Advanced Very High Resolution Radiometer Radiance Research Articles

Direct assimilation of AVHRR satellite radiance measurements in a reanalysis system

AbstractThe Advanced Very‐High‐Resolution Radiometer (AVHRR) is a broad‐band, five‐channel scanner sensing in the visible, near‐infrared and thermal infrared portions of the electromagnetic spectrum. AVHRR instruments onboard polar‐orbiting satellites have data records spanning 30 years. The radiances from the infrared channels of AVHRR have been directly assimilated over the ice‐free ocean in the NASA Goddard Earth Observing System (GEOS) since 2017 to constrain skin sea surface temperature (SST). The GEOS system already uses an advanced bulk SST and a skin SST model which incorporates diurnal warming and cool‐skin temperature on the surface of the ocean. The positive contribution of this effort to the Numerical Weather Prediction (NWP) makes it desirable to extend the skin SST data assimilation procedure to reanalysis applications. The AVHRR data from platforms that were previously unexplored for NWP applications are assimilated at the Global Modeling and Assimilation Office (GMAO) at NASA, for an upcoming reanalysis project. This study uses results from reanalysis experiments to assess the impact of assimilating AVHRR radiances over open ocean on the atmospheric state variables, focusing on the analyzed skin temperatures that include the active skin SST model. It is demonstrated that including the skin temperature variations in atmospheric analysis is generally beneficial. The addition of radiance measurements from AVHRR provides further improvements to the overall reanalysis system performance. by helping to correct the diurnal heating and reducing errors in the background departure of hyperspectral radiance observations, which are an essential component of atmospheric reanalyses.

Extension and statistical analysis of the GACP aerosol optical thickness record

The primary product of the Global Aerosol Climatology Project (GACP) is a continuous record of the aerosol optical thickness (AOT) over the oceans. It is based on channel-1 and -2 radiance data from the Advanced Very High Resolution Radiometer (AVHRR) instruments flown on successive National Oceanic and Atmospheric Administration (NOAA) platforms. We extend the previous GACP dataset by four years through the end of 2009 using NOAA-17 and -18 AVHRR radiances recalibrated against MODerate resolution Imaging Spectroradiometer (MODIS) radiance data, thereby making the GACP record almost three decades long. The temporal overlap of over three years of the new NOAA-17 and the previous NOAA-16 record reveals an excellent agreement of the corresponding global monthly mean AOT values, thereby confirming the robustness of the vicarious radiance calibration used in the original GACP product. The temporal overlap of the NOAA-17 and -18 instruments is used to introduce a small additive adjustment to the channel-2 calibration of the latter resulting in a consistent record with increased data density. The Principal Component Analysis (PCA) of the newly extended GACP record shows that most of the volcanic AOT variability can be isolated into one mode responsible for ~12% of the total variance. This conclusion is confirmed by a combined PCA analysis of the GACP, MODIS, and Multi-angle Imaging SpectroRadiometer (MISR) AOTs during the volcano-free period from February 2000 to December 2009. We show that the modes responsible for the tropospheric AOT variability in the three datasets agree well in terms of correlation and spatial patterns. A previously identified negative AOT trend which started in the late 1980s and continued into the early 2000s is confirmed. Its magnitude and duration indicate that it was caused by changes in tropospheric aerosols. The latest multi-satellite segment of the GACP record shows that this trend tapered off, with no noticeable AOT change after 2002. This result is consistent with the MODIS and MISR AOT records as well as with the recent gradual reversal from brightening to dimming revealed by surface flux measurements in many aerosol producing regions. Thus the robustness of the GACP record is confirmed, increasing our confidence in the validity of the negative trend. Although the nominal negative GACP AOT trend could partially be an artifact of increasing aerosol absorption, we argue that the time dependence of the GACP record, including the latest flat period, is more consistent with the actual decrease in the tropospheric AOT.

