44 research outputs found
On the absence of gravitational lensing of the cosmic microwave background
The magnification of distant sources by mass clumps at lower ()
redshifts is calculated analytically. The clumps are initially assumed to be
galaxy group isothermal spheres with properties inferred from an extensive
survey. The average effect, which includes strong lensing, is exactly
counteracted by the beam divergence in between clumps (more precisely, the
average reciprocal magnification cancels the inverse Dyer-Roeder
demagnification). This conclusion is in fact independent of the matter density
function within each clump, and remains valid for arbitrary densities of matter
and dark energy. When tested against the CMB, a rather large lensing induced
{\it dispersion} in the angular size of the primary acoustic peaks of the TT
power spectrum is inconsistent with WMAP observations. The situation is
unchanged by the use of NFW profiles for the density distribution of groups.
Finally, our formulae are applied to an ensemble of NFW mass clumps or
isothermal spheres having the parameters of galaxy {\it clusters}. The acoustic
peak size dispersion remains unobservably large, and is also excluded by WMAP.
For galaxy groups, two possible ways of reconciling with the data are proposed,
both exploiting maximally the uncertainties in our knowledge of group
properties. The same escape routes are not available in the case of clusters,
however, because their properties are well understood. Here we have a more
robust conclusion: neither of the widely accepted models are good description
of clusters, or important elements of physics responsible for shaping zero
curvature space are missing from the standard cosmological model. When all the
effects are accrued, it is difficult to understand how WMAP could reveal no
evidence whatsoever of lensing by groups and clusters.Comment: ApJ v628, pp. 583-593 (August 1, 2005
Harmonization of space-borne infra-red sensors measuring sea surface temperature
Sea surface temperature (SST) is observed by a constellation of sensors, and SST retrievals
are commonly combined into gridded SST analyses and climate data records (CDRs). Differential
biases between SSTs from different sensors cause errors in such products, including feature artefacts.
We introduce a new method for reducing differential biases across the SST constellation, by reconciling
the brightness temperature (BT) calibration and SST retrieval parameters between sensors. We use the
Advanced Along-Track Scanning Radiometer (AATSR) and the Sea and Land Surface Temperature
Radiometer (SLSTR) as reference sensors, and the Advanced Very High Resolution Radiometer
(AVHRR) of the MetOp-A mission to bridge the gap between these references. Observations across a
range of AVHRR zenith angles are matched with dual-view three-channel skin SST retrievals from
the AATSR and SLSTR. These skin SSTs act as the harmonization reference for AVHRR retrievals
by optimal estimation (OE). Parameters for the harmonized AVHRR OE are iteratively determined,
including BT bias corrections and observation error covariance matrices as functions of water-vapor
path. The OE SSTs obtained from AVHRR are shown to be closely consistent with the reference sensor
SSTs. Independent validation against drifting buoy SSTs shows that the AVHRR OE retrieval is stable
across the reference-sensor gap. We discuss that this method is suitable to improve consistency across
the whole constellation of SST sensors. The approach will help stabilize and reduce errors in future
SST CDRs, as well as having application to other domains of remote sensing
Comparisons of IASI-A and AATSR measurements of top-of-atmosphere radiance over an extended period
This study examines the trustworthiness of the Advanced Along-Track Scanning Radiometer (AATSR) and the Infrared Atmospheric Sounding Interferometer (IASI-A), as on-orbit reference instruments that are useful in re-calibrating the Advanced Very High Resolution Radiometer (AVHRR) series (Mittaz and Harris, 2011). To do this, a 39-month period (1 January 2008 to 31 March 2011) of AATSR and IASI-A inter-comparisons of top-of-atmosphere (TOA) radiance measurements is examined. Our inter-comparison reveals features of the AATSR and IASI-A bias with respect to scan angle, scene temperature, time and orbital maneuvers, and gives insight into their trustworthiness as an in-orbit reference instruments.
The first feature that our study reveals is that the AATSR (nadir view) and IASI-A are both stable (have no perceptible trends in the period of study). The second feature is that IASI-A is perhaps more accurate ( ∼  0.05 K) than its stated accuracy (0.5 K). In fact the AATSR and IASI-A bias is close to the AATSR pre-launch bias (plus a small offset of +0.07 K) implying that IASI-A can get close to pre-launch levels of accuracy. Third, a very small scan angular dependence of AATSR and IASI-A bias indicates that the IASI-A response vs. scan angle algorithm is robust, while the instrument is in orbit.
Inter-comparisons of AATSR with IASI-A further reveal the impact of orbital maneuvers of the ENVISAT, the platform carrying AATSR, done in October 2011 and not anticipated previously. Our study reveals that this maneuver introduced a temperature-dependent bias in the AATSR measurements for low temperatures (< 240 K) in the period following this maneuver (Cocevar et al., 2011). Our study also shows that the known AATSR 12 µm channel offset is in fact temperature dependent, grows up to 0.4 K, varies seasonally and is correlated with instrument temperature and cannot be corrected by shifting the spectral response function (SRF) of AATSR.
We also present a set of recommendations to help identify the parameters under which these instruments can provide the most trustworthy observations for the AVHRR re-calibration
Inter-calibration of HY-1B/COCTS thermal infrared channels with MetOp-A/IASI
The Chinese Ocean Color and Temperature Scanner (COCTS) on board the Haiyang-1B (HY-1B) satellite has two thermal infrared channels (9 and 10) centred near 11 μm and 12 μm respectively which are intended for sea surface temperature (SST) observations. In order to improve the accuracy of COCTS SSTs, the inter-calibration of COCTS thermal infrared radiance is carried out. The Infrared Atmospheric Sounding Interferometer (IASI) on board MetOp-A satellite is used as inter-calibration reference owing to its hyperspectral nature and high-quality measurements. The inter-calibration of HY-1B COCTS thermal infrared radiances with IASI is undertaken for data from the period 2009 to 2011 located in the northwest Pacific. Collocations of COCTS radiance with IASI are identified within a temporal window of 30 minutes, a spatial window of 0.12° and an atmospheric path tolerance of 3%. Matched IASI spectra are convolved with the COCTS spectral response functions, while COCTS pixels within the footprint of each IASI pixel are spatially averaged, thus creating matched IASI-COCTS radiance pairs that should agree well in the absence of satellite biases. The radiances of COCTS 11 and 12 μm channel are lower than IASI with relatively large biases, and a strong dependence of difference on radiance in the case of 11 μm channel. We use linear robust regression for different four detectors of COCTS separately to obtain the inter-calibration coefficients to correct the COCTS radiance. After correction, the mean values of COCTS 11 and 12 μm channel minus IASI radiance are -0.02 mW m-2 cm sr-1 and -0.01 mW m-2 cm sr-1 respectively, with corresponding standard deviations of 0.51 mW m-2 cm sr-1 and 0.57 mW m-2 cm sr-1. Striped noise is present in COCTS original radiance imagery associated with inconsistency between four detectors, and inter-calibration is shown to reduce, although not eliminate, the striping. The calibration accuracy of COCTS is improved after inter-calibration, that is potentially useful for improving COCTS SST accuracy in the future