as well as their monthly and seasonal variabilities are analyzed. The study shows greater variability of radiative forcing during springtime with respect to the other seasons. The mean daily cloudy periods have seasonal variation proportional to the seasonal variation of the cloud radiative forcing, with maximum values also occurring during springtime. The minimum values found for the daily-mean cloud radiative forcing are −139.5 and −198.4 W m−2 for MFRSR and Eppley data, respectively; the normalized values present about 40% of sample amplitude, both for MFRSR and Eppley. In addition, a quantitative relationship between the MFRSR and Eppley cloud radiative forcings applicable to other locations is proposed.

Assessment of Langley and NASA-GISS Calibration Techniques for MFRSR Aerosol Retrieval

Sun photometric measurements are an important method of measuring the column amount and optical properties of atmospheric aerosols and are frequently used to better understand the impact of aerosols on the Earth's radiative budget. This paper assesses two calibration techniques used for multifilter rotating shadowband radiometers (MFRSRs) and presents the results obtained at Long Island, NY, during the July 2011 Aerosol Life Cycle Intensive Operational Period (IOP) campaign. The instrument calibration constants are validated internally against each other and against an independent quantitative technique based on the solar irradiance at the top of the atmosphere, convolved with the MFRSR filter response. In addition, the aerosol optical parameters obtained from the MFRSR are assessed against the same parameters retrieved with a recently calibrated CIMEL sun/sky radiometer collocated with the MFRSR instrument. These comparisons indicate generally good agreement between the two instruments for both calibration techniques. The advantages and disadvantages of the two techniques are presented. Additionally, an analysis of the optical and physical properties of aerosols from the MFRSR and their chemical compositions obtained by an in situ high-resolution time-of-flight aerosol mass spectrometer, together with back trajectories, indicate that the principal source of high concentrations of fine aerosols observed during July 18-24 was forest fires in western Canada.

Retrievals and uncertainty analysis of aerosol single scattering albedo from MFRSR measurements

Aerosol single scattering albedo (SSA) can be retrieved from the ratio of diffuse horizontal and direct normal fluxes measured from multifilter rotating shadowband radiometer (MFRSR). In this study, the measurement channels at 415nm and 870nm are selected for aerosol optical depth (AOD) and Angstrom coefficient retrievals, and the measurements at 415nm are used for aerosol SSA retrievals with the constraint of retrieved Angstrom coefficient. We extensively assessed various issues impacting on the accuracy of SSA retrieval from measurements to input parameters and assumptions. For cloud-free days with mean aerosol loading of 0.13–0.60, our sensitivity study indicated that: (1) 1% calibration uncertainty can result in 0.8–3.7% changes in retrieved SSA; (2) without considering the cosine respond correction and/or forward scattering correction will result in underestimation of 1.1–3.3% and/or 0.73% in retrieved SSA; (3) an overestimation of 0.1 in asymmetry factor can result in an underestimation of 2.54–3.4% in retrieved SSA; (4) for small aerosol loading (e.g., 0.13), the uncertainty associated with the choice of Rayleigh optical depth value can result in non-negligible change in retrieved SSA (e.g., 0.015); (5) an uncertainty of 0.05 for surface albedo can result in changes of 1.49–5.4% in retrieved SSA. We applied the retrieval algorithm to the MFRSR measurements at the Atmospheric Radiation Measurements (ARM) Southern Great Plains (SGP) site. The retrieved results of AOD, Angstrom coefficient, and SSA are basically consistent with other independent measurements from co-located instruments at the site.

Areal-Averaged Spectral Surface Albedo from Ground-Based Transmission Data Alone: Toward an Operational Retrieval

We present here a simple retrieval of the areal-averaged spectral surface albedo using only ground-based measurements of atmospheric transmission under fully overcast conditions. Our retrieval is based on a one-line equation. The feasibility of our retrieval for routine determinations of albedo is demonstrated for different landscapes with various degrees of heterogeneity using three sets of measurements: (1) spectral atmospheric transmission from the Multi-Filter Rotating Shadowband Radiometer (MFRSR) at five wavelengths (415, 500, 615, 673, and 870 nm); (2) tower-based measurements of local surface albedo at the same wavelengths; and (3) areal-averaged surface albedo at four wavelengths (470, 560, 670 and 860 nm) from collocated and coincident Moderate Resolution Imaging Spectroradiometer (MODIS) observations. These integrated datasets cover both temporally long (2008–2013) and short (April–May 2010) periods at the Atmospheric Radiation Measurement (ARM) Southern Great Plains site and the National Oceanic and Atmospheric Administration (NOAA) Table Mountain site, respectively. The calculated root mean square error (RMSE), defined here as the root mean squared difference between the MODIS-derived surface albedo and the retrieved areal-averaged albedo, is quite small (RMSE ≤ 0.015) and comparable with that obtained previously by other investigators for the shortwave broadband albedo. Good agreement between tower-based measurements of daily-averaged surface albedo for completely overcast and non-overcast conditions is also demonstrated.