Retrieval of aerosol optical depth over land surfaces from AVHRR data

Abstract. The Advanced Very High Resolution Radiometer (AVHRR) provides a global, long-term, consistent time series of radiance data in several wavebands which are used for the retrieval of surface spectral reflectance, albedo and surface temperature. Long-term time series of such data products are necessary for studies addressing climate change, sea ice distribution and movement, and ice sheet coastal configuration. AVHRR radiances have also been used to retrieve aerosol properties over ocean and land surfaces. However, the retrieval of aerosol over land is challenging because of the limited information content in the data which renders the inversion problem ill defined. Solving the radiative transfer equations requires additional information to reduce the number of unknowns. In this contribution we utilise an empirical linear relationship between the surface reflectances in the AVHRR channels at wavelengths of 3.75 μm and 2.1 μm, which has been identified in the Moderate Resolution Imaging Spectroradiometer (MODIS) data. Following the MODIS dark target approach, this relationship is used to obtain the surface reflectance at 0.64 μm. The comparison of the estimated surface reflectances with MODIS reflectance products (MOD09) shows a strong correlation. Once this was established, the MODIS "dark-target" aerosol retrieval method was adapted to AVHRR data. A simplified look-up table (LUT) method, adopted from the Bremen AErosol Retrieval (BAER) algorithm, was used in the retrieval. The aerosol optical depth (AOD) values retrieved from AVHRR with this method compare favourably with ground-based measurements, with 71.8% of the points located within ±(0.1 + 0.15τ) (τ is the AOD) of the identity line. This method can be easily applied to other satellite instruments which do not have a 2.1 μm channel, such as those currently planned to be used on geostationary satellites.

Post calibration of channels 1 and 2 of long-term AVHRR data record based on SeaWiFS data and pseudo-invariant targets

Advanced very high resolution radiometer (AVHRR) data provide a global, long-term, consistent time series with favorable spectral and spatial resolution. Such measurements are necessary for climate change studies. However, significant problems with the AVHRR calibration have been seen in two separate sources of early AVHRR data — lack onboard calibration and observed trends and biases when the AVHRR radiances are compared with other accurate top-of-atmosphere (TOA) radiances sources. In this paper, new calibration coefficients for channels 1 and 2 of the AVHRR sensors onboard 16 NOAA satellites from TIROS-N, NOAA-6 to NOAA-19, and Metop-A, B are reported. The calibration slopes are derived based on several pseudo-invariant sites endorsed by the Committee on Earth Observation Satellites (CEOS). Sea-Viewing Wide Field-of-View Sensor (SeaWiFS) channels 6 and 8 radiance data are applied in characterizing the bidirectional reflectance distribution functions (BRDF) of the TOA reflectance over the employed targets, and MODTRAN simulations are carried out to parameterize the spectral channel adjustment factors (SBAFs) between SeaWiFS and each AVHRR sensor to eliminate the spectral response differences over different targets considering the impacts of observing geometries and gaseous absorption. The BRDFs and SBAFs are then applied to filtered AVHRR time-series to generate time series of calibration slopes presented in the form of quadratic polynomials as a function of time and applied in the recalibration of AVHRR data records. Comparisons with calibration coefficients generated from other products exhibit preferable validity and consistency, and notable incoherence and trend in the TOA reflectance time series of the L1B AVHRR data over each site from all the launched NOAA series polar orbiting satellites are largely eliminated after the recalibration. The long-term and inter-sensor consistent AVHRR data records after this post calibration can provide valuable insight into global climate changes since 1978.

Multi-Sensor Calibration Studies of AVHRR-Heritage Channel Radiances Using the Simultaneous Nadir Observation Approach

The European Space Agency project for studies of cloud properties in the Climate Change Initiative programme (ESA-CLOUD-CCI) aims at compiling the longest possible time series of cloud products from one single multispectral sensor—The five-channel Advanced Very High Resolution Radiometer (AVHRR) instrument. A particular aspect here is to include corresponding products based on other existing (Moderate Resolution Imaging Spectroradiometer (MODIS), Advanced Along-Track Scanning Radiometer (AATSR), MEdium Resolution Imaging Spectrometer (MERIS), Visible and Infrared Radiometer Suite (VIIRS)) and future Sea and Land Surface Temperature Radiometer (SLSTR) sensors measuring in similar (AVHRR-heritage) spectral channels. Initial inter-comparisons of the involved AVHRR-heritage channel radiances over a short demonstration period (2007–2009) were performed. Using Aqua-MODIS as reference, AVHRR (NOAA-18), AATSR, and MERIS channel radiances were evaluated using the simultaneous nadir (SNO) approach. Results show generally agreeing radiances within approximately 3% for channels at 0.6 µm and 0.8 µm. Larger deviations (+5%) were found for the corresponding AATSR channel at 0.6 µm. Excessive deviations but with opposite sign were also indicated for AATSR 1.6 µm and MERIS 0.8 µm radiances. Observed differences may largely be attributed to residual temporal and spatial matching differences while excessive AATSR and MERIS deviations are likely partly attributed to incomplete compensation for spectrally varying surface and atmospheric conditions. However, very good agreement was found for all infrared channels among all the studied sensors. Here, deviations were generally less than 0.2% for the measured brightness temperatures with the exception of some remaining non-linear deviations at extreme low and high temperatures.

