Roger Saunders

The radiometric characterization of AMSU-B

The Advanced Microwave Sounding Unit, AMSU, is being developed to fly on the new generation of NOAA polar orbiters due to be launched in the latter half of the 1990s. The UK Meteorological Office (UKMO) are procuring the high frequency component of AMSU (AMSU-B) with five channels in the range 88-191 GHz. In order to determine the radiometric performance and verify the method for calibration of AMSU-B an extensive series of tests have been performed by the UKMO on the engineering and three flight models. The instruments were placed in a 3 m thermal-vacuum chamber where their temperature could he controlled over the full range expected in orbit and an Earth target and a space target could be viewed. For the first flight model the measured Ne/spl Delta/T values were all >

Fast radiative transfer model for simulation of infrared atmospheric sounding interferometer radiances.

A fast radiative transfer model has been developed for prelaunch simulation studies of Infrared Atmospheric Sounding Interferometer (IASI) data and for the exploitation of IASI radiances within the framework of a numerical weather prediction variational analysis scheme. The model uses profile-dependent predictors to parameterize the atmospheric optical depths and is fast enough to cope with the processing of observations in near real time and with the several thousands of transmittance calculations required to simulate radiances from a full range of atmospheric conditions. The development of the model has involved the selection of a training set of atmospheric profiles, the production of a line-by-line transmittance database, the selection of optimal predictors for the gases considered in the study, and the production of regression coefficients for the fast transmittance scheme. The model fit to the line-by-line radiances shows that it can reproduce the line-by-line radiances to a degree of accuracy that is at or below the instrumental noise.

A 20 year independent record of sea surface temperature for climate from Along Track Scanning Radiometers

A new record of sea surface temperature (SST) for climate applications is described. This record provides independent corroboration of global variations estimated from SST measurements made in situ. Infra-red imagery from Along-Track Scanning Radiometers (ATSRs) is used to create a 20 year time series of SST at 0.1deg latitude- longitude resolution, in the ATSR Reprocessing for Climate (ARC) project. A very high degree of independence of in situ measurements is achieved via physics-based techniques. Skin SST and SST estimated for 20 cm depth are provided, with grid cell uncertainty estimates. Comparison with in situ datasets establishes that ARC SSTs generally have bias of order 0.1 K or smaller. The precision of the ARC SSTs is 0.14 K during 2003 to 2009, from three-way error analysis. Over the period 1994 to 2010, ARC SSTs are stable, with better than 95% confidence, to within 0.005 K/yr (demonstrated for tropical regions). The dataset appears useful for cleanly quantifying inter-annual variability in SST and major SST anomalies. The ARC SST global anomaly time series is compared to the in situ-based Hadley Centre SST dataset version 3 (HadSST3). Within known uncertainties in bias adjustments applied to in situ measurements, the independent ARC record and HadSST3 present the same variations in global marine temperature since 1996. Since the in situ observing system evolved significantly in its mix of measurement platforms and techniques over this period, ARC SSTs provide an important corroboration that HadSST3 accurately represents recent variability and change in this essential climate variable.

Simulated volcanic ash imagery: A method to compare NAME ash concentration forecasts with SEVIRI imagery for the Eyjafjallajökull eruption in 2010

[1] During volcanic eruptions that eject ash into the atmosphere Volcanic Ash Advisory Centers issue statements on the forecast dispersion of the ash so that the aviation industry can manage airspace to avoid aircraft encountering volcanic ash. Observations, such as those from satellites, are compared with the forecasts from an atmospheric dispersion model to assess the quality of the ash forecasts. A method has been developed to enable like-with-like comparison between satellite imagery of volcanic ash and simulated imagery using the forecast ash concentration data from an atmospheric dispersion model. The ash concentration and numerical weather prediction data are used as inputs to a radiative transfer model to simulate radiances. Simulated satellite images are created from these simulated radiances. Here, Spinning Enhanced Visible and Infrared Imager volcanic ash images based on infrared brightness temperatures for the Eyjafjallajokull eruption in 2010 are simulated. In addition to providing a useful tool for forecasters in a Volcanic Ash Advisory Center, the simulated images can be used to aid the understanding of how the ash affects the satellite imagery and also the physical properties of the ash.