Retrievals of aerosol optical depth and total column ozone from Ultraviolet Multifilter Rotating Shadowband Radiometer measurements based on an optimal estimation technique

A Bayesian optimal estimation (OE) retrieval technique was used to retreive aerosol optical depth (AOD), aerosol single scattering albedo (SSA), and an asymmetry factor (g) at seven ultraviolet wavelengths, along with total column ozone (TOC), from the measurements of the UltraViolet Multifilter Rotating Shadowband Radiometer (UV-MFRSR) deployed at the Southern Great Plains (SGP) site during March through November in 2009. The OE technique specifies appropriate error covariance matrices and optimizes a forward model (Tropospheric ultraviolet radiative transfer model, TUV), and thus provides a supplemental method for use across the network of the Department of Agriculture UV-B Monitoring and Research Program (USDA UVMRP) for the retrieval of aerosol properties and TOC with reasonable accuracy in the UV spectral range under various atmospheric conditions. In order to assess the accuracy of the OE technique, we compared the AOD retreivals from this method with those from Beer’s Law and the AErosol RObotic Network (AERONET) AOD product. We also examine the OE retrieved TOC in comparison with the TOC from the U.S. Department of Agriculture UV-B Monitoring and Research Program (USDA UVMRP) and the Ozone Monitoring Instrument (OMI) satellite data. The scatterplots of the estimated AOD from the OE method agree well with those derived from Beer’s law and the collocated AERONETAOD product, showing high values of correlation coefficients, generally 0.98 and 0.99, and large slopes, ranging from 0.95 to 1.0, as well as small offsets, less than 0.02 especially at 368 nm. The comparison of TOC retrievals also indicates the promising accuracy of the OE method in that the standard deviations of the difference between the OE derived TOC and other TOC products are about 5 to 6 Dobson Units (DU). Validation of the OE retrievals on these selected dates suggested that the OE technique has its merits and can serve as a supplemental tool in further analyzing UVMRP data.

Macrophysical and microphysical properties of monsoon clouds over a rain shadow region in India from ground‐based radiometric measurements

AbstractThe important radiative properties of clouds such as cloud optical depth (COD) and droplet effective radii (Re) are retrieved from the simultaneous measurements by ground‐based multifilter rotating shadowband radiometer (MFRSR) and microwave radiometric profiler (MWRP), colocated at Mahabubnagar, a rain shadow region in southern Indian peninsula. Min and Harisson's (1996) retrieval algorithm is used for the first time to derive monsoon cloud properties in India. COD and liquid water path (LWP) retrieved from two independent instruments of MFRSR and MWRP showed reasonably good correlation. During monsoon (July to September) and postmonsoon (October) months, the maximum probability of occurrence of COD for overcast sky is 20. The maximum probability of occurrence of LWP is 100 gm−2 for water clouds during monsoon months, while October showed maximum occurrence at a lower value of 50 gm−2, where most of the times the cloud bases are above freezing level indicating mixed phase clouds. Maximum Re varied from 14–16 µm (10–12%) to 12 µm (9%) during monsoon to postmonsoon transition with very less probability of occurrence indicating the characteristic feature of this region. A case study showed that the mean Re from ground‐based and aircraft measurements are 12.0 ± 3.7 µm and 8.14 ± 1.4 µm, respectively, indicating a fairly good agreement within the experimental constraints. Intercomparison of ground‐based and Moderate Resolution Imaging Spectroradiometer (MODIS)‐Terra and MODIS‐Aqua‐derived COD, LWP and Re over the observational site for overcast and warm clouds indicates that on an average, MODIS‐retrieved mean COD and LWP are underestimated, while mean Re is overestimated as compared to ground retrievals.