CLARA-A1: a cloud, albedo, and radiation dataset from 28 yr of global AVHRR data

Abstract. A new satellite-derived climate dataset – denoted CLARA-A1 ("The CM SAF cLoud, Albedo and RAdiation dataset from AVHRR data") – is described. The dataset covers the 28 yr period from 1982 until 2009 and consists of cloud, surface albedo, and radiation budget products derived from the AVHRR (Advanced Very High Resolution Radiometer) sensor carried by polar-orbiting operational meteorological satellites. Its content, anticipated accuracies, limitations, and potential applications are described. The dataset is produced by the EUMETSAT Climate Monitoring Satellite Application Facility (CM SAF) project. The dataset has its strengths in the long duration, its foundation upon a homogenized AVHRR radiance data record, and in some unique features, e.g. the availability of 28 yr of summer surface albedo and cloudiness parameters over the polar regions. Quality characteristics are also well investigated and particularly useful results can be found over the tropics, mid to high latitudes and over nearly all oceanic areas. Being the first CM SAF dataset of its kind, an intensive evaluation of the quality of the datasets was performed and major findings with regard to merits and shortcomings of the datasets are reported. However, the CM SAF's long-term commitment to perform two additional reprocessing events within the time frame 2013–2018 will allow proper handling of limitations as well as upgrading the dataset with new features (e.g. uncertainty estimates) and extension of the temporal coverage.

A Physical Method for the Calibration of the AVHRR/3 Thermal IR Channels. Part II: An In-Orbit Comparison of the AVHRR Longwave Thermal IR Channels on board MetOp-A with IASI

Abstract Obtaining stable and accurate satellite radiances for climate change research requires extremely high standards for satellite calibration. Many satellite sensors do not currently meet the accuracy criteria, especially heritage sensors such as the Advanced Very High Resolution Radiometer (AVHRR), which shows scene temperature–dependent trends and biases of up to 0.5 K. Recently, however, a detailed study of the AVHRR/3 prelaunch data showed significant problems with both the calibration algorithm and the prelaunch data and indicated that the inherent accuracy of the AVHRR may actually be quite high. A new approach has been suggested that fixed many of the issues with the current (operational) calibration, but has not yet been applied to the in-orbit case. In this paper the behavior of the AVHRR in orbit is examined and compared to the operational AVHRR radiances from the Meteorological Operation (MetOp)-A with those based on the new calibration to radiances derived from the Infrared Atmosphere Sounding Instrument (IASI). It is shown that the current AVHRR calibration does indeed introduce large (0.5 K) biases, but these biases are remarkably stable. It is further shown that, with some modification related to differences between the prelaunch test environment and the in-orbit environment, a physically based AVHRR calibration can match IASI to better than ~0.05 K, which is an order of magnitude better than what is currently available. Finally, it is shown that, while the new calibration is capable of providing accurate and stable radiances for the nadir view, off-nadir biases of up to 1.5 K still exist at the largest zenith angles and at the coldest scene temperatures (~210 K). For surface temperature determination, however, the scan angle bias is very small (<0.02 K), implying that the new AVHRR calibration will provide a significant improvement to, for example, sea surface temperature measurements, one of the Global Climate Observing System (GCOS)-designated essential climate variables.

Implementation of the Community Radiative Transfer Model in Advanced Clear‐Sky Processor for Oceans and validation against nighttime AVHRR radiances

