Our paper on data assimilation (DA) for Mars has just been published and this week we’re also speaking about it at the American Geophysical Union meeting in San Francisco. DA is a method for ‘fusing’ an atmospheric / climate model with data. If the model works well, DA allows the global extrapolation of observations that have incomplete or irregular coverage. If the model works poorly, DA provides a very efficient means of figuring out what’s wrong with the model and a framework for testing improvements. DA is widely used for Earth science for everything from recent climate reconstruction to weather forecasting. It’s only very rarely been used for other planets, though, because of the difficulty of developing and maintaining the DA capability (the software) and the computational demands. The DART DA system for Mars changes all of this by augmenting an open-source NCAR-developed DA system for planetary use. As a teaser, here’s an incomprehensible but colourful figure from the paper 
Data Assimilation (DA) is an enabling piece of your daily weather forecast. In this case, observations are used to drive a skillful (meaning it gets predictions of the current climate right) model of Earth’s atmosphere towards simulation of a specific day. What does this mean? A skillful model of Earth’s climate will get things like the seasonal temperature cycles at a location right, and will even get things like the ‘storminess’ of a given location right, but if you look at output for “August” from the model, what you get is a typical August, not August 2011. And you’re vanishingly unlikely to get the atmospheric state exactly as it was on 21 August 2011. For climate or atmospheric science, this doesn’t matter. But the way you get the weather forecast for 22 August 2011 is to take a global atmospheric state representing 21 August 2011 and integrate it forward by a day. In that case, you really need the specific global state in the model to represent 21 August 2011. The way this is done is by the ‘ingestion’ of data into the model with DA – basically the DA system pushes the model towards observations. The biggest errors for the ‘typical’ vs. ‘specific’ August day tend to be associated with the ‘phase’ of weather systems (is the eastward-moving low pressure storm system centered east of the UK over the Atlantic at this time or is it already over Sweden?) DA allows the model to “lock up” on the real timing and distribution of weather events for a real day.
Why do we care about this for Mars? We’re not really interested in weather forecasting for Mars. But we are interested in being able to find the global atmospheric state that corresponds to sparse and irregular spacecraft observations of Mars on specific days. Sparse and irregular data are difficult to handle and can cause errors in analysis if you’re not very careful. If this care has already gone in to globally extrapolating the spacecraft data to a nice, regular model grid, the data can be much more widely and easily used. This is the idea behind the creation of “reanalysis” data for Mars (reanalysis is another hold over from Earth weather forecasting terminology).
There is a lot more to be said about DA for Mars – we’ll have more blog posts on it in the future. For now, the DA system can be accessed via Ashima Research’s www.marsclimatecenter.com website.