5.6 Dierent starting regions grid
Analysis previous to this section has seen InTEM using a solution grid created using the county-based region grid (Figure 5.1). A listed description of all three grids used in InTEM is shown below. This section explores whether an alternative starting `regions grid' would have an eect on the resulting emission estimates.
NAME-output grid: This grid is the spatial resolution of the model output from the NAME model. The resolution and dimensions is also the same in the dilution matrix (Section 3.3.2). This is a regularly spaced grid of 1.5 x 1.5 km resolution.
Regional grid or starting regions grid: This grid sees the NAME-output grid boundary limits and resolution divided into irregular and arbitrarily de ned regions. An example of this is Figure 5.1, which shows regions based loosely on the UK county boundaries.
Solution grid: This grid also has the same boundary limits as the previous two grids, however it has a variable spatial resolution which ts around the regional grid and whose individual grid cells will never be ner than the NAME-output grid resolution. The solution grid is created using the methods described in Section 5.4 and Section 5.5. The solution grid resolution and arrangement of grid cells will be dierent if a dierent regional grid is used. NB: Post analysis of InTEM results (produced on the solution grid resolution) are sometimes aggregated back to the regional grid resolution to compare dierent model setups. This is because statistical data for each individual region is outputted with InTEM (as described at the beginning of this chapter on page 107).
This section assesses if the resulting methane emission estimates dier if an alternative regional grid is used to create the solutions grid. Figure 5.7 shows the 2012 NAEI on a more regularly de ned regional grid, as opposed to the county-based regions shown in Figure 5.1. Results from this section show analysis using both the NAME-based and the NAEI-based methods of producing solution grids (explained in Section 5.4), based on this new region format. These runs used a 150 solution grid box resolution.
Chapter 5 Development of the inversion approach
Figure 5.7: 2012 NAEI regridded to alternative region domains. Regions are more arbitrary. Two thick border regions exist (15 and 16) to remove excess emissions from baseline errors (Section 5.10). The inner regions are more regular shaped than the county boundaries region grid in Figure 5.1. The only region to appear in both grids tested is number 14. Measurement site locations shown as black points.
The resulting emission maps for both solution grid methods are shown in Figure 5.8 and the regional totals, along with the equivalent emissions of the NAEI, can be found in Table 5.3.
5.6 Dierent starting regions grid
A) −0.5 0.0 0.5 1.0 1.5 2.0
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TN WY TY
0 24.1 42.8 76 135.2 240.3 427.1 759.4 1350 2400
Max Value = 1287.939 ng s−1 m−2 Total Emissions = 547 kt yr−1
B) −0.5 0.0 0.5 1.0 1.5 2.0
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0 24.1 42.8 76 135.2 240.3 427.1 759.4 1350 2400
Max Value = 28025.17 ng s−1 m−2 Total Emissions = 486 kt yr−1
C) −0.5 0.0 0.5 1.0 1.5 2.0
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0 24.1 42.8 76 135.2 240.3 427.1 759.4 1350 2400
Max Value = 1286.235 ng s−1 m−2 Total Emissions = 571 kt yr−1
D) −0.5 0.0 0.5 1.0 1.5 2.0
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0 24.1 42.8 76 135.2 240.3 427.1 759.4 1350 2400
Max Value = 3803.819 ng s−1 m−2 Total Emissions = 465 kt yr−1
Figure 5.8: InTEM emission elds based on the alternative region structure showing emissions generated using A) NAEI-based solution grid ( ne spatial resolution) B) NAEI-based solution grid (coarse spatial resolution) C) NAME-based solution grid ( ne spatial resolution) D) NAME-based solution grid (coarse spatial resolution).
The major dierences and similarities between the NAME-based and NAEI-based emission maps formed from the alternative starting regional grid are summarised below.
Both solution grid methods produce similar emission distributions in the East Anglia area. This is observed within the county boundaries regional grid as well.
Most regions overlap with each other's standard deviations apart from three:
regions 4, 8 and 11.
The NAEI-based solution grid method shows a ne point source to the south east of Weybourne in region 4, which is not resolved in the NAME
Chapter 5 Development of the inversion approach
method. The result shows the NAME method emissions are more spread out over this region and over the nearby coast which underestimates region 4 with respect to the 2012 NAEI.
The NAEI-based grid emits more in region 8 (east of the Tacolneston site) resulting in a larger emission total for this region than both the 2012 NAEI and the NAME-based method. There is no obvious reason for this discrepancy between the two methods. The NAEI-based solution grid in the county boundaries regional grid also puts larger emissions to the east of Tacolneston than the NAME solution grid method.
Both solution grid methods produce lower estimates for regions 10 and 11 compared to the 2012 NAEI. The NAEI-based solution grid method puts almost all emissions in region 11, whereas the NAME-based technique has a more homogenous distribution, making both regions lower than the 2012 NAEI. This underestimation is also present in the nely resolved county based regions, although the overall county estimate is increased by including emissions from the south of region 5 (Figure 5.1).
London emissions (region 12) are slightly overestimated compared to the 2012 NAEI but the values still overlap within one standard deviation. This could be a further example of emissions being polarised in regions where they are too constrained.
This analysis shows that the starting regional grid can have an impact on the resulting solution grid. Taking the example of the point source to the south east of Weybourne, this point source is only resolved in the 16-box (the alternative, more arbitrarily de ned) regional grid using the NAEI-based solution grid method (Figure 5.8.C). This is not seen in either of the solution grids formed from the county-boundaries regional grid. This is because of how the regions are subdivided. A region is only divided if it reaches the de ned dilution threshold.
For the NAME-based solution grid this threshold is in the same units at the dilution matrix (s m-1). Areas nearer to the observation site will reach this threshold many times over and thus create nely resolved regions while areas where little information is known will remain more coarse. The NAEI-based method incorporates the 2012 NAEI as a multiplying factor to the dilution threshold to determine the solution grid resolution. Once this threshold is reached the region is split into two equally contributing subregions. This will dier depending on the area of the starting regional grid and thus can result in diering solution grids.