The fast Community Radiative Transfer Model (CRTM) has been integrated into National Environmental Satellite Data and Information Service's newly developed Advanced Clear‐Sky Processor for Oceans (ACSPO). CRTM is used in conjunction with the National Centers for Environmental Prediction (NCEP) Global Forecast System atmospheric profiles and Reynolds weekly version 2 sea surface temperatures (SST) to simulate clear‐sky brightness temperatures (BT). Model BTs are used to improve the ACSPO clear‐sky mask, monitor quality of advanced very high resolution radiometer (AVHRR) BTs, and explore physical SST retrievals. This paper documents CRTM implementation in ACSPO version 1 and evaluates nighttime “model minus observation” (M‐O) BT biases in three bands (3.7, 11, and 12 μm) of four AVHRR/3 instruments onboard NOAA‐16, NOAA‐17, NOAA‐18, and MetOp‐A. With careful treatment of input atmospheric and SST data, the agreement is generally good, showing only weak dependencies of M‐O biases on view zenith angle, column water vapor, and wind speed. The agreement improves if Reynolds weekly SST is used instead of NCEP SST. Including surface reflection also reduces the M‐O bias. After all optimizations, the M‐O biases are within several tenths of a Kelvin. Consistency between different platforms is ∼0.1K, except for NOAA‐16 channel 3B, which is biased low compared to other platforms by ∼0.4K. Our future plans include extending the analyses to daytime data and exploring physical SST retrievals. A web‐based tool is being established to continuously monitor the M‐O biases and physical SSTs. The validation methodology employed in this paper will be used to quantitatively measure the effect of each improvement on the M‐O bias and physical SST.

Aerosol retrievals from AVHRR radiances: effects of particle nonsphericity and absorption and an updated long-term global climatology of aerosol properties

The paper describes and discusses long-term global retrievals of aerosol properties from channel-1 and -2 Advanced Very High-Resolution Radiometer (AVHRR) radiances. We reconfirm the previously reached conclusion that the nonsphericity of dust-like and dry sea salt aerosols can lead to very large errors in the retrieved optical thickness if one mistakenly applies the scattering model for spherical particles. Comparisons of single-scattering albedo and A ̊ ngström exponent values retrieved from the AVHRR data and those measured in situ at Sable Island indicate that the currently adopted value 0.003 can be a reasonable choice for the imaginary part of the aerosol refractive index in the global satellite retrievals. Several unexpected features in the long-term satellite record indicate a serious problem with post-launch calibration of channel-2 radiances from the NOAA-11 spacecraft. We solve this problem by using a simple re-calibration procedure removing the observed artifacts and derive a global climatology of aerosol optical thickness and size over the oceans for the period extending from July 1983 to December 1999. The global monthly mean optical thickness and A ̊ ngström exponent of tropospheric aerosols show no significant trends over the entire period and oscillate around the average values 0.145 and 0.75, respectively. The Northern Hemisphere mean optical thickness systematically exceeds that averaged over the Southern Hemisphere. The AVHRR retrieval results during the period affected by the Mt. Pinatubo eruption are consistent with the retrievals of the stratospheric aerosol optical thickness based on Stratospheric Aerosol and Gas Experiment Data (SAGE). Time series of the aerosol optical thickness and A ̊ ngström exponent derived for four separate geographic regions exhibit varying degrees of seasonal variability controlled by local meteorological events and/or anthropogenic activities.

Global Two-Channel AVHRR Retrievals of Aerosol Properties over the Ocean for the Period ofNOAA-9Observations and Preliminary Retrievals UsingNOAA-7andNOAA-11Data

Abstract Described is an improved algorithm that uses channel 1 and 2 radiances of the Advanced Very High Resolution Radiometer (AVHRR) to retrieve the aerosol optical thickness and Angstrom exponent over the ocean. Specifically discussed are recent changes in the algorithm as well as the results of a sensitivity study analyzing the effect of several sources of retrieval errors not addressed previously. Uncertainties in the AVHRR radiance calibration (particularly in the deep-space count value) may be among the major factors potentially limiting the retrieval accuracy. A change by one digital count may lead to a 50% change in the aerosol optical thickness and a change of 0.4 in the Angstrom exponent. On the other hand, the performance of two-channel algorithms weakly depends on a specific choice of the aerosol size distribution function with less than 10% changes in the optical thickness resulting from replacing a power law with a bimodal modified lognormal distribution. The updated algorithm is applied t...

A Global Determination of Cloud Microphysics with AVHRR Remote Sensing

An algorithm is developed for determining the cloud optical thickness and effective particle radius simultaneously on a global scale using Advanced Very High Resolution Radiometer (AVHRR) multispectral radiance data. In the algorithm, the treatment of thermal radiation in Nakajima and Nakajima is improved by reformulating the thermal emission in the atmospheric layers. At the same time, the lookup table for thermal emission is parameterized in terms of the equivalent water vapor path in order to include the effect of various vertical water vapor profiles. The algorithm is applied to AVHRR radiance data corresponding to reported aircraft and balloon measurements of cloud microphysical parameters. A comparison shows a good agreement between in situ and satellite-retrieved values thus obtained. The algorithm is further applied to 4-month Global Area Coverage data of 1987 to generate global distributions of the cloud optical thickness and effective particle radius for every 0.5 83 0.58 box in a 2608‐608 latitudinal region. Similarities and differences in the global features of the effective particle radius and the optical thickness are found as compared with the previous studies.