Failure and Redemption of Multifilter Rotating Shadowband Radiometer (MFRSR)/Normal Incidence Multifilter Radiometer (NIMFR) Cloud Screening: Contrasting Algorithm Performance at Atmospheric Radiation Measurement (ARM) North Slope of Alaska (NSA) and Southern Great Plains (SGP) Sites

Well-known cloud-screening algorithms, which are designed to remove cloud-contaminated aerosol optical depths (AOD) from Multifilter Rotating Shadowband Radiometer (MFRSR) and Normal Incidence Multifilter Radiometer (NIMFR) measurements, have exhibited excellent performance at many middle-to-low latitude sites around world. However, they may occasionally fail under challenging observational conditions, such as when the sun is low (near the horizon) and when optically thin clouds with small spatial inhomogeneity occur. Such conditions have been observed quite frequently at the high-latitude Atmospheric Radiation Measurement (ARM) North Slope of Alaska (NSA) sites. A slightly modified cloud-screening version of the standard algorithm is proposed here with a focus on the ARM-supported MFRSR and NIMFR data. The modified version uses approximately the same techniques as the standard algorithm, but it additionally examines the magnitude of the slant-path line of sight transmittance and eliminates points when the observed magnitude is below a specified threshold. Substantial improvement of the multi-year (1999–2012) aerosol product (AOD and its Angstrom exponent) is shown for the NSA sites when the modified version is applied. Moreover, this version reproduces the AOD product at the ARM Southern Great Plains (SGP) site, which was originally generated by the standard cloud-screening algorithms. The proposed minor modification is easy to implement and its application to existing and future cloud-screening algorithms can be particularly beneficial for challenging observational conditions.

Cloud optical and microphysical properties derived from ground‐based and satellite sensors over a site in the Yangtze Delta region

AbstractComprehensive surface‐based retrievals of cloud optical and microphysical properties were made at Taihu, a highly polluted site in the central Yangtze Delta region, during a research campaign from May 2008 to December 2009. Cloud optical depth (COD), effective radius (Re), and liquid water path (LWP) were retrieved from measurements made with a suite of ground‐based and spaceborne instruments, including an Analytical Spectral Devices spectroradiometer, a multifilter rotating shadowband radiometer, a multichannel microwave radiometer profiler, and the Moderate Resolution Imaging Spectroradiometer (MODIS) on Terra and Aqua satellites. Retrievals from zenith radiance measurements capture better the temporal variation of cloud properties than do retrievals from hemispherical fluxes. Annual mean LWP, COD, and Re are 115.8 ± 90.8 g/m2, 28.5 ± 19.2, and 6.9 ± 4.2 µm. Over 90% of LWP values are less than 250 g/m2. Most of the COD values (>90%) fall between 5 and 60, and ~80% of Re values are less than 10 µm. Maximum (minimum) values of LWP and Re occur in summer (winter); COD is highest in winter and spring. Raining and nonraining clouds have significant differences in LWP, COD, and Re. Rainfall frequency is best correlated with LWP, followed by COD and Re. Cloud properties retrieved from multiple ground‐based instruments are also compared with those from satellite retrievals. On average, relative to surface retrievals, mean differences of satellite retrievals in cloud LWP, COD, and Re were −33.6 g/m2 (−26.4%), −5.8 (−31.4%), and 2.9 µm (29.3%) for 11 MODIS‐Terra overpasses and −43.3 g/m2 (−22.3%), −3.0 (−10.0%), and −1.3 µm (−12.0%) for 8 MODIS‐Aqua overpasses, respectively. These discrepancies indicate that MODIS cloud products still suffer from large uncertainties in this region.

Micropulse lidar‐derived aerosol optical depth climatology at ARM sites worldwide

