Christopher J. Merchant

The Global Ocean Data Assimilation Experiment High-resolution Sea Surface Temperature Pilot Project

A new generation of integrated sea surface temperature (SST) data products are being provided by the Global Ocean Data Assimilation Experiment (GODAE) High-Resolution SST Pilot Project (GHRSST-PP). These combine in near-real time various SST data products from several different satellite sensors and in situ observations and maintain the fine spatial and temporal resolution needed by SST inputs to operational models. The practical realization of such an approach is complicated by the characteristic differences that exist between measurements of SST obtained from subsurface in-water sensors, and satellite microwave and satellite infrared radiometer systems. Furthermore, diurnal variability of SST within a 24-h period, manifested as both warm-layer and cool-skin deviations, introduces additional uncertainty for direct intercomparison between data sources and the implementation of data-merging strategies. The GHRSST-PP has developed and now operates an internationally distributed system that provides operatio...

Multi-satellite measurements of large diurnal warming events

[1] Diurnal warming events between 5 and 7 K, spatially coherent over large areas (1000 km), are observed in independent satellite measurements of ocean surface temperature. The majority of the large events occurred in the extra-tropics. Given sufficient heating (from solar radiation), the location and magnitude of these events appears to be primarily determined by large-scale wind patterns. The amplitude of the measured diurnal heating scales inversely with the spatial resolution of the different sensors used in this study. These results indicate that predictions of peak diurnal warming using wind speeds with a 25 km spatial resolution available from satellite sensors and those with 50–100 km resolution from Numerical Weather Prediction models may have underestimated warming. Thus, the use of these winds in modeling diurnal effects will be limited in accuracy by both the temporal and spatial resolution of the wind fields. Citation: Gentemann, C. L., P. J. Minnett, P. Le Borgne, and C. J. Merchant (2008), Multi-satellite measurements of large diurnal warming events, Geophys. Res. Lett., 35, L22602,

Diurnal warm‐layer events in the western Mediterranean and European shelf seas

We characterize near-surface ocean diurnal warm-layer events, using satellite observations and fields from numerical weather forecasting. The study covers April to September, 2006, over the area 11 degrees W to 17 degrees E and 35 degrees N to 57 degrees N, with 0.1 degrees cells. We use hourly satellite SSTs from which peak amplitudes of diurnal cycles in SST (dSSTs) can be estimated with error similar to 0.3 K. The diurnal excursions of SST observed are spatially and temporally coherent. The largest dSSTs exceed 6 K, affect 0.01% of the surface, and are seen in the Mediterranean, North and Irish Seas. There is an anti-correlation between the magnitude and the horizontal length scale of dSST events. Events wherein dSST exceeds 4 K have length scales of = 40 km. From the frequency distribution of different measures of wind-speed minima, we infer that extreme dSST maxima arise where conditions of low wind speed are sustained from early morning to mid afternoon.

Toward the elimination of bias in satellite retrievals of sea surface temperature: 1. Theory, modeling and interalgorithm comparison

The along-track scanning radiometer (ATSR), launched in July 1991 on ERS-1, is an infrared radiometer designed to permit retrieval of skin sea surface temperature (SST) to the accuracy required for many climate research purposes. Using the prelaunch retrieval scheme, this accuracy (0.3 K) was achieved only when observations at 3.7 μm were available, i.e., SSTs derived from nighttime scenes before the failure of this channel in May 1992. Retrievals using only channels at 11 and 12 μm suffered significant biases. First, cold biases of up to 1.5 K arose from the radiative effects of the unanticipated presence of a significant loading of stratospheric aerosol following the eruption of Mount Pinatubo in June 1991. Second, cold biases of up to 0.4 K were associated with regions of high water vapor loading. We solve the first problem by choosing retrieval coefficients to be orthogonal to the modeled changes in brightness temperatures caused by variations in stratospheric aerosol optical depth. We attribute the second problem to deficiencies in radiative transfer modeling of water vapor continuum absorption and show that use of an updated parameterization reduces bias from wet atmospheres. Applying the new retrieval coefficients to ATSR data, we find good consistency between SSTs retrieved with and without the 3.7 μm channels, the global mean and standard deviation of differences between retrievals being of the order of 0.05 K and 0.25 K, respectively. We therefore anticipate that reprocessing ATSR data using our new retrieval scheme will result in a substantially improved record of ATSR SST, in that the following should be reduced to insignificant levels: (1) the artefactual trend (previously -0.25 K yr -1 in tropical regions) corresponding to the decaying load of post-Pinatubo aerosol, (2) the discontinuity in SST retrievals (previously up to 0.7 K) associated with the failure of the 3.7 μm channel, and (3) cold biases (previously ∼0.4 K) in wet tropical regions. Thus this work represents a significant advance in terms of the quality of ATSR SSTs for climate research. The techniques are also applicable to both the ATSR-2, flying on ERS-2, and the advanced ATSR (planned for launch on the Envisat platform in 2000). However, we note that even with the improved physical modeling on which the new retrieval coefficients are based, we do not yet meet the stringent requirement of 0. K decade -1 stability in retrievals for climate change detection purposes.