Stratospheric temperature changes during the satellite era

Satellite-based layer average stratospheric temperature (T) climate data records (CDRs) now span more than three decades and so can elucidate climate variability associated with processes on multiple time scales. We intercompare and analyze available published T CDRs covering at least two decades, with a focus on Stratospheric Sounding Unit (SSU) and Microwave Sounding Unit (MSU) CDRs. Recent research has reduced but not eliminated discrepancies between SSU CDRs developed by NOAA and the UK Meteorological Office. The MSU CDRs from NOAA and Remote Sensing Systems are in closer agreement than the CDR from the University of Alabama in Huntsville. The latter has a previously unreported inhomogeneity in 2005, revealed by an abrupt increase in the magnitude and spatial variability of T anomaly differences between CDRs. Although time-varying biases remain in both SSU and MSU CDRs, multiple linear regression analyses reveal consistent solar, El Nino–Southern Oscillation (ENSO), quasi-biennial oscillation, aerosol, and piecewise-linear trend signals. Together, these predictors explain 80 to 90% of the variance in the near-global-average T CDRs. The most important predictor variables (in terms of percent explained variance in near-global-average T) for lower stratospheric T measured by MSU are aerosols, solar variability, and ENSO. Trends explain the largest percentage of variance in observations from all three SSU channels. In MSU and SSU CDRs, piecewise-linear trends, with a 1995 break point, indicate cooling during 1979–1994 but no trend during 1995–2013 for MSU and during 1995–2005 for SSU. These observational findings provide a basis for evaluating climate model simulations of stratospheric temperature during the past 35 years.

Measurements of the AMSU-B antenna pattern

Measurements of the antenna patterns for the high frequency component of the Advanced Microwave Sounding Unit (AMSU-B), to fly on the NOAA-KLM polar orbiting satellites, are reported. They were made on the Compact Antenna Test Range at Queen Mary and Westfield College, London, at three frequencies within the AMSU-B receivers channels, centered at 89, 150, and 183.31 GHz. The main reflector of AMSU-Bs antenna is an offset parabola with a 219-mm diameter aperture. This paper describes the measurements that were made and presents the results of an analysis of them for the three AMSU-B flight models. Measurements of beamwidths, main beam efficiencies, and sensitivity to cross-polarization are all reported. These data are then used to compute simulated antenna temperatures for the space and Earth views and recommendations for correcting the space view and Earth view data due to antenna effects are proposed. The implications for the operational radiometric calibration of AMSU-B are also discussed.

Atmospheric transmittances for the AVHRR channels

Vertical path atmospheric transmittances from the surface to the top of the atmosphere have been computed for the five channels of the Advanced Very High Resolution Radiometer (AVHRR). Gaseous absorption computed from a line-by-line transmittance model and both molecular and aerosol scattering are included. Contributions from all gases with any significant absorption in each channel are listed together with transmittance spectra for the more important absorbers in each channel. The transmittance in channels 1 and 2 is dominated by aerosol scattering which is dependent on both aerosol type and concentration. For channel 3 water vapor line absorption is the principal component with water vapor continuum absorption and aerosol scattering having a smaller but significant affect. The absorption due to other minor atmospheric constituents (i.e. CH(4), N(2)O and N(2)) is also significant in this channel. For channels 4 and 5 water vapor continuum absorption dominates. Other atmospheric constituents having a significant absorption in these channels are CO(2), HNO(3), and the chlorofluorocarbons (CFCs) F11 and F12. The latter two only have a small effect on both channels at present but as the concentration of CFCs is increasing at ~5%/year this will increase significantly with time.

