Jerry Sullivan

Recalibration of microwave sounding unit for climate studies using simultaneous nadir overpasses

[1] The measurements from microwave sounding unit (MSU) on board different NOAA polar-orbiting satellites have been extensively used for detecting atmospheric temperature trend during the last several decades. However, temperature trends derived from these measurements are under significant debate, mostly caused by calibration errors. This study recalibrates the MSU channel 2 observations at level 0 using the postlaunch simultaneous nadir overpass (SNO) matchups and then provides a well-merged new MSU 1b data set for climate studies. The calibration algorithm consists of a dominant linear response of the MSU raw counts to the Earth-view radiance plus a smaller quadratic term. Uncertainties are represented by a constant offset and errors in the coefficient for the nonlinear quadratic term. A SNO matchup data set for nadir pixels with criteria of simultaneity of less than 100 s and within a ground distance of 111 km is generated for all overlaps of NOAA satellites. The simultaneous nature of these matchups eliminates the impact of orbital drifts on the calibration. A radiance error model for the SNO pairs is developed and then used to determine the offsets and nonlinear coefficients through regressions of the SNO matchups. It is found that the SNO matchups can accurately determine the differences of the offsets as well as the nonlinear coefficients between satellite pairs, thus providing a strong constraint to link calibration coefficients of different satellites together. However, SNO matchups alone cannot determine the absolute values of the coefficients because there is a high degree of colinearity between satellite SNO observations. Absolute values of calibration coefficients are obtained through sensitivity experiments, in which the percentage of variance in the brightness temperature difference time series that can be explained by the warm target temperatures of overlapping satellites is a function of the calibration coefficient. By minimizing these percentages of variance for overlapping observations, a new set of calibration coefficients is obtained from the SNO regressions. These new coefficients are significantly different from the prelaunch calibration values, but they result in bias-free SNO matchups and near-zero contaminations by the warm target temperatures in terms of the calibrated brightness temperature. Applying the new calibration coefficients to the Level 0 MSU observations, a well-merged MSU pentad data set is generated for climate trend studies. To avoid errors caused by small SNO samplings between NOAA 10 and 9, observations only from and after NOAA 10 are used. In addition, only ocean averages are investigated so that diurnal cycle effect can be ignored. The global ocean-averaged intersatellite biases for the pentad data set are between 0.05 and 0.1 K, which is an order of magnitude smaller than that obtained when using the unadjusted calibration algorithm. The ocean-only anomaly trend for the combined MSU channel 2 brightness temperature is found to be 0.198 K decade -1 during 1987-2003.

Deriving an inter-sensor consistent calibration for the AVHRR solar reflectance data record

A new set of reflectance calibration coefficients has been derived for channel 1 (0.63 μm) and channel 2 (0.86 μm) of the Advanced Very High Resolution Radiometer (AVHRR) flown on the National Oceanic and Atmospheric Administration (NOAA) and European Organization for the Exploitation of Meteorological Satellites (EUMETSAT) polar orbiting meteorological satellites. This paper uses several approaches that are radiometrically tied to the observations from National Aeronautics and Space Administrations (NASAs) Moderate Resolution Imaging Spectroradiometer (MODIS) imager to make the first consistent set of AVHRR reflectance calibration coefficients for every AVHRR that has ever flown. Our results indicate that the calibration coefficients presented here provide an accuracy of approximately 2% for channel 1 and 3% for channel 2 relative to that from the MODIS sensor.

Development of global drought-watch system using NOAA/AVHRR data

Abstract Recently, NOAA/NESDIS developed the Vegetation Condition Index (VCI) which has proved to be a good indicator of drought. This paper provides a background of VCI development, data processing and outlines a PC-based system designed for early-warning drought diagnostics. There are several examples showing the application of VCI during recent years for detecting and tracking droughts.

Intersatellite Radiance Biases for the High-Resolution Infrared Radiation Sounders (HIRS) on board NOAA-15, -16, and -17 from Simultaneous Nadir Observations

Abstract Intersatellite radiance comparisons for the 19 infrared channels of the High-Resolution Infrared Radiation Sounders (HIRS) on board NOAA-15, -16, and -17 are performed with simultaneous nadir observations at the orbital intersections of the satellites in the polar regions, where each pair of the HIRS views the same earth target within a few seconds. Analysis of such datasets from 2000 to 2003 reveals unambiguous intersatellite radiance differences as well as calibration anomalies. The results show that in general, the intersatellite relative biases are less than 0.5 K for most HIRS channels. The large biases in different channels differ in both magnitude and sign, and are likely to be caused by the differences and measurement uncertainties in the HIRS spectral response functions. The seasonal bias variation in the stratosphere channels is found to be highly correlated with the lapse rate factor approximated by the channel radiance differences. The method presented in this study works particularly...

