Thursday 28 June 2018

Sensitivity Analyses of Cape Town Atmospheric Inversion

The second part of the Cape Town inversion work has been accepted for review in ACP Discussions. Several components of the inversion were tested, which included alternative prior information products for fossil fuel fluxes (ODIAC) and biospheric fluxes, increasing and decreasing uncertainty estimates for both the observations and the prior fluxes, and removing error correlations. Those sensitivity tests which altered the uncertainty and correlations specified for the prior NEE fluxes had the largest impact on the inversion flux results, in terms of both the posterior flux estimates, their uncertainties, and the spatial distribution of these fluxes.
We also considered alternative control vectors. This dramatically reduced the computational time of the inversion. When solving for a mean weekly flux compared to individual weekly fluxes, the uncertainty reduction was substantially greater. Therefore disaggregating the fluxes into slow and fast components may be a way of capitalising on the observations to achieve greater uncertainty reductions.
By combining the information regarding the posterior flux estimates and their uncertainties from inversions with different prior information, the estimate of the posterior flux could be further refined.

Time Series Plot of Weekly Fluxes, Prior in Black with 95% Confidence Limits in Grey, and Posterior Estimates in Green. See Nickless et al. 2018


Percentage Uncertainty Reduction in CO2 Flux Estimates Using Different Control Vectors. 
The reference inversion solved for separate weekly fluxes within each month. See Nickless et al. 2018

Brief Summary of Cape Town CO2 Inversion Work

Earlier in the year the publication came where we presented the first attempt at using Bayesian Inverse Modelling to estimate CO2 flux estimates for the Cape Town metropolitan area (Cape Town ACP Paper). We obtained initial estimates of anthropogenic emissions from a bespoke fossil fuel emission inventory, and estimates of the natural fluxes from the CABLE land atmosphere exchange model. We were able to substantially reduce the uncertainty around the posterior flux estimates for many of the regions in Cape Town, specifically percentage uncertainty reduction was highest over the Table Mountain National Park area and the city area adjacent to this natural area. For these regions we had percentage uncertainty reduction between 60 and 90%. The majority of the Cape Town city area had an uncertainty reduction of about 20 to 40%. 

Although we achieve relatively large percentage uncertainty reduction in our flux estimates, because a large uncertainty was assigned to the natural fluxes (estimated from the Net Primary Productivity), the posterior uncertainty limits were still large in absolute terms; certainly too wide for the estimates to be useful for monitoring, reporting and verification. In a second paper which came out recently in ACP Discussions on the sensitivity analyses for this inversion, we demonstrate some of the ways that improved estimates could be obtained, and highlight the need for improving estimates of Net Ecosystem Exchange and its uncertainty.

Our first Cape Town inversion paper is summarised in a recent mini article in the Clean Air Journal: Estimates of CO2 fluxes for Cape Town


Percentage Uncertainty Reduction in CO2 flux estimates for May 2012

Monday 9 April 2018

Obtaining CO2 fluxes for the City of Cape Town through Inverse Modelling

Well it's been a long time in the making, but I have finally published the results for obtaining CO2 flux estimates for the city of Cape Town! I will tell you more about this is due course!
https://www.atmos-chem-phys.net/18/4765/2018/

Robben Island Lighthouse - Where much of the action happened!