Friday, 11 April 2014

Collaborative agreement with JAXA

Within their 5th Research Announcement of opportunity for the Global Change Observation Mission, JAXA have accepted an SST CCI proposal "SST Algorithms for AMSR-2 Intended for Use in long-term Climate Data Records". We look forward to this collaboration.

The collaborative agreement is being finalised. The objectives are:

1.     To carry out development and testing of optimal estimation to generate SSTs from passive microwave satellite instruments using optimal estimation retrievals.
2. To ensure that the optimal estimation of SSTs from the microwave measurements is done in a way consistent with infrared methods.

Friday, 4 April 2014

Nadir - Forward Shifts in ATSR Multi-mission Level 1b archive v2.1

This summarises some results from Owen Embury looking at whether, with the latest ATSR multi-mission archive, there is still a need (as in Phase 1) to shift the forward view to better line up with the nadir view for the ATSRs. We always knew that our empirical shifts in Phase 1 were only approximate and hoped that the v2.1 archive would have resolved the issue completely. Bottom line: it is an improvement, but not perfect.

Here is a scene with the v2.0 nadir-forward collocation. Note that in the Nad-Fwd differences there are red and blue (warm and cold) differences along SST fronts (wavy lines) in the thermal (middle panel) and the island have a bright edge and dark shadow. Both these effects imply the two views are not perfectly collocated.

Here is the equivalent from v2.1. The effects are much less, but the opposite effects can just be discerned.

Here is the same scene with a shift of a pixel. The fronts are less obvious and the shadowing of the land is not really much different. For SST this is suggests an improvement.

Owen did a further analysis, minimising nadir-forward variances across many small scene extracts, to see if the offsetting effect is constant in time. It appears not to be, as per the example below:

At the beginning of the AATSR mission, the best shift of the forward view is -1 pixel along track and 0 across track on the left side of the swath -- but from 2007, it appears better to shift across track by +1. For other sensors, the equivalent curves suggest the instrument geometries are even less stable.

However, the good news is that pretty much all of the required shifts are no greater than 1 pixel, which is an improvement over v2.0, where shifts up to 3 pixels were required.

Wednesday, 2 April 2014

Datasets from SST CCI and their DOIs

The datasets generated to date by ESA's Climate Change Initiative project for Sea Surface Temperature  are available from the Centre for Environmental Data Archival via the page Possibly it is most useful to go to the dataset pages using the DOIs of the datasets:

For SSTs derived only from the Advanced Very High Resolution Radiometers:

For SSTs derived only from the Along-Track Scanning Radiometers:
(This points to v1.0.  V1.1, addressing a few problems in v1.0, will be coming along soon.)

For a gap-filled, daily blend ("SST CCI analysis") of the above datasets (with good feature resolution):

To get data, you need to register just your e-mail address with the data centre. If you try to access data without logging in, you get a page that says "access to the dataset is restricted". This is misleading: there is no restriction on obtaining SST CCI data, you simply need to be logged in to the data centre.

Below is an example day from the SST CCI analysis. This image represents the sea surface temperature (SST) across the global oceans on 5th January, 2010. It is made by blending satellite-derived estimates of SST and filling gaps by an optimal interpolation -- a process referred to as 'analysis'. This SST analysis was created using data from the Along-Track Scanning Radiometers (ATSR) and Advanced Very High Resolution Radiometers (AVHRR), analysed using the Met Office OSTIA system. 

The particular value of the SST CCI dataset is that it has relatively high feature resolution, refers to a well-defined type of SST (temperature at 20 cm depth, daily average) and, unlike most satellite SSTs, is independent of in situ observations. The full dataset covers 1991 to 2010 and was created in Phase 1 of ESA's SST Climate Change Initiative (SST CCI). 

Tuesday, 1 April 2014

What is a sustainable system?

The project has two aims. The most obvious is to produce "climate data records" for sea surface temperature. SST is an "essential climate variable", which means that to understand and track climate variability and change, we need high quality records of how SST changes over time. With sufficient remote-sensing know-how, this can be achieved using infra-red imagery from Earth observing satellites.

The project has a limited lifetime, but the need for climate data records won't disappear. Therefore we have a second aim: in the process of delivering data, we will build a software system that can then be sustained to provide data in future. We don't know how that will be funded, at present, but nonetheless over the next 3 years the project will develop its software to a stage of being, in the jargon, "pre-operational".

 The system won't be able simply to be run untended, even in an sustained (or "operational") mode, without maintenance of the science, and cyclic improvement to our techniques. This is because satellites come and go, and to get new observations to the standard required for inclusion in the SST CDR requirements ongoing Earth observation development work. Moreover, overtime, we discover how to make CDRs better, and therefore should reprocess the whole dataset consistently in the improved manner.

Thus, the system includes software and human experts, in a cycle of sustained production of data (as the observations come in) and periodic reprocessing. Conceptually it is like this: