Earth Observation Data Group

Abstract:

The most widely used satellite products are averages of data onto a regular spatiotemporal grid, known as Level 3 data. Some atmospheric variables can vary rapidly in response to changing conditions. Over the scales of Level 3 averaging, the combination of observations across different conditions may result in data that is not normally distributed, such that a simple mean is not representative. The problem is illustrated by the distribution of aerosol optical depth from different sensors and algorithms. A simple statistical technique is proposed to better convey the diversity of satellite observations to users whereby a multimodal log-normal distribution is fit to the distribution of data observed within each grid cell. Allowing multiple modes within each cell is shown to improve the agreement between satellite products by highlighting regions of significant variability and isolating systematic differences between instruments.

Abstract:

Radiative transfer models used in remote sensing and hazard assessment of volcanic ash require knowledge of ash optical parameters. Here, we characterise the bulk and glass compositions of a representative suite of volcanic ash samples with known complex refractive indices (n + ik: where n is the real and k is the imaginary part). Using a linear regression model, we develop a new parameterization allowing the complex refractive index of volcanic ash to be estimated from ash SiO2 content or ratio of non-bridging oxygens to tetrahedrally-coordinated cations (NBO/T). At visible wavelengths, n correlates better with bulk than glass composition (both SiO2 and NBO/T), and k correlates better with SiO2 content than NBO/T. Over a broader spectral range (0.4–19 μm), bulk correlates better than glass composition, and NBO/T generally correlates better than SiO2 content for both parts of the refractive index. In order to understand the impacts of our new parameterization on satellite retrievals, we compared IASI satellite (wavelengths 3.62–15.5 μm) mass loading retrievals using our new approach with retrievals that assumed a generic (Eyjafjallajökull) ash refractive index. There are significant differences in mass loading using our calculated indices specific to ash type rather than a generic index. Where mass loadings increase, there is often improvement in retrieval quality (corresponding to cost function decrease). This new parameterization of refractive index variation with ash composition will help to improve remote sensing retrievals for the rapid identification of ash and quantitative analysis of mass loadings from satellite data on operational timescales.

Abstract:

A very large ensemble is used to identify subgrid-scale parameter settings for the HadCM3 model that are capable of best simulating the ocean state over the recent past (1980–2010). A simple particle filtering technique based upon the agreement of basin mean sea surface temperature (SST) and upper 700-m ocean heat content with EN3 observations is applied to an existing perturbed physics ensemble with initial conditions perturbations. A single set of subgrid-scale parameter values was identified from the wide range of initial parameter sets that gave the best agreement with ocean observations for the period studied. The parameter set, different from the standard model parameters, has a transient climate response of 1.68 K. The selected parameter set shows an improved agreement with EN3 decadal-mean SST patterns and the Atlantic meridional overturning circulation (AMOC) at 26°N as measured by the Rapid Climate Change (RAPID) array. Particle filtering techniques as demonstrated here could have a useful role in improving the starting point for traditional model-tuning exercises in coupled climate models.