Corrections for detector nonlinearities and calibration inconsistencies of the infrared channels of the advanced very high resolution radiometer

The data received from the thermal infrared channel 4 (∼10.3–11.3 μm) and channel 5 (∼11.5–12.5 μm) of the advanced very high resolution radiometer (AVHRR) flown on the National Oceanic and Atmospheric Administration (NOAA) Polar-Orbiting Operational Environmental Satellites provide only a linear estimate of the actual radiance. We describe here a simple procedure, which incorporates data from prelaunch calibration tests, to correct the linear estimates for the nonlinear response of the channels 4 and 5 Mercury-Cadmium-Telluride (Hg-Cd-Te) sensors. The procedure applies a “nonzero radiance of space” concept to specify the form of the linear radiance estimate. This linear radiance is nearly independent of the operating temperature of the AVHRR and is the sole input to the correction algorithm. Additionally, it is demonstrated with NOAA 14 data that this calibration procedure resolves discrepancies found in the prelaunch data which can affect the calibration accuracy of channel 3 (∼3.55–3.95 μm), which possesses a linear response, as well as channels 4 and 5. When applied to independent sets of prelaunch calibration data, this procedure reproduces the laboratory-measured temperature data to within an accuracy of 0.1°–0.2°K. Comparison with nonlinearity corrections based on different procedures points to the superior applicability of the present results over the entire range of Earth-scene temperatures measured by the AVHRR in orbit. This accuracy is particularly important when these three infrared channels are used in multichannel algorithms to generate environmental parameters such as sea surface temperature. The algorithm coefficients and values of the nonzero radiance of space required to calculate the nonlinearity radiance corrections are given for the AVHRRs on NOAA 7, 9, 10, 11, 12, and 14 spacecraft.

AVHRR-based vegetation and temperature condition indices for drought detection in Argentina

Abstract The AVHRR-based Vegetation Condition Index (VCI) and Temperature Condition Index (TCI) have been developed and successfully used for monitoring drought in the USA., the Former Soviet Union, Zimbabwe, and China. This research was designed to apply and validate those indices for drought detection and impact assessment on agricultural yields in Cordoba province of Argentina. Seventy one percent of corn yield variability was explained by the spectral indices averaged over January and February. The VCI and TCI were useful to assess the spatial characteristics, the duration and severity of drought, and were in a good agreement with precipitation patterns.

Solar contamination effects on the infrared channels of the advanced very high resolution radiometer (AVHRR)

Solar impingement on the advanced very high resolution radiometers (AVHRRs) near the terminator can contaminate the onboard radiometric calibration system and degrade AVHRR data. The solar contamination causes disagreement between the sensor-measured radiometric output of the onboard blackbody versus its bulk temperature measured by the platinum resistance thermometers. Despite the sun shield installed on the latest AVHRR unit, solar contamination can still contribute errors of more than 0.5 K in the 3.7 μm channel and 0.25 K in the long-wave infrared channels. The timescale and spectral characteristics of the contamination are analyzed to find the possible causes. In addition, stray light in Earth scenes is found whenever the radiometric calibration is contaminated. These effects occur as the spacecraft moves out of the shadow of the Earth at spacecraft sunrise. The intensity of the effects varies by orbit and season and is related to the solar zenith and azimuth angles at the spacecraft.

Calibration of visible and near-infrared channels of the NOAA-12 AVHRR using time series of observations over deserts

Time series of desert sites are used to derive a post-launch calibration of the visible (ch1—0.63 µm) and near-infrared (ch2—0.86 µm) channels on the NOAA-12 AVHRR. This work extends the techniques that have been applied to NOAA satellites in afternoon orbits to a satellite in a morning orbit. An analysis of the bidirectional reflectance distribution function effects apparent in the data was used to limit the time period used to analyse the calibration slope of ch1 and ch2. Three desert sites were used to compute the relative degradation rates of the calibration slope of ch1 and ch2. Measurements from NOAA-9 over the Libyan Desert and from aircraft flights over White Sands, New Mexico were used to produce an absolute calibration. The resulting absolute calibrations for ch1 and ch2 agree with previous results to within 2%. The degradation results indicated that the calibration slope decreases by 3.14% per year for ch1 and 3.19% per year for ch2.

New radiance-based method for AVHRR thermal channel nonlinearity corrections

Thermal channels 4 and 5 of the Advanced Very High Resolution Radiometer (AVHRR) on National Oceanic and Atmospheric Administration (NOAA) polar-orbiting satellites have an onboard calibration process that provides data from which incoming scene radiance is linearly related to AVHRR count output. However, prelaunch calibration tests show that the radiance is more accurately modelled as a quadratic in count value and that the actual quadratic fit depends upon the operating temperature of the AVHRR itself. NOAA has developed a new method to provide prelaunch information to operational data users that is both concise and accurate. It corrects the linear radiance estimate instead of correcting equivalent blackbody temperature values. The nonlinear correction to the linear radiance estimate is provided by a single quadratic equation, independent of the AVHRR temperature. The new method was first applied to the NOAA-14 AVHRR. When corrected radiance estimates for the NOAA-14 AVHRR are compared to precise prelau...

Post-launch calibration of meteorological satellite sensors

Abstract Vicarious calibration techniques which are used to characterize the post-launch performance of meteorological satellite sensors in the visible and near-infrared regions of the spectrum in the absence of onboard calibration devices are briefly reviewed. Their application is illustrated with examples drawn from the post-launch calibration of the visible (channel 1: ≈0.58–0.68μm) and near-infrared (channel 2: ≈0.72–1.1μm) of the Advanced Very High Resolution Radiometer (AVHRR), flown on NOAAs Polar-orbiting Operational Environmental Satellite (POES), and of the visible channel (≈0.52–0.72μm) of the imager flown on NOAAs Geostationary Operational Environmental Satellite (GOES). The basic elements of a calibration strategy to address the blended geophysical data needs of the future are presented.