A dynamic water vapor correction method for the retrieval of land surface temperatures from the advanced very high resolution radiometer

We present a method which permits retrievals of land surface temperatures (LSTs) from AVHRR (advanced very high resolution radiometer) radiances sensed through atmospheres which may contain a large and strongly varying water vapor content. This new method is an extension of the dynamic water vapor (DWV) algorithm which was designed to retrieve sea surface temperatures. The generalization to LST retrievals recognizes that in general, land emissivities are unknown, may be spectrally dependent, and are less than unity. Because the LST retrieval problem is inherently underconstrained (there are more unknowns than radiative transfer equations), some knowledge of surface emissivity is required in order to establish upper and lower bounds on surface temperature. We demonstrate our method by comparing DWV‐LST retrievals with point surface measurements made by a cluster of eight infrared thermometers (IRTs) deployed over a grasslands prairie site in eastern Kansas in July and August 1989; this deployment was part of the First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE). We find that several of the AVHRR images supplied on FIFE CD‐ROM contain navigation errors of ∼30 km, consistent with a misidentification of the Tuttle Reservoir ground control point. After correcting the navigation, we identified the IRT pixels and computed the bias and rms errors for a DWV‐IRT comparison. For night passes we obtained agreement to.+0.39±1.11 K, while for day passes the comparison yielded +4.09±3.10 K. The large daytime bias is probably the result of the IRT readings not being representative of the ∼1 km2‐scale areas sensed by AVHRR (the IRT views vegetation; the AVHRR field of view includes warmer, less well vegetated surfaces). Our results show that while a fixed‐coefficient, global split‐window algorithm does not perform well in the relatively moist FIFE atmosphere, it is quite feasible to use the DWV‐LST to derive a local split‐window algorithm whose coefficient is tuned on a per‐pass basis.

A chronology of the 1991 to 1993 Mount Etna eruption using advanced very high resolution radiometer data: Implications for real‐time thermal volcano monitoring

Between December 1991 and March 1993 a continuous effusive eruption at Mount Etna built a 7.6 km2 lava flow field. Flows extended to within 1 km of the town of Zafferana before a successful artificial diversion was carried out higher up the volcano. During this eruption the spaceborne advanced very high resolution radiometer (AVHRR) acquired 308 images on which the activity could be detected. Since these data can be freely and directly available, such coverage potentially allows regular, real‐time monitoring. Ground observations and a flow map that we produced using a SPOT image and electronic distance measurement allowed us to develop and test data extraction techniques. AVHRR radiance maps were consistent with known locations of surface activity. These documented the transition from channel to tube fed phases, the changing threat to Zafferana, and flow diversion. Quantitative analysis of the AVHRR data enabled estimation of active lava area, thermal flux, effusion rates, and total flow field volume. Our estimates for eruption rate and total flow field volume, 5.6 to 7.6 m3/s and 220×106 to 300×106 m3, respectively, are in agreement with published ground‐based estimates of 5.8 m3/s and 235×106 m3. These correlations demonstrate the high degree of confidence that can now be placed in interpretations of AVHRR time series for eruptions where ground‐based data are scanty.

An automated scheme for the removal of cloud contamination from AVHRR radiances over western Europe

Abstract Various cloud-detection schemes are applied to 1.1 km Advanced Very High Resolution Radiometer (AVHRR) day- and night-time data to determine an optimum automated scheme for deriving cloud-free radiances over both land and sea. A combination of the spatial coherence method at infrared wavelengths (11 μm) and dynamic visible threshold methods proved to be the most effective scheme for day-time use. Uniform thin cirrus (i.e. reflectance less than 15 per cent) was difficult to detect with all methods. Problems were also encountered over regions with a changing underlying surface type (e.g. coastal areas) where the automated scheme was not as effective as over uniform surfaces. At night a combination of the spatial coherence method and a scheme based on the differences in brightness temperatures between the three infrared channels at 37, II and 12 μm wavelength was successfully used. Results obtained by applying these algorithms to AVHRR data are presented and the different problems encountered with e...