Retrieval of Sea Surface Temperature from Space, Based on Modeling of Infrared Radiative Transfer: Capabilities and Limitations

Abstract The retrieval (estimation) of sea surface temperatures (SSTs) from space-based infrared observations is increasingly performed using retrieval coefficients derived from radiative transfer simulations of top-of-atmosphere brightness temperatures (BTs). Typically, an estimate of SST is formed from a weighted combination of BTs at a few wavelengths, plus an offset. This paper addresses two questions about the radiative transfer modeling approach to deriving these weighting and offset coefficients. How precisely specified do the coefficients need to be in order to obtain the required SST accuracy (e.g., scatter <0.3 K in week-average SST, bias <0.1 K)? And how precisely is it actually possible to specify them using current forward models? The conclusions are that weighting coefficients can be obtained with adequate precision, while the offset coefficient will often require an empirical adjustment of the order of a few tenths of a kelvin against validation data. Thus, a rational approach to defining r...

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.

Direct observations of skin‐bulk SST variability

Skin sea-surface temperatures from the first Along Track Scanning Radiometer (ATSR) are compared with coincident bulk temperatures from the Tropical Atmosphere Ocean (TAO) moored buoy array in the equatorial Pacific Ocean. The response of the skin-bulk sea-surface temperature difference (ΔT) to variations in wind speed and surface heat flux is examined. The use of remotely-sensed skin temperatures for this purpose is enabled by ATSRs unique design which permits the independent retrieval of ocean skin temperature to an accuracy of 0.3 K. For the four-year period considered (August 1991–August 1995), almost 6000 coincident skin and bulk sea surface temperature (SST) measurements were available; at night, the mean value of ΔT is −0.20±0.46K, with a daytime mean value of +0.05 ± 0.51 K. ΔT is found to depend on both net heat flux and local wind speed as predicted by the Saunders [1967] model and other formulations, and an estimate of the Saunders λ parameter is obtained.

Surface water temperature observations of large lakes by optimal estimation

Optimal estimation (OE) and probabilistic cloud screening were developed to provide lake surface water temperature (LSWT) estimates from the series of (advanced) along-track scanning radiometers (ATSRs). Variations in physical properties such as elevation, salinity, and atmospheric conditions are accounted for through the forward modelling of observed radiances. Therefore, the OE retrieval scheme developed is generic (i.e., applicable to all lakes). LSWTs were obtained for 258 of Earths largest lakes from ATSR-2 and AATSR imagery from 1995 to 2009. Comparison to in situ observations from several lakes yields satellite in situ differences of −0.2 ± 0.7 K for daytime and −0.1 ± 0.5 K for nighttime observations (mean ± standard deviation). This compares with −0.05 ± 0.8 K for daytime and −0.1 ± 0.9 K for nighttime observations for previous methods based on operational sea surface temperature algorithms. The new approach also increases coverage (reducing misclassification of clear sky as cloud) and exhibits greater consistency between retrievals using different channel–view combinations. Empirical orthogonal function (EOF) techniques were applied to the LSWT retrievals (which contain gaps due to cloud cover) to reconstruct spatially and temporally complete time series of LSWT. The new LSWT observations and the EOF-based reconstructions offer benefits to numerical weather prediction, lake model validation, and improve our knowledge of the climatology of lakes globally. Both observations and reconstructions are publically available from http://hdl.handle.net/10283/88.

Exposures to power-frequency magnetic fields in the home

Power-frequency magnetic fields in homes come from a variety of sources, internal (appliances and domestic wiring) and external (electricity distribution and transmission circuits). The authors present results from a survey of the fields encountered at home by 258 adults over one week each. Information on the major electrical features of each of the homes was collected and related to the exposures incurred. The strongest identified factor influencing exposure at home was the presence or absence of overhead lines at voltages of 132 kV or above within 100 m of the home (geometric-mean TWA field encountered by participants 208 nT near lines, 54 nT not near lines). Occupants of homes near overhead lines or supplies from 415 V to 66 kV did not on average encounter fields significantly different to those in homes without such lines (50 and 54 nT, respectively). Occupants of flats incurred greater exposures than those incurred by occupants of semi-detached and terraced houses, which were in turn greater than those incurred by occupants of detached houses (109, 60, 56 and 43 nT, respectively).

NOAA's sea surface temperature products from operational geostationary satellites

Abstract NOAAs National Environmental Satellite, Data, and Information Service (NESDIS) has generated sea surface temperature (SST) products from Geostationary Operational Environmental Satellite (GOES)-East (E) and GOES-West (W) on an operational basis since December 2000. Since that time, a process of continual development has produced steady improvements in product accuracy. Recent improvements extended the capability to permit generation of operational SST retrievals from the Japanese Multifunction Transport Satellite (MTSAT)-1R and the European Meteosat Second Generation (MSG) satellite, thereby extending spatial coverage. The four geostationary satellites (at longitudes of 75°W, 135°W, 140°E, and 0°) provide high temporal SST retrievals for most of the tropics and midlatitudes, with the exception of a region between ∼60° and ∼80°E. Because of ongoing development, the quality of these retrievals now approaches that of SST products from the polar-orbiting Advanced Very High Resolution Radiometer (AVH...