This paper focuses on climatology of the vertical distribution of aerosol optical depth (AOD (z)) from micropulse lidar (MPL) observations for climatically different locations worldwide. For this, a large data set obtained by MPL systems operating at 532 nm during the 4 year period 2007–2010 was used to derive vertical profiles of AOD (z) by combining the corresponding AOD data as an input from an independent measurement using nearly colocated multifilter rotating shadowband radiometer (MFRSR) systems at five different U.S. Department of Energy Atmospheric Radiation Measurement (ARM) Program sites—three permanent sites (SGP in north‐central Oklahoma, at 36.6°N, 97.5°W, 320 m; TWP‐Darwin in the tropical western Pacific, at 12.4°S, 130.9°E, 30 m; and NSA at Barrow on the North Slope of Alaska, at 71.3°N, 156.6°W, 8 m) and two mobile facility sites (GRW at Graciosa Island in the Azores, at 39°N, 28°W, 15 m; and FKB in the Black Forest of Germany, at 48.5°N, 8.4°E, 511 m). Therefore, amount of data used in this study is constrained by the availability of the MFRSR data. The MPL raw data were averaged for 30 s in time and 30 m in altitude. The diurnally averaged AOD (z) profiles from 4 years were combined to obtain a multiyear vertical profile of AOD (z) climatology at various ARM sites, including diurnal, day‐to‐day, and seasonal variabilities. Most aerosols were found to be confined to 0–2 km (approximately the planetary boundary layer region) at all sites; however, all sites exhibited measurable aerosols well above the mixed layer, with different height maxima. The entire data set demonstrates large day‐to‐day variability at all sites. However, there is no significant diurnal variation in AOD (z) at all sites. Significant interannual variability was observed at the SGP site. Clear seasonal variations in AOD (z) profiles exist for all five sites, but seasonal behavior was distinct. Moreover, the different seasonal variability for the lower level (0 to ~2 km) versus the level above indicates a contribution of different types of air masses from different sources. The lower annual mean AOD (z) values (0.093 ± 0.033 for daytime and 0.093 ± 0.05 for nighttime) observed near the surface at GRW are not unexpected for maritime aerosols (mostly sea salt), and the corresponding higher values at SGP (0.118 ± 0.038 for daytime and 0.11 ± 0.05 for nighttime), FKB (0.124 ± 0.042 for daytime and 0.127 ± 0.047 for nighttime), and TWP (0.13 ± 0.078 for daytime and 0.14 ± 0.073 for nighttime) are usual for continental aerosols. The annual mean AOD (z) values observed near the surface during daytime and nighttime for NSA were 0.1 ± 0.042 and 0.09 ± 0.037, respectively. These results will aid the scientific community in understanding aerosol properties and boundary layer dynamics and in improving the incorporation of aerosol radiative effects into global climate models.

Aerosol Transport and Source Attribution Using Sunphotometers, Models and In-Situ Chemical Composition Measurements

Understanding of chemical, physical, and radiative processes-emissions, transport, deposition, and modification of aerosol optical properties due to ageing-is of major importance to global and regional climate simulations and projections as well as health impairment. This paper presents aerosol optical properties retrieved with the Multifilter Rotating Shadowband Radiometers (MFRSRs) and the source attribution based on back trajectories and in situ aerosol chemical composition analysis obtained during the Aerosol Life Cycle Intensive Observational Period at Brookhaven National Laboratory on Long Island, NY, during July and August 2011. The aerosol optical properties retrieved with the MFRSR exhibit excellent agreement with those obtained with a colocated Cimel sunphotometer. Apportioning aerosol optical depths by size modes reveals several episodes of high loading of fine aerosol (diameter less than 2.5 μm). Analysis of optical and physical properties of aerosols as well as their chemical composition obtained by an in situ high-resolution time-of-flight aerosol mass spectrometer together with back trajectories indicates that the principal source of high concentrations of fine aerosols observed during July 18-24 was forest fires in western Canada, consistent with reports by the Canadian Forest Service and satellite observations by the Moderate Resolution Imaging Spectroradiometer (MODIS).

Fifteen‐year aerosol optical depth climatology for Salt Lake City

Aerosol optical depth (AOD) and its wavelength dependence have been measured for the past 15 years in the Salt Lake City metropolitan area using a multifilter rotating shadowband radiometer. The instrument has not experienced a major hardware failure. It has been continuously field calibrated for extraterrestrial responses in its five aerosol channels. The instrument's cosine response was measured in 1996 and again in 2012. In our analysis of this 15 year data set, linear interpolation of these two cosine responses was used to approximate the angular response between the two characterizations. The Salt Lake City aerosol burden increased through the mid‐2000s, but has dropped to its lowest level of the record since that time despite a population increase of approximately 25%. Annually, the aerosol burden is highest in midspring and midsummer with relatively coarse aerosols during the spring peak and fine aerosols during the summer peak. There is no indication of a diurnal cycle in AOD. There is a significant, but low, correlation between PM2.5 and 500 nm AOD, and a slightly lower correlation between PM10 and 500 nm AOD. The correlations between the surface‐based measurements and total column AOD explain only 13% and 9% of the variance, respectively. Measurements are continuing to track future trends.