Monitoring Satellite Radiance Biases Using NWP Models

Radiances measured by satellite radiometers are often subject to biases due to limitations in their radiometric calibration. In support of the Global Space-based Inter-Calibration System project, to improve the quality of calibrated radiances from atmospheric sounders and imaging radiometers, an activity is underway to compare routinely measured radiances with those simulated from operational global numerical weather prediction (NWP) fields. This paper describes the results obtained from the first three years of these comparisons. Data from the High-resolution Infrared Radiation Sounder, Spinning Enhanced Visible and Infrared Imager, Advanced Along-Track Scanning Radiometer, Advanced Microwave Sounding Unit, and Microwave Humidity Sounder radiometers, together with the Atmospheric Infrared Sounder, a spectrometer, and the Infrared Atmospheric Sounding Interferometer, an interferometer, were included in the analysis. Changes in mean biases and their standard deviations were used to investigate the temporal stability of the bias and radiometric noise of the instruments. A double difference technique can be employed to remove the effect of changes or deficiencies in the NWP model which can contribute to the biases. The variation of the biases with other variables is also investigated, such as scene temperature, scan angle, location, and time of day. Many of the instruments were shown to be stable in time, with a few exceptions, but measurements from the same instrument on different platforms are often biased with respect to each other. The limitations of the polar simultaneous nadir overpasses often used to monitor biases between polar-orbiting sensors are shown with these results due to the apparent strong dependence of some radiance biases on scene temperature.

Mineral dust aerosol net direct radiative effect during GERBILS field campaign period derived from SEVIRI and GERB

Colocated Spinning Enhanced Visible and Infrared Imager (SEVIRI) retrieved dust optical depths at 0.55 microns, τ055, and Geostationary Earth Radiation Budget (GERB) fluxes at the top of atmosphere are used to provide, for the first time, an observationally based estimate of the cloud-free net direct radiative effect (DRE) of mineral dust aerosol from geostationary satellite observations, providing new insights into the influence of time of day on the magnitude and sign of the shortwave, longwave, and overall net effect during sunlit hours. Focusing on the Geostationary Earth Radiation Budget Intercomparison of Longwave and Shortwave radiation (GERBILS) campaign over North Africa during June 2007, the presence of mineral dust aerosol reduces the outgoing longwave radiation at all times of day with the peak reduction clearly following the diurnal cycle of surface temperature. The instantaneous shortwave DRE shows strong dependencies on pristine sky albedo and solar zenith angle such that the same dust loading can induce a positive or negative value dependent on time of day. However, the area mean net DRE over the GERBILS period is dominated by the longwave component at all sampled times of day, with mineral dust inducing a reduction in outgoing net flux of the order of 10W m−2. Hence, in the mean sense, Saharan dust is found to warm the Earth-atmosphere system over northern Africa and the Middle East.

Validation of the ATSR Reprocessing for Climate (ARC) Dataset Using Data from Drifting Buoys and a Three-Way Error Analysis

AbstractThe Along-Track Scanning Radiometer (ATSR) Reprocessing for Climate (ARC) project aims to create an independent climate data record of sea surface temperatures (SSTs) covering recent decades that can be used for climate change analysis. Here, the ARC SSTs are assessed using comparisons with collocated drifting buoy observations and a three-way error analysis that also includes Advanced Microwave Scanning Radiometer for Earth Observing System (AMSR-E) data. The SSTs using the three-channel nighttime retrievals in the ARC data at 1-m depth are found to have a warm bias of 0.054 K (standard deviation 0.151 K) with respect to the drifting buoy data for the 1995–2009 time period using ATSR-2 and Advanced Along-Track Scanning Radiometer (AATSR) instrument data. However, when studying the two-channel retrievals, the ATSR-1 data are found to be less stable and with more extreme values than in later years. Some dependence on latitude, season, and fields such as total column water vapor is found in the ATSR...