The calibration methods for Multi-Filter Rotating Shadowband Radiometer: a review

The continuous, over two-decade data record from the Multi-Filter Rotating Shadowband Radiometer (MFRSR) is ideal for climate research which requires timely and accurate information of important atmospheric components such as gases, aerosols, and clouds. Except for parameters derived from MFRSR measurement ratios, which are not impacted by calibration error, most applications require accurate calibration factor(s), angular correction, and spectral response function(s) from calibration. Although a laboratory lamp (or reference) calibration can provide all the information needed to convert the instrument readings to actual radiation, in situ calibration methods are implemented routinely (daily) to fill the gaps between lamp calibrations. In this paper, the basic structure and the data collection and pretreatment of the MFRSR are described. The laboratory lamp calibration and its limitations are summarized. The cloud screening algorithms for MFRSR data are presented. The in situ calibration methods, the standard Langley method and its variants, the ratio-Langley method, the general method, Alexandrov’s comprehensive method, and Chen’s multi-channel method, are outlined. The reason that all these methods do not fit for all situations is that they assume some properties, such as aerosol optical depth (AOD), total optical depth (TOD), precipitable water vapor (PWV), effective size of aerosol particles, or angstrom coefficient, are invariant over time. These properties are not universal and some of them rarely happen. In practice, daily calibration factors derived from these methods should be smoothed to restrain error.

Climatology of aerosol optical properties at ACRF sites in the tropical warm pool region

AbstractThe long‐term multifilter rotating shadowband radiometer measurements at three Atmospheric Radiation Measurements Climate Research Facility sites of Darwin, Nauru, and Manus have been processed to develop the climatology of aerosols in the tropical warm pool region at the interannual, seasonal, and diurnal temporal scales. Due to their unique geolocations and associated large‐scale circulation patterns, aerosols at the Nauru site exhibit background oceanic characteristics (strongly correlated with the sea surface wind), aerosols at the Darwin site show strong influences by biomass‐burning aerosols, particularly in the dry season, and aerosols at the Manus site have climatologic characteristics in between the Darwin and Nauru sites. There are no obvious trends of aerosol loading for past decades at all three sites. El Niño/Southern Oscillation has its impacts on aerosol optical depth, as well as particle size and composition, at all three sites. Madden‐Julian Oscillation modulates aerosol optical depth at the Manus and Nauru sites along the equator but has no apparent impact at the Darwin site. The annual or seasonal variation of aerosols is closely linked with Indo‐Australian monsoons, exhibiting wet and dry season differences. The aerosol loading is significant lower with relatively larger particles in the wet season than in the dry season. There are significant diurnal cycles in both aerosol optical depth and Angstrom exponent at the Darwin site: low values of aerosol optical depth and Angstrom exponent in the midday and the two peaks in the early morning and late afternoon. There are noticeable changes between the dry and wet seasons. The amplitude of diurnal variation during La Niña periods is greater than that during El Niño periods. However, there are no significant diurnal variations of aerosol loading at the Manus and Nauru sites.

Retrievals of vertical profiles of stratus cloud properties from combined oxygen A‐band and radar observations

A synergetic algorithm of retrieving vertical distribution of cloud drop effective radius (Re), liquid water content (LWC), and optical depth has been developed by combining oxygen A‐band spectral measurements with radar reflectivity. The underlying physics is to utilize the photon path length information from oxygen A‐band measurements to constrain the retrievals of cloud microphysical properties from radar reflectivity. Through a radiation closure of the rotating shadowband spectrometer (RSS) diffuse‐horizontal irradiances in the oxygen A‐band (which is sensitive to vertical distribution of the 2nd moment of particle size distribution), vertical distributions of cloud microphysical property (the 3rd moment of particle size distribution) are optimized and retrieved from Millimeter Wave Cloud Radar (MMCR) reflectivity (the 6th moment of particle size distribution). The critical issues in the synergetic cloud retrievals of combining passive and active instruments are discussed and carefully taken into account. Evaluation and validation through case studies show that the retrieved column mean values of cloud optical depth, effective radius, and liquid water path agree well with independent measurements from the microwave radiometer (MWR) and independent retrievals from a combined algorithm of multifilter rotating shadowband radiometer (MFRSR) and MWR and the column physical characterization (CPC) of MMCR. These studies demonstrate that this active‐passive synergetic retrieval algorithm is feasible and accurate for retrievals of vertical distribution of cloud Re and LWC for stratus clouds.